TECHNICAL FIELD

[0001 ] The present invention is generally applicable to differential

mechanisms for rotational power transmission systems such as axle assemblies, and more specifically relates to locking mechanisms for differentials.

BACKGROUND

[0002] There are many known types of differential locking mechanism which selectively lock various parts together to prevent the connected half shafts from rotating at different speeds. Such differential locking mechanisms include: rings for locking a pinion gear to the carrier; rings or sleeves for locking a half shaft to the carrier; or dog clutches, splined sleeve-and-shaft and other arrangements for locking half-shaft to half-shaft in the centre of the differential. These mechanisms can be actuated by a variety of arrangements including a linear side actuator and fork or an annular actuator concentric around the half shaft. Most of these arrangements can be hydraulically, pneumatically or electro-magnetically operated.

[0003] For example, the locking differential assembly in United States Patent Application Publication Number US 2012/0244987 utilises an axially moveable ring having an internal ring of teeth engaged with the carrier to selectively also engage part of the internal ring of teeth with an external ring of teeth on one of the pinion (or side or half-shaft) gears. The ring includes a channel forming part of an annular fluid pressure chamber to provide an axial force on the ring when pressurised into the engaged or locked position and return springs to return the ring to the dis-engaged or unlocked position. This requires the pinion (or side or half-shaft) gear that the ring engages with to include the external ring of teeth that the other pinion gears in the differential do not require. Therefore that pinion gear is different to the other pinion gears. The pinion gears in a differential are generally high quality forged gears, so providing a different pinion gear in manufacturing of the original differential increases the number of different high quality parts required and makes after-market fitment of such a locking

mechanism dependent on the supply of a forged pinion gear of the same high quality as the original pinion gears.

[0004] Chinese Patent Publication Number CN2814026 discloses a similar locking mechanism with detail of the supply of fluid pressure to the rotation differential assembly using a fluid passageway collar rotating on the carrier and communicating with a fluid passageway through the carrier to the annular fluid pressure chamber.

[0005] International Patent Application Publication Number WO 2008/094586 and United States Patent Application Publication Number 2010/0255947 show examples of electro-magnetic actuation of a similar axially movable ring. Again the axially moveable ring is rotatably fixed to, but axially slidable relative to the carrier and able to be selectively engaged or disengaged with the pinion (side or half-shaft) gear.

[0006] United States patent number 4167881 discloses the use of a collar sliding along an input axle to a differential to enable the axle to be locked to a splined portion on the differential housing. Chinese utility patent number

202251802 shows a similar arrangement using axially extending teeth as in a dog clutch. European Patent number 1 171727 uses a collar splined to and moveable along a half-shaft, the collar again having axially extending teeth to engage with the crown wheel of the differential, as does Russian utility patent number 134268. None of these arrangements are compact so could not be retro-fitted to an existing differential and would require significant alterations to the differential casing.

[0007] Japanese Patent Number H05-221250 is typical of differentials having both limited slip and locking functions. It uses the external linear side actuator and fork arrangement to provide the locking function which is both bulky and integrated into the differential casing. [0008] It would therefore be desirable to provide a differential locking mechanism that is space efficient.

[0009] It would also be desirable to provide a differential locking mechanism that does not require modification or replacement of any of the pinion gears of a basic open differential.

[0010] It would also be desirable to provide a differential locking mechanism that is suitable for after-market retro-fit to existing differentials of both the open and the limited slip types.

SUMMARY OF INVENTION

[001 1 ] According to a first aspect of the invention there is provided a lockable differential device including a carrier having a primary axis, a first side gear and a second side gear for connection to a respective first and second shaft axially aligned with the primary axis of the carrier, at least one pinion gear and a locking arrangement, the carrier including a first flange portion (fixed to or integral with the carrier) adjacent the first side gear, wherein the locking arrangement includes an actuator and a locking gear, the locking gear housed within the first flange portion, the actuator being provided to move the locking gear in an axial direction between an engaged position and a disengaged position, in the engaged position the locking gear interlocking between the first flange portion and teeth on the first shaft preventing relative rotation between the carrier and the first shaft, and in the disengaged position the locking gear being disengaged from the first flange portion and/or the teeth on the first shaft permitting relative rotation between the carrier and the first shaft.

