1. A tractor comprising: a three-point link mechanism connected to a rear portion of a vehicle body so as to be able to pivot up and down; a lifting drive unit of a hydraulic type that drives a tractor-drawn tillage machine attached to the three-point link mechanism to raise and lower the tillage machine together with the three-point link mechanism; and a mechanical linkage unit for automatic lifting that converts an amount of change in a traction load to a lifting operation amount and transmits the lifting operation amount to the lifting drive unit, wherein the mechanical linkage unit comprises: a change amount converting mechanism that has a load detection member that pivots back and forth in accordance with the traction load transmitted via a top link of the three-point link mechanism; and a link mechanism that connects the change amount converting mechanism and the lifting drive unit in an interlocking manner, and the change amount converting mechanism has a pivot member that is pivotably supported by the load detection member and is connected to the lifting drive unit via the link mechanism in an interlocking manner, and is configured to amplify, by means of relative pivoting of the load detection member and the pivot member, an amount of change in the traction load obtained by back-and-forth pivoting of the load detection member, and thereafter converts the amplified amount of change in the traction load to the lifting operation amount.

2. The tractor according to claim 1, wherein the change amount converting mechanism is switchable between a first converting state of converting the amount of change in the traction load to the lifting operation amount without amplifying the amount of change in the traction load, and a second converting state of amplifying the amount of change in the traction load and thereafter converting the amplified amount of change in the traction load to the lifting operation amount, when the change amount converting mechanism is in the first converting state, the amount of change in the traction load is transmitted, without being amplified, as the lifting operation amount to the lifting drive unit, by means of the load detection member and the pivot member integrally pivoting, and when the change amount converting mechanism is in the second converting state, the amount of change in the traction load is amplified, and the amplified amount of change in the traction load is transmitted as the lifting operation amount to the lifting drive unit, by means of the load detection member and the pivot member relatively pivoting.

3. The tractor according to claim 2, wherein the mechanical linkage unit comprises an operation tool for switching the change amount converting mechanism between the first converting state and the second converting state.

4. The tractor according to claim 3, wherein the change amount converting mechanism comprises a holding mechanism that biases the pivot member to a predetermined orientation relative to the load detection member, the operation tool comprises a receiving member that moves between a non-receiving position and a receiving position, when the receiving member is located at the non-receiving position, the load detection member and the pivot member integrally pivot due to a holding effect of the holding mechanism as a result of the receiving member being out of a pivoting area of the pivot member and not receiving the pivoting member, and when the receiving member is located at the receiving position, the load detection member and the pivot member are allowed to relatively pivot against the holding effect of the holding member as a result of the receiving member being located in the pivoting area of the pivot member and receiving the pivoting member.

5. The tractor according to claim 4, wherein, when the receiving member is located at the receiving position, the load detection member and the pivot member integrally pivot due to the holding effect of the holding mechanism while the pivot member is not received by the receiving member, and the load detection member and the pivot member are allowed to relatively pivot against the holding effect of the holding mechanism while the pivot member is received by the receiving member.

6. The tractor according to claim 4 or 5, wherein a roller capable of rotating around an axis parallel to a pivot axis of the pivot member is provided at a portion of the receiving member at which the pivot member is received.

7. The tractor according to any one of claims 4 to 6, wherein the holding mechanism comprises a stopper that restricts the pivot member from pivoting in a direction in which the three-point link mechanism is lowered, relative to the load detection member, and a spring that pivots and biases the pivot member toward the stopper.

8. The tractor according to any one of claims 3 to 7, wherein the operation tool is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body.

9. The tractor according to any one of claims 1 to 8, wherein the change amount converting mechanism is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body.

10. A tractor capable of operating with a tillage machine attached thereto, comprising: a three-point link mechanism to which the tillage machine can be attached, the three-point link mechanism being connected to a rear portion of a vehicle body so as to be able to pivot up and down; a lifting drive unit of a hydraulic type that drives the three-point link mechanism to raise and lower the three-point link mechanism; and a mechanical linkage unit for automatic lifting that selectively converts one of an amount of change in a traction load and an amount of change in a tillage depth to a lifting operation amount and transmits the lifting operation amount to the lifting drive unit, wherein the mechanical linkage unit comprises: a change amount converting mechanism capable of switching between a first linkage converting state and a second linkage converting state, and a link mechanism that connects the change amount converting mechanism and the lifting drive unit in an interlocking manner, the change amount converting mechanism has a load detection member that pivots back and forth in accordance with the traction load transmitted via a top link of the three-point link mechanism, and a pivot member that pivots in accordance with the tillage depth of the tillage machine, if the change amount converting mechanism is switched to the first linkage converting state, the change amount converting mechanism can convert the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member to the lifting operation amount, and if the change amount converting mechanism is switched to the second linkage converting state, the change amount converting mechanism can convert the amount of change in the tillage depth obtained by pivoting of the pivot member to the lifting operation amount.

11. The tractor according to claim 10, wherein the mechanical linkage unit comprises an operation tool for switching the change amount converting mechanism between the first linkage converting state and the second linkage converting state.

12. The tractor according to claim 11, wherein the pivot member is pivotably supported by the load detection member, the operation tool comprises a contact member that moves between a contact position at which the contact member comes into contact with the load detection member to inhibit the load detection member from pivoting back and forth, and a non-contact position at which the contact member does not come into contact with the load detection member and allows the load detection member to pivot back and forth, when the contact member is at the non-contact position, the amount of change in the traction load is transmitted as the lifting operation amount to the lifting drive unit by mean of the load detection member pivoting back and forth, and when the contact member is at the contact position, the amount of change in the tillage depth is transmitted as the lifting operation amount to the lifting drive unit by means of the pivot member pivoting independently in a state where the load detection member does not pivot back and forth.

13. The tractor according to claim 12, wherein a first converting state and a second converting state are provided as the first linkage converting state, the change amount converting mechanism comprises a holding mechanism that can be switched between the first converting state and the second converting state by operating the operation tool, and biases the pivot member to a predetermined orientation relative to the load detection member, when the change amount converting mechanism is in the first converting state, the load detection member and the pivot member integrally pivot due to a holding effect of the holding mechanism, and the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member is transmitted, without being amplified, as the lifting operation amount to the lifting drive unit, and when the change amount converting mechanism is in the second converting state, the load detection member and the pivot member are allowed to relatively pivot against the holding effect of the holding mechanism, and the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member is amplified, and the amplified amount of change in the traction load is transmitted as the lifting operation amount to the lifting drive unit.

14. The tractor according to claim 13, wherein the holding mechanism comprises a stopper that restricts the pivot member from pivoting in a direction in which the three-point link mechanism is lowered, relative to the load detection member, and a spring that pivots and biases the pivot member toward the stopper.

15. The tractor according to any one of claims 11 to 14, wherein the operation tool is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body.

16. The tractor according to any one of claims 10 to 15, wherein the change amount converting mechanism is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body.

17. The tractor according to any one of claims 10 to 16, wherein the load detection member comprises a first connecting portion to which a long top link is connected, and a second connecting portion to which a bracket for supporting a short top link is connected.

Technical Field
[0001] The present invention relates to a tractor that includes a three-point link mechanism connected to a rear portion of a vehicle body so as to be able to pivot up and down, a lifting drive unit of a hydraulic type that drives a tractor-drawn tillage machine attached to the three-point link mechanism to raise and lower the tillage machine together with the three-point link mechanism, and a mechanical linkage unit for automatic lifting that converts an amount of change in a traction load to a lifting operation amount and transmits the lifting operation amount to the lifting drive unit.
Background Art [0002] [1]
Some tractors such as that described above include a draft adjusting lever for setting a tillage depth, a link mechanism (cylindrical shaft, arm etc.) for interlocking a spool of a control valve with the draft adjusting lever, a load detection member (top link hinge) that pivots back and forth in accordance with a traction load, a second link mechanism (draft feedback link mechanism) for interlocking the spool of the control valve with the load detection member, and so on, thereby being able to perform draft control to keep the traction load at a set value determined based on a tillage depth that is set using the draft adjusting lever (e.g. see Patent Document 1). [0003] [2]
Some tractors such as those described above include a mechanical linkage unit for draft control (draft feedback link mechanism) that converts an amount of change in a traction load to a lifting operation amount and transmits the lifting operation amount to a lifting drive unit (hydraulic lifting apparatus) during a tillage operation during which a plow, which is an example of a tractor-drawn tillage machine, is attached to a three-point link mechanism (e.g. see Patent Document 1).
[0004] Also, some tractors such as those described above include a mechanical linkage unit for automatic tillage depth control (pivot link, interlocking link, sensor wire) that converts an amount of change in a tillage

depth that is obtained as a result of a rear cover, which is provided on a rotary tillage machine so as to be able to pivot up and down, pivoting and being displaced to a lifting operation amount, and transmits the lifting operation amount to a lifting drive unit (control hydraulic circuit), during a tillage operation during which a rotary tillage machine, which is an example of a tractor-drawn tillage machine, is attached to a three-point link mechanism (e.g. see Patent Document 2).
[0005] The tractor described in Patent Document 1, which includes a mechanical linkage unit for draft control, can perform draft control to allow the plow to be automatically raised and lowered in accordance with the traction load when a tillage operation is carried out using the plow. Thus, in this tractor, the traction load can be kept constant, and stalling of the engine due to an increase in the traction load can be avoided.
[0006] The tractor described in Patent Document 2, which includes a mechanical linkage unit for automatic tillage depth control, can perform automatic tillage depth control to enable the rotary tillage machine to be automatically raised and lowered in synchronization with up-down pivoting of the rear cover according to the tillage depth, when a tillage operation is carried out using the rotary tillage machine. Thus, this tractor realizes an accurate tillage operation in which the tillage depth is kept constant.
Prior Art Documents Patent Documents
[0007] Patent Document 1: JP 2006-109802A Patent Document 2: JP 2005-198665A
Disclosure of the Invention
Problem to be Solved by the Invention
[0008] [1]
A problem corresponding to the background art [1] is as follows.
The tractor described in Patent Document 1 raises and lowers a tillage machine, such as a plow or a subsoiler, at a standard speed in accordance with the traction load, regardless of, for example, a case of carrying out a tillage operation with draft control in a standard field in which the traction load is unlikely to increase or drastically change due to the hardness or the like of the soil, and a case of carrying out a tillage operation with draft control in a field

in which the traction load is likely to increase and drastically change due to the hardness or the like of the soil.
Thus, even if the traction load suddenly increases, the tillage machine is raised at a standard speed based on an increase in the traction load.
For this reason, if the traction load suddenly increases, the increase in the traction load cannot be favorably suppressed, and there is a concern that the engine will stall due to the increase in the traction load. [0009] That is to say, it is desired to be able to avoid stalling of the engine due to an increase in the traction load even if the traction load suddenly increases. [0010] [2]
A problem corresponding to the background art [2] is as follows. [0011] The tractor described in Patent Document 1, which does not include a mechanical linkage unit for automatic tillage depth control, cannot perform the aforementioned automatic tillage depth control when a tillage operation is carried out using the rotary tillage machine. Thus it is difficult to carry out an accurate tillage operation while the tillage depth is kept constant. [0012] The tractor described in Patent Document 2, which does not include a mechanical linkage unit for draft control, cannot perform the aforementioned draft control when a tillage operation is carried out using a plow. For this reason, there is a concern that the engine will stall due to an increase in the traction load.
[0013] That is to say, there is a demand for the development of a mechanical linkage unit that enables draft control, which is suitable for a tillage operation using a plow or the like, and automatic tillage depth control, which is suitable for a tillage operation using a rotary tillage machine.
Means for Solving Problem [0014] [1]
A solving means corresponding to the problem [1] is as follows. [0015] A tractor according to the present invention includes:
a three-point link mechanism connected to a rear portion of a vehicle body so as to be able to pivot up and down; a lifting drive unit of a hydraulic type that drives a tractor-drawn tillage machine attached to the three-point link mechanism to raise and lower the tillage machine together with the three-point link mechanism; and a mechanical linkage unit for automatic lifting that converts an amount of change in a traction load to a lifting

