Externally actuated locking differential assembly

[0002] 1. Field of the Invention

[0003] The present invention relates to a differential assembly, and more particularly to a locking differential assembly for motor vehicles, having an electro-mechanically externally actuated friction clutch assembly.

[0004] 2. Description of the Prior Art

[0005] Conventionally, differentials well known in the prior art, are arranged in a power transmission system of a motor vehicle to allow a pair of output shafts operatively coupled to an input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Conventional differentials include a differential case defining a gear chamber, and disposed therein, a differential gear set including at least one input pinion gear, and a pair of output side gears non-rotatably coupled to corresponding axle shafts. Such a device essentially distributes the torque provided by the input shaft between the output shafts. However, this type of differentials known in the art as an open differentials, i.e. a differential where movements of its various internal components are not restricted in any significant fashion, is unsuitable in slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel; for instance, when one wheel of a vehicle is located on a patch of ice or mud and the other wheel is on dry pavement. In such a condition, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a “spin out” of that wheel. Since the maximum amount of torque, which can be developed on the wheel with traction, is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. Thus, the necessity for a differential which limits the differential rotation between the output shafts to provide traction on slippery surfaces is well known.

[0006] Prior methods of limiting slippage between the side gears and the differential case usually employ a frictional clutch mechanism typically disposed between at least one of the side gears and an adjacent surface of the gear case, such that the clutch pack is operable to limit relative rotation between the gear case and the one side gear. A number of methods have been developed to limit wheel slippage under such conditions.

[0007] Limited slip differentials conventionally use the frictional clutch mechanism and a bias mechanism, usually a spring, to apply an initial preload between the frictional clutch mechanism and the differential casing. However, such preloaded clutches are usually always engaged, and thus are susceptible to wear, causing undesirable repair and replacement costs. Also, such a preloaded clutch mechanism may lock the output shafts together in situations where differential rotation is necessary.

[0008] Another method of limiting slippage involves the use of a selectively controllable frictional clutch mechanism as a differential locking device actuated manually by a vehicle operator or by a signal from an appropriate electronic control unit, to restrict the movements of the internal components of the differential. The frictional clutch mechanism may be actuated by various hydraulic or electromagnetic mechanisms, which conventionally constructed of elements disposed inside the differential casing. However, the locking differentials occupy bigger space and are often quite complex, cumbersome and expansive to manufacture.

[0009] Thus, there is a need for a lockable differential that is simple, compact and inexpensive to manufacture.

[0010] The present invention provides an improved electronically controlled locking differential assembly having an externally actuated friction clutch assembly.

[0011] The differential assembly in accordance with the preferred embodiment of the present invention includes a rotatable differential case forming a housing, a differential gearing rotatably supported in the case, and a pair of opposite side gears in meshing engagement with the differential gearing to permit differential rotation thereof. The differential assembly includes a friction disk clutch assembly disposed within the differential case and provided to lock the differential assembly. The friction clutch assembly includes a number of alternating outer friction plates non-rotatably coupled to the differential case and inner friction plates splined to one of the side gears. An annular actuator plate is arranged within the differential case between the friction clutch assembly and an adjacent surface of the differential case, and is adapted to axially reciprocate within the case for loading the friction clutch plates. An electronic selectively controllable actuator assembly is provided for axially displacing the actuator plate in order to load the friction assembly when needed, thus providing the differential assembly with a locking function. The actuator assembly comprises an electric motor mounted to an axle housing and provided for rotating a drive screw via a gear reducer. In turn, the drive screw axially drives a clutch actuating sleeve through a complimentary threaded pusher block. The clutch actuating sleeve is provided coaxially to the side gear to apply an axial force to the actuator plate in order to compress and, thus, actuate the friction clutch assembly.

[0012] Therefore, the locking differential assembly in accordance with the present invention provides a simple, compact and inexpensive locking differential assembly.

[0013] Other objects and advantages of the invention will become apparent from a study of the following specification when viewed in light of the accompanying drawings, wherein:

[0014] FIG. 1 is a sectional side view of a locking differential assembly in accordance with a first embodiment of a present invention;

[0015] FIG. 2 is a top view of the locking differential assembly of the present invention;

[0016] FIG. 3 is a sectional side view of the locking differential assembly in accordance with a second embodiment of the present invention.

[0017] The preferred embodiment of the present invention will now be described with the reference to accompanying drawings.

[0018] FIGS. 1 and 2 of the drawings illustrate in detail the preferred arrangement of the differential assembly 10 in accordance with the present invention. Reference numeral 12 defines a differential case rotatably supported in an axle housing 2 trough roller bearings (not shown), and defines an axis of rotation 14. The axle housing 2 has left and right axle shaft tubes 4 and 6 respectively, projecting coaxially from opposite sides.

[0019] The differential assembly 10 is provided with a set of pinion gears 16 rotatably supported on a pinion shaft 18 secured to the differential case 12. The pinion gears 16 engage a pair of opposite side gears 20 adapted to rotate about the axis 14. The side gears 20 are splined to output axle shafts (not shown) extending through a pair of opposite trunions 15 formed with the differential case 12.