[0012] The first and/or second shaft may be respective half-shafts, drive shafts, or any axle half-shaft, axle drive shaft, or any input/output shaft. [0013] The teeth on the first shaft may be integral with the first shaft (e.g. a portion of a spline) or may be teeth on a sleeve provided on the first shaft. Either way, the teeth on the first shaft are rotationally fixed relative to the first shaft.

[0014] The first flange portion may be fixed to or integral with the carrier.

[0015] The locking gear may include an outer locking portion for engagement with a carrier locking portion fixed to the carrier in at least the engaged position.

[0016] The carrier locking portion fixed to the carrier may be provided by a separate flange locking gear which is, in operation, connected or fixed to the carrier or the first flange portion.

[0017] The locking portions fixed to the carrier and on the locking gear can be any form of spline, radially toothed gear or axially toothed gear (for example a dog clutch). Ideally, when engaged, the first flange portion and the locking gear (which can otherwise rotate freely relative to each other) are coupled to each other by an interference, rather than a friction, type clutch.

[0018] If the teeth on the shaft are integral with the first shaft, then they may be part of a splined portion of the first shaft. In that case, the locking gear may include an internal splined portion which may be connected to the splined portion of the first shaft such that rotation between the locking gear and the shaft is substantially prevented and such that (at least a limited magnitude of) axial motion between the locking gear and the shaft is permitted (between the engaged and disengaged positions).

[0019] An actuator control line collar may be mounted on the carrier and able to rotate relative to the carrier, the actuator control line collar being in

communication with the actuator. For example, the first flange portion may include an axial tube portion on which the actuator control line collar is mounted. As the flange portion is part of or fixed to the carrier, so the axial tube portion is part of or fixed to the carrier.

[0020] The actuator may be or include a fluid actuator.

[0021 ] The actuator control line may be used to selectively supply pressurised fluid to energise the actuator and urge the locking gear into the engaged position.

[0022] The locking arrangement may further include at least one return spring for urging the locking gear to return to the disengaged position when the actuator is not energised.

[0023] The pressurised fluid may be or include compressed air or other compressed gas or a pressurised liquid.

[0024] The locking gear may include an annular piston portion or an annular piston may be provided separate to the locking gear. The carrier may further include an annular cylinder portion for at least partially housing the annular piston or piston portion of the locking gear, said annular cylinder portion and said annular piston or piston portion together forming an annular actuating chamber.

[0025] The actuator control line collar may encircle at least a portion of the axial tube portion fixed to the carrier. The axial tube portion fixed to the carrier may include a passage in a wall of the axial tube portion, the passage

communicating a fluid pressure region of the actuator collar with said annular actuating chamber, such that when pressurised fluid (for example, of at least a minimum actuating pressure) is supplied to the fluid pressure region of the actuator collar, the actuator is energised by pressurised fluid acting on the piston portion in the chamber.

[0026] Alternatively, the actuator may be or include an electro-magnetic actuator. An actuator control line may be used to selectively supply an electrical current to energise the actuator and urge the locking gear into the engaged position, the locking arrangement further including at least one return spring for urging the locking gear to return to the disengaged position when the actuator is not energised.

[0027] The differential device may further include at least one pinion gear rotatably mounted to the carrier to provide at least a portion of an interconnection between the first and second side gears such that rotation of the first side gear relative to the carrier generates a rotation in an opposite sense of the second side gear relative to the first side gear. For example, the first and second side gears and the at least one pinion gear may each be bevel gears, or alternately the at least one pinion gear may be at least one pair of pinion gears and the first and second side gears and the at least one pair of pinion gears may each be spur gears.

