1. A tractor comprising: a three-point linkage mechanism that is vertically pivotally connected to a rear portion of a vehicle body and that allows attachment of a utility implement including a cultivator; a hydraulic lift driving unit for liftably driving the three-point linkage mechanism; and a mechanical coupling unit for automatic lifting, the mechanical coupling unit being configured to convert a change amount of a towing load to a lift operational amount at time of a cultivation work in which the cultivator is attached to the three-point linkage mechanism and to transmit the lift operational amount to the lift driving unit; wherein the mechanical coupling unit includes: a load detection member pivotable back and forth according to a towing load transmitted via a top link of the three-point linkage mechanism, an urging mechanism for pivotally urging the load detection member in a decreasing direction of the towing load, a pivotal member pivotally supported to the load detection member, a restriction mechanism for restricting a pivotal movement of the pivotal member relative to the load detection member, and a link mechanism for operably coupling the pivotal member with the lift driving unit; and wherein the restriction mechanism includes: a receiving tool pivotable together with the load detection member, and an urging unit for pivotally urging the pivotal member for pressing the pivotal member against the receiving tool.

2. The tractor of claim 1, wherein: the urging unit includes: a first urging mechanism provided between the load detection member and the pivotal member or the link mechanism for suppressing pivotal movement of the pivotal member relative to the load detection member, and a second urging mechanism provided between a fixing member included in the vehicle body and the pivotal member or the link mechanism for suppressing pivotal movements of the load detection member and the pivotal member in the increase direction of the towing load.

3. The tractor of claim 2, wherein: the first urging mechanism includes a first tension spring provided from the load detection member to the pivotal member or the link mechanism and a second tension spring having a smaller diameter than the first tension spring; and the second tension spring extends through inside of the first tension spring to be provided from the load detection member to the pivotal member or the link mechanism.

4. The tractor of claim 2 or 3, wherein: the second urging mechanism includes a rod that is inserted and supported in/to the fixing member to be freely projectable into and withdrawable from the fixing member and that is operably coupled with the pivotal member, and a compression spring fitted on the rod; one end of the compression spring is placed in contact with the fixing member and the other end of the compression spring is supported to the rod; and a supporting position of the other end to the rod is variable.

TECHNICAL FIELD [0001]
This invention relates to a tractor including a three-point linkage mechanism that is vertically pivotally connected to a rear portion of a vehicle body and that allows attachment of a utility implement including a cultivator, a hydraulic lift driving unit for liftably driving the three-point linkage mechanism, and a mechanical coupling unit for automatic lifting, the mechanical coupling unit being configured to convert a change amount of a towing load to a lift operational amount at time of a cultivation work in which the cultivator is attached to the three-point linkage mechanism and to transmit the lift operational amount to the lift driving unit.
BACKGROUND ART [0002]
The above-described tractor, with inclusion of the mechanical coupling unit for automatic lifting, is configured to be capable of draft control at the time of cultivation work for maintaining a towing load constant by automatically lifting up/down the cultivator according to a towing load encountered thereby. [0003]
As a tractor capable of draft control, there is known one that includes e.g. a draft adjustment lever for setting a cultivation depth, a link mechanism (a tubular shaft and an arm, etc.) for coupling a spool of a control valve to the draft adjusting lever, a load detection member (a top link hinge) which is pivoted back and forth according to a towing load, a linkage mechanism (a draft feedback linkage mechanism) for operably coupling a spool of a control valve to the load detection member, etc., so that the towing load may be maintained at a set value determined by the cultivation depth of the draft adjustment lever (see Patent Document 1 for instance).

[0004]
The tractor disclosed in Patent Document 1 is configured such that the cultivator such as a plow, a sub-soiler, etc. is lifted up/down at a standard speed according to a towing load, regardless of whether a cultivation work using the draft control is effected in a standard field where the towing load hardly increases due to hardness of soil, thus providing less possibility of variation in the towing load becoming violent or whether such cultivation work using the draft control is effected in a field where the towing load tends to increase due to hardness of soil, thus providing violent variation in the towing load. [0005]
Thus, even when the towing load sharply increases, the cultivator will be lifted up at the standard speed based on the increase of the towing load. As a result, in the case of sharp increase of towing load, it is not possible to suitably suppress the increase of the towing load, so there is a risk that an engine stall due to towing load increase may be invited. [0006]
In view of the above, in order to avoid engine stall due to sharp increase of towing load, for the above-described tractor having a mechanical coupling unit for automatic lifting, the present applicant proposed to provide the mechanical coupling unit with a change amount conversion mechanism switchable between a standard conversion state and an amplified conversion state (see e.g. Patent Document 2). [0007]
According to the invention disclosed in Patent Document 2, in case a cultivation work using the draft control is effected in a standard field where the towing load hardly increases due to hardness of soil, thus providing less possibility of variation in the towing load becoming violent, the worker (operator) will set the change amount conversion mechanism to the standard conversion state in advance. With this, it becomes possible to allow the cultivator to be lifted up/down at the the standard speed suitable for this field. On the other hand, in case such cultivation work using the draft control is effected in a field where the towing load tends to increases due to hardness of soil, thus providing violent variation in the towing load, the worker will set the change amount

conversion mechanism to the amplified conversion state in advance. With this, it becomes possible to allow the cultivator to be lifted up/down at a higher speed suitable for this field. Consequently, in both the field that hardly provides violent change in the towing load and the filed that tends to provide such violent change in the towing load, engine stall due to increase in the towing load can be effectively avoided.
BACKGROUND AET DOCUMENTS
PATENT DOCUMENTS
[0008]
Patent Document V Japanese Unexamined Patent Application Publication No. 2006-109802
Patent Document 2- Japanese Patent Application No. 2016-182005.
SUMMARY OF THE INVENTION
OBJECT TO BE ACCOMPLISHED BY INVENTION
[0009]
In the invention disclosed in Patent Document 2, the mechanical coupling unit having the change amount conversion mechanism includes a load detection member which is pivoted back and forth according to change in the towing load which is transmitted via a top link of a three-point link mchanism, a pivotal member pivotally supported to the load detection member, a maintaining mechanism for urging the pivotal member to a predetermined posture relative to the load detection member, a receiving member position-switchable between an active position to act on the pivotal member and a retracted position relative to the same, a linkage mechanism for operably coupling the pivotal member to the lift driving unit, and so on. And, when the change amount conversion mechanism is under the standard conversion state, the receiving member is located at the retracted position, so that the load detection member and the pivotal member are pivoted together by the action of the maintaining mechanism. With this, the change amount of the towing load, without its amplification, will be transmitted as a lift operational amount to the lift driving unit. As a result, the cultivator will be lifted up/down at the standard speed. On the other hand, when the

change amount conversion mechanism is under the amplified conversion state, the receiving member is located at the active position, so that the greater the load detection member is pivoted in the increase direction of the towing load, the greater the load detection member and the pivotal member will be pivoted relative to each other against the action of the maintaining mechanism. With this, the change amount of the towing load, with its amplification, will be transmitted as a lift operational amount to the lift driving unit. As a result, the cultivator will be lifted up/down at the higher speed. [0010]
Notwithstanding the above, according to the invention disclosed in Patent Document 2, if the variation in the towing load becomes violent at the time of cultivation work using the draft control, in association with this variation in the towing load and under its effect, the load detection member will be violently pivoted back and forth. Due to the effect of this, if the load detection member is pivoted in the increase direction of the towing load, there tends to occur excessive pivotal movement of the pivotal member relative to the pivotal motion of the load detection member.
On the other hand, if the load detection member is pivoted in the decrease direction of the towing load, there tends to occur a delay in the pivotal movement of the pivotal member relative to the pivotal motion of the load detection member. Then, with occurrence of such excessive pivotal movement or pivotal movement delay of the pivotal member relative to the load detection member, due to such excessive pivotal movement or pivotal movement delay, a chattering phenomenon can sometimes occur in the mechanical coupling unit. And, with occurrence of such chattering phenomenon, accurate draft control according to a towing load becomes difficult. [0011]
Namely, there is a need for achieving suppression of chattering phenomenon at the time of cultivation work using draft control, thus preventing deterioration in the control accuracy in the draft control due to such chattering.
SOLUTION

