GB2483683A

GB2483683A - Differential having gear pumps/motors and an additional pressure source

Info

Publication number: GB2483683A
Application number: GB1015504.2A
Authority: GB
Inventors: John Morton
Original assignee: DRIVE SYSTEM DESIGN Ltd; Drive Systems Ltd
Current assignee: DRIVE SYSTEM DESIGN Ltd; Drive Systems Ltd
Priority date: 2010-09-16
Filing date: 2010-09-16
Publication date: 2012-03-21
Also published as: GB201015504D0

Abstract

A torque distributing or vectoring differential comprises a gear assembly (8, fig 2) having at least two gears (11-14, fig 10) arranged to rotate in substantially the same plane. The differential has at least two reservoir portions 18, 19 which are coupled to one another by at least two openings (26A, 26B, fig 8) disposed at points adjacent to where teeth of a first of the at least two gears (11-14) meshes with teeth of another of said at least two gears (11-14). In use, at least one of said at least two gears (11-14) acts as a hydraulic gear pump or hydraulic gear motor. The differential further comprises an additional fluid pressure source, e.g. a pump, coupled via pipes 44, 48 and valve 40 to the at least two reservoir portions 18, 19 for altering a pressure in a fluid contained in the first or second reservoir portions 18, 19 for encouraging or discouraging differential rotation between two outputs.

Description

TORQUE DiSTRIBUTING DIFFERENTIAL

FIELD OF INVENTION

S The invention relates to the field of differentials, more particularly but not exclusively to differential which can controllably distribute torque between a pair of output shafts.

BACKGROUND OF THE INVENTION

A differential is a device used largely in the automobile industry to distribute torque from a single rotating input to two outputs whilst allowing them to rotate at different speeds. This is useful when, for example, the torque is to be distributed to the wheels of the driven axle of a vehicle turning a corner. The wheel on the inside of the corner should rotate at a different speed to the wheel on the outside of the corner; as the two wheels are travelling different distances in the same time. Without a differential, both wheels would turn at the same speed, causing either the inner wheel to spin or the outer wheel to drag. in either case, this can create undesirable characteristics in the handling of the vehicle, Furthermore, damage to the tyres or roads can result.

A double epicyclic differential is illustrated in Figures 1 and 2. The differential comprises a crown wheel 1 fixed to an annulus gear 2 of the epicyclic gear set. A planet carrier 3 carries at least one pair of planet gears 4, 5, which are allowed to rotate freely on axes fixed to the planet carrier 3. The planet carrier 3 is also fixed to an output spline (not shown) for transmitting torque to one of two drive shafts (not shown). A sun gear 6 is in contact with the inner planet gear(s) 5, and the sun gear 6 has an output spline 7 for transmitting torque to a drive shaft (not shown). The arrows show an example of the direction of rotation of the gears. As long as there is an equal torque reaction from both drives shafts, the planet carrier 3 and sun gear 6 rotate at the same speed as the annulus 2. When there is an unequal torque reaction from the drive shafts, the plant carrier 2 and sun gear 6 are allowed to rotate at different speeds.

A limited sUp differential is a modified form of differential that distributes torque to two rotating outputs; but when there is speed difference between these outputs, a torque is generated between them. There are certain conditions under which a limited slip differential gives an advantage over a standard differential. For example, if one wheel of the driven axle of a vehicle rests on ice whilst the vehicle is trying to pull away, a standard differential would allow this one to spin and would be unable to supply torque to the other wheels and the whe& on ice cannot generate any torque. A imited slip differential ensures that by generating torque between the two wheels as one spins; some torque will be transmitted to the wheel on the surface which provides the most traction and that the vehicle will be able to pull away.

A common type of limited slip differential is a speed sensitive limited slip differential. One type of speed sensitive limited slip differential relies on a viscous fluid that changes its physical properties when subjected to shear forces to generate torque. An example of such a fluid is a silicon fluid. One type of viscous fluid limited slip differential comprises a cylindrical chamber filled with a stack of perforated discs rotating with the normal motion of the output shafts. An inside surface of the chamber is coupled to a drive shaft, and an outside surface of the chamber is coupled to the differential carrier. Differential motion causes the perforated discs to move through the fluid against each other. The greater the relative speed of the discs, the more resistance they will encounter as the viscous fluid thickens discouraging differential rotation of the outputs.

it is known to use the gears of the differential itself to pump fluid and generate torque. For example, US 6;402;656 discloses a limited slip differential that uses gears to pump fluid.

However, owing to the layout of the design, the fluid flow cannot easily be channelled and controlled by valves, The pressure of the fluid is governed by controlling the clearance between the pump gears and the housing.

US 6,031,040 disdoses a hydraulicafly operated limited slip differential that uses differential gears to pump fluid, but his device drives a clutch to generate torque. A disadvantage of devices that use a clutch to generate torque is they are bulky, which often has an effect on the weight and cost of the device, and suffer from wear and so require regular maintenance.

Furthermore, differentials which rely on friction to generate torque can generate inconsistent torque due to the difference in the values of static and dynamic friction coefficients between the plates which can be disconcerting to a driver of the vehicle.

US 6,048,286 discloses a differential that uses a separate pump to drive a fluid. The pump is driven in accordance with the speed difference between two drive shafts.