operation amount and transmits the lifting operation amount to the lifting drive unit,
wherein the mechanical linkage unit includes: a change amount converting mechanism that has a load detection member that pivots back and forth in accordance with the traction load transmitted via a top link of the three-point link mechanism; and a link mechanism that connects the change amount converting mechanism and the lifting drive unit in an interlocking manner, and
the change amount converting mechanism has a pivot member that is pivotably supported by the load detection member and is connected to the lifting drive unit via the link mechanism in an interlocking manner, and is configured to amplify, by means of relative pivoting of the load detection member and the pivot member, an amount of change in the traction load obtained by back-and-forth pivoting of the load detection member, and thereafter converts the amplified amount of change in the traction load to the lifting operation amount.
[0016] With this means, if the traction load changes during a tillage operation during which a tractor-drawn tillage machine such as a plow or a subsoiler is attached to the three-point link mechanism, the amount of change in the traction load is amplified as a result of the load detection member pivoting back and forth in accordance with the amount of change in the traction load at this time, as well as the load detection member and the pivot member relatively pivoting. Then, the amplified amount of change in the traction load is converted to the lifting operation amount and is transmitted to the lifting drive unit.
With this configuration, the tractor-drawn tillage machine can be promptly raised and lowered in accordance with a change in the traction load. As a result, even if the traction load suddenly increases, stalling of the engine due to an increase in the traction load can be avoided.
[0017] In addition, as a result of the load detection member and the pivot member relatively pivoting, the amount of change in the traction load can be greatly amplified while narrowing the pivoting area of the load detection member and the pivot member. Thus, the length in the front-rear direction of the space needed to install a change amount converting mechanism that has the load detection member and the pivot member can be reduced.
As a result, stalling of the engine due to a sudden increase in the

traction load can be avoided while suppressing an increase in the size of the vehicle body that leads to an increase in the entire length of the vehicle body. [0018] As a means for making the present invention more preferable,
the change amount converting mechanism is switchable between a first converting state of converting the amount of change in the traction load to the lifting operation amount without amplifying the amount of change in the traction load, and a second converting state of amplifying the amount of change in the traction load and thereafter converting the amplified amount of change in the traction load to the lifting operation amount,
when the change amount converting mechanism is in the first converting state, the amount of change in the traction load is transmitted, without being amplified, as the lifting operation amount to the lifting drive unit, by means of the load detection member and the pivot member integrally pivoting, and
when the change amount converting mechanism is in the second converting state, the amount of change in the traction load is amplified, and the amplified amount of change in the traction load is transmitted as the lifting operation amount to the lifting drive unit, by means of the load detection member and the pivot member relatively pivoting. [0019] With this means, in the case of, for example, carrying out a tillage operation using a plow, a subsoiler, or the like in a standard field in which the traction load is unlikely to increase or drastically change due to the hardness or the like of the soil, if an operator switches the change amount converting mechanism to the first converting state, a tillage machine such as a plow or a subsoiler can be raised and lowered in accordance with the traction load at a standard speed suitable for this field. As a result, stalling of the engine due to an increase in the traction load can be avoided while performing draft control under which the tillage depth changes moderately.
Also, for example, in the case of carrying out a tillage operation using a plow, a subsoiler, or the like in a field in which the traction load is likely to increase and drastically change due to the hardness or the like of the soil, if the operator switches the change amount converting mechanism to the second converting state, a tillage machine such as a plow or a subsoiler can be raised and lowered in accordance with the traction load at a high speed suitable for the field. As a result, even if the traction load suddenly increases, stalling of the engine due to an increase in the traction load can be avoided.

[0020] As a means for making the present invention more preferable,
the mechanical linkage unit includes an operation tool for switching the change amount converting mechanism between the first converting state and the second converting state.
[0021] With this means, the operator can conveniently switch the change amount converting mechanism between the first converting state and the second converting state by operating the operation tool. [0022] As a means for making the present invention more preferable,
the change amount converting mechanism includes a holding mechanism that biases the pivot member to a predetermined orientation relative to the load detection member,
the operation tool includes a receiving member that moves between a non-receiving position and a receiving position,
when the receiving member is located at the non-receiving position, the load detection member and the pivot member integrally pivot due to a holding effect of the holding mechanism as a result of the receiving member being out of a pivoting area of the pivot member and not receiving the pivoting member, and
when the receiving member is located at the receiving position, the load detection member and the pivot member are allowed to relatively pivot against the holding effect of the holding member as a result of the receiving member being located in the pivoting area of the pivot member and receiving the pivoting member.
[0023] With this means having a configuration in which, simply, the holding mechanism is provided in the change amount converting mechanism, and the receiving member is provided in the operation tool, the first converting state where the load detection member and the pivot member integrally pivot in the change amount converting mechanism can be achieved when the receiving member is at the non-receiving position. Also, the second converting state where the load detection member and the pivot member are allowed to relatively pivot in the change amount converting mechanism can be achieved when the receiving member is at the receiving position.
As a result, the change amount converting mechanism can be reliably switched between the first converting state and the second converting state, while simplifying the configuration. [0024] As a means for making the present invention more preferable,

when the receiving member is located at the receiving position, the load detection member and the pivot member integrally pivot due to the holding effect of the holding mechanism while the pivot member is not received by the receiving member, and the load detection member and the pivot member are allowed to relatively pivot against the holding effect of the holding mechanism while the pivot member is received by the receiving member.
[0025] With this means, even if the receiving member is at the receiving position, the load detection member and the pivot member integrally pivot until the amount of pivoting displacement of the load detection member based on the traction load reaches a predetermined amount with which the pivot member is received by the receiving member, and accordingly, the amount of change in the traction load is transmitted, without being amplified, as the lifting operation amount to the lifting drive unit. Upon the amount of pivoting displacement of the load detection member reaching or exceeding the predetermined amount, the pivot member is received by the receiving member, and the load detection member and the pivot member relatively pivot. Accordingly, the amount of change in the traction load is transmitted, after being amplified, as the lifting operation amount to the lifting drive unit.
Thus, in an operational situation where the traction load only slightly exceeds a set value and there is no concern that the engine will stall, the lifting drive unit drives a tillage machine such as a plow or a subsoiler to raise and lower the tillage machine in accordance with the traction load at a standard speed suitable for the operational situation at the time, even if the change amount converting mechanism is in the second converting state.
In an operational situation where the traction load greatly exceeds the set value and the engine is more likely to stall, the lifting drive unit drives a tillage machine such as a plow or a subsoiler to raise and lower the tillage machine in accordance with the traction load at a high speed suitable for the operational situation at the time.
As a result, even in an operational situation where the traction load only slightly exceeds the set value, stalling of the engine due to a sudden increase in the traction load can be avoided while avoiding the possibility that the durability of the lifting drive unit will be degraded due to the lifting drive unit driving the tillage machine to raise and lower the tillage machine at an unnecessarily high speed.

[0026] As a means for making the present invention more preferable,
a roller capable of rotating around an axis parallel to a pivot axis of the pivot member is provided at a portion of the receiving member at which the pivot member is received.
[0027] With this means, when the change amount converting mechanism is in the second converting state, and the pivot member slides relative to the receiving member while the load detection member and the pivot member are relatively pivoting, the roller rotates, with the sliding of the pivot member, in the sliding direction.
Thus, the pivot member smoothly pivots relative to the receiving member, and the load detection member and the pivot member relatively pivot smoothly.
As a result, draft control can be smoothly performed when the change amount converting mechanism is in the second converting state. [0028] As a means for making the present invention more preferable,
the holding mechanism includes a stopper that restricts the pivot member from pivoting in a direction in which the three-point link mechanism is lowered, relative to the load detection member, and a spring that pivots and biases the pivot member toward the stopper.
[0029] With this means having a simple configuration in which only the stopper and the spring are provided, the load detection member and the pivot member can integrally pivot when the change amount converting mechanism is in the first linkage converting state, and the load detection member and the pivot member can relatively pivot when the change amount converting mechanism is in the second linkage converting state.
As a result, the change amount converting mechanism can be preferably switched between the first converting state and the second converting state, while simplifying the configuration. [0030] As a means for making the present invention more preferable,
the operation tool is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body.
[0031] With this means, the operator can operate the operation tool by stretching a hand toward the portion rearward of the operator seat while sitting on the operator seat, and can readily switch the converting state of the change amount converting mechanism. [0032] As a means for making the present invention more preferable,

the change amount converting mechanism is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body. [0033] With this means, the operator can readily check the operational state of the change amount converting mechanism by viewing the portion rearward of the operator seat while sitting on the operator seat.
In addition, since the portion rearward of the operator seat is not covered from above by a cover or the like, maintenance can be readily carried out on the change amount converting mechanism. [0034] [2]
A solving means corresponding to the problem [2] is as follows. [0035] A tractor according to the present invention is a tractor capable of operating with a tillage machine attached thereto, including:
a three-point link mechanism to which the tillage machine can be attached, the three-point link mechanism being connected to a rear portion of a vehicle body so as to be able to pivot up and down; a lifting drive unit of a hydraulic type that drives the three-point link mechanism to raise and lower the three-point link mechanism; and a mechanical linkage unit for automatic lifting that selectively converts one of an amount of change in a traction load and an amount of change in a tillage depth to a lifting operation amount and transmits the lifting operation amount to the lifting drive unit,
wherein the mechanical linkage unit includes: a change amount converting mechanism capable of switching between a first linkage converting state and a second linkage converting state, and a link mechanism that connects the change amount converting mechanism and the lifting drive unit in an interlocking manner,
the change amount converting mechanism has a load detection member that pivots back and forth in accordance with the traction load transmitted via a top link of the three-point link mechanism, and a pivot member that pivots in accordance with the tillage depth of the tillage machine,
if the change amount converting mechanism is switched to the first linkage converting state, the change amount converting mechanism can convert the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member to the lifting operation amount, and
if the change amount converting mechanism is switched to the second linkage converting state, the change amount converting mechanism can

convert the amount of change in the tillage depth obtained by pivoting of the pivot member to the lifting operation amount.
[0036] With this means, when a tillage operation using a plow, a subsoiler, or the like is carried out, the plow, the subsoiler, or the like is attached to the three-point link mechanism. Thus, the mechanical linkage unit enters a draft controlling state (first linking state). In this draft controlling state, if the operator switches the change amount converting mechanism to the first linkage converting state, draft control is performed during a tillage operation so that the plow, the subsoiler, or the like is automatically raised and lowered in accordance with the traction load. As a result, stalling of the engine due to an increase in the traction load can be avoided.
In the case of carrying out a tillage operation using a rotary tillage machine or the like, the mechanical linkage unit enters the automatic tillage depth controlling state (second linking state) as a result of the rotary tillage machine or the like being attached to the three-point link mechanism. In this automatic tillage depth controlling state, if the operator switches the change amount converting mechanism to the second linkage converting state, automatic tillage depth control is performed during a tillage operation so that the rotary tillage machine or the like is automatically raised and lowered in accordance with the tillage depth. As a result, an accurate tillage operation in which the tillage depth is kept constant can be carried out.
That is to say, since the mechanical linkage unit that has the above-described configuration is provided, the tractor can perform the above-described draft control when a tillage operation is carried out using a plow, a subsoiler, or the like. Also, the tractor can perform the above-described automatic tillage depth control while a tillage operation is being carried out using a rotary tillage machine or the like. [0037] As a means for making the present invention more preferable,
the mechanical linkage unit includes an operation tool for switching the change amount converting mechanism between the first linkage converting state and the second linkage converting state. [0038] With this means, the operator can conveniently switch the change amount converting mechanism between the first linkage converting state and the second linkage converting state by operating the operation tool. [0039] As a means for making the present invention more preferable,
the pivot member is pivotably supported by the load detection

