The invention relates to a differential drive apparatus having an input member and two output members.

It is known, especially in wheel-driven vehicles, to introduce an unequal torque division between the output members of a differential by applying braking torque to one of the members. The torque transferred across the differential is then equal to that carried by the brake. In addition, if the rotation speeds of the output members are similar, then the power transferred across the differential is approximately equal to that absorbed by the brake.

In systems of this type, the braking is usually under the automated control of a controller which may be a computer. The controller controls the brakes in response to data from sensors, whose nature depends upon the application of the differential drive apparatus.

In previously known limited slip differentials the braking on a slipping wheel extracts energy from the drive to the axle and wastes it.

One object of the invention is to provide a differential drive apparatus which recirculates the braking power.

A further object of the invention is to . provide an active differential which can employ much lighter brakes with lower torque and power handling capacity. This in turn can lead to better response speed of the brakes and to improved sensitivity of operation.

Summary of the Invention

A preferred form of the present invention is based on a differential in which a brake is linked both to its output member and also to the input member, which may be a differential case, in such a way that the torque applied by the brake to the output member is augmented by a reaction torque from the input member. The torque applied to the output member is equal to the brake torque plus the reaction torque. The reaction torque is recirculated via the input member and the differential gearing to the opposite output member. The torque that may be transferred from the brake to the opposite output member may thus be considerably more than the brake torque itself.

In one aspect the invention provides a differential drive assembly comprising a rotary input member, two rotary output members, differential gearing interconnecting the input member and the output members, a rotary element, braking means for applying a braking torque to the rotary element and torque distributing means linked to a first said output member, to the input member and to the said rotary element brake means, and for applying, on operation of the braking means, a torque to the first output member and a reaction torque to the input member, the said torque and the said reaction torque being multiples of the braking torque applied by the braking means to the rotary element.

The brake, the output member and the input member may be linked by epicyclic gearing forming a torque distributing means in which the sun gear is connected to the brake, the planet carrier to the output member and the ring gear to the input member. In this form of the invention, if the planet gears have, for example, half the radius of the sun gear.

the torque transferred to the opposite output member is three times the brake torque.

Further, especially if the differential is under computer control, if the rotation speeds of the output members are similar, then the speed of the rotary element of the brake will not be substantially greater than the output member's rotation speed. Therefore, not only will the torque transferred to the opposite output member be greater than that applied by the brake, but also the power transferred will be greater than that absorbed by the brake.

In a particular embodiment therefore a differential drive assembly according to the invention comprises an input rotary member and two output rotary members interconnected differentially, an element which is rotatable relative to said members, means for applying a brake torque to said rotatable element and an epicyclic gear train linking one of said output rotary members, said input member and said rotatable element.

Brief description of the drawings

Figure 1 shows diagrammatically a cross section of a differential drive according to one embodiment of the invention with its control system shown schematically;

Figure 2 shows diagrammatically a cross section of part of a dif erential drive according to a second embodiment of the invention; and

Figure 3 shows diagrammatically a cross section of part of a further embodiment of the invention.

Detailed description of preferred embodiments

The differential shown in the drawings is primarily intended for use in a wheel-driven vehicle and is contained in a housing 1. As shown in Figure 1, two rotatable output shafts 2, 3 extend from the housing 1 and drive road wheels (not shown) at their outer ends. Within the housing 1 the differential comprises a differential case or frame 4 which is rotatably mounted in bearings 5 and 5a in the housing 1 and carries a ring of gear teeth 4A. This ring of gear teeth constitutes an input drive member for the differential and is driven by any suitable transfer gearing (not shown) . The output shafts 2, 3 are coaxial and are located in bearings 6 and 6a in the case 4. Within the case 4 the output shafts 2, 3 carry bevel gear wheels 7 and 8 respectively. These wheels 7 and 8 mesh with two bevel pinions 9 and 9a rotatably mounted on pins (not shown) carried by the case 4.

A brake assembly 10 is associated with the output shaft 2.

The brake assembly is shown schematically as including brake pads 12 and disposed to act on a brake disc 13 which is

mounted for rotation on and about the output shaft 2. There is an epicyclic link between the brake disc 13 and the differential. In this embodiment the link comprises a sun gear 23 which forms part of the hub of the brake disc, a set of planet gears 14 rotatably mounted on a planet carrier 15 which is fixed to the output shaft 2 and a geared annulus 11 disposed on the case 4, each planet meshing with the sun gear and the annulus.