Another type of differential sometimes known as a "torque vectoring differential is a modified form of differential in which the torque from an input shaft can be controllably distributed between two output shafts, in a vehicle, the torque from the engine is preferably directed to a wheel that has the largest torque potential, in other words the wheel in contact with a surface having the greatest friction.

It is known to provide epicyclic gearing and clutches to effectively create a gear ratio between two output shafts. This may also have the effect of steering the vehicle.

The amount of torque delivered to the wheel with the greatest friction maybe higher than the torque that could be delivered in a limited slip differential.

When deployed in a vehicle a torque distributing differential can be used to improve the handling characteristics of the vehicle, the way the vehicle performs transversely to their direction of motion, particularly during cornering or swerving and may also improve stability when moving in a straight line.

Torque vectoring or torque distributing differentials can be used to reduce, or eliminate, or neutralise undesirable handling characteristics such as understeer or oversteer. Understeer results when application of a cornering force (lateral force) also applies a rotational torque (or moment), to the vehicle the opposite direction to the direction of the turn, thus the vehicle follows a less curved trajectory than the driver is trying to impose. Oversteer results when the application of a cornering force (lateral force) also applies a rotational torque (or moment) to the vehicle in same the direction as the direction of the turn.

n a vehicle having a four wheel drive, a differential can be provided on each axle, front axle and rear axle, and a centre differential can be provided to allow the front axle to rotate at a different speed to the rear axle. A torque distributing centre differential can be used to direct torque to the front or rear axle depending on which wheels have the greatest torque potential.

Typically, torque vectoring differentials function by engaging and disengaging a gear ratio between the outputs of the differential, when fully engaged, these systems have a fixed maximum steering moment. The present invention is not limited to particular predetermined gear ratio; the steering moment is a function the pressure and flow available and can be progressively altered or tuned to provide the desired handling characteristics.

Existing differentials use a clutch to engage and disengage the gear system which distributes the torque. Differences between the values of static and dynamic friction in the clutch plates can make changing the distribution of torque between the output shafts occur in a spasmodic or jerky manner.

The present invention allows the changes to be made smoothly and progressively. The present invention is simpler than existing designs and is more compact since there are fewer parts. It is envisages that this will result in cost savings. The present invention is flexible; it can be used as an open differential, a passively and actively controfled limited slip differential or as an actively controfled torque distributing differential.

SUMMARY OF INVENTION

The present invention seeks to overcome or at least mitigate the problems of known differentials.

According to a first aspect of the present invention there is provided a differential comprising, a gear assembly having at least two gears, each gear having plurality of teeth; a fluid reservoir comprising at least two reservoir portions; the at least two reservoir portions being coupled to one another by at least two openings; the openings being disposed at points adjacent to where the teeth of a first of said at least two gears meshes with the teeth of another of said at least two gears; a device coupled to the at least two reservoir portions for altering a pressure in a fluid contained the first or second reservoir portions; wherein in use, at least one of said at least two gears acts as a hydraulic gear pump or hydraulic gear motor.

Preferably, the at least two gears act as a hydraulic gear pump to create a pressure differential between the fluid in the first and the fluid in the second reservoir portions.

Preferably, the at least two gears act as a hydraulic gear motor in response to a pressure differential in the fluid between the first and second reservoir portions.

Preferably, said at least two gears are arranged to rotate in substantially the same plane.

Preferably, the gear assembly is an epicyclic gear assembly comprising a sun gear, an annulus at least one pair of intermeshing planet gears and a planet carrier to which the planet gears are attached wherein the annulus, sun gear and planet gears are arranged to rotate substantially on the same plane.

Preferably, the at least two openings are provided in the planet carrier, the openings being disposed at points adjacent to where the teeth of at east one of the planet gears meshes with the teeth of another gear the openings connecting the meshing teeth to one of the first and second fluid reservoir portions.

Preferably, the differential comprises a planetary gear assembly.

Preferably, the differential comprises a bevel gear assembly.

Preferably, the device is a pump, for example a gear pump or gerotor. More preferably, the pump is integrated with the differential.

Optionally, the pump is mechanically driven.

Optionally, the pump is electrically driven.

Preferably, the pump is a gear pump and comprises a pump gear assembly having a pump planet gear, pump planet gear carrier and pump annulus, the pump planet gear carrier comprising at least two openings disposed at a points adjacent where the teeth of the at least one pump planet gear meshes with the pump annulus wherein in use the pump gear assembly acts as a hydraulic gear pump to alter a pressure in the fluid contained in the first or second reservoir portions.

Preferably, the pump planet gear carrier is fixedly attached and the pump planet gears are rotationally attached to the differential.

Preferably, the pump annulus is rotationally attached to the differential.

Preferably, the pump gear assembly is coupled to a drive means.

Preferably, the pump annulus is coupled to said drive means.

Preferably, the drive means is an electric motor.

Preferably, an element of the pump is coupled to a motor stator or motor rotor of said electric motor.

Preferably, the pump annulus is fixedly attached to a motor stator of said electric motor.

Preferably, the pump planet carrier is fixedly attached to the motor rotor of said electric motor.

Preferably, the differential further comprises a housing surrounding the gear assembly. More preferably, an element of the pump the pump annulus is selectively coupled to the housing.

Optionally) the element of the pump is coupled to the differential housing by a clutch mechanism for selective engagement therewith.

Preferably, the pump annulus is coupled to the differential housing by a clutch mechanism for selective engagement therewith.