member,
the operation tool comprises a contact member that moves between a contact position at which the contact member comes into contact with the load detection member to inhibit the load detection member from pivoting back and forth, and a non-contact position at which the contact member does not come into contact with the load detection member and allows the load detection member to pivot back and forth,
when the contact member is at the non-contact position, the amount of change in the traction load is transmitted as the lifting operation amount to the lifting drive unit by mean of the load detection member pivoting back and forth, and
when the contact member is at the contact position, the amount of change in the tillage depth is transmitted as the lifting operation amount to the lifting drive unit by means of the pivot member pivoting independently in a state where the load detection member does not pivot back and forth. [0040] With this means, the change amount converting mechanism enters the aforementioned first linkage converting state when the contact member is at the non-contact position. The change amount converting mechanism enters the aforementioned second linkage converting state when the contact member is at the contact position.
Thus, in the automatic tillage depth control during a tillage operation using a rotary tillage machine or the like, the load detection member does not pivot back and forth in accordance with the traction load transmitted via the top link. Accordingly, there is no concern that the amount of change in the traction load will be input to the change amount converting mechanism due to back-and-forth pivoting of the load detection member.
As a result, in the automatic tillage depth control based on the amount of change in the tillage depth, it is possible to avoid the possibility that operation accuracy will decrease due to the amount of change in the traction load being input to the change amount converting mechanism. [0041] As a means for making the present invention more preferable,
a first converting state and a second converting state are provided as the first linkage converting state,
the change amount converting mechanism includes a holding mechanism that can be switched between the first converting state and the second converting state by operating the operation tool, and biases the pivot

member to a predetermined orientation relative to the load detection member,
when the change amount converting mechanism is in the first converting state, the load detection member and the pivot member integrally pivot due to a holding effect of the holding mechanism, and the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member is transmitted, without being amplified, as the lifting operation amount to the lifting drive unit, and
when the change amount converting mechanism is in the second converting state, the load detection member and the pivot member are allowed to relatively pivot against the holding effect of the holding mechanism, and the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member is amplified, and the amplified amount of change in the traction load is transmitted as the lifting operation amount to the lifting drive unit.
[0042] With this means, in the case of, for example, carrying out a tillage operation using a plow, a subsoiler, or the like in a standard field in which the traction load is unlikely to increase or drastically change due to the hardness or the like of the soil, if the operator operates the operation tool to switch the change amount converting mechanism to the first converting state, the plow, the subsoiler, or the like can be raised and lowered in accordance with the traction load at a standard speed suitable for this field. As a result, stalling of the engine due to an increase in the traction load can be avoided while performing draft control under which the tillage depth changes moderately.
Also, for example, in the case of carrying out a tillage operation using a plow, a subsoiler, or the like in a field in which the traction load is likely to increase and drastically change due to the hardness or the like of the soil, if the operator operates the operation tool to switch the change amount converting mechanism to the second converting state, the plow, the subsoiler, or the like can be raised and lowered in accordance with the traction load at a high speed suitable for this field. As a result, stalling of the engine due to a sudden increase in the traction load can be avoided. [0043] As a means for making the present invention more preferable,
the holding mechanism includes a stopper that restricts the pivot member from pivoting in a direction in which the three-point link mechanism is lowered, relative to the load detection member, and a spring that pivots and

[0044] With this means having a simple configuration in which only the stopper and the spring are provided, the load detection member and the pivot member can integrally pivot when in the first linkage converting state. Also, the load detection member and the pivot member are allowed to relatively pivot when in the second linkage converting state.
As a result, the change amount converting mechanism can be preferably switched between the first converting state and the second converting state, while simplifying the configuration. [0045] As a means for making the present invention more preferable,
the operation tool is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body.
[0046] With this means, the operator can operate the operation tool by stretching a hand toward the portion rearward of the operator seat while sitting on the operator seat, and can readily switch the converting state of the change amount converting mechanism. [0047] As a means for making the present invention more preferable,
the change amount converting mechanism is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body. [0048] With this means, the operator can readily check the operational state of the change amount converting mechanism by viewing the portion rearward of the operator seat while sitting on the operator seat.
In addition, since the portion rearward of the operator seat is not covered from above by a cover or the like, maintenance can be readily carried out on the change amount converting mechanism. [0049] As a means for making the present invention more preferable,
the load detection member includes a first connecting portion to which a long top link is connected, and a second connecting portion to which a bracket for supporting a short top link is connected.
[0050] With this means, in the case of, for example, carrying out a tillage operation using a rotary tillage machine, the specifications of the three-point link mechanism can be readily changed between standard link specifications in which the long top link and the left and right lower links are provided, and special link specifications in which the short top link and the left and right lower links are provided.
If the specifications of the three-point link mechanism are changed from the standard link specifications to the special link specifications, the

lifting driving amount of the rotary tillage machine relative to the lifting operation amount of the lifting drive unit increases, and the highest raising position of the rotary tillage machine is set higher.
As a result, in the case of, for example, carrying out a tillage operation using a rotary tillage machine in a field with a high ridge, if the specifications of the three-point link mechanism are changed from the standard link specifications to the special link specifications, the rotary tillage machine can readily avoid the possibility of coming into contact with a high ridge during travel to pass over the ridge or while turning around near the ridge, for example.
Brief Description of the Drawings
[0051] FIG. 1 is a right side view of a tractor in which a plow is attached to a
three-point link mechanism, according to a first embodiment.
FIG. 2 is a right side view of a tractor in which a rotary tillage machine is attached to the three-point link mechanism, according to the first embodiment.
FIG. 3 is a vertical right side view of a main portion illustrating a configuration of a lifting drive unit and a mechanical linkage unit according to the first embodiment.
FIG. 4 is a plan view of a main portion illustrating a configuration of the lifting drive unit and the mechanical linkage unit according to the first embodiment.
FIG. 5 is an exploded perspective view of a main portion illustrating a configuration of the lifting drive unit and the mechanical linkage unit according to the first embodiment.
FIG. 6 is a vertical expanded right side view of a main portion illustrating an operational state of the lifting drive unit when a tillage machine is not raised or lowered with the position of a height setting lever kept on a low-position setting side.
FIG. 7 is a vertical expanded right side view of a main portion illustrating an operational state of the lifting drive unit when the tillage machine is being raised in synchronization with the height setting lever being operated to pivot in a high-position setting direction.
FIG. 8 is a vertical expanded right side view of a main portion illustrating an operational state of the lifting drive unit when the tillage

machine is not being raised and lowered with the position of the height setting lever kept on a high-position setting side.
FIG. 9 is an exploded perspective view of a main portion illustrating a configuration of a change amount converting mechanism and so on, according to the first embodiment.
FIG. 10 is a back view of a main portion illustrating a configuration of the change amount converting mechanism and so on, according to the first embodiment.
FIG. 11 is a cross-sectional view taken along a line XI-XI in FIG. 4.
FIG. 12 is a vertical right side view of a main portion illustrating an operational state of the mechanical linkage unit and the lifting drive unit when the change amount converting mechanism according to the first embodiment is in a first converting state and a traction load has not exceeded a set value.
FIG. 13 is a vertical right side view of a main portion illustrating an operational state of the mechanical linkage unit and the lifting drive unit when the change amount converting mechanism according to the first embodiment is in the first converting state and the traction load has exceeded the set value.
FIG. 14 is a vertical right side view of a main portion illustrating an operational state of the mechanical linkage unit and the lifting drive unit when the change amount converting mechanism according to the first embodiment is in a second converting state and the traction load has not exceeded the set value.
FIG. 15 is a vertical right side view of a main portion illustrating an operational state of the mechanical linkage unit and the lifting drive unit when the change amount converting mechanism according to the first embodiment is in the second converting state and the traction load has exceeded the set value.
FIG. 16 is a vertical right side view of a main portion illustrating an operational state of the mechanical linkage unit and the lifting drive unit when the change amount converting mechanism according to the first embodiment is in a second linkage converting state and the tillage depth has reached a set tillage depth.
FIG. 17 is a vertical right side view of a main portion illustrating an operational state of the mechanical linkage unit and the lifting drive unit

when the change amount converting mechanism according to the first embodiment is in the second linkage converting state and the tillage depth is greater than the set tillage depth.
FIG. 18 is a right side view of a tractor in which a plow is attached to a three-point link mechanism, according to a second embodiment.
FIG. 19 is a vertical right side view of a main portion showing a configuration of a lifting drive unit and a mechanical linkage unit according to the second embodiment.
FIG. 20 is a vertical right side view of a main portion illustrating an operational state of the mechanical linkage unit and the lifting drive unit when the traction load has not exceeded a set value, according to the second embodiment.
FIG. 21 is a vertical right side view of a main portion illustrating an operational state of the mechanical linkage unit and the lifting drive unit when the traction load has exceeded the set value, according to the second embodiment.
Best Mode for Carrying out the Invention
First Embodiment
[0052] The first embodiment, which is an example of a mode for carrying out
the present invention, will be described below based on the drawings.
Note that the direction indicated by an arrow F shown in FIG. 1 is a forward direction relative to a tractor, and the direction indicated by an arrow U is an upward direction relative to the tractor.
[0053] As shown in FIGS. 1 and 2, a tractor described as an example in the first embodiment includes a front frame 1 arranged in front portion of a vehicle body, an engine 2 connected to a rear portion of the front frame 1, a clutch housing 3 connected to a lower portion of a rear end of the engine 2, an intermediate frame 4 connected to a rear end portion of the clutch housing 3, a transmission case (hereinafter, “T/M case”) 5 that is connected to a rear end portion of the intermediate frame 4 and also serves as a rear frame, left and right front wheels 6 arranged on the left and right sides of the front frame 1, left and right rear wheels 7 arranged on the left and right sides of the T/M case 5, left and right rear fenders 8 that cover the left and right rear wheels 7, a passenger-type operation unit 9 arranged in a rear portion of the vehicle body, and so on.