[0012]
As a solution to solve the above problem, a tractor according to the present invention comprises^
a three-point linkage mechanism that is vertically pivotally connected to a rear portion of a vehicle body and that allows attachment of a utility implement including a cultivator;
a hydraulic lift driving unit for liftably driving the three-point linkage mechanism; and
a mechanical coupling unit for automatic lifting, the mechanical coupling unit being configured to convert a change amount of a towing load to a lift operational amount at time of a cultivation work in which the cultivator is attached to the three-point linkage mechanism and to transmit the lift operational amount to the lift driving unit; wherein the mechanical coupling unit includes:
a load detection member pivotable back and forth according to a towing load transmitted via a top link of the three-point linkage mechanism,
an urging mechanism for pivotally urging the load detection member in a decreasing direction of the towing load,
a pivotal member pivotally supported to the load detection member,
a restriction mechanism for restricting a pivotal movement of the pivotal member relative to the load detection member, and
a link mechanism for operably coupling the pivotal member with the lift driving unit; and
wherein the restriction mechanism includes:
a receiving tool pivotable together with the load detection member, and
an urging unit for pivotally urging the pivotal member for pressing the pivotal member against the receiving tool. [0013]
According to the above-described solution, at the time of cultivation work using the draft control, when the load detection member is pivoted back and forth according to the towing load, the receiving tool is pivoted together with the load detection member. In addition to this,

the pivotal member is maintained under a state of it being pressed against the receiving tool under the action of the urging unit. With this, even if e.g. the load detection member is pivoted back and forth in a violent manner due to violet change in the towing load at the time of cultivation work, thanks to the action of the urging unit, excessive pivotal movement and pivotal delay of the pivotal member relative to the load detection member, etc. can be suppressed. [0014]
Consequently, even when there occurs violent change in the towing load at the time of cultivation work using the draft control, it is possible to suppress occurrence of the chattering phenomenon in the mechanical coupling unit due to e.g. such excessive pivotal movement or pivotal delay of the pivotal member relative to the load detection member. As a result, deterioration in the control accuracy in the draft control due to chattering can be effectively prevented. [0015]
According to one preferred solution proposed by the present invention^
the urging unit includes:
a first urging mechanism provided between the load detection member and the pivotal member or the link mechanism for suppressing pivotal movement of the pivotal member relative to the load detection member, and
a second urging mechanism provided between a fixing member included in the vehicle body and the pivotal member or the link mechanism for suppressing pivotal movements of the load detection member and the pivotal member in the increase direction of the towing load. [0016]
With the above-described solution, the first urging mechanism, when the load detection member and the pivotal member are pivoted together, provides a constant force for suppressing the pivotal movement of the pivotal member relative to the load detection member, irrespectively of the back and forth pivotal movement of the load detection member according to the towing load. Further, this first urging mechanism, when the load detection member and the pivotal

member are pivoted relative to each other, provides an increasing suppressing force for suppressing the pivotal motion of the pivotal member relative to the load detection member which force increases progressively in association with increase in the pivotal movement of the pivotal member relative to the load detection member. [0017]
On the other hand, the second urging mechanism, in both the case of pivotal movement of the load detection member and the pivotal member together and the case of pivotal movement of the load detection member and the pivotal member relative to each other, provides a progressively increasing force for suppressing pivotal motions of the load detection member and the pivotal member in the towing load increase direction, in association with increase in the pivotal movement of the pivotal member relative to the load detection member. Further, when the load detection member and the pivotal member are pivoted relative to each other, in association with a large pivotal movement of the load detection member in the towing load increase direction, the second urging mechanism provides an increasing force for suppressing the pivotal movements of the load detection member and the pivotal member in the increasing direction of the towing load. With this arrangement, the second urging mechanism provides a greater force for suppressing the pivotal movements of the load detection member and the pivotal member in the towing load increasing direction in the case of the relative pivotal movement between the load detection member and the pivotal member than the case of unitary pivotal movement of the load detection member and the pivotal member. [0018]
Namely, the second urging mechanism provides a progressively increasing force for suppressing pivotal movements of the load detection member and the pivotal member in the towing load increasing direction, thus acting as a damper for damping the pivotal movements of the load detection member and the pivotal member in the towing load increasing direction, irrespectively whether the load detection member and the pivotal member are pivoted together or whether the load detection member and the pivotal member are pivoted relative to each other. [0019]

With the above, regardless of whether the load detection member and the pivotal member are pivoted together or the load detection member and the pivotal member are pivoted relative to each other, if variation in the towing load becomes violent and the load detection member is pivoted violently back and forth, thanks to the action of the second urging mechanism, the excessive pivotal movement and pivotal delay of the pivotal member relative to the load detection member, etc. can be more effectively suppressed. [0020]
Further, in case the load detection member and the pivotal member are pivoted relative to each other, if the load detection member and the pivotal member are pivoted violently as the variation in the towing load becomes violent, thanks to the actions of the first urging mechanism and the second urging mechanism, the excessive pivotal movement of pivotal movement delay of the pivotal member relative to the load detection member, etc. can be suppressed even more effectively. [0021]
And, since there is no need for the action of the first urging mechanism alone to cope with the situation of violent variation in the towing load, it is possible to avoid occurrence of inconvenience of difficulty of the pivotal movement of the pivotal member relative to the load detection member, which would occur in case the measure thereof were provided solely by the function of the first urging mechanism. [0022]
Consequently, even if there occurs violent variation in the towing load at the time of cultivation work using the draft control, the chattering phenomenon in the mechanical coupling unit due to the excessive pivotal movement or pivotal delay of the pivotal member relative to the load detection member can be suppressed in a more reliable and favorable manner. Thus, deterioration in the control accuracy in the draft control due to chattering can be prevented even more reliably. [0023]
According to a further preferred solution proposed by the present invention^
the first urging mechanism includes a first tension spring

provided from the load detection member to the pivotal member or the link mechanism and a second tension spring having a smaller diameter than the first tension spring; and
the second tension spring extends through inside of the first tension spring to be provided from the load detection member to the pivotal member or the link mechanism. [0024]
With the above-described solution, it is possible to form the outer shape of the first urging mechanism with a smaller diameter than a case of constituting the first urging mechanism of a single tension spring. Thus, it becomes possible for the first urging mechanism to secure a strong tension required without inviting enlargement of the first urging mechanism. So, it becomes possible to avoid such inconvenience as the first urging mechanism interfering with a member present in its periphery. [0025]
According to a further preferred solution proposed by the present invention^
the second urging mechanism includes a rod that is inserted and supported in/to the fixing member to be freely projectable into and withdrawable from the fixing member and that is operably coupled with the pivotal member, and a compression spring fitted on the rod;
one end of the compression spring is placed in contact with the fixing member and the other end of the compression spring is supported to the rod; and
a supporting position of the other end to the rod is variable. [0026]
With the above-described solution, since the second urging mechanism employs a compression spring, in comparison with an arrangement of using a tension spring, there is provided a greater choices for the spring, so that the spring can be freely selected with ease to suit a field characteristics and a regional characteristics. Further, by varying the supporting position of the other end of the compression spring, the urging force of the second urging mechanism can be adjusted. Therefore, in accordance with e.g. soil hardness of a field which differs among regions, the urging force of the second urging mechanism can be

changed, so that the risk of chattering occurrence can be suppressed in a more reliable and favorable manner, irrespectively of soil hardness of the field. Further, if e.g. there occurs deterioration in the compression spring which reduces its resilient force, the spring force can be adjusted to an appropriate value, so that e.g. the use period of the compression spring can be extended advantageously. BRIEF DESCRIPTION OF THE DRAWINGS [0027]
[Fig. l] is a right side view showing a tractor with a plow being attached to a three-point linkage mechanism,
[Fig. 2] is a right side view showing the tractor with a rotary cultivator being attached to the three-point linkage mechanism,
[Fig. 3] is a right side view in vertical section of principal portions showing arrangements of a lift driving unit and a mechanical coupling unit,
[Fig. 4] is a plan view of principal portions showing the arrangements of the lift driving unit and the mechanical coupling unit,
[Fig. 5] is an exploded perspective view of principal portions showing the arrangements of the lift driving unit and the mechanical coupling unit,
[Fig. 6] is a right side view in vertical development of principal portions showing an operational state of the lift driving unit when a cultivator is stopped in its lifting movement by position maintenance of a height setting lever on a low position setting side,
[Fig. 7] is a right side view in vertical development of principal portions showing an operational state of the lift driving unit when the cultivator is elevated in operative association with a pivotal operation of the height setting lever to a high position setting direction,
[Fig. 8] is a right side view in vertical development of principal portions showing an operational state of the lift driving unit when the cultivator is stopped in its lifting movement by position maintenance of the height setting lever on a high position setting side,
[Fig. 9] is an exploded perspective view of principal portions showing e.g. an arrangement of a change amount conversion mechanism,
[Fig. 10] is a rear view of principal portions showing e.g. the arrangement of the change amount conversion mechanism,

