Limited slip differential having magnetorheological fluid brake

Details Limited slip differential having magnetorheological fluid brake Limited slip differential having magnetorheological fluid brake

2001-05-10

2003-03-04

Wright, Dirk (Department: 3681)

Planetary gear transmission systems or components

Electric or magnetic drive or control

Differential drive or control

Reexamination Certificate

active

06527661

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to differentials, and more particularly, to controllable, traction enhancing differentials.
Differentials are well known mechanisms and generally provide a means to transfer rotational torque, via an input shaft, i.e., a drive shaft, to a pair of output shafts, i.e., axle shafts. Conventional differential construction includes, typically, a fixed housing including a rotatable casing therein driven by the input shaft through a ring gear attached about the casing. The casing rotatably supports each output shaft which typically includes a side gear fixed thereto and positioned within the casing. The side gears intermesh with pinion gears which rotate about a pin fixed relative to the casing. Differentials are often utilized in conventional vehicle applications where the differential engages a pair of wheels which respectively mount to each output shaft to maintain traction with the road while the vehicle is turning. The differential essentially distributes torque, provided by the input shaft, to the output shafts. One type of differential, termed an “open” differential, includes a construction which distributes torque to the output shafts without implementing means to compensate for loss of traction. The open differential 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 the vehicle is located on a patch of ice and the other wheel is on dry pavement. 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 exerted 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. A number of methods have been developed to limit wheel slippage under such conditions.
Prior methods of limiting slippage between the side gears and the differential casing use a frictional clutch mechanism, either clutch plates or a frustoconical engagement structure, and a bias mechanism, usually a spring, to apply an initial preload between the side gears and the differential casing. By using a frictional clutch with an initial preload, for example a spring, a minimum amount of torque can always be applied to the wheel having traction, i.e. the wheel located on dry pavement. The initial torque generates gear separating forces which further act on the frictional clutch and develop additional torque. Examples of such limited slip differentials are disclosed in U.S. Pat. No. 4,612,825 (Engle), U.S. Pat. No. 5,226,861 (Engle) and U.S. Pat. No. 5,556,344 (Fox), which are assigned to the assignee of the present invention. The disclosures of these patents are each expressly incorporated herein by reference.
In a differential, the development of torque will create side gear separating forces which tend to move the side gears away from the pinion gears. In general, gear separating forces are forces induced on any set of meshing gears by the application of torque to the gears. Differentials were adapted to provide an initial preload to utilize side gear separating forces for further braking action between the side gears and the differential casing. In operation, when one wheel is in contact with a slippery surface, the initial preload creates contact and frictional engagement between the differential casing and a clutch mechanism. The clutch mechanism is disposed between the side gears and the differential casing to distribute engine torque to the wheel having traction. The torque transfer induces gear separating forces on the side gears tending to separate the side gears and further frictionally engage the clutch mechanism with the casing. The increased frictional engagement of the clutch allows more torque to be distributed between the side gears and the differential casing to effectively transfer torque to the wheel with traction. However, the clutches of such preloaded differentials are usually always engaged, and thus are susceptible to wear, causing undesirable repair and replacement costs. Additionally, such clutch mechanisms usually employ spring mechanisms which add to the cost and difficulty of manufacture.
An additional problem associated with preloaded clutch mechanisms are that they lock the output shafts together in situations where differential rotation between axle shafts is necessary. For example, if the vehicle is making a turn when the wheels are sufficiently engaged on the road surface and a sufficient amount of torque is developed, the differential will tend to lock up the output shafts due to the action of the side gear separating forces. This may occur, for example, during turns on surfaces with a high coefficient of friction while under acceleration. In such a case, even though differential rotation is required, the two output shafts lock up causing one wheel to drag and slide along the road surface. This problem is evident in rear drive vehicles during turns under acceleration as the portion of the vehicle near the dragging wheel may tend to bounce up and down.
Another method of limiting slippage involves engaging a frictional clutch mechanism between the side gears and the differential casing based on the difference in rotational speeds between the two output shafts. Limited slip differentials employing this method are classified as speed-sensitive differentials. The frictional clutch may be actuated by various hydraulic pump mechanisms which may be external to the differential casing or may be constructed of elements disposed inside the differential casing. However, such mechanisms usually are complicated and also add to the cost and difficulty of manufacture. Further, speed sensitive differentials are “reactive”, i.e., they react after a wheel has already lost traction.
Another known method of limiting slippage involves using a flyweight governor in combination with a clutch mechanism. The governor actuates the clutch mechanism when a predetermined differential rotation rate is detected. However, devices heretofore using such arrangements are configured such that the governor almost instantaneously applies extremely high clutch torque to the output shafts, which often leads to lock-up of the two output shafts. Distributing torque in such a manner applies very high stresses on the output shafts which may result in fracturing the output shafts.
In addition to actuating a clutch mechanism using mechanical or hydraulic arrangements, response and performance characteristics may be improved by controlling the actuation of a limited slip differential using electronic control methods. An example of such an electronically controlled differential is disclosed in U.S. Pat. No. 5,989,147 (Forrest), assigned to the assignee of the present invention, the disclosure of which is expressly incorporated herein by reference. Electronic control methods provide the advantage of accurate, reliable control within a narrow control band. Electronic control methods also allow operating parameters to be easily changed, for example by programming the electronic control systems to respond to a particular range of differentiation speeds or some other vehicle parameter such as throttle position.
The electronically controllable differential provides a clutch mechanism which transfers torque between a differential casing and a side gear in response to the application of an initiating force by an electronic actuator. The clutch mechanism, for example, may comprise a cone clutch element engageable with an insert disposed between the side gear and the rotatable casing. The clutch is engageable with the casing through camming portions provided between the side gear and clutch element. Alternatively, the camming portions may be substituted with a ball ramp assembly. The ball ramp assembly provides axial displacement of the clutch element when an initiating force is applied by the electronic

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