[0028] The carrier may be fixed to or integral with a ring gear (e.g. a crown wheel or drive gear). The first shaft may be splined to an internal splined portion of the locking gear and to the first side gear, the second shaft may be splined to the second side gear, whereby rotation of the first shaft relative to the carrier and therefore the ring gear and the second side gear and second shaft can selectively be permitted or prevented.

[0029] A limited slip differential (or similar friction pack) may be interposed between the first and/or second side gear (or the first and/or second shaft) and the carrier to thereby provide a differential having both locking and limited slip features.

[0030] Alternatively, the lockable differential device may include a limited slip device.

[0031 ] The lockable differential device may include a differential housing for at least substantially enclosing the carrier and gears. [0032] Another aspect of the present invention provides a locking device for selectively locking a differential, the locking device including: a locking gear housed within a flange portion; an actuator to move the locking gear in an axial direction between an engaged position and a disengaged position, in the engaged position the locking gear interlocking between the flange portion and teeth on a first shaft preventing relative rotation between the flange portion and the first shaft, and in the disengaged position the locking gear being disengaged from the flange portion and/or teeth on the first shaft permitting relative rotation between the flange and the first shaft.

[0033] Another aspect of the present invention provides a locking device for selectively locking a differential (such as for example an automotive differential), the differential including a carrier having a primary axis, a first side gear for connection to a first shaft axially aligned with the primary axis of the carrier, a second side gear for connection to a second shaft axially aligned with the primary axis of the carrier, at least one pinion gear and a locking arrangement, the locking device including: a locking gear housed within a flange portion fixed to the carrier; an actuator to move the locking gear in an axial direction between an engaged position and a disengaged position, in the engaged position the locking gear interlocking between the flange portion and teeth on a first shaft preventing relative rotation between the flange portion and the first shaft, and in the disengaged position the locking gear being disengaged from the flange portion and/or teeth on the first shaft permitting relative rotation between the flange and the first shaft.

[0034] The flange part may be attached to or fixable to or integral with or directly or indirectly connected to a drive gear (for example, a crown wheel) and/or a carrier (for example, for supporting planetary bevel or spur gears) of the differential.

[0035] The locking gear may include an outer locking portion for engagement in at least the engaged position with a locking portion fixed to the flange portion. [0036] The locking portion of the flange portion may be provided by a separate flange locking gear which is, in operation, connected or fixed to the flange part.

[0037] The flange portion may include an axial tube portion. The locking device may further include an annular piston. The locking device may further include at least one resilient means. The flange portion may include an annular cylinder portion for at least partially housing the annular piston, said annular cylinder portion and said annular piston together forming an annular actuating chamber. The flange portion may further include a passage in a wall of the axial tube portion, the passage communicating a port on an outer end of the axial tube portion with said annular actuating chamber, such that when pressurised fluid is supplied to the port, the pressurised fluid acting on the piston in the chamber loads the locking gear towards the engaged position in which the resilient means are compressed, and such that when pressurised fluid is released from the chamber, the resilient means loads the locking gear towards the disengaged position.

[0038] The locking device may further include a fluid pressure transmission ring having a central bore to receive at least a portion of the axial tube portion of the flange portion, the fluid pressure transmission ring further including an external port.

[0039] The fluid pressure transmission ring may encircle at least a portion of the axial tube portion of the flange portion. Two seals may be provided between the fluid pressure transmission ring and the flange part, an annular volume being created therebetween, the annular volume being in fluid communication with the external port of the fluid pressure transmission ring and the passage in the wall of the axial tube portion, to thereby provide fluid communication between the external port of the fluid pressure transmission ring and the annular actuating chamber regardless of rotation of the flange part about a major axis of the axial portion. [0040] The pressurised fluid may be or include compressed air or other compressed gas or a pressurised liquid.

[0041 ] One or more forms of the present invention may provide a differential gear assembly including any of the forms of locking device described above. Such a differential gear assembly may further include a carrier, first and second side gears and at least one pinion gear. The two side gears and the at least one pinion gear may each be bevel gears. Alternatively, the at least one pinion gear may be at least one pair of pinion gears in which case the first and second side gears and the at least one pair of pinion gears are each spur gears.