[Fig. 11] is a section view taken along XI-XI in Fig. 4,
[Fig. 12] is a right side view in vertical section of principal portions showing operational states of the mechanical coupling unit and the lift driving unit when a towing lead does not exceed a set value under a standard conversion state of the change amount conversion mechanism,
[Fig. 13] is a right side view in vertical section of principal portions showing operational states of the mechanical coupling unit and the lift driving unit when a towing lead exceeds the set value under the standard conversion state of the change amount conversion mechanism,
[Fig. 14] is a right side view in vertical section of principal portions showing operational states of the mechanical coupling unit and the lift driving unit when a towing lead does not exceed a set value under an amplified conversion state of the change amount conversion mechanism,
[Fig. 15] is a right side view in vertical section of principal portions showing operational states of the mechanical coupling unit and the lift driving unit when a towing lead exceeds the set value under the amplified conversion state of the change amount conversion mechanism,
[Fig. 16] is a right side view in vertical section of principal portions showing operational states of the mechanical coupling unit and the lift driving unit when a cultivation depth reaches a set cultivation depth under a second conversion state of the change amount conversion mechanism, and
[Fig. 17] is a right side view in vertical section of principal portions showing operational states of the mechanical coupling unit and the lift driving unit when a cultivation depth is deeper than a set cultivation depth under the second conversion state of the change amount conversion mechanism.
EMBODIMENT [0028]
Next, an embodiment of the present invention will be explained with reference to the accompanying drawings.
Incidentally, a direction indicated by an arrow with a mark F shown in Figs. 1 and 2 is the front side of a tractor and a direction

indicated by an arrow with a mark U is the upper side of the tractor. [0029]
As shown in Figs. 1 and 2, the tractor illustrated in this embodiment includes a front frame 1 disposed at a 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 rear end lower portion of the engine 2, an intermediate frame 4 connected to a rear end portion of the clutch housing 3, a transmission case (to be referred to as a "T/M case" hereinafter) 5 acting also as a rear frame and connected to a rear end portion of the intermediate frame 4, left and right front wheels 6 disposed on the left and right sides of the front frame 1, left and right rear wheels 7 disposed on the left and right sides of the T/M case 5, left and right rear fenders 8 covering the left and right rear wheels 7, a riding type driving section 9 disposed at a rear portion of the vehicle body, etc. [0030]
Though not shown, power from the engine 2 is transmitted via a main clutch housed in the clutch housing 3, a transmission shaft covered by the intermediate frame 4, etc. to a main speed changer device incorporated in the T/M case 5. And, power speed-changed by the main speed changer device is transmitted to the left and right front wheels 6 and the left and right rear wheels 7 via an auxiliary speed changer device, etc. incorporated in the T/M case 5. [0031]
As shown in Figs. 1 and 2, the driving section 9 includes a steering wheel 10 for steering the front wheels, a driver's seat 11 disposed between the left and right rear fenders 8, etc. [0032]
To a rear portion of the T/M case 5, there is vertically pivotally connected a three-point linkage mechanism 12 for allowing attachment of a utility implement. The three-point linkage mechanism 12 includes a single top link 13, left and right lower links 14, etc. With this, when a cultivation work is to be effected by this tractor, a towing type cultivator 15, as an example of the utility implement, can be attached to the three-point linkage mechanism 12. [0033]

Incidentally, in this embodiment, there are disclosed a case when a plow 15A as one example of the towing type cultivator 15 is attached to the three-point linkage mechanism 12 (see Fig.l) and a further case when a rotary cultivator 15B as another example of the towing type cultivator 15 is attached to the three-point linkage mechanism 12 (see Fig.2). However, other towing type cultivators 15 such as a disc harrow, a cultivator implement, and sub-soiler, etc. can also be attached to the three-point linkage mechanism 12. Further, to this three-point linkage mechanism 12, aside from such towing type cultivators, a utility implement such as a mower, a sowing unit, etc. can be attached also. [0034]
As shown in Figs. 1-8, the tractor includes a mechanical coupling type hydraulic lift device 16.
The hydraulic lift device 16 includes a hydraulic lift driving unit 17 for liftably driving the cultivator 15 together with the three-point linkage mechanism 12, and a mechanical coupling unit 18 for automatic lifting configured to covert a selected one of a towing load change amount and a cultivation depth change amount into a lift operational amount and then to transmit the resultant lift operational amount to the lift driving unit 17, at the time of a cultivation work with the cultivator 15 being attached to the three-point linkage mechanism 12. [0035]
The lift driving unit 17 includes left and right lift arms 20 for suspendingly supporting the left and right lower links 14 via left and right support members 19, a hydraulic cylinder 21 for pivotally driving the left and right lift arms 20 in the vertical direction, a control valve 22 for controlling an operation of the hydraulic cylinder 21, a height setting lever 23 for setting a control target height of the cultivator 15, a friction type maintaining mechanism 24 for maintaining the height setting lever 23 at a desired operational position, a first link mechanism 25 for operably coupling a spool 22A of the control valve 22 with the height setting lever 23, a feedback link mechanism 26 for operably coupling the spool 22A with the left and right lift arms 20, and so on. The control valve 22 incorporates an urging means (not shown) for urging the spool 22A to return to its lowered position on the front side of the vehicle body. [0036]

The first link mechanism 25 includes a first pivotal arm 27 which comes into contact with a coupling portion 23A of the height setting lever 23 from the vehicle body rear side (the high position side of the height setting lever 23), a balance arm 28 supported to the spool 22A of the control valve 22 to be pivoted back and forth, a first crank shaft 29 which extends from a pivot center of the first pivotal arm 27 to an upper end portion of the balance arm 28, and so on. In response to a pivotal operation of the height setting lever 23 to the high position setting direction (vehicle body rear direction), the first link mechanism 25, in association with this operation, moves the spool 22A of the control valve 22 from a neutral positon to an elevated position, against the action of the urging means. Whereas, in response to a pivotal operation of the height setting lever 23 to the low position setting direction (vehicle body front direction), the first link mechanism 25, in association with this operation, allows the spool 22A of the control valve 22 to move from the neutral position to a lowered position under (with the aid of) the action of the urging means. [0037]
The feedback link mechanism 26 includes a coupling rod 30 which extends from the right lift arm 20 to the vehicle body front side, a second pivotal arm 31 which is operably associated with the left and right lift arms 20 via the coupling rod 30, a second crank arm 32 which extends from the pivotal center of the second pivotal arm 31 to the lower end of the balance arm 28, and so on. In operation, when the cultivator 15 reaches a control target height, the feedback link mechanism 26, in association with this arrival, moves the spool 22A of the control valve 22 from either the elevated position or the lowered position to the neutral position. [0038]
With the above-described arrangement, if an operator sets a high control target height for the cultivator 15 by operating the height setting lever 23, in correspondence with this operation, the first link mechanism 25 moves the spool 22A of the control valve 22 from the neutral position to the elevated position (see Fig. 7). With this, the cultivator 15 is elevated together with the left and right lift arms 20. And, in this elevation, when the cultivator 15 reaches the control target height, in

association with this, the feedback link mechanism 26 moves the spool 22A of the control valve 22 from the elevated position to the neutral position (see Fig. 8). With this, the cultivator 15 together with the left and right lift arms 20 stops its elevation. [0039]
On the other hand, if the operator sets a low control target height for the cultivator 15 by operating the height setting lever 23, in correspondence with this operation, the first link mechanism 25 allows the spool 22A to be moved from the neutral position to the lowered position and the spool 22A is moved from the neutral position to the lowered position under the action of the urging means. With this, the cultivator 15 together with the left and right lift arms 20 is lowered. And, in this descent, when the cultivator 15 reaches the control target height, in association with this, the feedback link mechanism 26 moves the spool 22A of the control valve 22 from the lowered position to the neutral position. With this, the cultivator 15 together with the left and right lift arms 20 stops its lowering (descent). [0040]
Namely, with this tractor, with the provision of the lift driving unit 17 described above, position control for liftably displacing the cultivator 15 to a desired control target height set by an operation of the height setting lever 23 can be carried out in a favorable manner. And, with this position control, the cultivation depth at the time of cultivation work can be set to a desired depth. [0041]
As show in Figs. 1-5 and Figs. 9-17, the mechanical coupling unit 18 includes a change amount conversion mechanism 33 switchable between a first conversion state and a second conversion state, an operational tool 35 for switching the change amount conversion mechanism 33 to the first conversion state or the second conversion state, etc. The change amount conversion mechanism 33 includes a load detection member 36 which is pivoted back and forth in accordance with a towing load transmitted via the top link 13, an urging mechanism 49 for pivotally urging the load detection member 36 to a vehicle body rear direction which comprises a decreasing direction of the towing load, a pivotal member 38 pivotally supported to the load detection member 36,

a restriction mechanism 52 for restricting the pivotal movement of the pivotal member 38 relative to the load detection member 36, a second link mechanism 34 for operably coupling the pivotal member 38 with the lift driving unit 17, and so on. The pivotal member 38 is pivoted in accordance with the cultivation depth of the cultivator 15 in case the member is operably coupled with a ground contact body 37 (see Fig. 2, Figs. 16-17) for detecting a cultivation depth of the cultivator 15. [0042]
Under the first conversion state of the change amount conversion mechanism 33, the pivotal movement of the pivotal member 38 relative to the load detection member 36 is restricted by the restriction mechanism 52 while a back-and-forth pivotal movement of the load detection member 36 according to a towing load is allowed. Under the second conversion state of the change amount conversion mechanism 33, the pivotal movement of the pivotal member 38 relative to the load detection member 36 is restricted by the restriction mechanism 52 while the back-and-forth pivotal movement of the load detection member 36 according to a towing load is inhibited. [0043]
Among the various kinds of cultivators 15, the plow 15A (see Fig. l), the sub-soiler, etc. do not have the ground contact body 37. The rotary cultivator 15B (see Fig. 2) has a vertically pivotable rear cover functioning as the ground contact body 37. Therefore, in e.g. a cultivation work with the plow 15A being attached to the three-point linkage mechanism 12, the mechanical coupling unit 18 is set under a first coupling state in which operative coupling between the pivotal member 38 and the ground contact body 37 is released (see Fig. 1, Figs. 3-5, Figs. 9-15). Further, in a cultivation work with the rotary cultivator 15B being attached to the three-point linkage mechanism 12, the mechanical coupling unit 18 is set under a second coupling state in which the pivotal member 38 and the ground contact body 37 are operably coupled with each other via a third link mechanism 39 (see Fig. 2, Figs. 16-17). [0044]
Under the first coupling state of the mechanical coupling unit 18, if the change amount conversion mechanism 33 is switched into the first

