1. A tractor comprising: a lift arm for hang-supporting a traction type implement; a hydraulic actuator for pivotally driving the lift arm in a vertical direction; a control valve for controlling operation of the hydraulic actuator; an urging means for urging a spool of the control valve to return to its lowered position; a height setting lever for setting a control target height of the implement; a first link mechanism for operably linking the spool to the height setting lever; a feedback link mechanism for operably linking the spool to the lift arm; a load detecting member movable back and forth in accordance with a traction load; a second link mechanism for operably linking the spool to the load detecting member; and a sensitivity adjusting mechanism for varying operational sensitivity of the spool when this spool is operably linked to the load detecting member; wherein when the traction load exceeds a set value, the load detecting member is displaced to a vehicle body front side in association with rise of the traction load and the load detecting member is displaced to a vehicle body rear side in association with drop of the traction load; wherein the first link mechanism includes a pivot arm that comes into contact with the height setting lever from a high-position setting side of the height setting lever; the first link mechanism operates the spool toward an elevated position side when the pivot arm is pivoted in a high-position setting direction in association with a pivotal operation on the height setting lever in a high-position setting direction,; the first link mechanism allows movement of the spool toward a lowered positon side in association with a pivotal operation on the height setting lever in a low-position setting direction; and wherein the second link mechanism includes an operational arm pivotally supported to the pivot arm; the second link mechanism operates the spool toward the elevated position side when the operational arm is pivoted in the high-position setting direction to press the pivot arm in association with a movement of the load detecting member toward the vehicle body front side; and the second link mechanism allows a movement of the spool toward the lowered-position side when the operational arm is pivoted in the low-position setting direction to move away from the pivot arm in association with a movement of the load detecting member toward the vehicle body rear side; wherein the feedback link mechanism operates the spool to a neutral position in response to arrival of the implement at the control target height or stop of the pivotal movement of the load detecting member; and wherein the sensitivity adjusting mechanism adjusts the operational sensitivity by varying a gap between the pivot arm and the operational arm in response to an operation on a sensitivity adjusting lever disposed in a driving section.

2. The tractor according to claim 1, wherein the second link mechanism includes a relay arm which is disposed under a vertical posture and is balance-pivoted in the front/rear direction about a pivotal support shaft which is disposed under a left/right oriented posture, a first linking member extending from the load detecting member to an upper end portion of the relay arm and a second linking member extending from a lower end portion of the relay arm to the operational arm.

3. The tractor according to claim 2, wherein: the first linking member is disposed on one left/right side of the relay arm and connected to an upper end portion of the relay arm from the one left/right side of the relay arm; and the second linking member is disposed on the other left/right side of the relay arm and connected to a lower end portion of the relay arm from the other left/right side of the relay arm.

4. The tractor according to claim 2 or 3, wherein: the pivotal support shaft is supported to a supporting member provided on the vehicle body side, with the pivotal support shaft being position-changeable relative to the supporting member; the sensitivity adjusting mechanism includes a linking member extending from the sensitivity adjusting lever to the pivotal support shaft: and the sensitivity adjusting mechanism varies a gap between the pivot arm and the operational arm in association with change of a fixed position of the pivotal support shaft by an operation on the sensitivity adjusting lever.

WE CLAIM:
1. A tractor comprising:
a lift arm for hang-supporting a traction type implement;
a hydraulic actuator for pivotally driving the lift arm in a vertical direction;
a control valve for controlling operation of the hydraulic actuator;
an urging means for urging a spool of the control valve to return to its lowered position;
a height setting lever for setting a control target height of the implement;
a first link mechanism for operably linking the spool to the height setting lever;
a feedback link mechanism for operably linking the spool to the lift arm;
a load detecting member movable back and forth in accordance with a traction load;
a second link mechanism for operably linking the spool to the load detecting member; and
a sensitivity adjusting mechanism for varying operational sensitivity of the spool when this spool is operably linked to the load detecting member;
wherein when the traction load exceeds a set value, the load detecting member is displaced to a vehicle body front side in association with rise of the traction load and the load detecting member is displaced to a vehicle body rear side in association with drop of the traction load;
wherein the first link mechanism includes a pivot arm that comes into contact with the height setting lever from a high-position setting side of the height setting lever;
the first link mechanism operates the spool toward an elevated position side when the pivot arm is pivoted in a high-position setting direction in association with a pivotal operation on the height setting lever in a high-position setting direction,;
the first link mechanism allows movement of the spool toward a lowered positon side in association with a pivotal operation on the height setting lever in a low-position setting direction; and
wherein the second link mechanism includes an operational arm pivotally supported to the pivot arm;
the second link mechanism operates the spool toward the elevated position side when the operational arm is pivoted in the high-position setting direction to press the pivot arm in association with a movement of the load detecting member toward the vehicle body front side; and
the second link mechanism allows a movement of the spool toward the lowered-position side when the operational arm is pivoted in the low-position setting direction to move away from the pivot arm in association with a movement of the load detecting member toward the vehicle body rear side;

wherein the feedback link mechanism operates the spool to a neutral position in response to arrival of the implement at the control target height or stop of the pivotal movement of the load detecting member; and
wherein the sensitivity adjusting mechanism adjusts the operational sensitivity by varying a gap between the pivot arm and the operational arm in response to an operation on a sensitivity adjusting lever disposed in a driving section.
2. The tractor according to claim 1, wherein the second link mechanism includes a relay arm which is disposed under a vertical posture and is balance-pivoted in the front/rear direction about a pivotal support shaft which is disposed under a left/right oriented posture, a first linking member extending from the load detecting member to an upper end portion of the relay arm and a second linking member extending from a lower end portion of the relay arm to the operational arm.
3. The tractor according to claim 2, wherein:
the first linking member is disposed on one left/right side of the relay arm and connected to an upper end portion of the relay arm from the one left/right side of the relay arm; and
the second linking member is disposed on the other left/right side of the relay arm and connected to a lower end portion of the relay arm from the other left/right side of the relay arm.
4. The tractor according to claim 2 or 3, wherein:
the pivotal support shaft is supported to a supporting member provided on the vehicle body side, with the pivotal support shaft being position-changeable relative to the supporting member;
the sensitivity adjusting mechanism includes a linking member extending from the sensitivity adjusting lever to the pivotal support shaft: and
the sensitivity adjusting mechanism varies a gap between the pivot arm and the operational arm in association with change of a fixed position of the pivotal support shaft by an operation on the sensitivity adjusting lever.