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A tractor has hydraulically driven wheels at a cab end and castor wheels at an engine end. It can be driven cab forward in a working mode with a header on the forward end. It is rotated to engine forward in the transport position for more stable higher speed travel. The driven wheels are driven by h

A tractor has hydraulically driven wheels at a cab end and castor wheels at an engine end. It can be driven cab forward in a working mode with a header on the forward end. It is rotated to engine forward in the transport position for more stable higher speed travel. The driven wheels are driven by hydraulic motors each having their own drive pump the output of which is controlled by a cam plate. There is provided an automatic steering system having a guidance controller arranged to receive GPS position information and feed back signals and to generate output for controlling a hydraulic control valve for controlling flow in hydraulic fluid for a double acting hydraulic control cylinder arranged to actuate pivotal movement of the steering link member independently of the steering shaft.

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The invention claimed is: 1. A tractor comprising: a tractor frame; an engine mounted on the tractor frame; a cab mounted on the tractor frame; a first and a second driven wheels of the tractor mounted at a driven end of the tractor, the first driven wheel including a first hydraulic drive motor fo

The invention claimed is: 1. A tractor comprising: a tractor frame; an engine mounted on the tractor frame; a cab mounted on the tractor frame; a first and a second driven wheels of the tractor mounted at a driven end of the tractor, the first driven wheel including a first hydraulic drive motor for driving the first wheel and the second driven wheel including a second hydraulic drive motor for driving the second wheel; two non-driven castor wheels of the tractor mounted at a second end of the tractor; the driven end having mounting assemblies for supporting a crop cutting header at a position in front of the driven end and in front of the cab when the driven end is forwards; a first and a second hydraulic pump each driven by the engine for generating a source of hydraulic fluid under pressure, with the first hydraulic pump arranged for driving the first hydraulic motor of the first driven wheel and the second hydraulic pump arranged for driving the second hydraulic motor of the second driven wheel; the first hydraulic pump including a first cam plate movable to vary the displacement of the pump to vary the volume of fluid generated as the pump is driven and the second hydraulic pump including a second cam plate movable to vary the displacement of the pump to vary the volume of fluid generated as the pump is driven; a manually operable steering control movable between left and right positions through a center position; the manually operable steering control including a steering wheel and a steering shaft rotatable by the steering wheel; a manually operable speed control movable between forward and reverse positions through a neutral position; the manually operable speed control being arranged to control a rate of supply of hydraulic fluid from the first hydraulic pump to the first drive motor and from the second hydraulic pump to the second drive motor to control a common speed of rotation of the driven wheels and thus a speed of movement of the tractor; the manually operable steering control being arranged to control a differential in rate of supply of hydraulic fluid from the first hydraulic pump to the first drive motor and from the second hydraulic pump to the second drive motor to control a relative speed of rotation of the driven wheels and thus a turning direction of the tractor; a linkage including a first link for controlling the first cam plate and a second link for controlling the second cam plate; the linkage including a speed control member operable by the manually operable speed control with the speed control member arranged, when actuated in an actuation direction, to move both the first and second links for moving the first and second cam plates; the linkage including a steering link member pivotally mounted relative to the speed control member so as, when actuated, to pivot to respective sides so as to move the first and second links relative to the speed control member to effect differential movement of the first and second cam plates; the steering link member being pivoted by input caused by rotation of the steering shaft; and an automatic steering system for providing a steering action in response to position information so as to steer the tractor to required positions comprising: a guidance controller arranged to receive the position information and to generate output signals for controlling the steering action; a hydraulic control valve arrangement responsive to the signals from the guidance controller for controlling flow in hydraulic fluid for supply of the fluid to first and second supply ducts; a double acting hydraulic control cylinder arrangement for receiving the hydraulic fluid from the first and second supply ducts and including a drive shaft of the cylinder arrangement which is driven in first and second directions in response to supply of fluid from the first and second ducts; the drive shaft being arranged to actuate pivotal movement of the steering link member independently of the steering shaft. 2. The tractor according to claim 1 wherein the steering link member is connected to an arm actuated by the steering shaft for actuating the pivotal movement of the steering link member and wherein the drive shaft of the cylinder is connected to the arm. 3. The tractor according to claim 1 wherein the cylinder is connected to a fixed point on the tractor. 4. The tractor according to claim 3 wherein the cylinder is arranged such that the drive shaft extends in a direction generally at right angles to the direction of actuation of the speed control member. 5. The tractor according to claim 1 wherein the cylinder is connected to a point movable with the speed control and is arranged to move the steering link member relative to the point. 6. The tractor according to claim 1 wherein the hydraulic control valve arrangement includes a proportional flow valve arranged to cause a flow of fluid to the first and second ducts which flow is proportional to an amplitude value of the output signals from the guidance controller. 7. The tractor according to claim 1 wherein the hydraulic control valve arrangement includes a bypass valve to allow hydraulic fluid in the cylinder to by-pass the cylinder when the cylinder is driven by movement of the steering link member from the steering shaft caused by a manual over-ride of the steering. 8. The tractor according to claim 7 wherein the hydraulic control valve arrangement is arranged such that the bypass valve includes a movable valve member which is operated by detection by a component of the hydraulic control valve arrangement of manual override of the steering generated by forces applied to the steering wheel. 9. The tractor according to claim 8 wherein the component of the hydraulic control valve arrangement is arranged for detection of the manual override of the steering by detecting a temporary increase in pressure in the hydraulic fluid supplied to the cylinder caused by the forces from the manual override. 