This invention relates to tractor suspension and in particular to arrangements for improving rear axle suspension on a tractor.

It is well known to provide a suspension of front axle of a tractor and typically such a front axle suspension is provided by mounting a rigid front axle on the chassis of the tractor for pivoting about a first axis generally parallel to the longitudinal axis of the chassis to allow vertical movement of front wheels carried on the ends of the front axle. The rear axle of the tractor is normally rigidly attached to the chassis and ride quality for the driver is improved by vibrationally isolating the driver's cab from the chassis.

Whilst this arrangement may cope with small ground irregularities it does not cope well with larger bumps and obstacles nor with sideway movements of the cabin and driver.

There is therefore a requirement to provide a simply robust and relative cheap form of suspension for the rear axle of a tractor to further improve ride quality.

Thus according to the present invention there is provided a tractor having a chassis, rear axle mounting means for mounting a rigid rear axle on the chassis for pivoting about a first axis generally parallel to the longitudinal axis of the chassis to allow vertical movement of rear wheels carried by the rear axle, first spring/damping means acting between the rear axle and the chassis, a front axle mounted on the chassis by a front axle mounting means for pivoting about a second axis also generally parallel to the longitudinal axis of the chassis to allow vertical movement of the front wheels carried by the front axle, second spring/damping means act between the front axle and the chassis, and steering means act to steer the front wheels relative to the front axle.

A lock-out means may be provided to prevent pivoting of the rear axle about the first axis when desired. Such an arrangement may be particularly desirable when ploughing and may be achieved by locking-up the spring damping means or a mechanical lock between the axle and chassis. The rear wheels may be provided with steering means for steering the rear wheels relative to the rear axle to provide a four wheel steering capability when desired.

In some vehicles it may be possible for the front and rear axles to be identical or very similar in construction thus achieving further economies of production.

The front or rear axle mounting means may also allow vertical movement of the front or rear axle relative to the chassis.

The first or second damping means referred to above may be passive, semi-active or fully active depending on the operational requirements of the vehicle.

One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 shows diagrammatically a perspective view of a tractor rear suspension in accordance with the present invention;

Figure 2 shows the rear axle used in the suspension of Figure 1 ;

Figures 3 and 4 shows perspective and end views respectively of an actual tractor rear suspension of the form shown in Figure 1 provided with a wheel steering function;

Figure 5 shows diagrammatically a hydraulic passive suspension circuit for use in the damping of the movement of the axles of Figures 1 to 4;

Figure 6 shows diagrammatically a hydraulic semi-active suspension circuit for semi- active damping of the movement of the axles of Figures 1 to 4;

Figure 7 shows diagrammatically a hydraulic active suspension circuit for the fully active damping of the movement of the axles of Figures 1 to 4, and Figure 8 shows diagrammatical Iy a tractor chassis provided with similar suspended rear and front axles.

Referring to the drawings a tractor chassis 10 has a rigid rear axle casing I l which is mounted on the chassis 10 for pivoting about a longitudinally extending axis X-X.

Drive into axle 11 is via an input shaft 9 into a differential 13 and hence through drive shafts 12 to final drive assemblies 14 and hence to the rear wheels mounted on bolts 15.

The chassis includes a transversely extending cut-out 16 which allows the pivoting of axle 11 about axis X-X. Pivoting of axle 11 is controlled by one or more springs/dampers 17 which act between the axle 11 and the chassis 10. These springs/dampers may be hydraulic or pneumatic piston type actuators or could be steel spring with hydraulic or pneumatic damping.

In the arrangement shown diagrammatically in Figures 1 and 2, the wheels are not steerable relative to the axle 11 but in the arrangement shown in Figures 3 and 4 the rear wheels are steerable about axes Y by a control hydraulic steering cylinder 18 and steering rod 18a to provide a tractor with a four wheel steering capability.

When desired, the pivoting of axle 11 relative to chassis 10 can be locked-out using springs/dampers 17 or a mechanical lock between the axle 11 and the chassis 10.

