GB2245328A

GB2245328A - Torque-transmitting coupling for releasably connecting drive to one axle in a four-wheel drive vehicle

Info

Publication number: GB2245328A
Application number: GB9109200A
Authority: GB
Inventors: Wolfgang Beigang; Frank Boettger
Original assignee: Viscodrive GmbH
Current assignee: Viscodrive GmbH
Priority date: 1990-04-30
Filing date: 1991-04-29
Publication date: 1992-01-02
Anticipated expiration: 2011-04-29
Also published as: IT1247290B; FR2661375A1; FR2661375B1; DE4013855C2; ITMI911089A0; DE4013855A1; JPH0789376A; GB2245328B; ITMI911089A1; JP2973374B2; GB9109200D0

Description

Title: Torque-transmitting coupling and method for its release.

Ibis invention relates to a motor vehicle transmission having a drive unit, e.g. an internal combustion engine and gearbox, and a plurality of driven axles wherein a first said axle is permanently connected to the drive unit and a second said axle is releasably connectable to the drive unit through a torque transmitting slip coupling and a further releasable torque transmitting coupling. The invention is concerned with a method of disconnecting said second axle from the drive unit by means of said releasable torque transmitting coupling. The invention also relates to a releasable torque transmitting coupling for use in a motor vehicle having a transmission of the above type and wherein said coupling is of the type comprising a first shaft having a plurality of circumferentially distributed recesses therein, a second shaft comprising a sleeve having a plurality of circumferentially distributed apertures therein associated with said recesses, coupling elements radially displaceably carried in said apertures and being displaceable to engage in said recesses for connecting said shafts together in nonrotatable engagement relative to one another, and an actuating member coaxial with and movable axially relative to said shafts for radially displacing said coupling elements and for retaining said coupling elements in the position in which the shafts are non-rotatably connected together in a locked condition of said coupling.

A transmission of the above type may be provided in a four-wheel drive system of a motor vehicle wherein a second axle, usually the rear axle, is torque-loaded through a slip coupling when there is a positive speed differential between a first permanently driven axle and the second axle. Slip couplings.suitable for this purpose may comprise fluid friction couplings of the type known as viscous couplings. Other couplings such as those of the pump type are also suitable. In other types of four-wheel drive transmission there may be provided two slip couplings arranged in a drivable differential carrier in an axle differential a 2 in the second axle which is not permanently driven. Such second axle is normally the rear axle with the respective halves of the couplings being connected to the axle half-shafts.

If, in the transmissions referred to above, the axles were to be connected permanently via the torque-transmitting coupling, then torque would also be transmitted from the second axle to the first axle if there occurred a negative speed differential during braking between a locked first axle and the second axle.

Speed differentials between front and rear wheels also occur during actuation of anti-lock braking systems. It is already known during braking to actuate a releasable torque transmitting coupling which, when the brake is actuated, disconnects the axles from one another. Such disconnection may be sudden and therefore undesirable from the point of view of the vehicle occupants especially if the first axle is a front axle which transmits torque to the second or rear axle when the brake is actuated. This can occur when bends in the road are being negotiated wherein the front axle rotates to a greater extent than the rear axle thereby causing torsion in the driveline.

It is an object of the present invention to provide an improved method of disconnecting a second axle from a drive unit in a transmission of the type described above and also to provide a releasable torque transmitting coupling suitable for use in such method.

In accordance with a first aspect of the invention there is provided, in a motor vehicle having a drive unit and a plurality of driven axles wherein a first said axle is permanently connected to the drive unit and a second said axle is releasably connectable to the drive unit through a torque-transmitting slip coupling and a lockable and unlockable releasable torque-transmitting coupling,.a method of disconnecting said second axle from the drive unit wherein said releasable torque- transmitting coupling is unlockable to a release condition in response to the driving condition of the vehicle such as braking or negative acceleration and is automatically releasable, when in said release condition, from 3 its torque-transmitting condition only in response to a reversal of torque from said second axle to the drive unit.

Said releasable torque-transmitting coupling may be unlockable to said release condition in response to release of the vehicle accelerator, or in response to depression of the vehicle brake pedal, or in response to a signal from an antilock braking system of the vehicle.