[0054] Although not shown in the diagrams, power from the engine 2 is transmitted to a main transmission contained in the T/M case 5 via a main clutch contained in the clutch housing 3, a transmission shaft covered by the intermediate frame 4, and so on. Power output after the speed is changed by the main transmission is transmitted to the left and right front wheels 6 and the left and right rear wheels 7 via a sub-transmission contained in the T/M case 5, and so on.
[0055] As shown in FIGS. 1 and 2, the operation unit 9 includes a steering wheel 10 for steering the front wheels, an operator seat 11 arranged between the left and right rear fenders 8, and so on.
[0056] A three-point link mechanism 12 that enables attachment of a work machine is connected to a rear portion of the T/M case 5 so as to be able to pivot up and down. The three-point link mechanism 12 includes a single top link 13, left and right lower links 14, and so on. This configuration allows a tractor-drawn tillage machine 15, which is an example of the work machine, to be attached to the three-point link mechanism 12 when a tillage operation is carried out using this tractor.
[0057] Note that the first embodiment illustrates, as examples, a state where a plow 15A, which is an example of the tractor-drawn tillage machine 15, is attached to the three-point link mechanism 12 (see FIG. 1), and a state where a rotary tillage machine 15B, which is an example of the tractor-drawn tillage machine 15, is attached to the three-point link mechanism 12 (see FIG. 2). However, tractor-drawn tillage machines 15 such as a disk harrow, a cultivator, and a subsoiler can be attached to the three-point link mechanism 12.
[0058] As shown in FIGS. 1 to 8, the tractor includes a mechanical linkage-type hydraulic lifting apparatus 16. The hydraulic lifting apparatus 16 includes a hydraulic lifting drive unit 17 that drives the tillage machine 15 to raise and lower the tillage machine 15 together with the three-point link mechanism 12, and a mechanical linkage unit 18 for automatic lifting that selectively converts one of an amount of change in a traction load and an amount of change in a tillage depth to a lifting operation amount and transmits the lifting operation amount to the lifting drive unit 17. [0059] The lifting drive unit 17 includes left and right lifting arms 20 that suspend and support the left and right lower links 14 via left and right support members 19, a hydraulic cylinder 21 that drives the left and right

lifting arms 20 to pivot the left and right lifting arms 20 in an up-down direction, a control valve 22 for controlling operations of the hydraulic cylinder 21, a height setting lever 23 for setting a control target height of the tillage machine 15, a friction-type holding mechanism 24 that keeps the height setting lever 23 at an arbitrary operation position, a first link mechanism 25 that interlocks a spool 22A of the control valve 22 with the height setting lever 23, a feedback link mechanism 26 that interlocks the spool 22A with the left and right lifting arms 20, and so on. The control valve 22 contains a biasing means (not shown) for biasing and restoring the spool 22A to a lowering position on a vehicle body front side.
[0060] The first link mechanism 25 includes a first pivot arm 27 that comes into contact with the height setting lever 23 from a vehicle body rear side (high-position setting side of the height setting lever 23), a balance arm 28 that is supported by the spool 22A of the control valve 22 so as to be able to pivot back and forth, a first crankshaft 29 that spans the pivot center of the first pivot arm 27 and an upper end portion of the balance arm 28, and so on. Upon the height setting lever 23 being operated to pivot in a high-position setting direction (vehicle body rearward direction), the first link mechanism 25 moves the spool 22A of the control valve 22 from a neutral position to a raising position against the effect of the biasing means, in synchronization with the pivot operation. Upon the height setting lever 23 being operated to pivot in a low-position setting direction (vehicle body forward direction), the first link mechanism 25 allows the spool 22A of the control valve 22 to move from the neutral position to a lowering position under the effect of the biasing means, in synchronization with the pivot operation.
[0061] The feedback link mechanism 26 includes a linkage rod 30 that extends from the right lifting arm 20 forward of the vehicle body, a second pivot arm 31 that is interlocked with the left and right lifting arms 20 via the linkage rod 30, a second crankshaft 32 that spans the pivot center of the second pivot arm 31 and a lower end portion of the balance arm 28, and so on. Upon the tillage machine 15 reaching the control target height, the feedback link mechanism 26 moves the spool 22A of the control valve 22 from the raising position or the lowering position to the neutral position in synchronization with the control target height being reached. [0062] With the above configuration, if an operator operates the height setting lever 23 to increase the control target height of the tillage machine 15,

the first link mechanism 25 moves the spool 22A of the control valve 22 from the neutral position to the raising position in accordance with this operation (see FIG. 7). Thus, the tillage machine 15 is raised together with the left and right lifting arms 20. Upon the tillage machine 15 reaching the control target height as a result of thus being raised, the feedback link mechanism 26 moves the spool 22A of the control valve 22 from the raising position to the neutral position in synchronization with the control target height being reached (see FIG. 8). Thus, the tillage machine 15 together with the left and right lifting arms 20 stops rising.
If the operator controls the height setting lever 23 to decrease the control target height of the tillage machine 15, the first link mechanism 25 allows the spool 22A to move from the neutral position to the lowering position in accordance with this operation, and the spool 22A moves from the neutral position to the lowering position due to the effect of the biasing means. Thus, the tillage machine 15 is lowered together with the left and right lifting arms 20. Upon the tillage machine 15 reaching the control target height as a result of thus being lowered, the feedback link mechanism 26 moves the spool 22A of the control valve 22 from the lowering position to the neutral position in synchronization with the control target height being reached. Thus, the tillage machine 15 together with the left and right lifting arms 20 stops lowering.
[0063] That is to say, since the tractor includes the aforementioned lifting drive unit 17, position control can be favorably performed to displace the tillage machine 15 so as to be raised and lowered to any control target height that is set by operating the height setting lever 23. This position control enables setting of the tillage depth during a tillage operation to any depth. [0064] As shown in FIGS. 1 to 5 and 9 to 17, the mechanical linkage unit 18 includes a change amount converting mechanism 33 capable of switching between a first linkage converting state and a second linkage converting state, a second link mechanism 34 that connects the change amount converting mechanism 33 to the lifting drive unit 17 in an interlocking manner, an operation tool 35 for switching the change amount converting mechanism 33 between the first linkage converting state and the second linkage converting state, and so on. The change amount converting mechanism 33 has a load detection member 36 that pivots back and forth in accordance with a traction load transmitted via the top link 13, a pivot member 38 that is connected to a

grounded body 37 (see FIGS. 2, 16, and 17) for detecting the tillage depth of the tillage machine 15 in an interlocking manner, and thus pivots in accordance with the tillage depth of the tillage machine 15, and so on. [0065] Of various tillage machines 15 connectable to the three-point link mechanism 12, the plow 15A (see FIG. 1), a subsoiler, and the like do not have a grounded body 37. The rotary tillage machine 15B (see FIG. 2) has a rear cover that functions as the grounded body 37, so that the rear cover can pivot up and down. For this reason, in a state where the plow 15A is attached to the three-point link mechanism 12, the mechanical linkage unit 18 is in a draft controlling state (first linking state) in which the interlocking connection between the pivot member 38 and the grounded body 37 is cancelled (see FIGS. 1, 3 to 5, and 9 to 15). In a state where the rotary tillage machine 15B is attached to the three-point link mechanism 12, the mechanical linkage unit 18 is in an automatic tillage depth controlling state (second linking state) in which the pivot member 38 and the grounded body 37 are connected in an interlocking manner via a third link mechanism 39 (see FIGS. 2, 16, and 17). [0066] If the change amount converting mechanism 33 is switched to the aforementioned first linkage converting state when the mechanical linkage unit 18 is in the draft controlling state, then the change amount converting mechanism 33 can convert the amount of change in the traction load that is obtained by back-and-forth pivoting of the load detection member 36 to a lifting operation amount (see FIGS. 1, 3 to 5, and 9 to 15).
If the change amount converting mechanism 33 is switched to the aforementioned second linkage converting state when the mechanical linkage unit 18 is in the automatic tillage depth controlling state, the change amount converting mechanism 33 can convert an amount of change in the tillage depth that is obtained by pivoting of the pivot member 38 to a lifting operation amount (see FIGS. 2, 16, and 17).
Thus, when a tillage operation is carried out with this tractor using the plow 15A, the mechanical linkage unit 18 is in the draft controlling state (first linking state) due to the plow 15A being attached to the three-point link mechanism 12. In this draft controlling state, if the operator operates the operation tool 35 to switch the change amount converting mechanism 33 to the first linkage converting state, draft control is performed during the tillage operation so that the plow 15A is automatically raised and lowered in accordance with the traction load. As a result, stalling of the engine due to

an increase in the traction load can be avoided.
When a tillage operation is carried out with this tractor using the rotary tillage machine 15B, the mechanical linkage unit 18 is in the automatic tillage depth controlling state (second linking state) due to the rotary tillage machine 15B being attached to the three-point link mechanism 12, and the pivot member 38 and the grounded body 37 are connected in an interlocking manner via the third link mechanism 39. In this automatic tillage depth controlling state, if the operator operates the operation tool 35 to switch the change amount converting mechanism 33 to the second linkage converting state, automatic tillage depth control is performed during the tillage operation so that the rotary tillage machine 15B is automatically raised and lowered in synchronization with up-and-down pivoting of the grounded body (rear cover) 37 according to the tillage depth. As a result, an accurate tillage operation in which the tillage depth is kept constant can be carried out.
That is to say, since the mechanical linkage unit 18 that has the above-described configuration is provided, the tractor can perform the above-described draft control when a tillage operation is carried out using the plow 15A. Also, the tractor can perform the above-described automatic tillage depth control when a tillage operation is carried out using the rotary tillage machine 15B.
[0067] As shown in FIGS. 3 to 5, 9, 10, and 12 to 17, the pivot member 38 is pivotably supported by the load detection member 36. The operation tool 35 includes a contact member 40 that moves between a contact position, at which the contact member 40 comes into contact with the load detection member 36 to inhibit the load detection member 36 from pivoting back and forth, and a non-contact position, at which the contact member 40 does not come into contact with the load detection member 36 and allows the load detection member 36 to pivot back and forth. When the contact member 40 is at the non-contact position, the load detection member 36 pivots back and forth, and thus the amount of change in the traction load is transmitted as a lifting operation amount to the lifting drive unit 17. When the contact member 40 is at the contact position, the pivot member 38 independently pivots with the load detection member 36 not pivoting back and forth, and thus the amount of change in the tillage depth is transmitted as a lifting operation amount to the lifting drive unit 17.
That is to say, the change amount converting mechanism 33 is in the

above-described first linkage converting state when the contact member 40 is at the non-contact position, and the change amount converting mechanism 33 is in the above-described second linkage converting state when the contact member 40 is at the contact position.
With this configuration, in the automatic tillage depth control during a tillage operation using the rotary tillage machine 15B, the load detection member 36 does not pivot back and forth in accordance with the traction load transmitted via the top link 13. Accordingly, there is no concern that the amount of change in the traction load will be input to the change amount converting mechanism 33 due to back-and-forth pivoting of the load detection member 36.
As a result, in the automatic tillage depth control based on the amount of change in the tillage depth, it is possible to avoid the possibility that operation accuracy will decrease due to the amount of change in the traction load being input to the change amount converting mechanism 33. [0068] As shown in FIGS. 2, 16, and 17, the grounded body 37 of the rotary tillage machine 15B is spring biased in a downward direction (grounding direction). The third link mechanism 39 includes an inversion arm 41 supported by the rotary tillage machine 15B, a linkage rod 42 that spans the grounded body 37 and the inversion arm 41, a control cable 43 that spans the inversion arm 41 and the pivot member 38, and so on. One end portion of the control cable 43 is detachably pin-connected to a portion of the pivot member 38 upward of a pivot fulcrum thereof.
[0069] A plurality of connection holes 38A for the control cable are formed on an upper side of the pivot member 38. The position at which the control cable 43 is connected to the pivot member 38 can thus be changed. This change makes it possible to change the amount of pivoting displacement of the pivot member 38 relative to the amount of pivoting displacement of the grounded body 37. As a result, in the automatic tillage depth control, it is possible to adjust the responsiveness of the rotary tillage machine 15B when being raised and lowered in synchronization with up-and-down pivoting of the grounded body 37 according to the tillage depth.
Although the first embodiment describes an example of a mode in which two connection holes 38A are formed as the plurality of connection holes 38A in the pivot member 38, three or more connection holes 38A may be formed in the pivot member 38.