conversion state described above, the back-and-forth pivotal movement of the load detection member 36 according to a towing load is allowed and the unitary pivotal movements of the load detection member 36 and the pivotal member 38 under the action of the restriction mechanism 52 is made possible. With this, it becomes possible for the change amount conversion mechanism 33 to convert a change amount of towing load obtained by the back-and-forth pivotal movement of the load detection member 36 into an lift operational amount (see Fig. 1, Figs. 3-5, Figs. 9-15). [0045]
Under the second coupling state of the mechanical coupling unit 18, if the change amount conversion mechanism 33 is switched into the second conversion state described above, the back-and-forth pivotal movement of the load detection member 36 according to a towing load is inhibited and the pivotal movement of the pivotal member 38 relative to the load detection member 36 against the action of the restriction mechanism 52 is made possible. With this, it becomes possible for the change amount conversion mechanism 33 to convert a change amount of towing load obtained by the pivotal movement of the pivotal member 38 operably associated with the ground contact body 37 into a lift operational amount (see Fig. 2, Figs. 16-17). [0046]
With the above-described configuration, when this tractor is to effect a cultivation work by the plow 15A, with attachment of the plow 15A to the three-point linkage mechanism 12, the mechanical coupling unit 18 is rendered into the first coupling state. And, under this first coupling state, if the operator operates the operational tool 35 to switch the change amount conversion mechanism 33 to the first conversion state, the draft control for automatically lifting up/down the plow 15A according to a towing load can be effected. As a result, occurrence of engine stall due to rise in a towing load can be avoided. [0047]
Further, when this tractor is to effect a cultivation work by the rotary cultivator 15B, the rotary cultivator 15B is attached to the three-point linkage mechanism 12 and also the pivotal member 38 and the ground contact body 37 are operably coupled with each other via the

third link mechanism 39, whereby the mechanical coupling unit 18 is rendered into the second coupling state. And, under this second coupling state, if the operator operates the operational tool 35 to switch the change amount conversion mechanism 33 to the second conversion state, the automatic cultivation depth control in which the rotary cultivator 15B is automatically lifted up/down in association with a vertical pivotal movement of the ground contact body (rear cover) according to a cultivation depth can be effected. As a result, it becomes possible to effect a high-accuracy cultivation work with maintaining the cultivation depth constant. [0048]
Namely, with this tractor, with the inclusion of the mechanical coupling unit 18 having the above-described arrangement, when a cultivation work by the plow 15A is to be carried out, a draft control suitable for such cultivation work by the plow 15A can be effected. Further, when a cultivation work by the rotary cultivator 15B is to be carried out, an automatic cultivation depth control suitable for such cultivation work by the rotary cultivator 15B can be effected. [0049]
Though not shown, when this tractor is to effect, as a work other than a cultivation work, a grass mowing work with a mower being attached to the three-point linkage mechanism 12 or a sowing work with a sowing device being attached to the three-point linkage mechanism 12, etc., the mechanical coupling unit 18 will be set to the first coupling state since such utility implement as the mower, the sowing device, etc., does not have the ground contacting body 37 for cultivation depth detection. And, in such grass mowing work or sowing work, the operator will operate the operational tool 35 to switch the change amount conversion mechanism 33 into the second conversion state, whereby the back-and-forth pivotal movement of the load detection member 36 according to a towing load is inhibited. As a result, in the course of a grass mowing work or sowing work, the risk of the mower or the sowing device being automatically lifted up/down according to a towing load can be avoided. [0050]
As shown in Figs. 3-5, Figs. 9-10 and Figs. 12-17, the operational

tool 35 includes a plate-like contact member 40 which is movable between an active position for coming into contact with the load detection member 36 thus inhibiting the back-and-forth pivotal movement of the load detection member 36 and a retracted position not coming into contact with the load detection member 36 thus allowing the back-and-forth pivotal movement of the load detection member 36. And, when the contact member 40 is at the retracted position, the back-and-forth pivotal movement of the load detection member 36 according to a towing load is allowed and the unitary pivotal movement of the load detection member 36 and the pivotal member 38 under the action of the restriction mechanism 52 is made possible, so that a change amount of the towing load will be transmitted as a lift operational amount to the lift driving unit 17. On the other hand, when the contact member 40 is at the active position, the back-and-forth pivotal movement of the load detection member 36 according to a towing load is inhibited and the pivotal movement of the pivotal member 38 relative to the load detection member 36 against the action of the restriction mechanism 52 is made possible, whereby a change amount in the cultivation depth will be transmitted as a lift operational amount to the lift driving unit 17. [0051]
Namely, when the contact member 40 is at the retracted position, the change amount conversion mechanism 33 assumes the above-described first conversion state. Whereas, when the contact member 40 is at the active position, the change amount conversion mechanism 33 assumes the above-described second conversion state. [0052]
As shown in Fig. 2 and Figs. 16-17, the ground contact body 37 of the rotary cultivator 15B is spring-urged in a lowering direction (ground contacting direction). The third link mechanism 39 includes a reversing arm 41 supported to the rotary cultivator 15B, a coupling rod 42 extending between the ground contact body 37 and the reversing arm 41, a control cable 43 extending between the reversing arm 41 and the pivotal member 38, etc. And, one end of the control cable 43 is detachably pin-connected to a portion of the pivotal member 38 upwardly of its pivot.

[0053]
As shown in Fig. 3, Fig. 5, Fig. 9 and Figs. 12-17', at an upper side of the pivotal member 38, there are formed a plurality of connecting holes 38A for the control cable. With this, it is possible to change the connecting position of the control cable 43 relative to the pivotal member 38. And, with such change, a pivotal displacement amount of the pivotal member 38 relative to a pivotal displacement amount of the ground contact body 37 can be changed. As a result, in the automatic cultivation depth control, response in lifting up/down of the rotary cultivator 15 in association with a vertical pivotal movement of the ground contact body 37 according to a cultivation depth can be adjusted. [0054]
Incidentally, in the instant embodiment, there is disclosed an example in which the pivotal member 38 defines two connection holes 38A as the plurality of connection holes 38A. However, the pivotal member 38 can define three or more connection holes 38A. [0055]
As shown in Fig. 1, Figs. 3-5 and Figs. 10-15, in the three-point linkage mechanism 12 to which the plow 15A is attached, the front end portion of the top link 13 is connected to the load detection member 36 via a first connection pin 44. Further, the front end portions of the left and right lower links 14 are connected via second connection pins 46 to left and right brackets 45 provided at a rear end portion of the T/M case 5. With this connection arrangement, a towing load at the time of a cultivation work is applied to the load detection member 36 via the top link 13. [0056]
As shown in Figs. 3-5 and Figs. 9-17, the load detection member 36 is supported via a first support shaft 48 to a support bracket 347 fixed to the rear end of the T/M case 5 to be pivotally displaceable back and forth. The mechanical coupling unit 18 includes a restriction mechanism 50 for restricting the range of back-and-forth pivotal movement of the load detection member 36. The load detection member 36 is retained, by the actions of the urging mechanism 49 and the restriction mechanism 50 described above, under a standard posture where the load detection member 36 extends perpendicularly upwards from the first

support shaft 48. And, when a towing load exceeds a set value, the load detection member 36 is pivotally displaced from the standard posture to the vehicle body front side in association with the rise of the towing load, against the action of the urging mechanism 49. And also, in association with decrease in the towing load, the load detection member 46 is pivotally displaced to the vehicle body rear side under the action of the urging mechanism 49 to be returned to the standard posture. At a free end portion of the load detection member 36, there is provided a second support shaft 51 which pivotally supports the pivotal member 38. [0057]
As shown in Figs. 3-10 and Figs. 12-17, by the action of the restriction mechanism 52, the pivotal member 38 is maintained under a predetermined posture relative to the load detection member 36. The pivotal member 38 is configured such that when it is operably connected to the ground contact body 37, the predetermined posture of the pivotal member 38 may correspond to the maximally lowered position of the ground contact body 37. Further, in the pivotal member 38, at portion of this pivotal member 38 lower than its pivot (second support shaft 51), the above-described second link mechanism 34 is pin-connected. The second link mechanism 34 includes an operational arm 53 supported to a coupling portion 27A of the first pivotal arm 27 to be pivotable back and forth, a reversing arm 54 for reversing the operational direction, a first coupling member 55 extending between the operational arm 53 and one end portion of the reversing arm 54, a second coupling member 56 extending between the other end portion of the reversing arm 54 and the pivotal member 38, and so on. The coupling portion 27A of the first pivotal arm 27 is formed in a U-shape as seen in a plan view. The operational arm 53, when pivoted in the vehicle body rear direction (the high position setting side of the height setting lever 23), comes into contact with the coupling portion 2 7A of the first pivotal arm 27 from the vehicle body front side (low position setting side of the height setting lever 23). [0058]
With the above-described arrangement, if the change amount conversion mechanism 33 is switched to the first conversion state while the mechanical coupling unit 18 is under the first coupling state, when