The brake assembly may have a variety of forms and may be a multiplate brake or an electromagnetic brake.

In this embodiment of the invention a brake assembly 10a is associated with the output shaft 3 and is epicyclically coupled to the output shaft and the differential frame in the same way as described with reference to the assembly 10 and the shaft 2, corresponding parts being denoted by the suffix "a".

The differential also comprises sensors 16, 16a and 17 which in Figure 1 are intended to sense the speeds of rotation of each brake disc 13 and of the frame 4. In the embodiment shown these are magnetic sensors. The data from these sensors are sent to an electronic controller 18 which uses the data to control the brake assemblies 12 and 12a in accordance with its programming. The controller 18 may in addition use data from other sensors such as an accelerometer 19 for sensing lateral acceleration of the housing 1, a steering angle sensor 20, and a vehicle speed sensor 21. The controller will not be described in detail because controllers for anti-skid or anti-spin braking are well known in the art and the particular form of the controller is not important for the present invention.

The use of an electronic controller to control such an anti-wheel spin device is intended mainly for use in racing and sporting vehicles where the side brakes are intended to be controlled automatically through sensors and electrohydraulic actuators. It is possible to envisage, however, an embodiment of the present invention being used for example in an agricultural tractor, where the side brakes may be operated by a driver's foot pedal and achieve the same purpose: that is, to transfer torque from a slipping wheel to the wheel on the other side where there is more adhesion.

In normal operation, when neither road wheel is spinning under power, neither of the brakes 10 and 10a operates and the differential acts in its usual manner. While always providing equal torques to the output shafts, the differential allows, for example, the shafts to rotate at different speeds during cornering.

If one wheel spins under power, this is detected by the controller 18 on the basis of information from the sensors comparing, for example, signals representing the speed of the shaft and the vehicle speed. The brake 10 linked to that output shaft, for example output shaft 2, is actuated. Thus, a braking torque ^~~ „ is applied to the combined brake disc and sun gear 13. If the planet gears 14 in each epicyclic gear train are of radius r and the radius of the sun gear 13 is R, then the epicyclic train ratio E is (1 + 2r/R), and a torque of (2 + 2r/R)T _β or (E + 1) is thereby applied to the planet carrier 15 and so to the corresponding bevel wheel on the inner end of the corresponding output shaft. In addition a reaction torque of (1 + 2r/R)T _B or ET_ is applied to the annular gear and the case 4. Prior art differentials give a 1:1 reverse

rotation if one side is turned with the main casing held still. In the present invention, turning the side-brake pinion gives a reverse rotation relative to input of 1/E, a torque increase to the differential pinions of (E + 1)T_ and a recirculating torque of ET _β .

In the embodiment shown in Figure 1 in which the sun gear radius R is twice the planet gear radius r, the torque applied by the brake 10 is multiplied by the epicyclic gearing so that three times the braking torque is transmitted to the output shaft 2 to which the brake 10 is linked, and a reaction torque of twice the braking torque is transmitted to the differential case 4. Three times the brake torque is thus transferred by the differential to the second output shaft 3 to which the non-spinning wheel is attached.

In Figure 1, when the side brake is completely locked up, the speed differential transferred is not 100 per cent as with a brake direct on to the wheel but (1 - E ); or in

E + 1 the illustrated embodiment, one third. This proportion can be varied by different values of E in the epicyclic train, or by using a different train.

The reaction torque ET _β which is transmitted to the differential case 4 when a braking torque is applied provides economy of driving power by recirculating the torque to the input instead of dissipating power as heat in a direct brake on the wheel.

In the event of one wheel spinning under power, braking torque may be applied progressively to that wheel under the control of the controller 18, so that that wheel is slowed

down and maximum available tractive contact with the road surface is re-established and then maintained. In addition the driving force applied to the vehicle by both the driven wheels is optimised because the torque applied by the brake increases the torque applied to the non-spinning wheel. This may be increased in a limiting case until both driven wheels are on the point of spinning, at which stage the power input to the differential must be limited. This limiting of input power may also be controlled by the controller 18.