[0042] For either the spur gear or bevel gear types (and other types) of differential, the carrier may be fixed to or integral with the flange portion and a ring gear (for example, a crown wheel or drive gear). A first shaft may be splined to an internal splined portion of the locking gear and to the first side gear, a second shaft may be splined to the second side gear, whereby rotation of the first shaft relative to the carrier and therefore the ring gear and the second side gear (and therefore second shaft) can selectively be permitted or prevented.

[0043] Optionally, a limited slip device such as, for example, a friction pack may be interposed between the first side gear (or the first shaft) and the carrier to thereby provide a differential having both locking and limited slip features.

[0044] One or more forms of the present invention may provide a limited slip differential gear assembly including any of the locking devices described above.

[0045] It will be convenient to further describe the invention by reference to the accompanying drawings which illustrate preferred aspects of the invention.

[0046] Other embodiments of the invention are possible and consequently particularity of the accompanying drawings is not to be understood as

superseding the generality of the preceding description of the invention. BRIEF DESCRIPTION OF DRAWINGS

[0047] In the drawings:

[0048] Figure 1 is a partial cross-sectional view of a differential including a locking mechanism according to the present invention.

[0049] Figure 2 is an exploded view of the locking mechanism of Figure 1 .

[0050] Figure 3 is an end view of the locking mechanism of Figure 2.

[0051 ] Figure 4 is a cross section through the locking mechanism of Figure 3.

[0052] Figure 5 is an exploded view of another locking mechanism according to the present invention.

[0053] Figure 6 is a cross section through the locking mechanism of Figure 5.

[0054] Figure 7 is an exploded view of another locking mechanism and a limited slip switching mechanism according to the present invention.

[0055] Figure 8 is a cross section through the locking mechanism and limited slip switching mechanism of Figure 7.

[0056] Figure 9 is a cross section of another locking mechanism according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0057] Referring initially to Figure 1 , there is shown a portion of a differential assembly 1 including a locking mechanism 2. Differential assemblies include a carrier within which are mounted pinion gears. In the example shown, the side gear 3 is one of the pinion gears and is connected to a spline 4 on the end of a half-shaft 5. The flange part 6 forms one end of the carrier of the differential and a bearing 7 is shown for locating the flange part relative to a differential housing (not shown), for example forming part of an automotive drive train.

[0058] The locking mechanism 2 is formed in and around the flange part 6 and the locking mechanism of Figure 1 is also shown in exploded view in Figure 2. In the drawings like or equivalent components and features are given common reference numerals.

[0059] Locking gear 10 has an internal spline 1 1 for engaging with the spline 4 on the half-shaft 5, so the locking gear 10 rotates with the half-shaft 5 and the side gear 3. The locking gear 10 also includes a ring protruding axially, forming an annular piston portion 12.

[0060] An annular groove 13 formed into the flange part 6 receives the annular piston portion 12 forming an annular actuating chamber 14 sealed by inner and outer seals 15.

[0061 ] An actuator control line collar 20 is a fluid pressure transmission ring rotatably mounted onto an axial tube portion 17 of the flange part 6. The actuator control line collar 20 can be stationary relative to the housing while the differential carrier rotates about the primary axis 18 of the half-shafts (and of the carrier, not shown) and allows control line 19 to be in fluid communication with the annular actuating chamber 14 for the locking gear via the annular chamber 21 inside the actuator control line collar 20, a port 22 on the axial tube portion 17 of the flange part and a passageway 23 in that axial tube portion 17.

[0062] The passageway 23 in the axial tube portion 17 of the flange part 6 can be a blind axial drilling from the annular actuating chamber end or a through- drilling capped by a plug 24. [0063] The annular chamber 21 is positioned between two seals 25 to prevent loss of control pressure at the interface between the actuator control line collar 20 and the flange part 6.