the towing load increases, in association with this increase, the load detection member 36 and the pivotal member 38 are pivoted together to the vehicle body front side under the action of the restriction mechanism 52. Conversely, when the towing load decreases, in association with this decrease, the load detection member 36 and the pivotal member 38 are pivoted together to the vehicle body rear side under the action of the restriction mechanism 52. [0059]
And, when the load detection member 36 and the pivotal member 38 are pivoted together to the vehicle body front side, in association with this unitary pivotal movement, the operational arm 53 is pivoted in the vehicle body rear direction and with this pivotal movement, the operational arm 53 presses the coupling portion 27A of the first pivotal arm 27 in the vehicle body rear direction. With this, the first pivotal arm 27 is pivoted in the vehicle body rear direction and the spool 22A of the control valve 22 operably coupled with the first pivotal arm 27 is moved from the neutral position to the elevated position against the action of the urging means (see Fig. 13). As a result, the plow 15A together with the left and right lift arms 20 is elevated. [0060]
And, when the load detection member 36 and the pivotal member 38 are pivoted together to the vehicle body rear side, in association with this unitary pivotal movement, the operational arm 53 is pivoted in the vehicle body front direction, and with this pivotal movement, the operational arm 53 is removed from the the coupling portion 27A of the first pivotal arm 27 in the vehicle body front direction. With this, the pivotal movement of the first pivotal arm 27 in the vehicle body front direction is allowed and the spool 22A of the control valve 22 is moved from the neutral position to the lowered position under the action of the urging means. As a result, the plow 15A together with the left and right lift arms 20 is lowered. [0061]
On the other hand, if the change amount conversion mechanism 33 is switched to the second conversion state while the mechanical coupling unit 18 is under the second coupling state, when the cultivation depth becomes deeper and the ground contact body 37 is elevated. In

association with this elevation, the pivotal member 38 is independently pivoted in the rear inclining direction relative to the load detection member 36 against the action of the restriction mechanism 52. On the other hand, when the cultivation depth becomes shallower and the ground contact body 37 is lowered, in association with this lowering, the pivotal member 38 is independently pivoted in the front inclining direction relative to the load detection member 36 under the action of the restriction mechanism 52. [0062]
And, when the pivotal member 38 is independently pivoted in the rear inclining direction, in association with this independent pivotal movement, the operational arm 53 is pivoted in the vehicle body rear direction, and with this pivotal movement, the operational arm 53 presses the coupling portion 27A of the first pivotal arm 27 in the vehicle body rear direction. With this, the first pivotal arm 27 is pivoted in the vehicle body rear direction and the spool 22A of the control valve 22 operably coupled with the first pivotal arm 27 is moved from the neutral position to the elevated position against the action of the urging means (see Fig. 17). As a result, the rotary cultivator 15B together with the left and right lift arms 20 is elevated. [0063]
And, when the pivotal member 38 is independently pivoted in the front inclining direction, in association with this independent pivotal movement, the operational arm 53 is pivoted in the vehicle body front direction, and with this pivotal movement, the operational arm 53 is removed from the the coupling portion 27A of the first pivotal arm 27 in the vehicle body front direction. With this, the pivotal movement of the first pivotal arm 27 in the vehicle body front direction is allowed and the spool 22A of the control valve 22 is moved from the neutral position to the lowered position under the action of the urging means. As a result, the rotary cultivator 15B together with the left and right lift arms 20 is lowered. [0064]
Referring now to the feedback control mechanism 26, when the mechanical coupling unit 18 is under the first coupling state, if the unitary pivotal movement of the load detection member 36 and the

pivotal member 38 in accordance with elevation or lowering of the plow 15A is stopped, in association with this stop of pivotal movement, the feedback control mechanism 26 causes the spool 22Aof the control valve 22 to move from the elevated position or the lowered position to the neutral position. With this, the plow 15A together with the left and right lift arms 20 stops its elevation or lowering. [0065]
Referring further to the feedback control mechanism 26, when the mechanical coupling unit 18 is under the second coupling state, if the independent pivotal movement of the pivotal member 38 according to a cultivation depth due to elevation or lowering of the rotary cultivator 15B is stopped, in association with this stop of pivotal movement, the feedback control mechanism 26 causes the spool 22Aof the control valve 22 to move from the elevated position or the lowered position to the neutral position. With this, the rotary cultivator 15B together with the left and right lift arms 20 stops its elevation or lowering. [0066]
With the above-described configuration, in this tractor, with the provision of the mechanical coupling unit 18 having the above-described arrangements, when a cultivation work using the plow 15A is to be carried out, the above-described draft control can be effected in a favorable manner. Further, when a cultivation work using the rotary cultivator 15B is to be carried out, the above-described automatic cultivation depth control can be effected in a favorable manner. [0067]
As shown in Figs. 3-5, Figs. 9-10 and Figs. 12-15, the change amount conversion mechanism 33 includes, as the first conversion state, a standard conversion state (see Figs. 12-13) and an amplified conversion state (see Figs. 14-15). And, in response to an operation of the operational tool 35 described above, the change amount conversion mechanism 33 is switched over between the standard conversion state and the amplified conversion state. And, when the change amount conversion mechanism 33 is under the standard conversion state, under the action of the restriction mechanism 52, the load detection member 36 and the pivotal member 38 are pivoted together. And, with this unitary pivotal movement, a change amount of towing load obtained by the

back-and-forth pivotal movement of the load detection member 36 is transmitted without amplification as a lift operational amount to the lift driving unit 17 (see Figs. 12-13). On the other hand, when the change amount conversion mechanism 33 is under the amplified conversion state, the relative pivotal movement between the load detection member 36 and the pivotal member 38 against the action of the restriction mechanism 52 is allowed. And, with this relative pivotal movement, a change amount of towing load obtained by the back-and-forth pivotal movement of the load detection member 36 is amplified and this is transmitted as a lift operational amount to the lift driving unit 17 (see Figs. 14-15). [0068]
With the above, in case a cultivation work using the plow 15Ais to be carried out in a standard field where the towing load due to e.g. soil hardness hardly increases and change in the towing load hardly becomes violent, the operator will operate the operational tool 35 to switch the change amount conversion mechanism 33 to the standard conversion state in advance. Thus, it becomes possible to lift the plow 15A up/down according to the towing load at a standard speed suitable for this field. As a result, with execution of a draft control for gentle change in the cultivation depth, engine stall due to rise in the towing load can be avoided. [0069]
On the other hand, in case a cultivation work using the plow 15A is to be carried out in a field where the towing load due to e.g. soil hardness tends to increase and change in the towing load tends to become violent, the operator will operate the operational tool 35 to switch the change amount conversion mechanism 33 to the amplified conversion state in advance. Thus, it becomes possible to lift the plow 15A up/down according to the towing load at a high speed suitable for this field. As a result, with execution of a draft control for gentle change in the cultivation depth, engine stall due to sharp rise in the towing load can be avoided. [0070]
As shown in Figs. 3-5, Figs. 9-10 and Figs. 12-17, the operational tool 35 incudes a roller 60 as a "receiving member" which is movable

between a retracted position and an active position. And, when the roller 60 is located at the retracted position, as the roller 60 is moved away from the pivotal range of the pivotal member 38, the load detection member 36 and the pivotal member 38 are pivoted together under the action of the restriction mechanism 52. On the other hand, when the roller 60 is located at the active position, the roller 60 enters the pivotal range of the pivotal member 38. And, in the period until the pivotal member 38 comes to be received by the roller 60, the load detection member 36 and the pivotal member 38 are pivoted together under the action of the restriction mechanism 52. Further, during the period when the pivotal member 38 is received by the roller 60, the load detection member 36 and the pivotal member 38 are pivoted relative to each other against the action of the restriction mechanism 52. [0071]
Namely, with inclusion of the roller 60 in the operational tool 35, when the roller 60 is located at the retracted position, it is possible to obtain the standard conversion state of the change amount conversion mechanism 33 wherein the load detection member 36 and the pivotal member 38 are pivoted together. Whereas, when the roller 60 is located at the active position, it is possible to obtain the amplified conversion state of the change amount conversion mechanism 33 wherein the relative pivotal movement between the load detection member 36 and the pivotal member 38 is allowed. [0072]
As shown in Fig. 14, when the change amount conversion mechanism 33 is under the amplified conversion state, if the load detection member 36 assumes the above-described standard posture, there is secured a gap between the pivotal member 38 and the roller 60. Therefore, until the pivotal displacement amount of the load detection member 36 from the standard posture based on a towing load reaches a predetermined amount at which the pivotal member 38 comes into contact with the roller 60, the load detection member 36 and the pivotal member 38 are pivoted together as the roller 60 does not receive the pivotal member 38. And, when the pivotal displacement amount of the load detection member 36 from the standard posture exceeds the predetermined amount, as the roller 60 receives the pivotal member 38,