[0070] As shown in FIGS. 1, 3 to 5, and 10 to 15, in the three-point link mechanism 12 to which the plow 15A is attached, a front end portion of the top link 13 is connected to the load detection member 36 via a first connecting pin 44. Front end portions of the left and right lower links 14 are connected to left and right brackets 45, which are provided in a rear end portion of the T/M case 5, via left and right second connecting pins 46. With this connection structure, the traction load during a tillage operation acts on the load detection member 36 via the top link 13.
[0071] As shown in FIGS. 3 to 5 and 9 to 17, the load detection member 36 is supported by a support member 47 fixed to a rear end of the T/M case 5, so as to be able to pivot and be displaced via a first support shaft 48 in a front-rear direction. The mechanical linkage unit 18 includes a biasing mechanism 49 that pivots and biases the load detection member 36 in a direction (vehicle body rearward direction) against the traction load applied to the load detection member 36, and a restriction mechanism 50 that restricts the front-rear pivoting area of the load detection member 36. The load detection member 36 is supported in a reference orientation in which the load detection member 36 vertically extends upward from the first support shaft 48, due to the effects of the biasing mechanism 49 and the restriction mechanism 50. If the traction load exceeds a set value, the load detection member 36 pivots and is displaced toward the vehicle body front side from the reference orientation in synchronization with an increase in the traction load, against the effect of the biasing mechanism 49. Also, the load detection member 36 pivots and is displaced toward the vehicle body rear side in synchronization with a decrease in the traction load due to the effect of the biasing mechanism 49, and is restored to the reference orientation. A second support shaft 51 for pivotably supporting the pivot member 38 is provided at a free end portion of the load detection member 36.
[0072] As shown in FIGS. 3 to 10 and 12 to 17, the change amount converting mechanism 33 includes a holding mechanism 52 that biases and restores the pivot member 38 to a predetermined orientation relative to the load detection member 36. The pivot member 38, while connected to the grounded body 37 in an interlocking manner, is configured to be connected so that the predetermined orientation of the pivot member 38 corresponds to the lowest lowering position of the grounded body 37. The aforementioned second link mechanism 34 is pin-connected to a portion of the pivot member 38 downward

of the pivot fulcrum thereof. The second link mechanism 34 includes an operating arm 53 that is supported by a linking portion 27A of the first pivot arm 27 so as to be able to pivot back and forth, an inversion arm 54 for inverting the operating direction, a first linking member 55 that spans the operating arm 53 and one end portion of the inversion arm 54, a second linking member 56 that spans the other end portion of the inversion arm 54 and the pivot member 38, and so on. The linking portion 27A of the first pivot arm 27 is formed to have a U-shape when seen in a plan view. The operating arm 53 comes into contact with the linking portion 27A of the first pivot arm 27 from the vehicle body front side (low-position setting side of the height setting lever 23) as a result of pivoting in the vehicle body rearward direction (i.e. high-position setting direction of the height setting lever 23).
With this configuration, if the change amount converting mechanism 33 is switched to the first linkage converting state when the mechanical linkage unit 18 is in the draft controlling state, upon the traction load increasing, the load detection member 36 and the pivot member 38 integrally pivot toward the vehicle body front side in synchronization with the increase in the traction load due to the holding effect of the holding mechanism 52. Upon the traction load decreasing, the load detection member 36 and the pivot member 38 integrally pivot toward the vehicle body rear side in synchronization with the decrease in the traction load due to the holding effect of the holding mechanism 52.
If the load detection member 36 and the pivot member 38 integrally pivot toward the vehicle body front side, the operating arm 53 pivots in the vehicle body rearward direction in synchronization with the integral pivoting of the load detection member 36 and the pivot member 38. As a result of this pivoting, the operating arm 53 presses the linking portion 27A of the first pivot arm 27 in the vehicle body rearward direction. Thus, the first pivot arm 27 pivots in the vehicle body rearward direction, and the spool 22A of the control valve 22 linked to the first pivot arm 27 moves from the neutral position to the raising position against the effect of the biasing means (see FIG. 13). As a result, the plow 15A is raised together with the left and right lifting arms 20.
If the load detection member 36 and the pivot member 38 integrally pivot toward the vehicle body rear side, the operating arm 53 pivots in the vehicle body forward direction in synchronization with the integral pivoting of

the load detection member 36 and the pivot member 38. As a result of this pivoting, the operating arm 53 moves away from the linking portion 27A of the first pivot arm 27 in the vehicle body forward direction. Thus, the first pivot arm 27 is allowed to pivot in the vehicle body forward direction, and the spool 22A of the control valve 22 moves from the neutral position to the lowering position due to the effect of the biasing means. As a result, the plow 15A is lowered together with the left and right lifting arms 20.
Meanwhile, if the change amount converting mechanism 33 is switched to the second linkage converting state when the mechanical linkage unit 18 is in the automatic tillage depth controlling state, upon the tillage depth increasing and the grounded body 37 being raised, the pivot member 38 independently pivots in a rearward tilting direction relative to the load detection member 36 in synchronization with the rising of the grounded body 37, against the biasing by the holding mechanism 52. Upon the tillage depth decreasing and the grounded body 37 being lowered, the pivot member 38 independently pivots in a forward tilting direction relative to the load detection member 36 as a result of being biased by the holding mechanism 52 in synchronization with the lowering of the grounded body 37.
When the pivot member 38 independently pivots in the rearward tilting direction, the operating arm 53 pivots in the vehicle body rearward direction in synchronization with the independent pivoting of the pivot member 38. As a result of this pivoting, the operating arm 53 presses the linking portion 27A of the first pivot arm 27 in the vehicle body rearward direction. Thus, the first pivot arm 27 pivots in the vehicle body rearward direction, and the spool 22A of the control valve 22 linked to the first pivot arm 27 moves from the neutral position to the raising position against the effect of the biasing means (see FIG. 17). As a result, the rotary tillage machine 15B is raised together with the left and right lifting arms 20.
When the pivot member 38 independently pivots in the forward tilting direction, the operating arm 53 pivots in the vehicle body forward direction in synchronization with the independent pivoting of the pivot member 38. As a result of this pivoting, the operating arm 53 moves away from the linking portion 27A of the first pivot arm 27 in the vehicle body forward direction. Thus, the first pivot arm 27 is allowed to pivot in the vehicle body forward direction, and the spool 22A of the control valve 22 moves from the neutral position to the lowering position due to the effect of the biasing means. As a
26

result, the rotary tillage machine 15B is lowered together with the left and right lifting arms 20.
[0073] Upon the integral pivoting of the load detection member 36 and the pivot member 38 in accordance with the traction load stopping due to the plow 15A being raised or lowered when the mechanical linkage unit 18 is in the above-described draft controlling state, the feedback link mechanism 26 moves the spool 22A of the control valve 22 from the raising position or the lowering position to the neutral position in synchronization with the stoppage of the pivoting. Thus, the plow 15A together with the left and right lifting arms 20 stops rising or lowering.
Upon the independent pivoting of the pivot member 38 in accordance with the tillage depth stopping due to the rotary tillage machine 15B being raised or lowered when the mechanical linkage unit 18 is in the above-described automatic tillage depth controlling state, the feedback link mechanism 26 moves the spool 22A of the control valve 22 from the raising position or the lowering position to the neutral position in synchronization with the stoppage of the pivoting. Thus, the rotary tillage machine 15B together with the left and right lifting arms 20 stops rising or lowering. [0074] With the above configuration, since a mechanical linkage unit 18 that has the above-described configuration is provided, the tractor can favorably perform the above-described draft control when a tillage operation using the plow 15A is carried out. Also, the tractor can favorably perform the above-described automatic tillage depth control when a tillage operation using the rotary tillage machine 15B is carried out.
[0075] As shown in FIGS. 3 to 5, 9, 10, and 12 to 17, the holding mechanism 52 includes a stopper 57 that restricts the pivot member 38 from pivoting in a direction in which the three-point link mechanism 12 is lowered, relative to the load detection member 36, and a spring 58 that pivots and biases the pivot member 38 toward the stopper 57.
That is to say, a simple configuration in which only the stopper 57 and the spring 58 are provided allows the load detection member 36 and the pivot member 38 to integrally pivot when the change amount converting mechanism 33 is in the first linkage converting state, and allows the pivot member 38 to independently pivot relative to the load detection member 36 when the change amount converting mechanism 33 is in the second linkage converting state.

As a result, the change amount converting mechanism 33 can be favorably switched between the first linkage converting state and the second linkage converting state, while simplifying the configuration. [0076] The stopper 57 is supported by the load detection member 36. The spring 58 spans the load detection member 36 and the second linking member 56.
[0077] As shown in FIGS. 3 to 5, 9, 10, and 12 to 15, the change amount converting mechanism 33, as the first linkage converting state, includes a first converting state (see FIGS. 12 and 13) and a second converting state (see FIGS. 14 and 15). The change amount converting mechanism 33 is switched between the first converting state and the second converting state by operating the aforementioned operation tool 35.
When the change amount converting mechanism 33 is in the first converting state, the load detection member 36 and the pivot member 38 integrally pivot due to the holding effect of the holding mechanism 52. As a result of this integral pivoting, an amount of change in the traction load obtained by back-and-forth pivoting of the load detection member 36 is transmitted, without being amplified, as a lifting operation amount to the lifting drive unit 17 (see FIGS. 12 and 13).
When the change amount converting mechanism 33 is in the second converting state, the load detection member 36 and the pivot member 38 are allowed to relatively pivot against the holding effect of the holding mechanism 52. As a result of this relative pivoting, an amount of change in the traction load obtained by back-and-forth pivoting of the load detection member 36 is amplified and is then transmitted as a lifting operation amount to the lifting drive unit 17 (see FIGS. 14 and 15).
With this configuration, for example, in the case of carrying out a tillage operation using the plow 15A in a standard field in which the traction load is unlikely to increase or drastically change due to the hardness or the like of soil, if the operator operates the operation tool 35 to switch the change amount converting mechanism 33 to the first converting state, the plow 15A can be raised and lowered in accordance with the traction load at a standard speed suitable for this field. As a result, stalling of the engine due to an increase in the traction load can be avoided while performing draft control under which the tillage depth changes moderately.
Also, for example, in the case of carrying out a tillage operation using

the plow 15A in a field in which the traction load is likely to increase and drastically change due to the hardness or the like of soil, the plow 15A can be raised and lowered according to the traction load at a high speed suitable for this field by the operator operating the operation tool 35 to switch the change amount converting mechanism 33 to the second converting state. As a result, stalling of the engine due to a sudden increase in the traction load can be avoided.
[0078] As shown in FIGS. 3 to 5, 9, 10, and 12 to 17, the operation tool 35 includes a receiving member 59 that moves between a non-receiving position and a receiving position. When the receiving member 59 is located at the non-receiving position, the receiving member 59 is out of the pivoting area of the pivot member 38, and thus, the load detection member 36 and the pivot member 38 integrally pivot due to the holding effect of the holding mechanism 52. When the receiving member 59 is located at the receiving position, the receiving member 59 is located in the pivoting area of the pivot member 38, and the load detection member 36 and the pivot member 38 integrally pivot due to the holding effect of the holding mechanism 52 while the pivot member 38 is not received by the receiving member 59. The load detection member 36 and the pivot member 38 are allowed to relatively pivot against the holding effect of the holding mechanism 52 while the pivot member 38 is received by the receiving member 59.
That is to say, with the configuration in which only the receiving member 59 is provided in the operation tool 35, in addition to the aforementioned holding mechanism 52, the first converting state of the change amount converting mechanism 33 in which the load detection member 36 and the pivot member 38 integrally pivot can be achieved when the receiving member 59 is at the non-receiving position. The second converting state of the change amount converting mechanism 33 in which the load detection member 36 and the pivot member 38 are allowed to relatively pivot can be achieved when the receiving member 59 is at the receiving position.
As a result, the change amount converting mechanism 33 can be favorably switched between the first converting state and the second converting state, while simplifying the configuration.
[0079] When the change amount converting mechanism 33 is in the second converting state and the load detection member 36 assumes the reference orientation, a gap is secured between the pivot member 38 and the receiving