[0064] The locking tooth portion 29 of the locking gear 10 can be selectively engaged with a locking tooth portion on the flange part and these locking tooth portions can comprise radial or axial protrusions or teeth which can interlock with each other. In the example shown, an external ring of radial teeth form the locking tooth portion 29 on the locking gear 10.

[0065] The locking tooth portion on the flange is formed by an internal ring of radial teeth 30 on a flange locking gear 31 which is fixed to the flange 6 in operation.

[0066] Alternatively, the locking tooth portion on the flange may be formed as an integral part of the flange if desired. If separate as shown, the flange locking gear 31 can be rotationally located by either protrusions that mate with recesses in the flange or by bolts or both.

[0067] An end plate 35 is bolted onto the flange 6 by bolts 36 and prevents axial motion of the flange locking gear 31 . The end plate 35 includes recesses 37 to locate return springs 38 that return the locking gear to the disengaged position.

[0068] In operation, whenever an actuating pressure is supplied to the actuator control line 19, the annular piston portion 12 in the annular actuating chamber 14 is actuated towards an engaged position in which the locking tooth portion 29 of the locking gear 10 is engaged with the locking tooth portion 30 fixed to the flange 6. In that engaged position the carrier (including the flange part 6) is locked to the half-shaft, i.e. the differential is locked. [0069] The actuating pressure is required to be sufficient pressure to overcome the return springs 38 and move the locking gear 10 into the locked or engaged position.

[0070] When the actuating pressure is removed, for example when the control line 19 is vented to atmosphere, the return springs are able to return the locking gear to the unlocked or disengaged position.

[0071 ] The fluid is preferably air (such as from an on-board air compressor) so the pressurised fluid is then compressed air, although any fluid including liquids (such as hydraulic oil) could alternatively be used.

[0072] Figure 3 shows an end view of the locking mechanism components of Figure 2 with the cut line 4 indicating the plane through which the locking mechanism is cut to form the view in Figure 4.

[0073] Figure 5 shows a similar arrangement to Figure 3, but incorporating a few modifications. Firstly, a seal housing collar 41 is shown and can be used to provide axial and/or rotational location of the collar 20 relative to the differential housing (not shown).

[0074] As can be seen in the section view in Figure 6, inside the actuator control line collar 20 the annular chamber 21 is now formed by the clearance between the collar 20 and the axial tube portion 17 of the flange part 6, bounded by the O-rings or other seals 25.

[0075] An adjuster nut 42 is shown for enabling adjustment of the taper roller bearing 7. The bolts 36 holding the end plate 35 on to the flange part 6 are reversed, now threading into the end plate.

[0076] The locking gear in this example has been split into three parts: a separate piston 12, a sleeve 45 and the locking gear 10. The internal spline 1 1 is on the sleeve 45 which is thereby splined onto the spline 4 of the half -shaft 5 as shown in Figure 6. This sleeve part 45 does not need to move axially along the half-shaft spline for the locking mechanism to engage or disengage, so is beneficial where an original equipment half-shaft has insufficient spline length to accommodate the one-piece locking gear of Figures 1 and 2. The piston portion is now a separate piston 12 which acts on the locking gear 10, both parts moving axially with operation of the locking mechanism. As can be seen in Figure 5, the splined sleeve part 45 incorporates an external ring of gear teeth 46 to which an inner ring of gear teeth 47 on the locking gear mate when the locking gear external ring of teeth 29 are engaged with the inner ring of teeth 30 on the flange locking gear. All of these internal and external rings of mating teeth (46 and 47 or 29 and 30) can be any suitable mating locking portions.

[0077] As can be seen in the section view of Figure 6, the flange part 6 has been made in two parts, the main body, still numbered 6, and the press-in inner portion 43. Due to the depth of the annular groove 13 in which the piston 12 slides, using a one-piece flange part 6 with a face-machined annular groove can result in a poor surface finish where the seals 15 are located. So it can be beneficial to manufacture the flange part 6 in two parts, being the main body, still numbered 6, and the press-in inner portion 43.

