WO2009024814A1

WO2009024814A1 - Limited slip differential

Info

Publication number: WO2009024814A1
Application number: PCT/GB2008/050701
Authority: WO
Inventors: John Morton
Original assignee: John Morton
Priority date: 2007-08-17
Filing date: 2008-08-13
Publication date: 2009-02-26
Also published as: GB2451887A; GB0716015D0

Abstract

A limited slip 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. The annulus and gears are arranged to rotate substantially in the same plane. The differential further comprises a fluid reservoir comprising at least two reservoir portions, and at least two openings in the planet carrier disposed at points adjacent to where the teeth of the planet gears mesh with the teeth of another gear. The openings connect the meshing teeth to one of the first and second fluid reservoir portions. At least one valve connects the first fluid reservoir to the second fluid reservoir. In use, at least one gear of the epicyclic gear assembly acts as a hydraulic gear pump to alter pressure in a fluid contained in the reservoirs.

Description

Limited Slip Differential

Field of the Invention

The invention relates to the field of limited slip differentials.

Background to 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 or 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 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 drive 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 planet carrier 3 and sun gear 6 are allowed to rotate at different speeds.

A limited slip 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. Therefore, a differential rotation of the outputs requires an increase in the energy supplied to the device. 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 toque to the other wheels. The vehicle would not be able to pull away because equal torque is transmitted to both wheels and the wheel on ice cannot generate any torque. A limited 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, thereby 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 6,001 ,040 discloses a hydraulically operated limited slip differential that uses differential gears to pump fluid, but this device drives a clutch to generate torque. A disadvantage of devices that use a clutch to generate torque is that 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 4,838,120 discloses a limited slip differential that uses centrifugal force as the control element. This force is applied to a clutch pack, which provides direct drive between the casing and the side gears of the limited slip differential.

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.

Summary

The inventor has devised a planet carrier for a limited slip differential, and a limited slip differential, based on an epicyclic differential, that mitigates some of the problems of known limited slip differentials and does not rely on a clutch mechanism.

According to a first aspect of the invention, there is provided a limited slip differential. The limited slip differential comprises an epicyclic gear assembly. The epicyclic gear assembly comprises 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. The limited slip differential further comprises a fluid reservoir comprising at least two reservoir portions, and the planet carrier comprises at least two openings. The openings are disposed at points adjacent to where the teeth of at least one of the planet gears meshes with the teeth of another gear, and the openings connect the meshing gears with one of the first and second fluid reservoir portions. A valve connects the first fluid reservoir portion to the second fluid reservoir portion. In use, at least one gear of the epicyclic gear assembly acts as a hydraulic gear pump to alter a pressure in a fluid contained in the first and second reservoir portions, providing limited slip effect in the differential.

Whilst the description refers to fluid reservoirs, it will be apparent that this term encompasses any structure for holding a fluid, which may be a separate chamber, or simply fluid channels. For example, the first reservoir portion may be a first reservoir, and the second reservoir portion a second reservoir. Alternatively, the first and second reservoir portions may be fluid-carrying channels. As an example to show how the annulus, sun gear and planet gears can rotate substantially in the same plane, the sun gear may mesh to an inner planet gear, the inner planet gear may mesh with the sun gear and an outer planet gear, and the outer planet gear may mesh to the annulus.

In one embodiment, the first and second fluid reservoirs are disposed either side of the epicyclic gear assembly, and the valve passes through the planet carrier. However, note that the valve need not necessarily pass through the planet carrier; it may alternatively pass through the annulus and casings, or be used to restrict flow directly adjacent to the gear mesh.

It is preferred that the epicyclic gear assembly comprises a double epicyclic gear assembly, the double epicyclic gear assembly comprising at least one pair of planet gears.

The valve of the limited slip differential may be selected from one of a pressure relief valve, a shim type valve, a power steering type valve, electronically controlled valve and a centrifugal valve. By valve, it is meant any device suitable for controlling the flow of a fluid, and could also include a simple restrictor.

The first and second fluid reservoirs preferably contain hydraulic fluid.

The limited slip differential may be provided with means for maintaining a raised pressure on fluid in each reservoir portion. By maintaining a raised pressure on the fluid, cavitation of hydraulic fluid contained in the reservoir portions is less likely to occur.