Ille effect of the above method is such that, in the locked condition of the coupling, torque may be transmitted to the second axle in either direction of torque, i.e. even when reversing the vehicle drive is ensured for all wheels if there is a differential between the axle speeds. However, in contrast to prior art devices, when the vehicle is being driven forwards and a braking operation is initiated, the second axle is not effectively disconnected immediately upon actuating the brake but automatic disconnection only takes place upon reversal of the torque flow from the second axle to the first axle.

Also in accordance with the invention there is provided in or for a motor vehicle having a drive unit and a plurality of driven axles wherein a first said axle is permanently connected to the drive unit and a second said axle is releasably connectable to the drive unit through a torquetransmitting slip coupling and a lockable and unlockable releasable torque-transmitting coupling, said releasable torque-transmitting coupling comprising a first shaft having a plurality of circumferentially distributed recesses therein; a second shaft comprising a sleeve having a plurality of circumferentially distributed apertures therein associated with said recesses; coupling elements radially displaceably carried in said apertures and being displaceable to engage in said recesses for connecting said shafts together in non-rotatable engagement relative to one another; and an actuating member coaxial with and movable axially relative to said shafts for radially displacing said coupling elements and for retaining said coupling elements in the position in which the shafts are non-rotatably connected together in a locked condition of the coupling characterised in that a first group of said coupling elements is carried in afirst group of apertures and is retainable 4 in said locked condition by a cylindrical locking face on said actuating member, a first annular groove being provided on said member adjacent said cylindrical locking face; a second group of said coupling elements is carried in a second group of apertures and is retainable in said locked condition by a conical locking face on said actuating member, a second annular groove being provided on said member adjacent said conical locking face; the apertures of said first group thereof being axially spaced and circumferentially offset from the apertures of said second group thereof., a locking member is provided for restraining the actuating member against axial movement whereby, in said locked condition, under each direction of torque, only one group of coupling elements establishes contact at the flanks of its apertures and recesses and, upon release of said actuating member by said locking member, under torque in a first direction the coupling elements of the first group establish contact at the flanks of their apertures and recesses and are radially locked by said cylindrical locking face and, under torque in a second direction, the coupling elements of the second group establish contact at the flanks of their apertures and recesses and, through radial forces acting on said conical locking face, axial movement of the actuating member is effectible whereby the coupling elements may enter said annular grooves and release the shafts from their non-rotatable engagement with one another.

In accordance with a further aspect of the invention there is provided in or for a motor vehicle having a drive unit and a plurality of driven axles wherein a first said axle is permanently connected to the drive unit and a second said axle is releasably connectable to the drive unit through a torque-transmitting slip coupling and a lockable and unlockable releasable torque-transmitting coupling, said releasable torque- transmitting coupling comprising a first shaft -having a plurality of circumferentially distributed recesses therein; a second shaft comprising a sleeve having a plurality of circumferentially distributed apertures therein associated with said recesses; coupling elements radially displaceably carried in said apertures and being displaceable to engage in said recesses for 1 1 connecting said shafts together in non-rotatable engagement relative to one another; and an actuating member coaxial with and movable axially relative to said shafts for radially displacing said coupling elements and for retaining said coupling elements in the position in which the shafts are non-rotatably connected together in a locked condition of the coupling characterised in that a first group of said coupling elements is radially displaceable into a first group of recesses in said first shaft and is retainable in said locked condition by a cylindrical locking face on said actuating member, a first annular groove being provided on said member adjacent said cylindrical locking face; a second group of said coupling elements is radially displaceable into a second group of recesses in said first shaft and is retainable in said locked condition by a conical locking face on said actuating member, a second annular groove being provided on said member adjacent said conical locking face; the recesses of said first group thereof being axially spaced and circumferentially offset from the recesses of said second group thereof., a locking member is provided for restraining the actuating member against axial movement whereby, in said locked condition, under each direction of torque, only one group of coupling elements establishes contact at the flanks of its apertures and recesses and, upon release of said actuating member by said locking member, under torque in a first direction the coupling elements of the first group establish contact at the flanks of their apertures and recesses and are radially locked by said cylindrical locking face and, under torque in a second direction, the coupling elements of the second group establish contact at the flanks of their apertures and recesses and, through radial forces acting on said conical locking face, axial movement of the actuating member is effectible whereby the coupling elements may enter said annular grooves and release the shafts from their non-rotatable engagement with one another.

The said locking member may comprise an axially movable member which, in a first position, retains a plurality of locking balls in a radially outward position between opposed radial faces of the actuating member and a fixed counter-face and which, in a second or release position, permits radially inward 6 displacement of said locking balls out of the space between said radial faces of the actuating member and the counter-face.