Preferably, the differential comprises first and second covers surrounding the planet gears, sun gear, annulus and planet carrier. More preferably, the first and second covers are rotationally connected within the housing and a fluidic connection is provided between the housing and the first and second covers.

Preferably, the pump is coupled to the first and second reservoir portions by a valve system comprising one or more valves. Optionally, the valve system is integrated within the differential.

Preferably, the valve system comprises an inlet valve and an outlet valve.

Preferably, the differential comprises a crown wheel adapted to receive the valve system According to a second aspect of the present invention there is provided a crown wheel for a differential comprising first and second sides and being adapted to receive a valve system for coupling each side to a pressure source.

Preferably, the crown wheel comprises an inlet valve and an outlet valve.

Preferably, the crown wheel comprises a pressure source mounted thereon.

More preferably, the pressure source is a gear pump comprising a pump planet gear carrier and pump planet gears mounted on the crown wheel.

According to a third aspect of the present invention there is provided a differential comprising an epicyclic gear assembly, the epicyclic gear assembly comprising a sun gear, an annulus, at least one pair of intermeshing planet gears, and a planet carrier to which the planet gears are attached, wherein the annulus, sun gear and planet gears are arranged to rotate substantially in the same plane; a fluid reservoir comprising at least two reservoir portions; at least two openings in the planet carrier, the openings being disposed at points adjacent to where the teeth of at least one of the planet gears meshes with the teeth of another gear, the openings connecting the meshing teeth to one of the first and second fluid reservoir portions; at least one valve connecting the first fluid reservoir portion to the second fluid reservoir portion; wherein, in use, at least one gear of the epicyclic gear assemble acts as a hydraulic gear pump o motor to alter a pressure in a fluid contained in the first and second reservoir portion wherein there further comprises an additional pressure source for providing fluid under pressure to either of the first or second reservoir portions.

Preferably, the first reservoir portion is a first reservoir; and the second reservoir portion is a second reservoir.

Preferably, the first reservoir portion comprises at least one fluid-carrying channel and the second reservoir portion comprises at least one further fluid-carrying channel.

Preferably, the first and second fluid reservoir portions are disposed either side of the epicydic gear assembly, and the valve passes through the pa net carrier.

Preferably, the epicyclic gear assembly comprises a double epicyclic gear assembly, the double epicyclic gear assembly comprising at least one pair of planet gears.

Preferably, the valve is selected from one of a pressure reUef valve, a shim type valve, a power steering type valve, electronically controlled valve, a centrifugal valve and a simple flow restrictor.

Preferably, said first and second fluid reservoir portions contain hydraulic fluid.

Preferably, the differential further comprises means for maintaining a raised pressure on fluid in each of the reservoir portions.

Preferably, the means for maintain a pressure is selected from one of a spring and pressurised gas. l0

According to a fourth aspect of the present invention there is provided a vehicle comprising one or more differentials as herein before described.

BR1EF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which: FIGURE 1 illustrates schematically an open front view of a double epicyclic gear layout; FIGURE 2 illustrates schematically an isometric view of a differential according to the first embodiment of the present invention in which a cover has been removed; to FIGURE 3 illustrates schematically an isometric view of a differential according to the first embodiment of the present invention in which a cover and part of the planet carrier have been removed; FIGURE 4 illustrates schematically a differential according to the first embodiment showing section line XX; F1GURE S illustrates schematically a side cross-section view through section XX of the differential shown in Figure 4; F1GURE SA illustrates schematically an enlarged side cross-section view of the valve of Figure 5; ii FIGURE SB illustrates schematically an enlarged side cross-section view of the valve of Figure 5 in which the valve has been configured to bypass an external pressure source; FIGURE 6 illustrates schematically a side cross section view through section XX of the differential shown in Figure 4 in which the direction of fluid flow has been reversed; FIGURE 7 illustrates schematically an open front view of a differential according to the first embodiment of the invention; FIGURE 8 illustrates schematically the differential of Figure 7 showing section lines; FIGURE 9 illustrates schematically a side cross-section view through section AA of the differential shown in Figure 8; FIGURE 10 illustrates schematically a side cross-section view through section CC of the differential shown in Figure 8; FIGURE 11 illustrates schematically a side cross-section view through section BB of the differential shown in Figure 8; FIGURE 12 illustrates schematically an isometric view of the differential shown in Figure 8; FIGURE 13 illustrates schematically a side elevation view of the differential shown in Figure 8; FIGURE 14 illustrates schematically an open front view of a differential according to a second embodiment of the invention; and FIGURE 15 illustrates schematicaHy a side cross-section view of the differential shown in Figure 14.

FIGURE 16A illustrates schematically an isometric view of a differential according to a third embodiment of the invention.

FIGURE 16B illustrates schematically an isometric view of a differential of Figure 16A in which part of the cover and part of the pump planet carrier has been removed for illustrative purposes.

FIGURE 17 illustrates schematically a side cross-section view of a differential according to a third embodiment of the invention; and FIGURE 18 illustrates schematically a side cross-section view of a differential according to a fourth embodiment of the invention.

DETA1LED DESCR1PTION OF EXEMPLARY EMBOD1MENTS OF THE PRESENT INVENTiON Detailed descriptions of specific embodiments of the differential are disclosed herein. It will be understood that the disclosed embodiments are merely examples of the way in which certain aspects of the invention can be implemented and to no represent an exhaustive list of all of the ways the invention may be embodied. Indeed, it will be understood that the differential described herein may be embodied in various and alternative forms. The figures are not necessarily to scale and some features may be exaggerated or minimised to show details of particular components. Well-known components, material or methods are not necessarily described in great detail in order to avoid obscuring the present disclosure. Any specific structural and functional details disclosed herein are not to be interpreted as limiting, but A flI Ii fli flAOC merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the invention.