the load detection member 36 and the pivotal member 38 are pivoted
relative to each other.
[0073]
With the above, in a work situation wherein there is no risk of engine stall as the towing load exceeds the set value only slightly, even if the change amount conversion mechanism 33 is under the amplified conversion state, the plow 15A will be driven to be lifted at a standard speed according to the towing load. As a result, also in such work situation wherein the towing load exceeds the set value only slightly, it is possible to avoid the risk of deterioration in the durability of the lift driving unit 17 due to the plow 15A being lifted up/down at a high speed according to a towing load. [0074]
As shown in Fig. 15, when the change amount conversion mechanism 33 is under the amplified conversion state, if the pivotal member 38 is in contact with the roller 60, when the towing load increases, in association with this increase, the load detection member 36 is pivoted to the vehicle body front side and also the pivotal member 38 is relatively pivoted in the rear inclination direction against the action of the restriction mechanism 38. Further, when the towing load decreases, in association with this decrease, the load detection member 36 is pivoted to the vehicle body rear side and also the pivotal member 38 is relatively pivoted in the front inclination direction under the action of the restriction mechanism 52. [0075]
And, in case the load detection member 36 is pivoted to the vehicle body front side and also the pivotal member 38 is relatively pivoted in the rear inclination direction, in association with this relative pivotal movement, the operational arm 53 is pivoted in the vehicle body rear direction and with this pivotal movement, the operational arm 53 presses the coupling portion 27A of the first pivotal arm 27 in the vehicle body rear direction. With this, the first pivotal arm 27 is pivoted in the vehicle body rear direction and the spool 22A of the control valve 22 operably coupled with the first pivotal arm 27 is moved from the neutral position to the elevated position against the action of the urging means. As a result, the plow 15A together with the left and right lift arms 20 is

elevated. [0076]
And, in case the load detection member 36 is pivoted to the vehicle body rear side and also the pivotal member 38 is relatively pivoted in the front inclination direction, in association with this relative pivotal movement, the operational arm 53 is pivoted in the vehicle body front direction and with this pivotal movement, the operational arm 53 is removed from the coupling portion 27A of the first pivotal arm 27 in the vehicle body front direction. With this, the pivotal movement of the first pivotal arm 27 in the vehicle body front direction is allowed and the spool 22A of the control valve 22 is moved from the neutral position to the lowered position under the action of the urging means. As a result, the plow 15A together with the left and right lift arms 20 is lowered. [0077]
Referring now to the feedback link mechanism 26, when the above-described change amount conversion mechanism 33 is under the amplified conversion state, if the relative pivotal movement between the load detection member 36 and the pivotal member 38 according to the towing load is stopped due to elevation or lowering of the plow 15A, in association with this stop of pivotal movement, the feedback link mechanism 26 operates to the spool 22A of the control valve 22 from the elevated position or the lowered position to the neutral position. With this, the plow 15A together with the left and right lift arms 20 stops its elevation or lowering. [0078]
With the above-described configuration, with this tractor, due to the inclusion of the mechanical coupling unit 18 having the above-described arrangement, it is possible to select a draft control taking into consideration e.g. hardness of soil which differs from one field to another. As a result, irrespectively of soil hardness which differs from one field to another, the cultivation work using the plow 15Acan be carried out in an favorable manner. [0079]
As shown in Figs. 1-2 and Fig. 4, the change amount conversion mechanism 33 is disposed adjacent the driver' seat 11 at a rear portion of this driver's seat 11 on the vehicle body. With this arrangement, the

worker, as being kept seated at the driver's seat 11, can readily check the conversion state of the change amount conversion mechanism 33 by viewing a portion rearwardly of the driver's seat 11. Further, since the upper side of such portion rearwardly of the driver's seat 11 is not covered by a cover or the like, maintenance on the change amount conversion mechanism 33 can be easily carried out. [0080]
The operational tool 35 is disposed adjacent the driver' seat 11 at a rear portion of this driver's seat 11 on the vehicle body. With this arrangement, the worker, as being kept seated at the driver's seat 11, can operate the operational tool 35 by reaching his/her arm to a rear portion of the driver's seat 11, so that the change amount conversion mechanism 33 can be easily switched to the standard conversion state, the amplified conversion state or to the second conversion state. [0081]
As shown in Figs. 3-5, Figs. 9-10 and Figs. 12-17, the operational tool 35 includes a pivotal plate 61 pivotally supported to the support bracket 47 to be pivotable in the left-right direction and an operational handle 62 that extends forwardly from the pivotal plate 61. And, the above-described contact member 40 extends rearwards from a left portion of the pivotal plate 61. Further, left and right support members 59 supporting the above-described roller 60 extend rearwards from a right portion of the pivotal plate 61. The operational tool 35 is pivoted in the left-right direction about an axis X which extends in the front-rear direction. The operational tool 35 is selectively position-maintained at either a first operational position, a second operational position or a third operational position, by means of a detent mechanism 63 provided between the support bracket 47 and the pivotal plate 61. [0082]
When the operational tool 35 is position-maintained at the first operational positon on the right side, the contact member 40 and the roller 60 are located at the retracted position. With this, the change amount conversion mechanism 33 is set to the standard conversion state in the first conversion state for the draft control. [0083]
When the operational tool 35 is position-maintained at the

second operational positon on the left-right intermediate side, the contact member 40 is located at the retracted position and the roller 60 is located at the active position. With this, the change amount conversion mechanism 33 is set to the amplified conversion state in the first conversion state for the draft control. [0084]
When the operational tool 35 is position-maintained at the third operational positon on the left side, the contact member 40 is located at the active position and the roller 60 is located at the retracted position. With this, the change amount conversion mechanism 33 is set to the second conversion state for the automatic cultivation depth control. [0085]
Namely, as the worker switches over the operational position of the operational tool 35, the change amount conversion mechanism 33 can be readily switched to the standard conversion state for draft control, the amplified conversion state for draft control or to the second conversion state for automatic cultivation depth control. [0086]
As shown in Figs. 3-5, Figs. 9-10 and Figs. 12-17, the roller 60 is set such that when it is located at the active position, the rotational axis of this roller 60 may be parallel with the pivotal axis of the pivotal member 38. With this setting, under the amplified conversion state of the change amount conversion mechanism 33, when the pivotal member 38 is slid relative to the roller 60 in association with a relative pivotal movement between the load detection member 36 and the pivotal member 38, in association with this sliding movement, the roller 60 is rotated in the sliding direction of the pivotal member 38. With this, the pivotal member 38 smoothly effects a sliding movement relative to the roller 60, whereby the load detection member 36 and the pivotal member 38 are smoothly pivoted relative to each other. As a result, the draft control under the amplified conversion state of the change amount conversion mechanism 33 can proceed smoothly. [0087]
As shown in Fig. 3, Fig. 5 and Figs. 9-17, the load detection member 36 includes left and right side wall portions 36A having opposed through holes 36a. The support bracket 47 includes a vertical wall

portion 47A connected to a rear portion of the vehicle body, and a coupling portion 47B having U-shape in plan view and extending rearwards to enter between the left and right side wall portions 36A. At a portion of each side wall portion 36A opposed to the through hole 36a, there are formed left and right elongate holes 47a elongate in the front-rear direction. And, the restriction mechanism 50 described hereinbefore is comprised of e.g. the respective through holes 36a of the load detection member 36, the respective elongate holes 47a of the support bracket 47, coupling pins 64 to be inserted into the respective through holes 36a and the respective elongate holes 47a, and so on. Namely, the back-to-forth pivotal range of the load detection member 36 is restricted by the front-rear length of the respective elongate hole 47a defined in the support bracket 47. [0088]
The restriction mechanism 50 includes a rubber block 65 which is fitted in the gap between the vertical wall portion 47A and the coupling portion 47B of the support bracket 47. In the rubber block 65, at a portion thereof opposed to the elongate hole 47a of the support bracket 47, there is formed an elongate hole 65a which has a shorter front-rear length than the elongate hole 47a of the support bracket 47. And, in this elongate hole 65a, the coupling pin 64 is inserted. [0089]
With the above-described arrangement, when the restriction mechanism 50 restricts the front-rear pivotal movement of the load detection member 36, the coupling pin 64 of the restriction mechanism 50 hits the rubber block 65. Thus, it is possible to prevent generation of a noise due to the coupling pin 64 hitting the coupling portion 47B of the support bracket 47. [0090]
As shown in Figs. 3-6 and Figs. 12-17, the mechanical coupling unit 18 includes a sensitivity adjustment mechanism 66 for adjusting operational sensitivity when the lift driving unit 17 is operably coupled with the pivotal member 38. This sensitivity adjustment mechanism 66 includes a sensitivity adjustment lever 67 position-maintained at a desired operational position in the driving section 9, a coupling member 69 provided from the sensitivity adjustment lever 67 to a support shaft