Said first shaft may be a hollow shaft located around said second hollow shaft, and said actuating member may comprise a central shaft having said cylindrical and conical locking faces and said annular grooves on the exterior thereof. Alternatively said first shaft may be a shaft preferably a solid shaft located within said second shaft, and said actuating means may comprise an outer sleeve having said cylindrical and conical locking faces and said annular grooves on the interior thereof.

Said recesses in said first shaft may comprise axial grooves of partcircular cross-sectional form, or may be of part-spherical configuration, or may comprise bores having diameters slightly less than the respective dimensions of ball type coupling elements which are engagable in said recesses.

In all of the above aspects of the invention the coupling elements conveniently comprise a plurality of balls.

The locking member referred to above may be movable axially by pneumatic or electro-magnetic means in response to said braking or negative acceleration conditions of the vehicle.

Other features of the invention will become apparent from the following description given herein solely by way of example with reference to the accompanying drawings wherein:-

Figure 1 is a diagrammatic representation of a vehicle with a fourwheel drive transmission system and a Suid friction coupling arranged between the front and rear axle sets; Figure 2 is a diagrammatic representation of a vehicle with a fourwheel drive transmission system having a fluid friction coupling arranged in a rear.axle differential; Figures 3(1) to 3(4) illustrate a releasable torque-transmitting coupling constructed in accordance with the invention in four different operating conditions as follows:- 7 3(1) 3(2) 3(3) During forward drive of the vehicle (positive torque); After unlocking to the release condition following braking (positive torque); During drive reversal occurring on further braking (negative torque); 3(4) After automatic release during further braking (torque-free) Figure 4 is a longitudinal cross-sectional view through a releasable torque-transmitting coupling constructed in accordance with the invention for use in a single fluid friction coupling; and Figure 5 is a similar view to that of Figure 4 but showing a differential with two fluid friction couplings and two releasable torque-transmitting couplings.

Figure 1 illustrates a vehicle 21 having a drive unit 22 consisting of an engine and gearbox, a driven front axle 23 and a driven rear axle 24. At the gearbox of the drive unit 22 there is arranged a front axle differential drive 25 which, via first halfshafts 26, drives the wheels of the front axle 23. The input end of the differential gear 25 is connected to the input end of the viscous coupling 27 whose output end, via a propeller shaft 28, drives the rear axle differential 29 which, via second halfshafts 30, is connected to the wheels of the rear axle 24.

Figure 2, in principle, shows a vehicle 31 whose drive unit 32 consisting of an engine and gearbox drives the wheels of the front axle 33 and the rear axle 34. The gearbox of the drive unit 32 is connected to the front axle differential 35 which, directly via halfshafts 36, drives the wheels of the front axle 33. The input end of the front axle differential 35 is directly connected to a propeller shaft 38 which drives a differential carrier 37 as the input end of a double fluid friction coupling whose two output shafts 39, with second halfshafts 40, drive the wheels of the rear axle 34. First plates 41 are firmly connected to the housing part whereas second plates 42 are firmly connected to the output shafts 39.

The releasable torque-transmitting couplings of the invention which are associated with the fluid friction slip couplings as illustrated in Figures 1 and 2 are not shown diagrammatically, but are identifiable in Figures 4 and 5.

8 In Figures 3(1) to 3(4) corresponding parts have been given identical reference numbers. They each show a driving hollow shaft 1, a driven shaft 2 in the form of a sleeve and an actuating member 3 in the form of a journal. Shaft 1 is provided with recesses 4 in the form of oblong grooves whose cross-section has the shape of circle segments. These are associated with first oblong apertures 5 and second oblong apertures 6 in the sleeve shaft 2. Torque transmitting balls 7 are held in the apertures 5, with balls 8 being held in the apertures 6. The firstmentioned apertures 5 and balls 7 are provided in two circumferential rows, with the second apertures 6 and balls 8 being provided in one circumferential row. The central actuating member 3 comprises cylindrical locking faces 9 underneath the balls 7 and conical locking faces 10 underneath the balls 8, with circumferential grooves 11, 12 axially adjoining these. As the apertures 5, 6 are circumferentially offset relative to each other, in the configuration as shown in Figure 3(1) only the balls 7 are in contact with the end faces of the apertures 5, whereas the balls 8 are positioned with play relative to the two flanks of the apertures 6. When subjected to torque, the locking faces 9 are loaded purely radially by the balls 7 with forces F. The axial position of the actuating member 3 is secured by locking balls 15 which are locked by a locking member 13.