The differential of the present invention comprises a gear assembly which is arranged to act S upon a fluid as either a pump or motor, when acting as a pump the differential gear assembly can pressurize the fluid at acts a pressure source. The differential gear assembly can act as a motor in response to a pressure difference in the fluid. The differential comprises an additional pressure source such as a pump which can be used to encourage or discourage the operation of the differential gear assembly as a pump or motor. The pressure source is operable to encourage or discourage differential rotation between two outputs of the differential.

Figure 2 illustrates a differential according to a first embodiment of the present invention in which a cover 17, see Figure or 13, has been removed to show the gear assembly 8.

The gear assembly 8 is illustrated in more detail in Figure 7 which shows an open front view of the gear assembly 8.

The differential comprises a casing 30 surrounding the gear assembly. It is envisaged that in one application the casing 30 will be fixedly attached to a vehicle chassis (not shown).

Bearings 55, best illustrated in Figures 5 and 6, allow the differential to rotate within the casing 30.

An external pressure source (not shown) is coupled to the differential; the pressure source may be a pump such as a hydraulic pump such as those used in power steering assemblies. it is envisaged that the pump may be integral to the differential in alternative embodiments.

The pump may be powered either electronically or mechanically.

Left and right hand sides of the differential are coupled to the pressure source, best illustrated in Figure 5 and 6.

The pressure source is coupled to the differential via a valve 40. Valve 40 has four ports, P1, P2, P3, and P4, best illustrated in Figure 54.

Port P1 is coupled to the pressure source; Port P3 is also coupled to the pressure source.

Port P3 is coupled to the pressure source output by first pipe 46 and feeds fluid such as automatic transmission fluid or gear oil to the differential.

Port P1 is coupled to the pressure source return by second pipe 42 and may be coupled directly to a pump or to an external fluid reservoir.

Port P2 is coupled to a first fluid chamber 18 of the differential by third pipe 44. Port P4 is coupled to a second fluid chamber 19 of the differential by fourth pipe 48. ln alternative embodiments alternative fluidic coupling means may be provided for example conduits or channels may be provided in the housing 30 to fluidically couple the pressure source to the first and second chambers 18, 19 of the differential.

Valve 40 as illustrated in Figure 4 is shown in a first position in which the valve 40 couples the first fluid chamber 18 of the differential to the high pressure fluid feed and the second fluid chamber 19 is coupled to the low pressure return to the pump as illustrated in Figure 5.

Valve 40 is arranged such that port P1 is coupled to port P4 and port P2 is coupled to port PB.

Valve 40 can be controlled to change the direction of fluid flow, such that the first fluid chamber 18 of the differential is coupled to the low pressure return and the second fluid chamber 19 is coupled to the high pressure fluid feed as illustrated in Figure 6 wherein the valve 40 is shown in a second position. ln the illustrated embodiment this is achieved by rotating the valve 900.

Valve 40 is arranged such that port P1 is coupled to port P2 and port P3 is coupled to port P4.

S

The valve 40 may be configured to bypass the pressure source and directly couple first fluid chamber 18 to second fluid chamber 19 as shown in Figure 5B, the valve 40 has been rotated approximately 45 degrees from the position shown in Figure S or 6. In the fully open, unrestricted, position the differential functions as an open differential.

The valve 40 may be controUed to restrict the flow between the first and second chamber 18, 19 for example by rotating the valve more or less than 45 degrees such that the fluidic path through the vak,e 40 is not fully aligned with port P2 and port P4. In alternative embodiments alternative means for restricting fluid flow are envisaged for example reducing the cross-sectional area of the fluid path. In this way the differential may be configured to act as a limited slip differential which optionally may be actively controlled.

Connectors 50 couple third pipe 44 and fourth pipe 48 to the first fluid chamber 18 and second fluid chamber 19 of the differential respectively.

The pressure source can be controlled by a control system (not shown) to increase or decrease the fluid pressure on either of the fluid chambers 18, 19. The control system may receive data from one or more sensors.

When used in a vehicle the sensors may monitor a variety of vehicle characteristics such as but not limited to vehicle speed, wheel speed, gear position, steering angle, yaw rate and lateral G forces. Pressure sensors may be provided within the fluid chambers 18, 19 to monitor the fluid pressure therein.

n response to the data received from the sensors the control system controk the valve 40 and the pressure source to increase or decrease fluid pressure in one of the fluid chambers 18, 19 to create the desired vehicle handling characteristic.

in this way the differential can direct torque to either the left or right hand drive shaft.

Figure 9 illustrates as side cross-section view through section of the AA of the differential shown in Figure 8.

Figures 7 to 15 show illustrations of the differential in which the external pressure source and pipe connections have been omitted for clarity.

Referring to Figure 7, an open front view of a differential is illustrated in which the cover 17 and part of a planet carrier 16 have been removed for illustrative purposes. The gear assembly 8 of the differential comprises a crown wheel 10, an annulus 11, a pair of interconnecting planet gears 12, 13, a sun gear 14 and an output spline 15. A planet carrier 16 that supports the planet gears 12, 13 is disposed between the annulus 11 and the sun gear 14. The first planet gear 12 is in contact with the annulus 11 and the second planet gear 13. The second planet gear 13 is in contact with the first planet gear 12 and the sun gear 14. Rotational movement of the annulus 11 is transferred to the planet gears 12, 13 to the sun gear 14. Similarly) the planet carrier 16 counter-rotates with respect to the sun gear 14.

