This invention relates to vehicles with multiple driven axles and particularly to agricultural and industrial tractors with front and rear driven axles.

Such tractors conventionally have a clutch for connecting and disconnecting drive to the front axle to switch between two and four wheel drive. This clutch is necessary since it is essential to disconnect drive to the front wheels in certain operating conditions, for example, when the tractor is being driven at speed on a road in order to prevent excessive wear to the tyres due to unequal peripheral tyre speed .

It is also necessary to dis-engage the clutch when the tractor is required to turn very sharply. In these circumstances, the front wheels have to travel appreciably faster than the rear wheels and the turn is impeded if the fron wheels are mechanically driven at the same peripheral speed as the rear wheels.

It is well know on four wheel drive highway vehicles to use a differential between the front and rear axles, thus eliminating the excess tyre wear at high speed and the restriction of sharp turns. However, this system is unsatisfactory on a tractor due to the normal variation in front/rear weight distribution, which can vary between 80/20 and 20/80.

It is also known to be advantageous in a tractor provided with rear axle brakes only, to ensure that drive to the front wheels is directly connected to the rear wheels whenever such a tractor is braked in order to ensure that both front and rear axles can contribute to the braking effect via their contact with the ground.

Various systems have therefore been proposed which, for example, take account of the requirement to disconnect front wheel drive at speed on the road and which ensure that when both rear axle brakes are applied drive is automatically connected to the front axle.

Other systems have been proposed which change the gear ratio between the front and rear axles when the tractor is turning sharply; this causes the peripheral speed of the front wheels to increase relative to the rear wheels so that the front wheel drive aids rather than impedes the sharpness of the turn.

It is an object of the present invention to provide a vehicle with multiple dr ven axles which takes account of the operating conditions of an agricultural or industrial tractor and which obviates the need for a clutch to connect or disconnect drive to the front axle and for an inter-axle ratio changing device to facilitate sharp turns, as discussed above.

Thus according to the present invention there is provided a vehicle having front and rear drive axles driven from a prime mover via an inter-axle differential, left and right hand brakes for operation on the rear axle, a locking means for locking-up the inter-axle differential, an actuating means for the locking means and a control system for the actuating means, the control system being arranged to cause the actuating means to engage the locking means when both rear axle brakes are applied.

If the vehicle is a tractor with independently operable left and right hand brakes, the control system will be arranged not to engage the locking means if only one brake is applied to turn the tractor.

Preferably an operator control is provided to allow a vehicle operator to override the control means to engage or disengage the locking means when he determines that the operating conditions so require.

Preferably in a vehicle such as a tractor, in which the front to rear axle weight distribution is unequal, the inter-axle differential wi ll be of the unequal torque split type in which the torque distribution to the front and rear axles reflects the weight distribution.

If the vehicle is a tractor with implement attachment links movable between a lowered work position and a raised transport position, the control system preferably operates automati cally to engage the locking means of the inter-axle differential when the links are in their lowered work position and to disengage the locking means when the links are raised to their transport position.

The control system also preferably operates to disengage automatically the locking means when the vehicle is travelling above a preselected speed. A speed of the order of 14 k.p.h. has been found an appropriate preselected speed for tractors.

If the vehicle has an inter-wheel differential on each axle at least one of these inter-wheel differentials may also be provided with its own locking means which may be arranged to operate in unison with the inter-axle differential locking means under the control of the same control system.

The actuating means for the locking means of the inter-axle differential may include spring means to bias the locking

means into its engaged condition so that the locking means is automatically engaged should any fai lure occur in the control system or the actuating system.

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 is a schematic representation of a tractor embodying an inter-axle differential lock control arrangement in accordance with the present invention, and

Figure 2 s a schematic representation of an alternative intei—axle differential arrangement for use in the tractor of Figure 1.

Referring to Figure 1, this shows a tractor in which an engine E drives a front axle (represented by a crown wheel and pinion 10) and a rear axle (represented by a crown wheel and pinion 11) via an inter-axle differential 12, a gearbox 6 and a main drive clutch C. Each axle carries ground engaging wheels (not shown) with tyres.

The tractor is provided with right and left hand separately operable brakes (indicated schemat cally at 40) associated with the right and left hand sides of the rear axle of the tractor. In the conventional manner these brakes are operated by independent right and left hand brake pedals (not shown) to allow the application of one brake only in order to faci litate turning of the tractor. As is normal for road use of the tractor, these two brake pedals are connected so that both right and left hand rear axle brakes must be applied simultaneously.

The tractor is also provided with conventional rear implement attachment links (not shown).

The inter-axle differential 12 comprises an input sun gear 13 driven from a gearbox output shaft 14, an output sun gear 15, orbiting compound planet gears 16, meshing with sun gears 13 and 15 and a planet gear carrier 17, which carries a front axle drive gear 18 which meshes with a gear 19 on a front axle drive shaft 20. Drive to the rear crown wheel and pinion 11 is via a shaft 21 connected with output sun gea r 15.

The inter-axle differential gear 12 also includes a locking means in the form of a dog clutch member 22, which is axially slidable on splines 21a on shaft 21 and has teeth 23 engageable with corresponding dog teeth 24 formed on planet carrier 17. As wi ll be appreciated when the teeth 23 and 24 are engaged, the sun gear 15 is in fact locked against rotation relative to the planet carrier 17 so that the entire inter-axle differential 12 rotates as a single unit, thus providing a rigid drive connection between shafts 14, 21 and 20.