member 59. For this reason, until the amount of pivoting displacement of the load detection member 36 from the reference orientation based on the traction load reaches a predetermined amount with which the pivot member 38 comes into contact with the receiving member 59, the load detection member 36 and the pivot member 38 integrally pivot due to the receiving member 59 not receiving the pivot member 38. Upon the amount of pivoting displacement of the load detection member 36 from the reference orientation becoming the predetermined amount or more, the load detection member 36 and the pivot member 38 relatively pivot due to the receiving member 59 receiving the pivot member 38.
Thus, in an operational situation where the traction load only slightly exceeds a set value and there is no concern that the engine will stall, the plow 15A is driven to be raised and lowered in accordance with the traction load at a standard speed even when the change amount converting mechanism 33 is in the second converting state.
As a result, it is possible to avoid the possibility that the durability of the lifting drive unit 17 will deteriorate due to the plow 15A being raised and lowered at an unnecessarily high speed even in an operational situation where the traction load only slightly exceeds the set value. [0080] If the load detection member 36 and the pivot member 38 are allowed to relatively pivot when the change amount converting mechanism 33 is in the second converting state, upon the traction load increasing, the load detection member 36 pivots toward the vehicle body front side, and the pivot member 38 relatively pivots in the rearward tilting direction against the biasing of the holding mechanism 52, in synchronization with the increase in the traction load. Upon the traction load decreasing, the load detection member 36 pivots toward the vehicle body rear side, and the pivot member 38 relatively pivots in the forward tilting direction as a result of being biased by the holding mechanism 52, in synchronization with the decrease in the traction load.
If the load detection member 36 pivots toward the vehicle body front side, and the pivot member 38 relatively pivots in the rearward tilting direction, the operating arm 53 pivots in the vehicle body rearward direction in synchronization with this relative pivoting. Due to this pivoting, the operating arm 53 presses the linking portion 27A of the first pivot arm 27 in the vehicle body rearward direction. Thus, the first pivot arm 27 pivots in the vehicle body rearward direction, and the spool 22A of the control valve 22

linked to the first pivot arm 27 moves from the neutral position to the raising position against the effect of the biasing means (see FIG. 15). As a result, the plow 15A is raised together with the left and right lifting arms 20.
If the load detection member 36 pivots toward the vehicle body rear side, and the pivot member 38 relatively pivots in the forward tilting direction, the operating arm 53 pivots in the vehicle body forward direction in synchronization with this relative pivoting. Due to this pivoting, the operating arm 53 moves away from the linking portion 27A of the first pivot arm 27 in the vehicle body forward direction. Thus, the first pivot arm 27 is allowed to pivot in the vehicle body forward direction, and the spool 22A of the control valve 22 moves from the neutral position to the lowering position due to the effect of the biasing means. As a result, the plow 15A is lowered together with the left and right lifting arms 20.
[0081] Upon the relative pivoting of the load detection member 36 and the pivot member 38 in accordance with the traction load stopping due to the plow 15A being raised or lowered when the change amount converting mechanism 33 is in the aforementioned second converting state, the feedback link mechanism 26 operates the spool 22A of the control valve 22 to the neutral position from the raising position or the lowering position in synchronization with the stoppage of the pivoting. Thus, the plow 15A together with the left and right lifting arms 20 stops rising or lowering.
[0082] With the above configuration, since a mechanical linkage unit 18 that has the above-described configuration is provided, draft control can be selected in the tractor while giving consideration to the hardness or the like of soil, which differs between fields. As a result, a tillage operation using the plow 15A can be favorably carried out regardless of the hardness or the like of soil, which differs between fields.
[0083] A roller 60 capable of rotating around an axis parallel to the pivot axis of the pivot member 38 is provided at a portion of the receiving member 59 at which the pivot member 38 is received.
With this configuration, when the change amount converting mechanism 33 is in the second converting state, and the pivot member 38 slides relative to the receiving member 59 while the load detection member 36 and the pivot member 38 are relatively pivoting, the roller 60 rotates, with the sliding of the pivot member 38, in the sliding direction.
Thus, the pivot member 38 smoothly pivots relative to the receiving

member 59, and the load detection member 36 and the pivot member 38 relatively pivot smoothly.
As a result, draft control can be smoothly performed when the change amount converting mechanism 33 is in the second converting state. [0084] As shown in FIGS. 1 and 2, the change amount converting mechanism 33 is arranged next to the operator seat 11 in a portion rearward of the operator seat 11 in the vehicle body. Thus, the operator can readily check the operational state of the change amount converting mechanism 33 by viewing the portion rearward of the operator seat 11 while sitting on the operator seat 11.
In addition, since the portion rearward of the operator seat 11 is not covered from above by a cover or the like, maintenance can be readily carried out on the change amount converting mechanism 33.
[0085] The operation tool 35 is arranged next to the operator seat 11 in a portion rearward of the operator seat 11 in the vehicle body. Thus, the operator can operate the operation tool 35 by stretching a hand toward the portion rearward of the operator seat 11 while sitting on the operator seat 11, and can readily switch the change amount converting mechanism 33 between the first converting state, the second converting state, and the second linkage converting state.
[0086] As shown in FIGS. 3 to 5, 9, 10, and 12 to 17, the operation tool 35 includes a pivot plate 61 that is supported by the aforementioned support member 47 so as to be able to pivot in the left-right direction, and an operation handle 62 that extends forward from the pivot plate 61. The contact member 40 that has a plate-like shape is provided in a left side portion of the pivot plate 61, and the receiving member 59 is provided in a right side portion of the pivot plate 61. The operation tool 35 pivots in the left-right direction around an axis X that extends in the front-rear direction. The position of the operation tool 35 is selectively kept at one of a first operation position, a second operation position, and a third operation position, by a detent mechanism 63 that spans the support member 47 and the pivot plate 61.
When the position of the operation tool 35 is held at the first operation position on the right side, the contact member 40 is located at the non-contact position, and the receiving member 59 is located at the non-receiving position. Thus, the change amount converting mechanism 33 is in the first converting state for draft control.

When the position of the operation tool 35 is held at the second operation position that is an intermediate position between the left and right sides, the contact member 40 is located at the non-contact position, and the receiving member 59 is located at the receiving position. Thus, the change amount converting mechanism 33 is in the second converting state for draft control.
When the position of the operation tool 35 is held at the third operation position on the left side, the contact member 40 is located at the contact position, and the receiving member 59 is located at the non-receiving position. Thus, the change amount converting mechanism 33 is in the second linkage converting state for automatic tillage depth control.
That is to say, the operator can readily switch the change amount converting mechanism 33 between the first converting state for draft control, the second converting state for draft control, and the second linkage converting state for automatic tillage depth control, by switching the operation position of the operation tool 35.
[0087] As shown in FIGS. 3, 5, and 9 to 17, the load detection member 36 has left and right side wall portions 36A that include opposing through-holes 36a. The support member 47 includes a vertical wall portion 47A connected to a rear portion of the vehicle body, and a linking portion 47B having a U-shape, when seen in plan view, that extends rearward from the vertical wall portion 47A and enters a gap between the left and right side wall portions 36A. In the linking portion 47B, left and right long holes 47a that are elongated in the front-rear direction are formed in portions opposing the through-holes 36a of the side wall portions 36A. The aforementioned restriction mechanism 50 is constituted by the through-holes 36a in the load detection member 36, the long holes 47a in the support member 47, the linking pin 64 inserted into the through-holes 36a and the long holes 47a, and so on.
That is to say, the back-and-forth pivoting area of the load detection member 36 is restricted by the length in the front-rear direction of the long holes 47a formed in the support member 47.
[0088] The restriction mechanism 50 includes a rubber block 65 that is fitted into a space between the vertical wall portion 47A and the linking portion 47B of the support member 47. The rubber block 65 has a long hole 65a whose length in the front-rear direction is smaller than that of the long holes 47a in the support member 47, in a portion opposing the long holes 47a in the

support member 47. The linking pin 64 is inserted into this long hole 65a.
Thus, when the restriction mechanism 50 restricts the back-and-forth pivoting of the load detection member 36, the linking pin 64 in the restriction mechanism 50 collides with the rubber block 65. As a result, noise caused by the collision of the linking pin 64 with the linking portion 47B of the support member 47 can be prevented.
[0089] As shown in FIGS. 3 to 6 and 12 to 17, the mechanical linkage unit 18 includes a sensitivity adjustment mechanism 66 that adjusts operation sensitivity when the lifting drive unit 17 operates with the pivot member 38 in an interlocking manner. The sensitivity adjustment mechanism 66 includes a sensitivity adjustment lever 67 that is arranged so that the position thereof can be kept at any operation position in the operation unit 9, a linking member 69 that spans the sensitivity adjustment lever 67 and a support shaft 68 of the inversion arm 54, a support member 70 that supports the support shaft 68 of the inversion arm 54 in a displaceable manner, and so on. In the sensitivity adjustment mechanism 66, if the position at which the support shaft 68 is supported by the support member 70 is changed in the front-rear direction by operating the sensitivity adjustment lever 67 to pivot in the front-rear direction, a gap 71 between the linking portion 27A of the first pivot arm 27 and the operating arm 53 changes in synchronization with the change in the support position. This configuration makes it possible to adjust the operational sensitivity at the time when the lifting drive unit 17 operates the pivot member 38 in an interlocking manner.
[0090] In the sensitivity adjustment mechanism 66, the smaller the aforementioned gap 71, the higher the operational sensitivity at the time when the lifting drive unit 17 operates with the pivot member 38 in an interlocking manner, and the higher the responsiveness at the time when the traction load exceeds the set value and the plow 15A is raised.
In the sensitivity adjustment mechanism 66, the greater the aforementioned gap 71, the lower the operational sensitivity at the time when the lifting drive unit 17 operates with the pivot member 38 in an interlocking manner, and the lower the responsiveness at the time when the traction load exceeds the set value and the plow 15A is raised.
As a result, by decreasing the aforementioned operational sensitivity as the traction load changes more drastically due to, for example, a field condition such as that in a highly uneven field, the accuracy in a tillage

operation can be prevented from decreasing due to hunting by the plow 15A as a result of being frequently raised and lowered.
[0091] As shown in FIGS. 1 to 3, 5, 9, and 11 to 17, the load detection member 36 includes a first connecting portion 36B to which a long top link 13A (see FIGS. 1, 3, 5, 9, and 11 to 15) is connected, and second connecting portions 36C to one of which a bracket 72 (see FIGS. 2, 16, and 17) for supporting a short top link 13B is connected.
With this configuration, when, for example, a tillage operation using the rotary tillage machine 15B is carried out, the specifications of the three-point link mechanism 12 can be readily changed between standard link specifications in which the long top link 13A and the left and right lower links 14 are provided, and special link specifications in which the short top link 13B and the left and right lower links 14 are provided.
If the specifications of the three-point link mechanism 12 are changed from the standard link specifications to the special link specifications, the lifting driving amount of the rotary tillage machine 15B relative to the lifting operation amount of the lifting drive unit 17 increases, and the highest raising position of the rotary tillage machine 15B is set higher.
As a result, when, for example, a tillage operation using the rotary tillage machine 15B is carried out in a field with a high ridge, the specifications of the three-point link mechanism 12 are changed from the standard link specifications to the special link specifications, and the rotary tillage machine 15B can then readily avoid the possibility of coming into contact with a high ridge when traveling to pass over the ridge or while turning around near a ridge, for example.
[0092] The load detection member 36 includes a single connection hole that enables the long top link 13A to be pin-connected, as the first connecting portion 36B. The load detection member 36 also includes two, namely upper and lower connection holes that enable the bracket 72 to be pin-connected, as the second connecting portions 36C.
Second Embodiment
[0093] The second embodiment, which is an example of a mode for carrying
out the present invention, will be described below based on the drawings.
The tractor described as an example in the second embodiment differs from the tractor described as an example in the first embodiment only in the