68 of the reversing arm 54, a supporting member 70 for supporting the support shaft 68 of the reversing arm 54 while allowing its displacement in the front-rear direction, and so on. With the sensitivity adjustment mechanism 66 in operation, when the supporting position of the support shaft 68 by the supporting member 70 is changed in the front-rear direction by a pivotal operation of the sensitivity adjustment lever 67 in the front-rear direction, in association with this change, a gap 71 between the coupling portion 27A of the first pivotal arm 27 and the operational arm 53 is changed. With this, it is possible to adjust the operational sensitivity when the lift driving unit 17 is operably coupled with the pivotal member 38. [0091]
The sensitivity adjustment mechanism 66 is configured such that the smaller the gap 71 described above, the higher the operational sensitivity when the lift driving unit 17 is operably coupled with the pivotal member 38, and the higher the responsiveness when the plow 15Ais elevated when the towing load exceeds a set value. [0092]
Conversely, the sensitivity adjustment mechanism 66 is configured such that the greater the gap 71 described above, the lower the operational sensitivity when the lift driving unit 17 is operably coupled with the pivotal member 38, and the lower the responsiveness when the plow 15Ais elevated when the towing load exceeds a set value. [0093]
As a result, the greater change in the towing load due to field condition such as violent undulation of the field, etc., thus, the greater the change in towing load, the lower the operational sensitivity can be made, so that deterioration in the cultivation work accuracy due to hunching as result of frequent up/down movements of the plow 15Acan be effectively prevented. [0094]
As shown in Figs. 3-4, Fig. 6 and Figs. 12-17, the supporting member 70 is fixed to a right lower portion of a seat supporting member 73 attached to the rear end portion of the T/M case 5. As the supporting member 70 has a receded portion 70A which is elongate in the front-rear direction from the front end to the rear side of the supporting member 70,

this supporting member 70 supports the support shaft 68 of the reversing arm 54 while allowing change in position thereof in the front-rear direction. [0095]
As shown in Figs. 3-5, Fig. 9-10 and Figs. 12-17, the restriction mechanism 52 includes a receiving tool 57 which is pivotable together with the load detection member 36 and an urging unit 58 which pivotally urges the pivotal member 38 in direction for pressing this member against the receiving tool 57. [0096]
With the above-described arrangement, at the time of cultivation work using the draft control, when the load detection member 36 is pivoted back and forth according to a towing load, the receiving tool 57 is pivoted together with the load detection member 36 and also the pivotal member 38 is kept being pressed against the receiving tool 57 under the action of the urging unit 58. With this, for instance, at the time of cultivation work, even if there occurs violent change in the towing load causing a violent back-and-forth pivotal movement of the load detection member 36, thanks to the action of the urging unit 58, excessive pivotal movement or delay of pivotal movement of the pivotal member 38 relative to the load detection member 36, etc. can be suppressed. [0097]
Thus, even when there occurs violent change in the towing load at the time of cultivation work using the draft control, it is possible to lessen occurrence of chattering due to excessive pivotal movement or delay of pivotal movement of the pivotal member 38 relative to the load detection member 36, etc. in the mechanical coupling unit 18. Consequently, deterioration in the control accuracy in the draft control due to chattering can be prevented. [0098]
As shown in Figs. 3-5, Fig. 10 and Figs. 12-17, the urging unit 58 includes a first urging mechanism 74 provided from the load detection member 36 to the second coupling member 56 of the second link mechanism 34 for suppressing the pivotal movement of the pivotal member 38 relative to the load detection member 36 and a second urging mechanism 75 provided from the seat supporting member 73 as a fixing

member of the vehicle body to the second coupling member 56 for suppressing the pivotal movements of the load detection member 36 and the pivotal member 38 in the towing load increasing direction. [0099]
With the above-described arrangements, in operation of the first urging mechanism 74, when the load detection member 36 and the pivotal member 38 are pivoted together, the force for suppressing the pivotal movement of the pivotal member 38 relative to the load detection member 36 is maintained constant, irrespectively of the back-and-forth pivotal movement of the load detection member 36 according to the towing load. Further, in operation of the first urging means 74, when the load detection member 36 and the pivotal member 38 are pivoted relative to each other, the greater the pivotal movement of the pivotal member 38 relative to the load detection member 36 in association with increase of the towing load, the greater the force for suppressing the pivotal movement of the pivotal member 38 relative to the load detection member 36 in the towing load increasing direction. [0100]
On the other hand, in operation of the second urging mechanism 75, in both the case of the load detection member 36 and the pivotal member 38 being pivoted together and the case of the the load detection member 36 and the pivotal member 38 being pivoted relative to each other, the greater the pivotal movement of the load detection member 36 in association with increase of the towing load, the greater the force for suppressing the pivotal movement of the pivotal member 38 relative to the load detection member 36 in the towing load increasing direction. Further, in operation of the second urging mechanism 75, in the case of the the load detection member 36 and the pivotal member 38 being pivoted relative to each other, in association with large pivotal movement of the load detection member 36 in the towing load increasing direction, the greater the pivotal movement of the pivotal member 38 relative to the load detection member 36, the greater the force for suppressing the pivotal movement of the pivotal member 38 relative to the load detection member 36 in the towing load increasing direction. With this, according to the second urging mechanism 75, the force for suppressing the pivotal movement of the pivotal member 38 relative to the load detection

member 36 in the towing load increasing direction is greater in the case of the the load detection member 36 and the pivotal member 38 being pivoted relative to each other than the case of the load detection member 36 and the pivotal member 38 being pivoted together. [0101]
Namely, according to this second urging mechanism 75, irrespectively of the case of the load detection member 36 and the pivotal member 38 being pivoted together or the case of the load detection member 36 and the pivotal member 38 being pivoted relative to each other, the greater the pivotal movement of the load detection member 36 in the towing load increasing direction, the greater the force for suppressing the pivotal movement of the pivotal member 38 relative to the load detection member 36 in the towing load increasing direction, so the mechanism acts as a damper for damping the pivotal movements of the load detection member 36 and the pivotal member 38 in the towing load increasing direction. [0102]
With the above, irrespectively of the case of the load detection member 36 and the pivotal member 38 being pivoted together or the case of the load detection member 36 and the pivotal member 38 being pivoted relative to each other, when there occurs violent change in the towing load causing violent back-and-forth pivotal movement of the load detection member 36, thanks to the action of the second urging mechanism 75, the excessive pivotal movement and pivotal delay of the pivotal member 38 relative to the load detection member 36, etc. can be more effectively suppressed. [0103]
Further, in the case of the load detection member 36 and the pivotal member 38 being pivoted relative to each other, when change in the towing load becomes violet, thus causing violent relative pivotal movement between the load detection member 36 and the pivotal member 38, thanks to the actions of the first urging mechanism 74 and the second urging mechanism 75, the excessive pivotal movement and pivotal delay of the pivotal member 38 relative to the load detection member 36, etc. can be more effectively suppressed. [0104]

And, since there is no need for the action of the first urging mechanism 74 alone to cope with the situation of violent variation in the towing load, it is possible to avoid occurrence of inconvenience of difficulty of the pivotal movement of the pivotal member 38 relative to the load detection member 36, which would occur in case the measure thereof were provided solely by the function of the first urging mechanism 74. [0105]
Consequently, even if there occurs violent variation in the towing load at the time of cultivation work using the draft control, the chattering phenomenon in the mechanical coupling unit due to the excessive pivotal movement or pivotal delay of the pivotal member 38 relative to the load detection member 36 can be suppressed in a more reliable and favorable manner. Thus, deterioration in the control accuracy in the draft control due to chattering can be prevented even more reliably. [0106]
As shown in Fig. 3, Fig. 5, Figs. 9-10 and Figs. 12-17, the first urging mechanism 74 includes a first tension spring 76 provided from the load detection member 36 to the second coupling member 56 and a second tension spring 77 having a smaller diameter than the first tension spring 76. The second tension spring 77 extends through inside of the first tension spring 76 to be provided from the load detection member 36 to the second coupling member 56. [0107]
With the above-described solution, it is possible to form the outer shape of the first urging mechanism 74 with a smaller diameter than a case of constituting the first urging mechanism 74 of a single tension spring. Thus, it becomes possible for the first urging mechanism 74 to secure a strong tension required without inviting enlargement of the first urging mechanism 74. So, it becomes possible to avoid such inconvenience as the first urging mechanism 74 interfering with a member present in its periphery. [0108]
As shown in Figs. 3-5 and Figs. 12-17, the second urging mechanism 75 includes a first boss member 78 fixed to the seat

supporting member 73, a second boss member 79 supported to the second coupling member 56 to be pivotable back and forth, a rod 80 mounted between and to the first boss member 78 and the second boss member 79, a compression spring 81 fitted on the rod 80, a spring receiving tool 82 for receiving the rear end of the compression spring 81, a positioning pin 83 for positioning the spring receiving tool 82 relative to the rod 80, and so on. The first boss member 78 slidably supports a front portion of the rod 80 and also receives the front end of the compression spring 81. To the second boss member 79, the rear end of the rod 80 is pin-connected. At the rear end of the rod 80, there are formed three through holes 80a for allowing change of the attaching position of the positioning pin 83 relative to the rod 80. [0109]
With the above-described arrangement, the second urging mechanism 75 allows change of the position of the spring receiving tool 82 in three stages. With this, the urging force of the second urging mechanism 75 can be changed in three stages, in accordance with soil hardness of field which differs regionally, for instance. [0110]
As a result, irrespectively of soil hardness of field which differs regionally, the risk of occurrence of chattering in the mechanical coupling unit 18 at the time of cultivation work using the draft control can be suppressed in an even more reliable and favorable manner. [0111]
As shown in Figs. 1-3, Fig. 5, Fig. 9, and Figs. 11-17, the load detection member 36 includes a first coupling portion 36B to which a long top link 13A (see Fig. 1, Fig. 3, Fig. 5, Fig. 9, Figs. 11-15) is connected, and a second coupling portion 36C to which a bracket 72 (see Fig. 2, Figs. 16-17) supporting a short top link 13B is connected. [0112]
With the above, in case a cultivation work using the rotary cultivator 15B is to be carried out, the specification of the three-point linkage mechanism 12 can be readily changed to a standard linkage specification having the long top link 13A and the left and right lower links 14 or to a special linkage specification having the short top link 13B and the left and right lower links 14.