10. The tractor according to claim 8 wherein the component of the hydraulic control valve arrangement is arranged to provide an output arranged to effect shut off of the output signals from the guidance controller. 11. The tractor according to claim 1 wherein the hydraulic control valve arrangement includes a component for detecting manual override of the steering and for providing an output arranged to effect shut off of the output signals from the guidance controller. 12. The tractor according to claim 11 wherein the component of the hydraulic control valve arrangement is arranged for detection of the manual override of the steering by detecting a temporary increase in pressure in the hydraulic fluid supplied to the cylinder caused by the forces from the manual override. 13. The tractor according to claim 1 wherein the hydraulic control valve arrangement includes a pressure reducing valve for reducing pressure in a fluid supply to the hydraulic control valve arrangement. 14. The tractor according to claim 1 wherein there is provided a sensor for detecting turning movement of the tractor for feedback to the hydraulic control valve arrangement. 15. An automatic steering system for providing a steering action in response to position information so as to steer a tractor to required positions where the tractor includes: a first and a second driven wheels of the tractor mounted at a driven end of the tractor, the first driven wheel including a first hydraulic drive motor for driving the first wheel and the second driven wheel including a second hydraulic drive motor for driving the second wheel; a first and a second hydraulic pump each driven by the engine for generating a source of hydraulic fluid under pressure, with the first hydraulic pump arranged for driving the first hydraulic motor of the first driven wheel and the second hydraulic pump arranged for driving the second hydraulic motor of the second driven wheel; the first hydraulic pump including a first cam plate movable to vary the displacement of the pump to vary the volume of fluid generated as the pump is driven and the second hydraulic pump including a second cam plate movable to vary the displacement of the pump to vary the volume of fluid generated as the pump is driven; a manually operable steering control movable between left and right positions through a center position; the manually operable steering control including a steering wheel and a steering shaft rotatable by the steering wheel; a manually operable speed control movable between forward and reverse positions through a neutral position; the manually operable speed control being arranged to control a rate of supply of hydraulic fluid from the first hydraulic pump to the first drive motor and from the second hydraulic pump to the second drive motor to control a common speed of rotation of the driven wheels and thus a speed of movement of the tractor; the manually operable steering control being arranged to control a differential in rate of supply of hydraulic fluid from the first hydraulic pump to the first drive motor and from the second hydraulic pump to the second drive motor to control a relative speed of rotation of the driven wheels and thus a turning direction of the tractor; a linkage including a first link for controlling the first cam plate and a second link for controlling the second cam plate; the linkage including a speed control member operable by the manually operable speed control with the speed control member arranged, when actuated in an actuation direction, to move both the first and second links for moving the first and second cam plates; the linkage including a steering link member pivotally mounted relative to the speed control member so as, when actuated, to pivot to respective sides so as to move the first and second links relative to the speed control member to effect differential movement of the first and second cam plates; the steering link member being pivoted by input caused by rotation of the steering shaft; the automatic steering system comprising: a guidance controller arranged to receive the position information and to generate output signals for controlling the steering action; a hydraulic control valve arrangement responsive to the signals from the guidance controller for controlling flow in hydraulic fluid for supply of the fluid to first and second supply ducts; a double acting hydraulic control cylinder arrangement for receiving the hydraulic fluid from the first and second supply ducts and including a drive shaft of the cylinder arrangement which is driven in first and second directions in response to supply of fluid from the first and second ducts; the drive shaft being arranged to actuate pivotal movement of the steering link member independently of the steering shaft. 16. The automatic steering system according to claim 15 wherein the hydraulic control valve arrangement includes a proportional flow valve arranged to cause a flow of fluid to the first and second ducts which flow is proportional to an amplitude value of the output signals from the guidance controller. 17. The automatic steering system according to claim 15 wherein the hydraulic control valve arrangement includes a bypass valve to allow hydraulic fluid in the cylinder to by-pass the cylinder when the cylinder is driven by movement of the steering link member from the steering shaft caused by a manual over-ride of the steering. 18. The automatic steering system according to claim 17 wherein the hydraulic control valve arrangement is arranged such that the bypass valve includes a movable valve member which is operated by detection by a component of the hydraulic control valve arrangement of manual override of the steering generated by forces applied to the steering wheel. 19. The automatic steering system according to claim 18 wherein the component of the hydraulic control valve arrangement is arranged for detection of the manual override of the steering by detecting a temporary increase in pressure in the hydraulic fluid supplied to the cylinder caused by the forces from the manual override. 20. The automatic steering system according to claim 19 wherein the component of the hydraulic control valve arrangement is arranged to provide an output arranged to effect shut off of the output signals from the guidance controller. 21. The automatic steering system according to claim 15 wherein the hydraulic control valve arrangement includes a component for detecting manual override of the steering and for providing an output arranged to effect shut off of the output signals from the guidance controller. 22. The automatic steering system according to claim 21 wherein the component of the hydraulic control valve arrangement is arranged for detection of the manual override of the steering by detecting a temporary increase in pressure in the hydraulic fluid supplied to the cylinder caused by the forces from the manual override. 23. The automatic steering system according to claim 15 wherein the hydraulic control valve arrangement includes a pressure reducing valve for reducing pressure in a fluid supply to the hydraulic control valve arrangement. 24. The automatic steering system according to claim 15 wherein there is provided a sensor for detecting turning movement of the tractor for feedback to the hydraulic control valve arrangement.