Figure 3(2) substantially shows the same parts. By actuating the vehicle brake, the locking member 13 is disengaged towards the left. With an unchanged torque direction, the balls 7 continue to act with radial forces F on to the locking faces 9 so that the actuating member 3 remains axially force-free, with the torque flow being maintained.

In Figure 3(3), under conditions of torque reversal and as a result of an increased speed of the second axle relative to the first permanently driven axle, the outer shaft 1 is now rotated relative to the second sleeve-type shaft 2 in such -a way that the balls 8 establish contact with the flanks of the apertures 6, whereas the balls 7 are held with play and torque-free in the apertures 5. Under the influence of torque, a normal surface force F divided into a radial component and an axial component is applied to the conical locking face 10. When the locking 1 1 9 member 13 is withdrawn, the axial component, via the radial face 19 and in cooperation with a conical face 14, causes the locking balls 15 to enter a central annular space 16 which was previously occupied by the locking member 13.

As a result, the actuating member 3, as shown in Figure 3(4) assumes a position which is displaced towards the left in which both the balls 7 and the balls 8 may enter the annular grooves 11 and 12 of the actuating member 3 so that - as shown in cross-section - the shafts 1 and 2 are freely rotatable relative to each other, with the subsequent fluid friction coupling being uncoupled. When the locking member 13 carries out a reverse movement, the locking balls are pushed radially outwardly, as a result of which the actuating member 3 is axially displaced, with the balls 7 and 8 again being pushed into the recesses 4 via conical faces 17, 18.

Figure 4 shows a releasable torque-transmitting coupling in accordance with the invention to whose input end, i.e. preferably to the flange 43 of a driveshaft 101 a fluid friction slip coupling would be attached.

Within the driveshaft 101 there is shown a sleeve-type output shaft 102 within which the actuating member 103 is identifiable. The driveshaft 101 is provided with groove-shaped recesses 104. The inner shaft is provided with associated first apertures 105 radially displaceably holding balls 107 which may be locked by a cylindrical locking face 109 as shown in the lower half of the Figure or which enter an adjoining annular groove 11, as shown in the upper halL Ile shaft 102 is provided with further apertures 106 holding balls 108 which either may be locked by a conical locking face 110 when entering the recesses 104 or, as illustrated in the upper half, may enter an annular groove 112 in an adjoining position. 7le balls 107 of the first group are provided in two rows whereas the balls 108 of the second group are provided in one row. A locking 'journal 113 provided with an actuating device yet to be described, in a position displaced towards the left, holds locking balls 115 in a radially outer position between a conical face 114 and a radial face 119 at the actuating member. In a position as illustrated in the upper half of Figure 4, the balls move inwardly and enter an annular space 116 to assume a position close to the axis. 71e output end of the shaft 102 is connected to a flange 44 bolted to a counter flange 45 and forming an interior 46. The interior 46 is divided by a diaphragm 47 which, via reinforcement plate, is firmly connected to the locking element 113 which, via pressure spring 49 is supported on the shaft 102. Via two bearings 50, 51, the shaft 102 is held in a part which is firmly attached to the housing and which, at the same time, serves as a vacuum supply line. For this purpose there is inserted a vacuum sleeve 52 which, via an annular space 53, is in contact with radial bores 54 in the shaft 102, with seals 55, 56, 57, 58 sealing this system towards the outside.

Figure 5 shows part of a differential drive housing 60 in which a differential carrier 61 is rotatably held via two bevel roller bearings. The differential carrier forms the outer part of two fluid friction slip couplings whose chambers 64, 85 are separated from each other and each comprise inner coupling parts which form the drive sleeves 201, 301 of releasable torque-transmitting couplings in accordance with the invention. The coupling carrier 61 is connected to outer plates 68, 69, whereas inner plates 70, 71 are connected to the drive sleeves 201, 301. The details of the releasable coupling correspond to those of Figure 4, but with there being provided only one row of balls 207, 307 and one row of balls 208, 308. Halfshafts of the rear axle may be flanged to the outer flange ends 245, 345.