configuration of the mechanical linkage unit 18 for automatic lifting, and the other configurations are the same. Accordingly, only the configuration of the mechanical linkage unit 18 for automatic lifting will be described below.
In the second embodiment as well, the direction indicated by an arrow F shown in FIG. 18 is a forward direction relative to a tractor, and the direction indicated by an arrow U is an upward direction relative to the tractor.
[0094] As shown in FIGS. 18 to 21, the mechanical linkage unit 18 for automatic lifting described as an example in the second embodiment is configured so that, during a tillage operation during which a tractor-drawn tillage machine 15 such as the plow 15A or a subsoiler (not shown) is attached to the three-point link mechanism 12, the amount of change in the traction load is converted to a lifting operation amount and is transmitted to the hydraulic lifting drive unit 17.
That is to say, this mechanical linkage unit 18 for automatic lifting is configured for draft control that only enables draft control to automatically raise and lower the tillage machine 15 in accordance with the traction load during a tillage operation.
[0095] The mechanical linkage unit 18 includes the change amount converting mechanism 33 that has the load detection member 36 that pivots back and forth in accordance with a traction load transmitted via the top link 13 of the three-point link mechanism 12, the second link mechanism 34 that connects the change amount converting mechanism 33 to the lifting drive unit 17 in an interlocking manner, and so on. The configuration of the second link mechanism 34 is the same as that described as an example in the first embodiment.
[0096] The change amount converting mechanism 33 has the pivot member 38 that is pivotably supported by the load detection member 36 and is connected to the lifting drive unit 17 via the second link mechanism 34 in an interlocking manner. The change amount converting mechanism 33 is configured to amplify the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member 36, by means of relative pivoting of the load detection member 36 and the pivot member 38, and then convert the amplified amount of change in the traction load to the lifting operation amount. [0097] With this configuration, if, for example, the traction load changes

during a tillage operation during which the plow 15A, which serves as the tractor-drawn tillage machine 15, is attached to the three-point link mechanism 12, the load detection member 36 pivots back and forth in accordance with the amount of change in the traction load at this time, and the amount of change in the traction load is amplified as a result of the load detection member 36 and the pivot member 38 relatively pivoting. Then, the amplified amount of change in the traction load is converted to a lifting operation amount and is transmitted to the lifting drive unit 17.
The plow 15A can thus be promptly raised and lowered in accordance with the change in the traction load. As a result, even if the traction load suddenly increases, stalling of the engine due to an increase in the traction load can be avoided.
[0098] In addition, as a result of the load detection member 36 and the pivot member 38 relatively pivoting, the amount of change in the traction load can be greatly amplified while narrowing the pivoting area of the load detection member 36 and the pivot member 38. Thus, the length in the front-rear direction of the space needed to install the change amount converting mechanism 33 that has the load detection member 36 and the pivot member 38 can be reduced.
As a result, stalling of the engine due to a sudden increase in the traction load can be avoided while suppressing an increase in the size of the vehicle body that leads to an increase in the entire length of the vehicle body. [0099] As shown in FIGS. 19 to 21, the load detection member 36 is supported by the support member 47 fixed to a rear end of the T/M case 5, so as to be able to pivot and be displaced via the first support shaft 48 in a front-rear direction. The second support shaft 51 for supporting the pivot member 38 so as to be able to pivot back and forth is provided at a free end portion of the load detection member 36. The aforementioned second link mechanism 34 is pin-connected to a portion of the pivot member 38 located downward of the pivot fulcrum (second support shaft 51) thereof. [0100] The mechanical linkage unit 18 includes a biasing mechanism 49 that pivots and biases the load detection member 36 in a direction (vehicle body rearward direction) against the traction load applied to the load detection member 36, and a restriction mechanism 50 that restricts the front-rear pivoting area of the load detection member 36. The configuration of the biasing mechanism 49 and the restriction mechanism 50 is the same as that

described as an example in the first embodiment.
[0101] The load detection member 36 is supported in a reference orientation in which the load detection member 36 vertically extends upward from the first support shaft 48, due to effects of the biasing mechanism 49 and the restriction mechanism 50. If the traction load exceeds a set value, the load detection member 36 pivots and is displaced toward the vehicle body front side from the reference orientation in synchronization with an increase in the traction load, against the effect of the biasing mechanism 49. Also, the load detection member 36 pivots and is displaced toward the vehicle body rear side in synchronization with a decrease in the traction load due to the effect of the biasing mechanism 49, and is restored to the reference orientation. [0102] The change amount converting mechanism 33 includes a holding mechanism 52 that biases and restores the pivot member 38 to a predetermined orientation relative to the load detection member 36. The holding mechanism 52 includes a stopper 57 that restricts the pivot member 38 from pivoting in a direction in which the three-point link mechanism 12 is lowered, relative to the load detection member 36, and a spring 58 that pivots and biases the pivot member 38 toward the stopper 57. The stopper 57 is supported by the load detection member 36. The spring 58 spans the load detection member 36 and the second linking member 56 in the second link mechanism 34.
[0103] The change amount converting mechanism 33 has the receiving member 59 that is arranged in the pivoting area of the pivot member 38 so as to be able to receive the upper side of the pivot member 38 from the vehicle body front side. The receiving member 59 is supported by an upper end portion of the support member 47.
[0104] The change amount converting mechanism 33 is configured so that a gap is secured between the pivot member 38 and the receiving member 59 when the load detection member 36 assumes the reference orientation.
For this reason, until the amount of pivoting displacement of the load detection member 36 from the reference orientation based on the traction load reaches a predetermined amount with which the pivot member 38 comes into contact with the receiving member 59, the load detection member 36 and the pivot member 38 integrally pivot due to the holding effect of the holding mechanism 52, as a result of the receiving member 59 not receiving the pivot member 38. Thus, an amount of change in the traction load obtained by

back-and-forth pivoting of the load detection member 36 is transmitted, without being amplified, as a lifting operation amount to the lifting drive unit 17.
Upon the amount of pivoting displacement of the load detection member 36 from the reference orientation becoming the predetermined amount or more, the load detection member 36 and the pivot member 38 are allowed to relatively pivot against the holding effect of the holding effect of the holding mechanism 52, as a result of the receiving member 59 receiving the pivot member 38. Thus, the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member 36 is amplified by means of the relative pivoting of the load detection member 36 and the pivot member 38, and is then transmitted as the lifting operation amount to the lifting drive unit 17.
That is to say, in an operational situation where the traction load only slightly exceeds the set value and there is no concern that the engine will stall, the lifting drive unit 17 drives the plow 15A to raise and lower the plow 15A in accordance with the traction load at a standard speed suitable for the operational situation at the time.
In an operational situation where the traction load greatly exceeds the set value and an engine is more likely to stall, the lifting drive unit 17 drives the plow 15A to raise and lower the plow 15A in accordance with the traction load at a high speed suitable for the operational situation at this time.
As a result, even in an operational situation where the traction load only slightly exceeds the set value, stalling of the engine due to a sudden increase in the traction load can be avoided while avoiding the possibility that the durability of the lifting drive unit 17 will be degraded due to the lifting drive unit 17 driving the plow 15A to raise and lower the plow 15A at an unnecessarily high speed.
[0105] In the change amount converting mechanism 33, upon the traction load increasing, when the load detection member 36 and the pivot member 38 are allowed to relatively pivot, the load detection member 36 pivots toward the vehicle body front side, and the pivot member 38 relatively pivots in the rearward tilting direction against the biasing of the holding mechanism 52, in synchronization with the increase in the traction load, as shown in FIGS. 18 to 21. Upon the traction load decreasing, the load detection member 36 pivots toward the vehicle body rear side, and the pivot member 38 relatively

pivots in the forward tilting direction as a result of being biased by the holding mechanism 52, in synchronization with the decrease in the traction load.
If the load detection member 36 pivots toward the vehicle body front side, and the pivot member 38 relatively pivots in the rearward tilting direction, the operating arm 53 pivots in the vehicle body rearward direction in synchronization with this relative pivoting. Due to this pivoting, the operating arm 53 presses the linking portion 27A of the first pivot arm 27 in the vehicle body rearward direction. Thus, the first pivot arm 27 pivots in the vehicle body rearward direction, and the spool 22A of the control valve 22 linked to the first pivot arm 27 moves from the neutral position to the raising position against the effect of the biasing means. As a result, the plow 15A is raised together with the left and right lifting arms 20.
If the load detection member 36 pivots toward the vehicle body rear side, and the pivot member 38 relatively pivots in the forward tilting direction, the operating arm 53 pivots in the vehicle body forward direction in synchronization with this relative pivoting. Due to this pivoting, the operating arm 53 moves away from the linking portion 27A of the first pivot arm 27 in the vehicle body forward direction. Thus, the first pivot arm 27 is allowed to pivot in the vehicle body forward direction, and the spool 22A of the control valve 22 moves from the neutral position to the lowering position due to the effect of the biasing means. As a result, the plow 15A is lowered together with the left and right lifting arms 20.
Upon the relative pivoting of the load detection member 36 and the pivot member 38 in accordance with the traction load stopping due to the plow 15A being raised or lowered, the feedback link mechanism 26 in the lifting drive unit 17 operates the spool 22A of the control valve 22 from the raising position or the lowering position to the neutral position in synchronization with the stoppage of the pivoting. Thus, the plow 15A together with the left and right lifting arms 20 stops rising or lowering.
[0106] As shown in FIGS. 19 to 21, the roller 60 capable of rotating around an axis parallel to the pivot axis of the pivot member 38 is provided at a portion of the receiving member 59 at which the pivot member 38 is received.
With this configuration, in the change amount converting mechanism 33, when the pivot member 38 slides relative to the receiving member 59 while the load detection member 36 and the pivot member 38 are relatively pivoting, the roller 60 rotates, with the sliding of the pivot member 38, in the

sliding direction.
With this configuration, the pivot member 38 smoothly slides relative to the receiving member 59, and the load detection member 36 and the pivot member 38 relatively pivot smoothly. As a result, draft control can be smoothly performed.
[0107] As shown in FIG. 18, the change amount converting mechanism 33 is arranged next to the operator seat 11 in a portion rearward of the operator seat 11 in the vehicle body. Thus, the operator can readily check the operational state of the change amount converting mechanism 33 by viewing the portion rearward of the operator seat 11 while sitting on the operator seat 11.
In addition, since the portion rearward of the operator seat 11 is not covered from above by a cover or the like, maintenance can be readily carried out on the change amount converting mechanism 33.
[0108] As shown in FIGS. 20 and 21, the mechanical linkage unit 18 includes a sensitivity adjustment mechanism 66 that adjusts operation sensitivity when the lifting drive unit 17 operates with the pivot member 38 in an interlocking manner. The sensitivity adjustment mechanism 66 has the same configuration as that described as an example in the first embodiment. With this sensitivity adjustment mechanism 66, for example, by decreasing the aforementioned operational sensitivity as the traction load changes more drastically due to, for example, a field condition such as that in a highly uneven field, the accuracy in a tillage operation can be prevented from decreasing due to hunting by the plow 15A as a result of being frequently raised and lowered.
Other Embodiments
[0109] The present invention is not limited to the configurations described as
examples in the first and second embodiments. Representative modifications
related to the present invention will be described as examples below.
[0110] [1] Various modifications can be made to the configuration of the
tractor.
For example, the tractor may be of a semi-crawler type with left and right crawlers in place of the left and right rear wheels 7.
For example, the tractor may be of a full-crawler type with left and right crawlers in place of the left and right front wheels 6 and the left and