The teeth 23 and 24 are arranged to be biased into engagement by a spring 25 which acts on the clutch member 22 via a lever 26. The inter-axle differential is disengaged by the application of fluid under pressure to an actuator 27 acting on lever 26. The actuator 27 may be, for example, hydrau I i ca I ly or pneumatically operated.

In the i llustrated example, the actuator 27 is supplied with pressurised hydraulic fluid by a pump 28 via a solenoid operated control valve 29. A pressure relief valve 39 controls the level of hydraulic pressure in the pump circuit. Control valve 29 is itself controlled by an elec-

tronic control system CS shown d agramati ca I ly in Figure 1 whose control criteria are discussed below. If any failure occurs in actuator 27 or the associated hydraulic or electrical circuitry the teeth 23 and 24 are automat cally engaged by spring 25.

The tractor is also provided with an operator controlled press button B, which is of the push-on push-off type which allows the operator to override the control system CS to engage or disengage the inter-axle differential lock when he judges that the operating conditions of the tractor require him to intervene. An indicator light L is also provided to indicate to the operator when the inter-axle differential lock is engaged.

In addition to the input from switch B, the Control System also receives inputs from various sensors 30, 31, 32, 33, etc. which indicate various operating parameters of the tractor such as right and left brake application (sensors 30 and 31), tractor forward speed (sensor 32) and the posit on of the tractor implement attachment links (sensor 33). Since the actual construct onal details of sensors 30, 31, 32, 33, etc. form no part of the present invention, no further details of these sensors will be given. Any one of the many well-known sensors capable of sensing each parameter may be used.

The control system CS controls the inter-axle differential locking means in accordance with the following criteria.

When the engine E is started up the actuator 27 is depressuri sed, thus allowing spring 25 to engage the differential lock 22, 23, 24.

As previously ndicated, whenever the operator control

button B is pressed this enables the tractor operator to override the command currently being issued by the control system CS if he feels that an override is necessary.

Whenever both left and right hand rear axle brakes are applied si ultaneously (sensed by sensors 30 and 31) the actuator 27 is again depressurised to engage the differential lock 22, 23, 24 to provide a rigid drive connection between the front and rear axle crown wheel and pinion assemblies 10 and 11, so that both axles can contribute to the braking effect via the contact of their associated wheels with the ground. If either the right or left hand brake is operated independen ly (indicating a requirement for a tight turn) the differential lock is unlocked by pressuring actuator 27.

When the implement attachment links are in their working positions (sensed by sensor 33) the actuator 27 is again depressurised to engage the differential lock since under these working conditions it is desirable to lock the differential action between front and rear axle drive shaft 20 and 21 to prevent excessive wheelslip in bad ground conditions. When the implement attachment links are raised to their transport condition (again sensed by sensor 33) indicating that the implement is now no longer in actual use, the actuator 27 is pressurised to disengage the differential locking means.

Additionally, if the tractor is travelling above a pre¬ selected speed of say 14 k.p.h (sensed by sensor 32), actuator 27 is arranged to be pressurised if it is not already in this condition to prevent excessive tyre wear due to possible unequal peripheral tyre speed of the front and rear wheels which might arise if no differential action is allowed between the front and rear drive shafts 20 and 21.

As will be appreciated in the described tractor application, the front and rear axles have their own inter-wheel d fferentials provided by the crown wheel and pinion assemblies 10 and 11 respect ve y. One or both of these intei—wheel differentials may be provided with a locking means controlled in unison with the Locking means of the intei—axle differential 12 by the control system CS.

In a vehicle such as a tractor, the we ght distribution between the front and rear axles is normally far from equal with the majority of the weight being taken on the rear axle. In these circumstances, the inter-axle differential 12 is arranged to provide an unequal torque split to the front and rear crown wheel and pinion assemblies 10 and 11 respectively. This torque split is arranged to reflect the relative weight distribution on the front and rear axle. Typically the torque split may be 30-70 in favour of the rear axle.

If the tractor is fitted with a means for determining wheel slip relative to the ground (as for example provided on the Massey-Fe rguson "Datat roni c" tractors), the control system CS may be arranged to ensure depressuri sat i on of actuator 27 at slip Levels above a preset Limit (say 15% slip) to engage the Locking means of the intei—axle differential 12 and the inter—wheel differentials 10 and 11.

As wilL be appreciated, not all the above control criteria need be employed in any one system and the most basic system need only employ the brake application criteria.

Also the control system of the present invention may be integrated into a Larger control system controlling other parts of the vehicle, such as that provided on the

previously mentioned Massey-Fe rguson "Datatronic" tractors

Figure 2 shows an alternative inter-axle differential arrangement in which components of a simi lar function to those shown in Figure 1 have been simi larly numbered. As can be seen from Figure 2, the input gear 13 to the inter-axle differential ^* » s increased in diameter and is internally toothed to engage the planet gears 16 externally as is the output gear 15. This alternative gear arrangement is designed to minimise the loading on the teeth of the differential gear elements by maximising the radius of gears 13 and 15 respectively. It must be remembered here that that space avai lable for fitting the inter-axle differential 12 is often limited by the close proximity of other parts of the tractor and the use of this alternative gear arrangement maximises the radius of gears 13 and 15 within the avai lable space .