Figure 8 illustrates the differential of Figure 7 with section lines added, the cover 17 and part of the planet carrier 16 have been illustrated in phantom (dashed) lines, and Figures 10 and 11 illustrate side elevation cross-section views through sections CC and BB respectively. A cover 17 is located around the gear assembly 8. A first fluid chamber 18 and a second fluid chamber 19 are disposed on either side of the planet carrier 16, and are defined by gaps between the cover 17 and the gear assembly 8. The planet carrier 16 comprises a series of holes 26 which are disposed at the points where gear teeth of the first planet gear 12 and the annulus 11 mesh, at points where the gear teeth of the first planet gear 12 and the second planet gear 13 mesh, and at points where the gear teeth of the second planet gear 13 and the sun gear 14 mesh. The holes 26 provide a path through the planet carrier 16 that connect the first fluid chamber 18 to S each gear mesh and then to the second fluid chamber 19, allowing fluid to pass between the chambers.

Optionally, valves 22, 23 are also provided that allow flow of fluid from one chamber 18 to another 19, and vice versa. Rotational movement of the sun gear 14 is transmitted to a first drive shaft (not shown) via a sun gear coupling point 20, and rotational movement of the planet carrier 16 is transmitted to a second drive shaft (not shown) via a planet carrier coupling point 21. Valve 22, 23 when present are preferably provided for pressure relief, this may be useful as a safety feature, Optionally, the valves 22, 23 may provided either in addition or alternative to the valve 40 to allow a restricted fluid flow between the fluid chambers 18, 19 to provide a limited slip effect.

A fluid such as automatic transmission fluid or gear oil fills the device and all air and gas is excluded. The fluid is retained by the covers 17, which provide a seal to prevent the fluid from escaping from the differential. Figures 12 and 13 Ulustrate the differential with the cover in place. The planet carrier 16 provides a seal between the two fluid chambers 18, 19 and fluid can only pass between them through the holes 26 at the gear meshes or through the valves 22, 23.

In use, the gears 11, 12, 13, 14 in the differential may act as hydraulic pumps, to pump oil between the chambers 18, 19. The direction of fluid flow depends on the direction of motion of the gears, and is illustrated in Figures 10 and 11 by dark arrows. Every gear mesh can be used to create a high and low pressure area at the planet carrier 16. At each point where gear teeth come together, high pressure is generated, and at each point where gear teeth separate, low pressure is generated. Examples of a high pressure region 24 and a low pressure region 25 are illustrated in Figure 7. If the direction of movement of the differential is reversed, then the regions of high and low pressure will consequently be reversed.

In use, the gears 11, 12, 13, 14 in the differential may act as hydraulic motors, to create differential rotation between the two output shafts in response to pressure difference in the fluid between the chambers 18, 19. The gears 11, 12, 13, 14 are moved by the fluid; the direction of motion of the gears depends on the direction of fluid flow which is determined by the pressure difference between the fluid chamber 18, 19. Those gear meshes used to create a high and low pressure areas at the planet carrier 16 when operating as hydraulic pumps can be utilised to generate the differential rotation when acting as a hydraulic motor. In order to reverse the direction of movement of the differential the fluid chambers 18, 19 having high pressure and the fluid chamber 18, 19 having low pressure need to be reversed.

The planet carrier 16 fits closely to tips of the teeth and the side faces of all the gears. This limits the flow back from the high pressure regions enabling the gear meshes to generate pressure. The spaces between the teeth carry fluid, and the fluid cannot escape in any direction owing to the close fitting of the planet carrier 16. As the gears come into mesh, a gear tooth fills the tooth space and the fluid is expelled. The holes 26 in the planet carrier on either side of the differential are located at the areas where pressure is generated so that high pressure fluid can escape from within and low pressure fluid can be channeled in where the separating gear teeth require it. The flow through the holes changes direction when the differential reverses its direction of rotation.

By linking the low pressure areas to a fluid chamber 18, 19, the differential will always pump fluid whilst the gears are rotating.

If the flow away from the high pressure areas is restricted, then a significant pressure drop can be created and more energy in the form of torque will be required to turn the gears in the pump (See Figure 9). This provides the limited slip aspect of the differential.

When the flow of fluid between the first and second fluid chambers 18, 19 is unrestricted, valves 22, 23 are open and fluid flows freely between the fluid chambers 18, 19, the differential operates as an open differential, since there is no resistance to differential rotation.

The differential can be operated as a passively controlled limited slip differential and also as an actively controlled limited slip differential.

The restriction on the fluid flow created by the valve may be fixed or variable and may be actively controlled.

The differential may also be operated as an actively controlled torque vectoring differential.

Apertures 26A in the planet carrier 16 on one side of the differential are off-set from those apertures 26B in the planet carrier 16 on the other side of the differential, best illustrated in Figure 8. Depending on the direction of rotation of the planet gears 12, 13 the apertures 26A, 26B on either side of the planet carrier 16 may be inlets or outlets. if apertures 26A on the first side of the planet carrier are inlets, then the apertures 26B on other side of the planet carrier are outlets and vice versa.

n the illustrated embodiment of Figure 8, the aperture 26A is an outlet and the aperture 26B is an inlet. If the direction of rotation is reversed then aperture 26A becomes an inlet and aperture 26B becomes an outlet.