Since the speed of response of the controller 18, which is a computerised control system, may be made very rapid, the relevant brake for torque transfer may be applied rapidly and so its associated wheels need never spin to a significant degree. Since the torque and power handling capacity required of the brakes can be reduced compared with the brakes of the prior art, they may also have a higher speed of response and may be applied more progressively in response to signals from the output shaft rotation speed sensors. The stability of the vehicle, which may otherwise be detrimentally affected by abrupt braking and torque transfer, can be better maintained.

Figures 2 and 3 illustrate two further embodiments of the invention wherein modifications to the embodiment shown in Figure 1 provide alternative methods of applying the braking torque T_ to the brake disc 13 and thus to epicyclic sun 23 which rotates with the brake disc.

For simplicity. Figures 2 and 3 illustrate only the right-hand half of the differential mechanism: in each case the left-hand half corresponds to the right-hand half.

In Figures 2 and 3 the sensor 17 senses the speed of rotation of the case 4 as in the embodiment of Figure 1; but the sensor 16 senses the speed of the half shaft 2 or the wheel, whereas in the embodiment shown in Figure 1 the sensor 16 senses the speed of rotation of the brake disc 13. Furthermore, in Figures 2 and 3, the braking torque T _β on the sun gear 13A is taken by the differential case 4 instead of by the axle housing 1 as shown in Figure 1. Thus the sliding speed of the brake is only the small slip between the case 4 and the brake disc 13, which has been detected by the sensor 16 on half shaft 2, plus any small normal effect of cornering.

In the embodiment shown in Figure 1 the brake sliding is at full axle shaft speed since the braking torque is taken by the axle housing 1 which is stationary.

Additionally, in Figures 2 and 3 the differential unit can now be locked up completely into direct drive when the side brake is fully applied and there is no slip between the case 4 and the brake disc 13; whereas in Figure 1, locking the sun transfers a speed differential of (1 - E ) to the

E + 1 second output shaft 3 as discussed above.

In the embodiments shown in Figures 2 and 3 braking action has to be transferred from the stationary axle housing to the rotating differential case. In Figure 2 fluid pressure is transferred from the axle housing to the differential case and a piston 22 is employed to operate the brake 10. Hydraulic pressure for actuating the piston is provided to a chamber 24 by way of a hydraulic line 25 under the control of the controller 18, a passage 26 through the housing 1, an annular groove 27 in the differential case and a passage 28.

In the embodiment of Figure 3, the passage 25 coupled to line 25 provides hydraulic pressure to a chamber 29 between housing 1 and a slider 28. A thrust member is formed by a rod 31 having at one end a head 32 engaging the slides and at its other end a brake-pad 12 for engaging the brake disc 13.

Considering the system shown in Figure 1, since the response is rapid and the rotation speeds of an output shaft and the differential case may never differ substantially, the rotational speed of the brake need never substantially exceed that of the output shaft in any embodiment. The power transferred by the differential from the brake to the opposite output shaft will then be multiplied by approximately the same factor as the torque, substantially reducing the required power absorption capacity of the brakes and the power consumption of the differential.

Considering the systems shown in Figures 2 and 3, the relative slip between output shafts 2 and 3 and the case 4 is limited to the increment permitted by the controller before regulatory forces are effectively applied. The power

(torque x slip speed) wasted in the brake can be extremely small. However, the power transferred to the wheel is still the multiple of brake torque x casing rotational speed, this power being effectively transferred by the case itself. The factor already mentioned comparing the power transferred to a wheel with the brake power required to do so will be much greater for these two systems than for that shown in Figure 1.

The differential of the present invention also provides an efficient steering mechanism for tracked vehicles which wastes the minimum of main driving power, and requires smaller than usual braking/steering forces and equipment.

A well known alternative to a bevel gear differential as shown in Figures 1 to 3 is an epicyclic train which has idler planets of train ratio 2. If an input is made to the annulus of the epicyclic, the planet carrier and the sun will have equal and opposite rotations. For the sake of brevity further embodiments including epicyclic differentials will not be described in detail. However, it will be understood that references herein to differential gearing are intended to include epicyclic as well as bevel gear differentials.