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;
a load detecting member displaceably supported to a supporting member which is fixed to a vehicle body rear portion;
an urging mechanism for urging displacement of the load detecting member in a direction against a traction load applied to the load detecting member; and
a link mechanism for operably linking a spool of a the control valve to the load detecting member;
wherein the urging means includes a coil spring configured to allow displacement of the load detecting member according to the traction load when the traction load exceeds a set value.
2. The tractor according to claim 1, wherein:
the load detecting member is supported via a supporting shaft to the supporting member to be pivotally displaceable relative thereto; and
the urging mechanism is disposed between the supporting member and a free end portion of the load detecting member.
3. The tractor according to claim 2, wherein:
there is provided a restricting mechanism for restricting displacement of the load detecting member; and
the restricting mechanism is disposed at a position between the supporting shaft and the urging mechanism, which position is between the supporting member and the load detecting member.

4. The tractor according to claim 3, wherein the restricting mechanism includes an elongate hole which is formed in one of the supporting member and the load detecting member and formed elongate in a displacing direction of the load detecting member, and a pin attached to the other of the supporting member and the load detecting member and inserted into the elongate hole.
5. The tractor according to any one of claims 1-4, wherein the urging mechanism includes a spring force adjusting portion for allowing change of the set value.