[0078] As can be seen from the Figures, the locking mechanism of the present invention can be substantially located inside the flange part of a differential carrier.

[0079] In an automotive application, the flange part can for example include the crown wheel i.e. the ring or driving gear.

[0080] Alternatively the flange part forming a portion of the locking

arrangement can be located on the opposite side of the differential to the crown wheel, or the crown wheel can be located at any other practical position. [0081 ] The locking mechanism can also be provided in a differential having a limited slip function so both a limited slip and a selectable locking function are provided by the differential. For example, the limited slip components (such as a friction disc or cone arrangement) can be provided adjacent a first side gear which is connected to a first half shaft, the limited slip arrangement providing reduction in the rotation between the first half shaft and the carrier and the locking mechanism can be provided adjacent a second side gear which is connected to a second half shaft, the locking mechanism selectively preventing rotation between the second half shaft and the carrier.

[0082] The locking mechanism can also be used in differentials having other types of limited slip arrangement. For example most limited slip differentials utilise clutch packs adjacent both the first and second side gears. As either the friction discs or spacer discs in the clutch pack need to be rotationally locked into the carrier by radial features on their periphery, such radial features coincide with the locking mechanisms of prior differential locks that lock the side (pinion) gear to the carrier. The locking mechanism of the present invention can be provided on a limited slip differential having clutch packs on both side gears since it locks the half shaft to the carrier using the locking gear 10.

[0083] Locking a limited slip differential when it is under load can easily generate destructive forces that cause catastrophic failure of the differential. The arrangement in Figures 7 and 8 incorporates a switchable limited slip clutch pack 50 and a differential locking mechanism 2. The differential can be operated as an open differential, a limited slip differential or locked. To prevent the possibility of damage resulting from attempting to engage the locking mechanism while the differential is in limited slip mode and under load, ideally a three position control is used with the open differential being the centre position, so the open mode is engaged between switching from limited slip operation to locked operation.. This helps to ensure limited slip operation and locked operation do not happen at the same time. [0084] In Figures 7 and 8, the actuator control collar 20 is connected to two control lines, the locking mechanism control line 19 and a limited slip operation control line 51 . Three seals 25 form two annular chambers, 21 and 52. The locking control annular chamber 21 is again in fluid communication with the control line 19 and is connected by port 22 to a passageway 23 drilled or otherwise formed into the axial tube portion 17 of the carrier flange 6.

Passageway 23 in this example is formed by three cross drillings, each capped with a plug 24. The passageway is connected to the chamber 14 formed by the annular piston 12 in the annular grove 14. In Figure 8 it can be seen that the inner edge of the annular grove is formed by the outer surface of an insert 43 pressed into the flange part 6 in a similar manner to that in Figures 5 and 6 to enable good surface finish in a deep annular groove. As in Figures 5 and 6, the insert and flange part can alternatively be formed as one piece. When activated by pressurised fluid, the piston 12 engages the locking gear 10 with the flange locking gear 31 and the sleeve 45 splined onto the axle half shaft (not shown), compressing the return springs 38.

[0085] The limited slip operation control line 51 is in fluid communication with the annular chamber 52 in the actuator control collar 20. Port 53 provide communication between the annular chamber 52 and the passageway 54 drilled or otherwise formed in the axial tube portion 17 of the flange part 6. The passageway is capped by a plug 24, although it can be blind drilled from the opposite end. As the passageway 54 passes through the insert 43 pressed into the flange part 6, either the insert needs to be rotationally aligned to ensure that passageway 54 is continuous, or the passageway drilled after the insert is pressed into place.

[0086] An annular groove 56 is formed in the flange part 6, of smaller diameter and smaller depth than the locking actuator groove 13. Annular piston 57 slides in this groove 56 forming an annular limited slip actuator chamber 58, connected to the passageway 54. When actuated, the annular limited slip actuating piston 57 acts on the sleeve 45 splined on the axle half shaft, which moves axially along the primary axis 18. The splined sleeve 45 in turn acts on the end plate 35.