Typically the pivoting of the axle about axis X-X is in the range +/- 5degrees as shown by the angle 0 in Figure 4.

When hydraulic suspension is used the springs/dampers 17 can be arranged to act either passively, semi-actively or fully actively using the hydraulic connections shown diagrammatically in Figures 5, 6 and 7 respectively.

In the arrangement shown in Figure 5 each actuator 17 is connected with a source of fluid pressure (which could be hydraulic or pneumatic) from a pump 25 via fluid a flow control valve 26. Normally control valve 26 remains closed so that any movement between the axle 11 and the chassis 10 results in movement of a piston 20 of the actuator 17 and consequently the movement of fluid from one side to the other of the piston 20 and hence via fixed orifices 21 into associated accumulators 22. Thus a fixed damping and spring support characteristic is provided which does not vary with the operating conditions of the tractor. Solenoid locking valves 27 and 28 are provided which when closed isolate the actuator 17 from the accumulators 22 and hence lock the axle 1 1 relative to the chassis 10 when required.

Using this basic rear axle suspension arrangement it is possible to attenuate the roll of the chassis and, in combination with the customary cab and seat suspension used on tractors, control the vibrations experienced by the tractor driver.

The control valve 26 is provided so that fluid can be admitted either above or below the piston 20 to provide automatic level control of the axle 11 using a position sensor 23. Valve 26 can also be used to tilt the axle relative to the chassis when e.g. plough or operating across a sloping field. In this case an additional inclinometer (not shown) is needed to determine the inclination of the tractor.

The circuitry shown in Figure 6 is basically the same as that of Figure 5 with the exception that the fixed orifices 21 are replaced by variable orifices in the form of proportional fluid control valves 24 which receive control signals from an associated electronic control unit into which the tractor driver can enter his requirement for harder or softer axle suspension damping and which can also be controlled automatically by an electronic monitoring system in response to the speed and movement of the axle relative to the chassis. In this arrangement in addition to the position sensor 23 an axle inclinometer and/or or a gyroscope (or two accelerometers) are provided to generate the input signals for automatically controlling the size of the orifices provided by the valves 24 which, as explained above, will vary in accordance with the speed and degree of movement of the axle relative to the chassis. Again inclination of the axle when operating on a slope or when ploughing can be controlled by valve 26 by adjusting to adjust the amount of fluid above or below the piston 20 and automatic level control of the axle 11 is again provided.

Figure 7 shows a fully active spring and damping system in which the pressure above and below the piston 20 and the movement of piston 20 is controlled by a proportional flow valve 30 to provide the necessary force in response to the roll movements of the axle relative to the chassis. In this active control system the valve 25 will again receive operating signals from an associated electronic control unit into which the tractor driver can enter his requirement for harder or softer axle damping and which can also be controlled automatically by an electronic monitoring system in response to the speed and movement of the axle relative to the chassis. In this arrangement in addition to the position sensor 23 an axle inclinometer and/or or a gyroscope (or two accelerometers) are provided to generate the input signals for automatically controlling the size of the orifices provided by the valves 30 which, as explained above, will vary in accordance with the speed and degree of movement of the axle relative to the chassis. Again the inclination of the axle when operating on a slope or when ploughing can also be controlled by the valve 30 by adjusting the amount of fluid above or below the piston 20 and automatic level control of axle 11 is again provided.

With passive, semi-active and fully active arrangements of Figures 5 to 7 the axle suspension can also be arranged to stiffen during cornering.

Conveniently the tractor chassis may be provided with a similar (or even identical) suspended front axle 30 having front wheels 31 steered by rods 32 and spring/damper 33 similar to spring/damper 17. Such an arrangement is shown in Figure 8.

Also, although in the arrangement described above all the spring/dampers are hydraulic or pneumatic, the damping of the movements of the rear and front axles can be controlled by mechanical springs.

Also, the rear and front axles may be designed to allow vertical movement of the axle pivot.