The fluid may be circulated from one side of the device to the other; to maintain the flow and therefore any differential rotation of the outputs, the fluid must pass from the high pressure reservoir on one side of the differential back to the low pressure reservoir on the other side.

This can be achieved using the external pressure source and! or the valve 40 and/or by providing valves 22, 23 between the fluid chambers 18, 19.

Figure 10 shows how the fluid may be circulated from one side of the device to the other using valve 22, 23 for example to relieve pressure to prevent damage to the differential due to excess pressure in one of the chambers 18.

S Optionally, a single valve 22, 23 may control the flow of fluid across the differential in each direction for pressure control, as illustrated in Figures 10 and 11. The valve illustrated in Figures 10 and 11 is a simple Pressure Relief type valve controlling the pressure. However, other types of valve may be used. For example, a shim type valve as used in a damper or shock absorber may be used to create a complex pressure characteristic related to the flow of fluid. A power steering type valve may be used to govern the pressure characteristic relative to the torque in the drive train. A centrifugal valve may be used to govern the pressure with relation to the speed of rotation of the crown wheel (vehicle speed). An electronic valve may be used to govern the pressure with relation to any number of other parameters. ln any case, a valve or restrictor is required to allow fluid to flow back into the fluid chamber from which fluid is being pumped.

The number of gear meshes, the size of the gear teeth, the viscosity of the fluid and the face-width of the gears all have an influence on the capacity of the pump in the differential.

Compared with other hydraulically controlled differentials, it is estimated that a differential of this design represents a significant reduction in the packaging volume required per unit of torque generated. Notably, this is achieved mainly by a reduction in the axial dimension of the device.

The differential may be provided with a further chamber in contact with at least one of the first or second fluid chambers, 18, 19. The further chamber is filled with a pressurized gas. The pressurized gas ensures that fluid in the chamber under low pressure does not cavitate to form bubbles, when dissolved gases in the hydraulic fluid come out of solution at low pressure. A similar effect can be achieved by using a biasing means such as a coil spring to maintain a raised pressure on the fluid in the chamber. By a raised pressure, it is meant a pressure that is higher than the pressure on the fluid when the limited slip differential is originally assembled.

Turning now to Figures 14 and 15, a differential according to a second embodiment of the invention is illustrated. The differential 24 in this embodiment comprises channels 25, 26 that allow fluid to move between regions where gears mesh and valves 27, 28 that allow fluid to pass from one side of the differential to another. in this example, the fluid channels can be thought of as a reservoir even though no relativdy large body of fluid is provided or required.

The channe's and valves allow regions of high and low fluid pressure to be generated at various points in the differential 24.

Referring now to Figures iSA, 16B, 17 and 18 there is shown third and fourth embodiments of the present invention. In the third and fourth embodiment, like numerals have, where possible, been used to denote like parts, albeit with the addition of the prefix 100" or 200" and so on to indicate that these features belong to the second embodiment. The alternative embodiments share many common features with the first and second embodiments and therefore only the differences from the embodiments illustrated in Figures 1 to 15 will be described in any greater detail.

Turning now to Figures 16A, 16B and 17 there is illustrated a third embodiment of the present invention.

The differential incorporates an integrated high pressure supply pump 160.

The pump 160 is a gear pump; it comprises a plurality of pump planet gears 162, a pump planet gear carrier 164 and a pump annulus 166.

The pump planet gears 162, pump planet gear axles and pump planet gear carrier 164 are fixed to the differential and when employed in a vehicle rotate with the differential at the speed of the axle in which the differentia' is located.

S The pump annulus 166 is coupled to the differential casing 130 by a clutch mechanism 176.

The pump annulus 166 is free to rotate along with the pump planet gears 162 and pump planet gear carrier 164 when the pump is not required to pump fluid.

Clutch mechanism 176 is operable to engage or disengage the pump annuus 166 with the differential casing 130. When the pump annulus 166 is engaged with the differential casing 130 it is stationary or braked to prevent rotation or reduce the speed of rotation with respect to the speed rotation of the differential. ln the engaged state the pump planet gears 162 rotate with respect to the pump annulus 166 and pump fluid into the pump inlet valve 172.

Fluid from pump reservoir 163 is pumped through pump inlet 168 out through pump outlet 170 by the same mechanism used in the differential gear assembly 8, described above.

Fluid is pumped into pump inlet valve 172 which is controllable to select which side of the differential fluid will be pumped into, either fluid chamber 118 or fluid chamber 119.

A pump exhaust valve 174 is provided to allow fluid to flow from either fluid chamber 118 or fluid chamber 119 back into pump reservoir 163.

n the illustrated embodiment the fluid is being pumped into fluid chamber 118 and passes through the differential gear assembly 8 into fluid chamber 119. From fluid chamber 119 it then flows via pump exhaust valve 174 back into pump reservoir 163.

The pump inlet and pump exhaust valves 172, 174 are activated by an activation ring 180 which moves the pump inlet and exhaust valves 172, 174 to direct fluid into the desired fluid chamber 118, 119, S in the illustrated embodiment the exhaust valve 174 is open between fluid chamber 119 and pump reservoir 163 and closed between fluid chamber 118 and the pump reservoir 162. The pump inlet valve 172 is open between the pump outlet 170 and fluid chamber 118 and closed between the pump outlet 170 and fluid chamber 119.