right rear wheels 7.
For example, the tractor may be of an electric type that includes an electric motor in place of the engine 2.
For example, the tractor may also be of a hybrid type that includes the engine 2 and an electric motor.
[0111] [2] A work machine to be attached to the three-point link mechanism 12 may be a machine other than a tillage machine, such as a mower or a seeding machine.
During operating travel during which a mower, a seeding machine, or the like is attached to the three-point link mechanism 12, if the operator moves the operation tool 35 to the third operation position to switch the change amount converting mechanism 33 to the second linkage converting state, the load detection member 36 can be inhibited from pivoting back and forth in accordance with the traction load. It is thus possible to avoid the possibility that the mower, the seeding machine, or the like is raised and lowered due to the traction load during operation.
[0112] [3] Various modifications can be made to the configuration of the lifting drive unit 17.
For example, the lifting drive unit 17 may include a hydraulic motor or the like in place of the hydraulic cylinder 21.
For example, the lifting drive unit 17 may have a biasing means that biases and restores the spool 22A of the control valve 22 to the lowering position, outside the control valve 22.
[0113] [4] For example, the mechanical linkage unit 18 may include, as the pivot member 38, a pivot member 38 for draft control that is pivotably supported by the load detection member 36, and a pivot member 38 for automatic tillage depth control that is pivotably supported by the support member 47, and include, as the link mechanism 34, a link mechanism 34 for draft control that connects the pivot member 38 for draft control to the lifting drive unit 17 in an interlocking manner, and a link mechanism 34 for automatic tillage depth control that connects the pivot member 38 for automatic tillage depth control to the lifting drive unit 17 in an interlocking manner. This mechanical linkage unit 18 may be switched, by operating the operation tool 35, between a first converting state of allowing the load detection member 36 and the pivot member 38 for draft control to integrally pivot and inhibiting the pivot member 38 for automatic tillage depth control

from pivoting, a second converting state of allowing the load detection member 36 and the pivot member 38 for draft control to relatively pivot and inhibiting the pivot member 38 for automatic tillage depth control from pivoting, and a second linkage converting state of inhibiting the load detection member 36 and the pivot member 38 for draft control from pivoting and allowing the pivot member 38 for automatic tillage depth control to pivot. [0114] [5] Various modifications can be made to the configuration of the change amount converting mechanism 33.
For example, the change amount converting mechanism 33 may be provided with the first converting state and the second converting state, without being provided with the second linkage converting state.
For example, the change amount converting mechanism 33 may be switched between, by operating the operation tool 35, a first converting state in which the load detection member 36 is connected to the link mechanism 34, and a second converting state in which the pivot member 38 is connected to the link mechanism 34.
For example, the change amount converting mechanism 33 may be provided with only the first converting state as the first linkage converting state.
For example, the change amount converting mechanism 33 may be configured so that the pivot member 38 is pivotably supported by the support member 47 that supports the load detection member 36, and may be switched, by operating the operation tool 35, between a first linkage converting state in which the load detection member 36 is connected to the link mechanism 34 and a second linkage converting state in which the pivot member 38 is connected to the link mechanism 34.
For example, the change amount converting mechanism 33 may include, as the pivot member 38, a pivot member 38 for draft control that is pivotably supported by the load detection member 36, and a pivot member 38 for automatic tillage depth control that is pivotably supported by the support member 47, and may be switched, by operating the operation tool 35, between a first linkage converting state in which the pivot member 38 for draft control is connected to the link mechanism 34 and a second linkage converting state in which the pivot member 38 for automatic tillage depth control is connected to the link mechanism 34. With this configuration, the first converting state and the second converting state can be provided as the first linkage converting

state.
[0115] [6] Various modifications can be made to the configuration of the load
detection member 36.
For example, the load detection member 36 may be configured so that the upper end portion thereof is supported by the support member 47 via the first support shaft 48.
For example, the load detection member 36 may be supported so as to be able to pivot back and forth, by the support member 47 via a first support shaft 48 that is vertically elongated.
[0116] [7] The holding mechanism 52 may be constituted by, for example, a spring receiving member that is fixed at a position opposing the pivot member 38 assuming a predetermined orientation, in the axial direction of the second support shaft 51, and a torsion spring that pivots and biases the pivot member 38 to assume a predetermined orientation. In this configuration, a coil portion of the torsion spring is fitted onto the second support shaft 51, and the torsion spring is configured so that both end sides of the torsion spring sandwich the spring receiving member and the pivot member 38 assuming the predetermined orientation.
Industrial Applicability
[0117] The present invention is applicable to a tractor that includes a hydraulic lifting drive unit that drives a tractor-drawn tillage machine attached to a three-point link mechanism to raise and lower the tillage machine together with the three-point link mechanism, and a mechanical linkage unit for automatic lifting.
Description of Reference Signs [0118] 11: Operator seat
12: Three-point link mechanism
13: Top link
13A: Long top link
13B: Short top link
15: Tillage machine
17: Lifting drive unit
18: Mechanical linkage unit
33: Change amount converting mechanism

34: Link mechanism
35: Operation tool
36: Load detection member
36B: First connecting portion
36C: Second connecting portion
38: Pivot member
40: Contact member
52: Holding mechanism
57: Stopper
58: Spring
59: Receiving member
60: Rotor
72: Bracket

WE CLAIM:
1. A tractor comprising:
a three-point link mechanism connected to a rear portion of a vehicle body so as to be able to pivot up and down; a lifting drive unit of a hydraulic type that drives a tractor-drawn tillage machine attached to the three-point link mechanism to raise and lower the tillage machine together with the three-point link mechanism; and a mechanical linkage unit for automatic lifting that converts an amount of change in a traction load to a lifting operation amount and transmits the lifting operation amount to the lifting drive unit,
wherein the mechanical linkage unit comprises: a change amount converting mechanism that has a load detection member that pivots back and forth in accordance with the traction load transmitted via a top link of the three-point link mechanism; and a link mechanism that connects the change amount converting mechanism and the lifting drive unit in an interlocking manner, and
the change amount converting mechanism has a pivot member that is pivotably supported by the load detection member and is connected to the lifting drive unit via the link mechanism in an interlocking manner, and is configured to amplify, by means of relative pivoting of the load detection member and the pivot member, an amount of change in the traction load obtained by back-and-forth pivoting of the load detection member, and thereafter converts the amplified amount of change in the traction load to the lifting operation amount.
2. The tractor according to claim 1,
wherein the change amount converting mechanism is switchable between a first converting state of converting the amount of change in the traction load to the lifting operation amount without amplifying the amount of change in the traction load, and a second converting state of amplifying the amount of change in the traction load and thereafter converting the amplified amount of change in the traction load to the lifting operation amount,
when the change amount converting mechanism is in the first converting state, the amount of change in the traction load is transmitted, without being amplified, as the lifting operation amount to the lifting drive

unit, by means of the load detection member and the pivot member integrally pivoting, and
when the change amount converting mechanism is in the second converting state, the amount of change in the traction load is amplified, and the amplified amount of change in the traction load is transmitted as the lifting operation amount to the lifting drive unit, by means of the load detection member and the pivot member relatively pivoting.
3. The tractor according to claim 2,
wherein the mechanical linkage unit comprises an operation tool for switching the change amount converting mechanism between the first converting state and the second converting state.
4. The tractor according to claim 3,
wherein the change amount converting mechanism comprises a holding mechanism that biases the pivot member to a predetermined orientation relative to the load detection member,
the operation tool comprises a receiving member that moves between a non-receiving position and a receiving position,
when the receiving member is located at the non-receiving position, the load detection member and the pivot member integrally pivot due to a holding effect of the holding mechanism as a result of the receiving member being out of a pivoting area of the pivot member and not receiving the pivoting member, and
when the receiving member is located at the receiving position, the load detection member and the pivot member are allowed to relatively pivot against the holding effect of the holding member as a result of the receiving member being located in the pivoting area of the pivot member and receiving the pivoting member.
5. The tractor according to claim 4,
wherein, when the receiving member is located at the receiving position, the load detection member and the pivot member integrally pivot due to the holding effect of the holding mechanism while the pivot member is not received by the receiving member, and the load detection member and the pivot member are allowed to relatively pivot against the holding effect of the

holding mechanism while the pivot member is received by the receiving member.
6. The tractor according to claim 4 or 5,
wherein a roller capable of rotating around an axis parallel to a pivot axis of the pivot member is provided at a portion of the receiving member at which the pivot member is received.
7. The tractor according to any one of claims 4 to 6,
wherein the holding mechanism comprises a stopper that restricts the pivot member from pivoting in a direction in which the three-point link mechanism is lowered, relative to the load detection member, and a spring that pivots and biases the pivot member toward the stopper.
8. The tractor according to any one of claims 3 to 7,
wherein the operation tool is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body.
9. The tractor according to any one of claims 1 to 8,
wherein the change amount converting mechanism is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body.
10. A tractor capable of operating with a tillage machine attached thereto,
comprising:
a three-point link mechanism to which the tillage machine can be attached, the three-point link mechanism being connected to a rear portion of a vehicle body so as to be able to pivot up and down; a lifting drive unit of a hydraulic type that drives the three-point link mechanism to raise and lower the three-point link mechanism; and a mechanical linkage unit for automatic lifting that selectively converts one of an amount of change in a traction load and an amount of change in a tillage depth to a lifting operation amount and transmits the lifting operation amount to the lifting drive unit,
wherein the mechanical linkage unit comprises: a change amount converting mechanism capable of switching between a first linkage converting state and a second linkage converting state, and a link mechanism that

connects the change amount converting mechanism and the lifting drive unit in an interlocking manner,
the change amount converting mechanism has a load detection member that pivots back and forth in accordance with the traction load transmitted via a top link of the three-point link mechanism, and a pivot member that pivots in accordance with the tillage depth of the tillage machine,
if the change amount converting mechanism is switched to the first linkage converting state, the change amount converting mechanism can convert the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member to the lifting operation amount, and
if the change amount converting mechanism is switched to the second linkage converting state, the change amount converting mechanism can convert the amount of change in the tillage depth obtained by pivoting of the pivot member to the lifting operation amount.
11. The tractor according to claim 10,
wherein the mechanical linkage unit comprises an operation tool for switching the change amount converting mechanism between the first linkage converting state and the second linkage converting state.
12. The tractor according to claim 11,
wherein the pivot member is pivotably supported by the load detection member,
the operation tool comprises a contact member that moves between a contact position at which the contact member comes into contact with the load detection member to inhibit the load detection member from pivoting back and forth, and a non-contact position at which the contact member does not come into contact with the load detection member and allows the load detection member to pivot back and forth,
when the contact member is at the non-contact position, the amount of change in the traction load is transmitted as the lifting operation amount to the lifting drive unit by mean of the load detection member pivoting back and forth, and
when the contact member is at the contact position, the amount of change in the tillage depth is transmitted as the lifting operation amount to

the lifting drive unit by means of the pivot member pivoting independently in a state where the load detection member does not pivot back and forth.
13. The tractor according to claim 12,
wherein a first converting state and a second converting state are provided as the first linkage converting state,
the change amount converting mechanism comprises a holding mechanism that can be switched between the first converting state and the second converting state by operating the operation tool, and biases the pivot member to a predetermined orientation relative to the load detection member,
when the change amount converting mechanism is in the first converting state, the load detection member and the pivot member integrally pivot due to a holding effect of the holding mechanism, and the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member is transmitted, without being amplified, as the lifting operation amount to the lifting drive unit, and
when the change amount converting mechanism is in the second converting state, the load detection member and the pivot member are allowed to relatively pivot against the holding effect of the holding mechanism, and the amount of change in the traction load obtained by back-and-forth pivoting of the load detection member is amplified, and the amplified amount of change in the traction load is transmitted as the lifting operation amount to the lifting drive unit.
14. The tractor according to claim 13,
wherein the holding mechanism comprises a stopper that restricts the pivot member from pivoting in a direction in which the three-point link mechanism is lowered, relative to the load detection member, and a spring that pivots and biases the pivot member toward the stopper.
15. The tractor according to any one of claims 11 to 14,
wherein the operation tool is arranged next to an operator seat in a portion rearward of the operator seat in the vehicle body.
16. The tractor according to any one of claims 10 to 15,
wherein the change amount converting mechanism is arranged next

to an operator seat in a portion rearward of the operator seat in the vehicle body.
17. The tractor according to any one of claims 10 to 16,
wherein the load detection member comprises a first connecting portion to which a long top link is connected, and a second connecting portion to which a bracket for supporting a short top link is connected.