Examples of such means include a pressurized gas located adjacent to each reservoir portion, or a spring providing pressure on the reservoir portions. For example, a third and fourth sealed compartment may be provided in contact with the reservoir portions respectively, wherein the third and fourth compartments each contain either the pressurized gas or spring in order for a force to act on the fluid contained within the reservoir portions. According to a second aspect of the invention, there is provided a planet carrier for a limited slip differential The planet carrier comprises at least one attachment point for attaching a planet gear, a first cover and a second cover which, in use, surround the planet gear, at least one opening in each of the first and second covers, each opening disposed adjacent to a point where the teeth of the planet gear mesh with the teeth of another gear, and at least one valve passing through the planet carrier between the first and second covers, the valve arranged to allow the passage of a fluid.

Brief Description of the Drawings

Figure 1 illustrates schematically an open front view of a double epicyclic differential;

Figure 2 illustrates schematically an open front view of a limited slip differential according to an embodiment of the invention;

Figure 3 illustrates schematically the limited slip differential of Figure 2 showing section lines;

Figure 4 illustrates schematically a side cross-section view through section AA of the limited slip differential shown in Figure 3;

Figure 5 illustrates schematically a side cross-section view through section CC of the limited slip differential shown in Figure 3;

Figure 6 illustrates schematically a side cross-section view through section BB of the limited slip differential shown in Figure 3;

Figure 7 illustrates schematically an isometric view of the limited slip differential shown in Figure 3;

Figure 8 illustrates schematically a side elevation view of the limited slip differential shown in Figure 3; Figure 9 illustrates schematically an open front view of a limited slip differential according to a second embodiment of the invention; and

Figure 10 illustrates schematically a side cross-section view of the limited slip differential shown in Figure 9.

Detailed Description

Referring to Figure 3, an open front view of a limited slip differential is illustrated. The gear assembly 8 of the limited slip differential comprises a crown wheel 10, an annulus

1 1 , 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 1 1 and the sun gear 14. The first planet gear 12 is in contact with the annulus

1 1 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 1 1 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 4 illustrates the same view as Figure 3 with section lines added, and Figures 5 and 6 illustrate side elevation cross-section views through sections BB and CC respectively. A cover 17 is located around the gear assembly 8. A first fluid chamber 18 and a second fluid chamber 19 are disposed 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 1 1 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 each gear mesh and then to the second fluid chamber 19, allowing fluid to pass between the chambers. 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. A fluid such as 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 limited slip differential. Figures 7 and 8 illustrate the limited slip 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 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 figure 5 and 6 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 3. If the direction of movement of the differential is reversed, then the regions of high and low pressure will consequently 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 channelled in where the separating gear teeth require it. When the differential reverses direction, the flow through the holes changes direction.

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 4). This provides the limited slip aspect of the differential. Figure 5 shows how the fluid is 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. A single valve governs the flow across the differential in each direction for pressure control, as illustrated in Figure 5. The valve illustrated in Figure 5 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. In 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 power of the pump in the differential. Compared with other hydraulic 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 limited slip 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 9 and 10, a limited slip differential according to a second embodiment of the invention is illustrated. The limited slip 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 limited slip differential to another. In this example, the fluid channels can be thought of as a reservoir even though no relatively large body of fluid is provided or required. The channels and valves allow regions of high and low fluid pressure to be generated at various points in the limited slip differential 24.

It 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

CLAIMS:

1. A limited slip 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 assembly acts as a hydraulic gear pump to alter a pressure in a fluid contained in the first and second reservoir portion.

2. The limited slip differential according to claim 1 , wherein the first reservoir portion is a first reservoir, and the second reservoir portion is a second reservoir.

3. The limited slip differential according to claim 1 , 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.

4. The limited slip differential according to claim 1 , 2 or 3, 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.

5. The limited slip differential according to any one of claims 1 to 4, wherein the epicyclic gear assembly comprises a double epicyclic gear assembly, the double epicyclic gear assembly comprising at least one pair of planet gears.

6. The limited slip differential according to any one of claims 1 to 5, 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.

7. The limited slip differential according to any one of claims 1 to 6, wherein said first and second fluid reservoir portions contain hydraulic fluid.

8. The limited slip differential according to any one of claims 1 to 7, further comprising means for maintaining a raised pressure on fluid in each of the reservoir portions.

9. The limited slip differential according to claim 8, wherein the means for maintaining a pressure is selected from one of a spring and a pressurized gas.

10. A planet carrier for a limited slip differential comprising; at least one attachment point for attaching a planet gear; a first cover and a second cover which, in use, surround the planet gear; at least one opening in each of the first and second covers, each opening disposed adjacent to a point where the teeth of the planet gear mesh with the teeth of another gear; and at least one valve passing through the planet carrier between the first and second covers, the valve arranged to allow the passage of a fluid.