Fluid chamber 118 is supphed with fluid under pressure, the fluid flows across the differential from fluid chamber 118 to fluid chamber 119, the differential gears either acting as a gear pump or a gear motor. The fluid in fluid chamber 119 then returns to the pump reservoir 163 via the pump exhaust valve 174 The activation ring 180 can be activated by moving in a direction substantially parallel to the drive shafts so that the pump outlet is coupled to one of the fluid chamber 119 or the fluid chamber 118; concisely the pump reservoir 163 will be coupled to the other of the fluid chambers 119, 118.

Figure 18 illustrates a further embodiment of the present invention in which the differential comprises an integrated pump 260. Pump 260 is driven electrically by an electric motor, whereas in the previous embodiment it was mechanically driven by the rotation of the drive shafts.

The operation of the differential and the valve system is substantially the same as that of the third embodiment.

The electric motor is coupled to pump ann ulus 266. Preferably, the motor stator 292 is rigidly connected to the pump annulus 266.

The motor rotor 294 is rigidly mounted on the differential preferab'y on the cover 217.

Electrical contacts 290 are couped to an external source of electrica' power such as the battery or alternator of a vehicle.

When the electric motor is powered the pump annulus 266 rotates at a different speed to the differential, This in turn enables the gear pump; pump annulus 266 and pump planet gears 262 to produce fluid pressure that can be utilised for distributing torque between the drive shafts when the motor is in an unpowered state the pump annulus 266 is free to rotate with the differential and does not generate any fluid pressure.

An advantage of this embodiment over the third embodiment is that the gear pump can be used to generate fluid pressure event when the differenti& is stationary.

The differentials of the foregoing embodiments can be used to effectively steer the vehicle, when the differential is connected between the steering wheels for example on the front axle of a car. The differential gears 12, 13 can be caused to rotate by increasing the pressure on the side of the differentiaL A pressure differential between the fluid in the left and right hand fluid chambers 18, 19 can be created by pumping fluid into either fluid chamber 18, 19. The fluid under high pressure wifl flow into the chamber under lower pressure. The high pressure fluid forces its way through apertures 26A, 26B in the planet carrier 16, gears 12, 13 rotate as a consequence, which in turn causes the left and right hand drive shafts to rotate relative to one another.

When the differential is used in a vehicle and circumstances e.g. executing a turn, loss of traction with road surface, cause the left and right hand shafts to rotate relative to one another, the pump or pressure source may be used to either increase or decrease the pressure differential between the left and right hand fluid chambers 18, 19 caused by the relative rotation, such that torque can be directed to the desired wheel or axle.

When the differential is being used as a limited slip differential the differential may be passively controlled, when valves 22, 23 are provided to restrict flow between fluid chambers 18, 19.

The differential can also be operated as an actively controlled limited slip differential. The control system directs the pressure source to pump fluid into one of the fluid chambers 18, 19.

In some embodiments the differential is provided with both the valves 22, 23 and the additional pressure source, the differential may be actively controlled by the control system using the pressure source to enhance or reduce the limited slip effect produced by the valves 22, 23.

ln alternative embodiments the differential may employ different gear assemblies such as those used in bevel type differentials or planetary type differentials in particular but not exclusively those employing two sun gears in addition or replacement of the annulus.

lt will be appreciated by a person of skill in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention.