Within the driveshaft 201, 301 there is illustrated a sleeve-shaped output shaft 202, 302 inside which the actuating member 203, 303 is identifiable. Groove-shaped recesses 204, 304 are provided in the driveshaft 201, 301. The inner shaft comprises associated first apertures 205, 305 radially displaceably holding balls 207, 307 which may be locked by a cylindrical locking face 209, 309 -as illustrated in the lower half of the Figure or they may enter an adjoining annular groove 211, 311, as illustrated in the upper half. The shaft 202, 302 comprises further apertures 206, 306 holding balls 208, 308 which, when entering the recesses 204, 304, may be locked by a conical locking face 210, 310 or, as 11 illustrated in the upper half, they enter an annular groove 212,312 in an adjoining position. A locking member 213, 313 provided with an actuating device yet to be described in detail, in a position axially moved forward inwardly, holds locking balls 215, 315 in a radially outer position between a conical face 214, 314 and a radial face 219, 319 at the switching element. In a withdrawn position as illustrated in the upper half, the balls, radially inwardly, enter an annular space 216, 316 into a position near the axis. The output end of the shaft 202, 302 is connected to a flange 244, 344 bolted to the counter flange 245, 345 and forming an interior 246, 346. The interior 246, 346 is divided by a diaphragm 247, 347 which via a reinforcement plate 248, 348, is firmly connected to the locking member 213, 313 which, via a pressure spring 249, 349, is supported on the shaft 202, 302. The shaft 202, 302 is axially fixed via a retaining ring. Vacuum is supplied via a bore 252, 352 in the gearbox housing which bore, via an annular space 253, 353, is in contact with radial bores 254, 354 in the shaft 202, 302.

Seals 255, 355, 256, 356, 258, 358 seal this system towards the outside.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

3 12

Claims

1. In a motor vehicle having a drive unit and a plurality of driven axles wherein a first said axle is permanently connected to the drive unit and a second said axle is releasably connectable to the drive unit through a torque- transmitting slip coupling and a lockable and unlockable releasable torque- transmitting c) coupling, a method of disconnecting said second axle from the drive unit wherein said releasable torque-transmitting coupling is unlockable to a release condition in response to the driving condition of the vehicle such as braking or negative acceleration and is automatically releasable, when in said release condition, from its torque-transmitting condition only in response to a reversal of torque from said second axle to the drive unit.

2. A method as claimed in claim 1 wherein said releasable torquetransmitting coupling is unlockable to said release condition in response to release of the vehicle accelerator.

3. A method as claimed in claim 1 wherein said releasable torquetransmitting coupling is unlockable to said release condition in response to depression of the brake pedal.

4. A method as claimed in claim 1 wherein said releasable torquetransmitting coupling is unlockable to said release condition in response to a signal from an anti-lock braking system of the vehicle.

5. In or for a motor vehicle having a drive unit and a plurality of driven axles wherein a first said axle is permanently connected to the drive unit and a second said axle is releasably connectable to the drive unit through a torquetransmitting slip coupling and a lockable and unlockable releasable torquetransmitting coupling, said releasable torquetransmitting coupling comprising a P 13 first shaft having a plurality of circumferentially distributed recesses therein; a second shaft comprising a sleeve having a plurality of circumferentially distributed apertures therein associated with said recesses; coupling elements radially displaceably carried in said apertures and being displaceable to engage in said recesses for connecting said shafts together in non-rotatable engagement relative to one another; and an actuating member coaxial with and movable axially relative to said shafts for radially displacing said coupling elements and for retaining said coupling elements in the position in which the shafts are nonrotatably connected together in a locked condition of the coupling characterised in that a first group of said coupling elements is carried in a first group of apertures and is retainable in said locked condition by a cylindrical locking face on said actuating member, a first annular groove being provided on said member adjacent said cylindrical locking face; a second group of said coupling elements is carried in a second group of apertures and is retainable in said locked condition by a conical locking face on said actuating member, a second annular groove being provided on said member adjacent said conical locking face; the apertures of said first group thereof being axially spaced and circumferentially offset from the apertures of said second group thereof., a locking member is provided for restraining the actuating member against axial movement whereby, in said locked condition, under each direction of torque, only one group of coupling elements establishes contact at the flanks of its apertures and recesses and, upon release of said actuating member by said locking member, under torque in a first direction the coupling elements of the first group establish contact at the flanks of their apertures and recesses and are radially locked by said cylindrical locking face and, under torque in a second direction, the coupling elements of the second group establish contact at the flanks of their apertures and recesses and, through radial forces acting on said conical locking face, axial movement of the actuating member is effectible whereby the coupling elements may enter said annular grooves and release the shafts from their non-rotatable engagement with one another.