[0113]
And, if the specification of the three-point linkage mechanism 12 is changed from the standard linkage specification to the special linkage specification, the lift driving amount of the rotary cultivator 15B relative to a lift operational amount of the lift driving unit 17 is increased and the uppermost elevated position of the rotary cultivator 15B becomes higher. [0114]
As a result, in case a cultivation work using the rotary cultivator 15B is to be carried out in a work land having high ridge, by changing the specification of the three-point linkage mechanism 12 from the standard linkage specification to the special linkage specification in advance, it becomes readily possible to avoid the risk of the rotary cultivator 15B coming into contact with the high ridge when a ridge climbing run or ridge turning run is effected for instance. [0115]
The load detection member 36 includes, as the first coupling portion 36B, a single connecting hole that allows the pin-connection of the long top link 13A. Further, the load detection member 36 includes, as the second coupling portion 36C, a pair of upper and lower connecting holes that allow the pin-connection of the bracket 72.
[0116]
[Other Embodiments]
The present invention is not limited to the arrangements illustrated in the foregoing embodiment. Some typical further embodiments of the present invention will be illustrated below. [0117]
[l] The tractor can employ alternative arrangements illustrated below.
For instance, the tractor can be configured as a semi-crawler type having left and right crawlers in place of the left and right rear wheels 7.
For instance, the tractor can be configured as a full-crawler type having left and right crawlers in place of the left and right front wheels 6 and left and right crawlers in place of the left and right rear wheels 7.
For instance, the tractor can be configured as an electrically powered type having an electric motor in place of the engine 2.

For instance, the tractor can be configured as a hybrid type having the engine 2 and an electric motor. [0118]
[2] The configuration of the lift driving unit 17 can vary in many ways.
For instance, the lift driving unit 17 can include a hydraulic motor or the like in place of the hydraulic cylinder 21.
For instance, the lift driving unit 17 can include, outside the control valve 22, an urging means for urging this spool 22A of the control valve 22 to return to the lowered position. [0119]
[3] The configuration of the mechanical coupling unit 18 can vary in many ways.
For instance, the mechanical coupling unit 18 may include, as the pivotal member 38, a pivotal member 38 for draft control pivotally supported to the load detection member 36, and a pivotal member 38 for draft control pivotally supported to the support bracket 47. Further, the mechanical coupling unit 18 may include, as the link mechanism 34, a link mechanism 34 for draft control for operably coupling the draft control pivotal member 38 to the lift driving unit 17 and a link mechanism 34 for automatic cultivation depth control for operably coupling the automatic cultivation depth control pivotal member 38 to the lift driving unit 17. In operation, in accordance with an operation on the operational tool 35, the mechanical coupling unit 18 is configured to be switched to a standard conversion state of a first conversion state for causing the load detection member 36 and the draft control pivotal member 38 to be pivoted together, an amplified conversion state of the first conversion state for causing the load detection member 36 and the pivotal member 38 to be pivoted relative to each other and also preventing a pivotal movement of the automatic cultivation depth control pivotal member 38, or to a second conversion state for preventing pivotal movements of the load detection member 36 and the draft control pivotal member 38 and also pivoting the automatic cultivation depth control pivotal member 38.
For instance, the mechanical coupling unit 18 may be configured to allow switchover between the standard conversion state and the

amplified conversion state of the change amount conversion mechanism 33 and to inhibit switchover of the change amount conversion mechanism 33 to the second conversion state.
For instance, the mechanical coupling unit 18 may be configured to transmit the change amount of towing load obtained by the back-and-forth pivotal movement of the load detection member 36 to the lift driving unit 17 only under a state wherein the change amount is converted into a lift operation amount with amplification thereof by a relative pivotal movement between the load detection member 36 and the pivotal member 38. [0120]
[4] The configuration of the load detection member 36 can vary in many ways.
For instance, the load detection member 36 can be configured such that its upper end portion is supported to the support bracket 47 via the first support shaft 48.
For instance, the load detection member 36 can be configured to be supported to the support bracket 47 to be pivotable back and forth, via a vertically long first support shaft 48. [0121]
[5] The configuration of the restriction mechanism 52 can vary in many ways.
For instance, the restriction mechanism 52 can be configured such that the first urging mechanism 74 of the urging unit 58 is mounted from the load detection member 36 to the pivotal member 38.
For instance, the restriction mechanism 52 can be configured such that the first urging mechanism 74 of the urging unit 58 is constituted of a single tension spring mounted from the load detection member 36 to the link mechanism 34.
For instance, the restriction mechanism 52 can be configured such that the first urging mechanism 74 of the urging unit 58 includes a tension spring mounted from the load detection member 36 to the link mechanism 34 and a torsion spring mounted from the load detection member 36 to the pivotal member 38.
For instance, the restriction mechanism 52 can be configured such that the second urging mechanism 75 of the urging unit 58 is

mounted from the fixing member 73 of the vehicle body to the pivotal member 38.
For instance, the restriction mechanism 52 can be configured such that the second urging mechanism 75 of the urging unit 58 is constituted of a gas damper mounted from the vehicle body side fixing member 73 to the pivotal member 38 or the link mechanism 34. [0122]
[6] The vehicle body side fixing member 73 to which the second urging mechanism 75 is mounted can be not only the seat supporting member 73, but also a special member attached to the rear end portion of the T/M case 5 for the sake of mounting of the second urging mechanism 75.
INDUSTRIAL APPLICABILITY [0123]
The present invention can be applied to a tractor including a three-point linkage mechanism that is vertically pivotally connected to a rear portion of a vehicle body and that allows attachment of a utility implement including a cultivator, a hydraulic lift driving unit for liftably driving the three-point linkage mechanism, and a mechanical coupling unit for automatic lifting, the coupling unit being configured to convert a change amount of a towing load to a lift operational amount at time of a cultivation work in which the cultivator is attached to the three-point linkage mechanism and to transmit the lift operational amount to the lift driving unit.
DESCRIPTON OF REFERENCE MARKS/NUMERALS [0124]
12: three-point linkage mechanism
13: top link
15: cultivator
17: lift driving unit
18: mechanical coupling unit
34: second link mechanism (link mechanism)
36: load detection member
38: pivotal member

49^ urging mechanism
52: restriction mechanism
57: receiving tool
58: urging unit
73: fixing member
74: first urging mechanism
75: second urging mechanism
76: first tension spring
77: second tension spring

WE CLAIM:
1. A tractor comprising:
a three-point linkage mechanism that is vertically pivotally connected to a rear portion of a vehicle body and that allows attachment of a utility implement including a cultivator;
a hydraulic lift driving unit for liftably driving the three-point linkage mechanism; and
a mechanical coupling unit for automatic lifting, the mechanical coupling unit being configured to convert a change amount of a towing load to a lift operational amount at time of a cultivation work in which the cultivator is attached to the three-point linkage mechanism and to transmit the lift operational amount to the lift driving unit; wherein the mechanical coupling unit includes:
a load detection member pivotable back and forth according to a towing load transmitted via a top link of the three-point linkage mechanism,
an urging mechanism for pivotally urging the load detection member in a decreasing direction of the towing load,
a pivotal member pivotally supported to the load detection member,
a restriction mechanism for restricting a pivotal movement of the pivotal member relative to the load detection member, and
a link mechanism for operably coupling the pivotal member with the lift driving unit; and
wherein the restriction mechanism includes:
a receiving tool pivotable together with the load detection member, and
an urging unit for pivotally urging the pivotal member for pressing the pivotal member against the receiving tool.
2. The tractor of claim 1, wherein:
the urging unit includes:
a first urging mechanism provided between the load detection member and the pivotal member or the link mechanism for

suppressing pivotal movement of the pivotal member relative to the load detection member, and
a second urging mechanism provided between a fixing member included in the vehicle body and the pivotal member or the link mechanism for suppressing pivotal movements of the load detection member and the pivotal member in the increase direction of the towing load.
3. The tractor of claim 2, wherein:
the first urging mechanism includes a first tension spring provided from the load detection member to the pivotal member or the link mechanism and a second tension spring having a smaller diameter than the first tension spring; and
the second tension spring extends through inside of the first tension spring to be provided from the load detection member to the pivotal member or the link mechanism.
4. The tractor of claim 2 or 3, wherein:
the second urging mechanism includes a rod that is inserted and supported in/to the fixing member to be freely projectable into and withdrawable from the fixing member and that is operably coupled with the pivotal member, and a compression spring fitted on the rod;
one end of the compression spring is placed in contact with the fixing member and the other end of the compression spring is supported to the rod; and
a supporting position of the other end to the rod is variable.