Claims

CLAIMSI. A differential comprising, a gear assembly having at least two gears, each gear having plurality of teeth; a fluid reservoir comprising at least two reservoir portions; the at least two reservoir portions being coupled to one another by at least two openings; the openings being disposed at points adjacent to where the teeth of a first of said at least two gears meshes with the teeth of another of said at least two gears; a device coupled to the at least two reservoir portions for altering a pressure in a fluid contained the first or second reservoir portions; wherein in use, at least one of said at least two gears acts as a hydraulic gear pump or hydraulic gear motor.
2. The differential according to claim I wherein the at least two gears act as a hydraulic gear pump to create a pressure differential between the fluid in the first and the fluid in the second reservoir portions.
3, The differential according to claim 1 wherein the at least two gears act as a hydraulic gear motor in response to a pressure differential in the fluid between the first and second reservoir portions.
4. The differential according to claim 1 wherein said at least two gears are arranged to rotate in substantially the same plane.
S. The differential according to any preceding claim wherein the gear assembly is an epicyclic gear assembly comprising a sun gear, an annulus at least one pair of intermeshing planet gears and a planet carrier to which the planet gears are attached wherein the annulus, sun gear and planet gears are arranged to rotate substantiafly on the same plane.
6. The differential according to claim S wherein the at least two openings are provided in the planet carrier, the openings being disposed at points adjacent to where the teeth of at least one of the planet gears meshes with the teeth of another gear the openings connecting the meshing teeth to one of the first and second fluid reservoir portions.
7. The differential according to claim I wherein the differential comprises a planetary gear assembly.
8, The differential according to claim 1 wherein the differential comprises a bevel gear assembly.
9, The differential according to any preceding claim wherein the device is a pump for example a gear pump, gerotor, impeller or vane type pump.
10. The differential according to claim 9 wherein the pump is integrated with the differential,
11. The differential according to either of claims 9 or 10 wherein the pump is mechanically driven.
12. The differential according to either of claims 9 or 10 wherein the pump is electrically driven.
13. The differential according the any of claims 9 to 12 wherein the pump is a gear pump and comprises a pump gear assembly having a pump planet gear, pump planet gear carrier and pump annulus, the pump planet gear carrier comprising at least two openings disposed at a points adjacent where the teeth of the at least one pump planet gear meshes with the pump annulus wherein in use the pump gear assembly acts as a hydraulic gear pump to alter a pressure in the fluid contained in the first or second reservoir portions.
14. The differential according to daim 13 wherein the pump planet gear carrier is fixedly attached and the pump planet gears are rotationally attached to the differential.
15. The differential according to either of claims 13 or 14 wherein the pump annulus is rotationally attached to the differential.
16. The differential according to claim 13 wherein the pump gear assembly is coupled to a drive means.
17. The differential according to claim 16 wherein the pump annulus is coupled to said drive means.
18. The differential according to claim 16 or 17 wherein the drive means is an electric motor.
19. The differential according to claim 15 wherein an element of the pump is coupled to a motor stator or motor rotor of said electric motor.
20. The differential according to claim 19 wherein the pump annulus is fixedly attached to a motor stator of said electric motor.
21. The differential according to claim 19 wherein the pump planet carrier is fixedly attached to the motor rotor of said electric motor.
22. The differential according to claim 1 further comprising a casing surrounding the gear assembly.
23. The differential according to claim 12 to 15, when dependent on claim 21 wherein an element of the pump the pump annulus is selectively coupled to the casing.
24. The differential according to claim 23 wherein the element of the pump is coupled to the differential casing by a clutch mechanism for selective engagement therewith.
25. The differential according to either of claims 23 or 24 wherein the pump annulus is coupled to the differential casing by a clutch mechanism for selective engagement therewith.
26. The differential according to either of claim 5 or 6 wherein the differential comprises first and second covers surrounding the planet gears, sun gear, annulus and planet carrier.
27. The differential according to claim 26 dependent upon claim 22 wherein the first and second covers are rotationally connected within the casing and a fluidic connection is provided between the casing and the first and second covers.
28. The differential according to claim 9 wherein the pump is coupled to the first and second reservoir portions by a valve system comprising one or more valves.
29. The differential according to claim 28 wherein the valve system is integrated within the differential.
30. The differential according to claim 29 wherein the valve system comprises an inlet valve and an outlet valve.
31. The differential according to either of claims 28 to 30 wherein the differential comprises a crown wheel adapted to receive the valve system
32. A crown wheel for a differential comprising first and second sides and being adapted to receive a valve system for coupling each side to a pressure source.
33. The crown wheel according to claim 32 comprising an inlet valve and an outlet valve.
34. The crown wheel according to either of claims 32 or 33 comprising a pressure source mounted thereon.
35. The crown wheel according to any of claims 32 to 34 wherein the pressure source is a gear pump comprising a pump planet gear carrier and pump planet gears mounted on the crown wheel.
36. A differential comprising an epicyclic gear assembly, the epicyclic gear assembly comprising a sun gear, an annulus, at least one pair of intermeshing planet gears, and a planet carrier to which the planet gears are attached, wherein the annulus, sun gear and planet gears are arranged to rotate substantially in the same plane; a fluid reservoir comprising at least two reservoir portions; at least two openings in the planet carrier, the openings being disposed at points adjacent to where the teeth of at least one of the planet gears meshes with the teeth of another gear, the openings connecting the meshing teeth to one of the first and second fluid reservoir portions; at least one valve connecting the first fluid reservoir portion to the second fluid reservoir portion; wherein, in use, at east one gear of the epicyclic gear assembly acts as a hydraulic gear pump or motor to alter a pressure in a fluid contained in the first and second reservoir portion wherein there further comprises art additional pressure source for providing fluid under pressure to either of the first or second reservoir portions.
37. The differential according to claim 36, wherein the first reservoir portion is a first reservoir; and the second reservoir portion is a second reservoir.
38. The differential according to claim 36, wherein the first reservoir portion comprises at least one fluid-carrying channel and the second reservoir portion comprises at least one further fluid-carrying channel.
39. The differential according to claims 36, 37 and 38, wherein the first and second fluid reservoir portions are disposed either side of the epicyclic gear assembly, and the valve passes through the planet carrier.
40. The differential according to any one of claims 36 to 39, wherein the epicyclic gear assembly comprises a double epicyclic gear assembly, the double epicyclic gear assembly comprising at least one pair of planet gears.
41. The differential according to any one of claims 36 to 40, wherein the valve is selected from one of a pressure relief valve, a shim type valve, a power steering type valve, electronically controlled valve, a centrifugal valve and a simple flow restrictor.
42. The differential according to any one of claims 36 to 41, wherein said first and second fluid reservoir portions contain hydraulic fluid.
43. The differential according to any one of claims 36 to 42, further comprising means for maintaining a raised pressure on fluid in each of the reservoir portions.
44. The differential according to claim 43, wherein the means to maintain a pressure is selected from one of a spring and pressurised gas.
45. A vehicle comprising one or more differentials according to any preceding claim.
46. A differential for distributing torque between two outputs substantially as described herein with reference to and/or as illustrated by any of Figures 2 to 18.
47. A vehicle comprising the differential substantially as described herein with reference to and/or as illustrated by any of Figures 2 to 18.