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6. In or for a motor vehicle having a drive unit and a plurality of driven axles wherein a first said axle is permanently connected to the drive unit and a second said axle is releasably connectable to the drive unit through a torquetransmitting slip coupling and a lockable and unlockable releasable torquetransmitting coupling, said releasable torquetransmitting coupling comprising a first shaft having a plurality of circumferentially distributed recesses therein; a second shaft comprising a sleeve having a plurality of circumferentially distributed apertures therein associated with said recesses; coupling elements radially displaceably carried in said apertures and being displaceable to engage in said recesses for connecting said shafts together in non-rotatable engagement relative to one another; and an actuating member coaxial with and movable axially relative to said shafts for radially displacing said coupling elements and for retaining said coupling elements in the position in which the shafts are nonrotatably connected together in a locked condition of the coupling characterised in that a first group of said coupling elements is radially displaceable into a first group of recesses in said first shaft and is retainable in said locked condition by a cylindrical locking face on said actuating member, a first annular groove being provided on said member adjacent said cylindrical locking face; a second group of said coupling elements is radially displaceable into a second group of recesses in said first shaft and is retainable in said locked condition by a conical locking face on said actuating member, a second annular groove being provided on said member adjacent said conical locking face; the recesses of said first group thereof being axially spaced and circumferentially offset from the recesses of said second group thereof., a locking member is provided for restraining the actuating member against axial movement whereby, in said locked condition, under each direction of torque, only one group of coupling elements establishes contact at the flanks of its apertures and recesses and, upon release of said actuating member by said locking member, under torque in a first direction the coupling elements of the first group establish contact at the flanks of their apertures and recesses and are radially locked by said cylindrical locking face and, under torque in a second is direction, the coupling elements of the second group establish contact at the flanks of their apertures and recesses and, through radial forces acting on said conical locking face, axial movement of the actuating member is effectible whereby the coupling elements may enter said annular grooves and release the shafts from their non-rotatable engagement with one another.

7. A releasable torque-transmitting coupling as claimed in either one of claims 5 or 6 wherein said locking member comprises an axially movable member which, in a first position, retains a plurality of locking balls in a radially outward position between opposed radial faces of the actuating member and a fixed counter-face and which, in a second or release position, permits radially inward displacement of said locking balls out of the space between said radial faces of the actuating member and the counter-face.

8. A releasable, torque-transmitting coupling as claimed in any one of claims 5 to 7 wherein said first shaft is a hollow shaft located around said second hollow shaft, and said actuating means comprises a central shaft having said cylindrical and conical locking faces and said annular grooves on the exterior thereof.

9. A releasable torque-transmitting coupling as claimed in any one of claims 5 to 7 wherein said first shaft is a shaft preferably a solid shaft located within said second shaft, and said actuating means comprises an outer sleeve having said cylindrical and conical locking faces and said annular grooves on the interior thereof.

10. A releasable torque-transmitting coupling as claimed in any one of claims 5 to 9 wherein said recesses in said first shaft comprise axial grooves of part-circular cross-sectional form.

16

11. A releasable torque-transmitting coupling as claimed in any one of claims 5 to 9 wherein said recesses in said first shaft are of partspherical configuration.

T,

12. A releasable torque-transmitting coupling as claimed in any one of claims 5 to 9 wherein said recesses in said first shaft comprise bores having diameters slightly less than the respective dimensions of ball type coupling elements which are engagable in said recesses.

13. A releasable torque-transmitting coupling as claimed in any one of claims 5 to 12 wherein said coupling elements are balls.

14. A releasable torque-transmitting coupling as claimed in any one of claims 5 to 13 wherein said locIcing member is movable axially by pneumatic or electro-magnetic means.

is. A method of disconnecting a second axle from the drive unit of a motor vehicle substantially as hereinbefore described with reference to the accompanying drawings.

16. A releasable torque transmitting coupling constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

17. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.

Published 1991 at The Patent Office, Concept House. Cardff Road. Newport. Gwent NP9 IRH. Further copies may be obtained from Saks Branch, Unit 6. Nine Mile Point Cwmfehnfach, Cross Keys, Newport. NP1 7H7_ Printed by Multiplex techniques Itd. St Mary Cray. Kent.