GB2451887A

GB2451887A - Limited slip differential

Info

Publication number: GB2451887A
Application number: GB0716015A
Authority: GB
Inventors: John Morton
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-08-17
Filing date: 2007-08-17
Publication date: 2009-02-18
Also published as: GB0716015D0; WO2009024814A1

Abstract

A limited slip differential comprises an epicyclic gear assembly 8 having a sun gear 14, an annulus 11, planet gears 11, and a planet carrier 16 to which the planet gears 11 are attached. The differential further comprises a first 18 and a second 19 fluid reservoir, and at least two openings 26 in the planet carrier 16 disposed at points adjacent to where teeth of the planet gears 11 mesh with the teeth of another gear. The openings 26 connect the meshing teeth to one of the first 18 and second 19 fluid reservoirs. At least one valve (22, 23, fig 6) connects the first fluid reservoir 18 to the second fluid reservoir 19. In use, at least one gear of the epicyclic gear assembly acts as a hydraulic gear pump to alter pressure in hydraulic fluid contained in the reservoirs 18, 19. The valve/s (22, 23) may be a pressure relief valve, a shim type valve, power steering valve, a centrifugal valve or a simple flow restrictor. The differential may comprise third and fourth reservoirs connected to the first and second reservoirs wherein the third and fourth reservoirs contain pressurized gas or a spring.

Description

Limited Slip Differential

Field of the invention

The invention relates to the field oIliniited 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 I 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 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 diflerential 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 transniitted 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 chaniber is coupled to a drive shall, and an outside surface of the chamber is coupled to the diflerential 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 punip 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 suffor 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.

Sumniary 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 dif'ferentials 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 planet gear, and a planet carrier to which the planet gear is attached, wherein the annulus, sun gear and planet gear are arranged to rotate substantially in the same plane. The limited slip differential further comprises a first and a second fluid reservoir d, and the planet carrier comprises at least two openings. The openings are disposed at points adjacent to where the teeth of the planet gear meshes with the teeth of another gear, and the openings connect the nieshing gears with one of the first and second fluid reservoir\. A 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 punip to alter a pressure in a fluid contained in the first and second reservoirs, 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.

It is prefCrred that the first and second fluid reservoirs are disposed either side of the epicyclic gear assembly, and the valve passes through the planet carrier.

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

The valve of the liniited slip differential may be selected from one of a pressure relief valve, a shini 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.

In order to prevent cavitation of the hydraulic fluid in the fluid reservoirs, the limited slip differential may further comprise comprising a third and a fourth compartment in contact with the first and second fluid reservoirs respectively, wherein the third and fourth compartments are each arranged to contain one of a pressurised gas and a spring.

According to a second aspect of the invention, there is provided a planet carrier for a limited slip differentia. 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 lirsi 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 dilTerential according to an enibodinient of the invention; Figure 3 illustrates schematically the limited slip differential of Figure 3 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 diflrential shown in Figure 3; and Figure 8 illustrates schematically a side elevation view of the limited slip differential shown in Figure 3.

Detailed Description

Referring to Figure 3, an open front view of a limited slip differential is illustrated. The gear assembly 8 oithe limited slip differential comprises a crown wheel 10, an annulus II, 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 I I is transferred 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 1 7 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 I I 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 chanibers. 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 transniitted 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 1 7, 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 II, 12, 13, 14 in the dilierential act as a 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 niesh 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 cany 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 niore 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 froni 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 daniper 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.

In an alternative embodiment of the invention, the limited slip differential comprises 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 chaniber under low pressure does not cavitate to form bubbles, when dissolved gases in the hydraulic fluid come out of solution at low pressure.

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

Claims

CLAIMS: I. A limited slip differential comprising; an epicyclic gear assembly, the epicyclic gear assembly comprising a sun gear, an annulus, at least one planet gear, and a planet carrier to which the planet gear is attached, wherein the annulus, sun gear and planet gear are arranged to rotate substantially in the sanie plane; a first and a second fluid reservoir; at least two openings in the planet carrier, the openings being disposed at points adjacent to where the teeth of the planet gear meshes with the teeth of another gear, the openings connecting the meshing teeth to one of the first and second fluid reservoirs; at least one valve connecting the first fluid reservoir to the second fluid reservoir; 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 reservoirs.
2. A limited slip differential according to claim I, wherein the first and second fluid reservoirs are disposed either side of the epicyclic gear assembly, and the valve passes through the planet carrier.
3. A limited slip diuierential according to claim I or 2, wherein the epicyclic gear assembly comprises a double epicyclic gear assembly, the double epicyclic gear assembly comprising at least one pair of planet gears.
4. A limited slip differential according to claim I, 2 or 3, wherein the valve is selected from one of a pressure relief valve, a shini type valve, a power steering type valve, electronically controlled valve, a centrifugal valve and a simple flow restrictor.
5. A limited slip differential according to any one fo the preceding claims, wherein said first and second fluid reservoirs contain hydraulic fluid.
6. A limited slip differential according to any one of the preceding claims, further comprising a third and a fourth reservoir in contact with the first and second fluid reservoirs respectively, wherein the third and fourth reservoirs are each arranged to contain one of a pressurized gas and a spring.
7. A planet carrier for a limited slip differential comprising; at least one attachment point for attaching a planet gear; a iirst 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.
8. A limited slip differential substantially as described herein with reference to figures 2 to 8 of the accompanying drawings.
9. A planet carrier for a limited slip differential, the planet carrier substantially as described herein with reference to figures 2 to 8 of the acconipanying drawings.