[0020] A friction clutch assembly 22 is provided within the differential case 12. The friction clutch assembly 22, well known in the prior art, includes sets of alternating outer friction plates 24 and inner friction plates 26. Conventionally, an outer circumference of the outer friction plates 24 is provided with projections that non-rotatably engages corresponding grooves 28 formed in the differential case 12. At the same time, the outer friction plates 24 are slideable in axial direction. The inner friction plates 26 are splined to the side gear 20 so that the inner friction clutch plates 26 are non-rotatably, but axially slidably mounted on the side gear 20.

[0021] An annular actuator plate 30 is provided to axially load the friction clutch plates 24 and 26 in order to actuate the friction clutch assembly 22. When the friction clutch assembly 22 is actuated, it completely restricts the movement of internal components of the differential, thereby providing full torque from the engine to both driving wheels.

[0022] As further illustrated in FIGS. 1 and 2, in order to selectively actuate the friction clutch assembly 22 when necessary, the differential assembly 10 further comprises a linear actuator assembly 40. The linear actuator assembly 40 includes a linear actuator motor 41 and a non-rotatable pusher block 48 coupled to the linear actuator motor 41 so that an actuating motion of the actuator motor 41 is transformed to a linear motion of pusher block 48. It will be appreciated that any types of appropriate linear motors that provide linear axial motion of the pusher block 48 are within the scope of the present invention.

[0023] The preferred embodiment of the actuator motor 41 comprises an electric servomotor 42 mounted to an exterior surface of the axle housing 2. Alternatively, hydraulic or pneumatic rotary motors may be employed instead of the electric motor. The servomotor 42 drives a drive screw 46 via a gear reducer 44. The electric motor 42 and the gear reducer 44 are mounted outside the differential case 12 to the axle housing 2, preferably to the axle tube 6.

[0024] The drive screw 46 is provided, preferably, with an acme screw thread. The drive screw 46 is partially disposed in the pusher block 48 provided with a thread that complements the thread in the drive screw 46. As a result rotation of the drive screw 46 causes corresponding axial movement of the pusher block 48 between a retracted position and an extended position.

[0025] It will be appreciated that any other appropriate types of the screw drives may be employed, such as helically grooved ball screws, as illustrated in FIG. 3. FIG. 3 shows a helically grooved ball screw 46′ disposed in a pusher block 48′ and an inner portion of the pusher block 48′ is provided with a helical groove that complements the groove in the ball screw 46′ so as to receive a plurality of balls 50. As a result rotation of the ball screw 46′ causes corresponding axial movement of the pusher block 48′.

[0026] Alternatively, other types of the linear actuator motor 41 may be employed, such as hydraulic or pneumatic power cylinders, electromagnetic linear actuator, and a manual linear actuator actuated by a vehicle operator.

[0027] The pusher block 48 is axially slideable but rotary constrained, so that the rotational motion of the drive screw 46 is transformed to the linear motion of the pusher block 48.

[0028] Furthermore, the preferred embodiment of the actuator assembly 40 of the present invention includes a clutch actuating sleeve 52 disposed at least partially within the trunnion 15 coaxially to the axis 14. The actuating sleeve 52 is secured to the pusher block 48 by welding or any other appropriate means well known in the prior art. Alternatively, the pusher block 48 and the actuating sleeve 52 may be manufactured as an integral single-piece part. The pusher block 48 is extending between the drive screw 46 and the actuating sleeve 52 through an elongated slot 8 in the axle tube 6. Such an arrangement allows axial displacement of the pusher block, but prevents rotation thereof. A sleeve bearing 54 press-fitted within the trunnion 15 is employed to guide the actuating sleeve 52.

[0029] The actuating sleeve 52 is provided to apply an axial force to the annular actuator plate 30 in order to load the friction clutch assembly 22. An axial thrust bearing 56 is provided between the actuator plate 30 and the actuating sleeve 52 to reduce the friction because the actuator plate 30 rotates with the side gear 20.

[0030] With the arrangement of the locking differential assembly 10 described above, when the motor 42 is actuated by a signal from any appropriate control unit (not shown), the gear reducer 44 rotates the drive screw 46. The rotary motion of the drive screw 46 is transmitted to the pusher block 48 which travels linearly along the drive screw 46 leftward in FIG. 1 from the retracted position and the extended position. The linear axial motion of the pusher block 48 is transmitted to the actuator plate 30 via the actuating sleeve 52 thereby to compress the friction clutch plates 24 and 26 in order to actuate the friction clutch assembly 22 and lock the differential assembly 10.

[0031] Therefore, the externally actuated differential assembly in accordance with the present invention represents a novel arrangement of the locking differential assembly that is simple, compact and inexpensive in manufacturing.

[0032] The foregoing description of the preferred embodiments of the present invention has been presented for the purpose of illustration in accordance with the provisions of the Patent Statutes. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments disclosed hereinabove were chosen in order to best illustrate the principles of the present invention and its practical application to thereby enable those of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated, as long as the principles described herein are followed. Thus, changes can be made in the above-described invention without departing from the intent and scope thereof. It is also intended that the scope of the present invention be defined by the claims appended thereto.