[0087] While the end plate 35 is not bolted rigidly to the flange part 6 in this embodiment, it does not have a large range of axial motion, only requiring sufficient motion to load or unload the clutch pack 50. Springs 61 unload the clutch pack 50 when the annular piston 57 is not actuated. The springs are held in the carrier portion 62 which is bolted to the flange portion by bolts 36. The carrier portion 62 includes recesses 63 into which is mounted the cross axle or spider of the differential (not shown) when the carrier is assembled using bolts 67. The carrier portion may also include features to locate the tabs 65 around the periphery of the friction discs 64. In making the differential lock able to be retrofitted to a standard limited slip differential, the original clutch pack can be used, or at least most of the friction discs 64 and spacer discs 65. To supply a retro-fit kit of parts to convert an original limited slip differential to the arrangement of the present invention, the kit would include all of the components shown in Figures 7 and 8 except for the side (pinion) gear 3 and clutch pack 50.

[0088] The axial motion required for switching the limited slip function can be around 1 .5mm, whereas the locking tooth portion of the locking function is ideally much larger, for example 6mm. As the stroke of the annular piston 57 can be much smaller than the stroke of the annular piston 12, the groove 56 can be much shallower than the groove 13, so groove 56 can be more easily face- machined into the flange part 6 or in this case, its insert or inner portion 43.

[0089] Actuating the limited slip function of the differential by applying a force (the actuating pressure acting over the actuating piston area) rather than by applying a fixed displacement allows wear of the clutch pack to be compensated. So the stroke of the annular piston is preferably slightly greater than the stroke required to actuate the limited slip function of the clutch pack. [0090] Being compact, the locking mechanism is versatile in its application, for example it can be provided as a kit for fitment to existing differential gear assemblies of the open or limited slip type or it can be included within a

differential device design and manufactured as part of the differential gear assembly.

[0091 ] An electro-magnetic actuator can be used instead of the or each fluid actuator, the actuator control line(s) then being electrical leads used to supply an electrical current via contacts inside the actuator control line collar to the electromagnets) mounted on the carrier.

[0092] Alternatively, if the actuator control line collar is a larger diameter and positioned closer to the flange portion of the carrier, the electro-magnets can be within the actuator control line collar, i.e. then the electro-magnets can be substantially stationary relative to the differential housing so the carrier can rotate relative to the actuator control line collar and the electro-magnets.

[0093] Figure 9 shows another possible arrangement using electro-magnetic actuation. The locking gear 10 is shown as a single part including a piston portion 12, although the piston can be formed separately to allow a different material to be used from the piston 12 to the locking gear 10. The electro-magnet 71 is mounted to the casing or housing 70 of the differential by a mounting cover 72, so does not rotate with the carrier including the flange 6. When the electromagnet is energised, it repels the piston 12 which moves the locking gear 10 to engagement between the flange locking gear 31 and the teeth on the shaft, in this example being the external radial teeth on the sleeve 45 which is in turn splined onto the shaft 5.

[0094] In relation to the embodiment shown in Figure 9, the locking gear 10 is always engaged with the teeth (46, Figs 5 & 7) on splined sleeve 45 and selectively engaged with the teeth (30, Figs 5 & 7) on flange locking gear 31 . [0095] Electro-magnetic actuation of other types of differential locking mechanism are known, such as the example described in United States Patent Number US 8,109,853 details of which are incorporated herein by reference.

[0096] Further modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention. For example, the differential assembly may be of the spur gear type instead of the bevel gear type illustrated in the drawings.

[0097] Spur gear type differential devices are well known and widely used, many being suitable for the application of the locking mechanism of the present invention that locks the half-shaft to the carrier using an locking gear sliding axially along the half-shaft or along a sleeve splined to the half-shaft to engage or disengage a flange locking gear or a locking tooth portion fixed to or formed as part of the carrier.