Planetary gear transmission systems or components – Electric or magnetic drive or control – Differential drive or control
Reexamination Certificate
2002-03-05
2003-06-24
Wright, Dirk (Department: 3681)
Planetary gear transmission systems or components
Electric or magnetic drive or control
Differential drive or control
Reexamination Certificate
active
06582336
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to limited slip differentials, and more particularly to limited slip differentials having an electromagnetically actuated clutch.
Differentials are well known in the prior art and allow each of a pair of output shafts or axles operatively coupled to a rotating 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. Such a device essentially distributes the torque provided by the input shaft between the output shafts.
The completely open differential, i.e., a differential without clutches or springs which restrict relative rotation between the axles and the rotating differential casing, is not well suited to slippery conditions in which one driven wheel experiences a much lower coefficient of friction than the other driven wheel: for instance, when one wheel of a vehicle is located on a patch of ice and the other wheel is on dry pavement. Under such conditions, 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. A number of methods have been developed to limit wheel slippage under such conditions.
Prior means for limiting slippage between the axles and the differential casing use a frictional clutch mechanism, either clutch plates or a frustoconical engagement structure, operatively located between the rotating case and the axles. Certain embodiments of such prior means provide a clutch element attached to each of the side gears, and which frictionally engages a mating clutch element attached to the rotating casing or, if the clutch is of the conical variety, a complementary interior surface of the casing itself. Such embodiments may also include a bias mechanism, usually a spring, to apply an initial preload between the clutch and the differential casing. By using a frictional clutch with an initial preload, a minimum amount of torque can always be applied to a wheel having traction, e.g., a wheel located on dry pavement. The initial torque generates gear separating forces between the first pinion gears and the side gears intermeshed therewith. The gear separating forces urge the two side gears outward, away from each other, causing the clutch to lightly engage and develop additional torque at the driven wheels. Examples of such limited slip differentials which comprise cone clutches 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), each of which is assigned to Auburn Gear, Inc., the disclosures of which are all expressly incorporated herein by reference.
Certain prior art limited slip differentials provide, between the first of the two side gears and its associated clutch element, interacting camming portions having ramp surfaces or ball/ramp arrangements. In response to an initiating force, this clutch element is moved towards and into contact with the surface against which it frictionally engages, which may be a mating clutch element attached to the casing, or an interior surface of the casing itself, as the case may be, thereby axially separating the clutch element and its adjacent first side gear, the interacting camming portions slidably engaging, the rotational speed of the clutch element beginning to match that of the differential casing due to the frictional engagement. Relative rotational movement between the ramp surfaces induces further axial separation of the clutch element and the first side gear. Because the clutch element is already in abutting contact with the surface against which it frictionally engages, the first side gear is forced axially away from the clutch element by the camming portions.
Certain embodiments of such limited slip differentials utilize an electromagnet having an electrical coil to effect the initiating force and actuate the clutch, as disclosed in U.S. Pat. No. 5,989,147 (Forrest et al.), U.S. Pat. No. 6,019,694 (Forrest et al.), and U.S. Pat. No. 6,165,095 (Till et al.), each of which is assigned to Auburn Gear, Inc., the disclosures of which are all expressly incorporated herein by reference. Each of these references discloses that the differential casing, in which the clutches are disposed, rotates within the housing and is rotatably supported by a pair of bearings. An electromagnet, which actuates a primary cone clutch element, is mounted in fixed relationship to the axle housing and is rotatably supported by the differential casing. Alternatively, as disclosed in pending U.S. patent application Ser. No. 09/484,967, filed Jan. 18, 2000, which is assigned to Auburn Gear, Inc., the disclosure of which is expressly incorporated herein by reference, the electromagnet may be fixedly supported by the axle housing. In either case, activation of the electromagnet draws a primary cone clutch element into frictional engagement with the rotating differential housing.
The camming portions, described above, act between the primary cone clutch element and the first side gear to axially separate them, forcing the first side gear into abutment with a transfer block located intermediate the first and second side gears. Responsive to this force, the transfer block is moved into abutment with the second side gear, which is rotatably fixed to a secondary cone clutch element, which frictionally engages a mating interior surface of the rotating differential casing. The frictional engagement of the secondary cone clutch element and the differential casing effects further clutched engagement between the axles and the differential casing, enhancing the locking capability of the limited slip differential. Notably, the load carrying capability of the secondary cone clutch mechanism is usually significantly greater than that of the primary cone clutch mechanism, owing to a greater axial engagement force exerted thereon. Examples of prior limited slip differentials are described in more detail below, with reference to
FIGS. 1 and 2
.
FIG. 1
depicts an embodiment of prior axle assembly
10
having electrically or electromagnetically actuated limited slip differential assembly
12
. Axle assembly
10
may be a conventional axle assembly or comprise part of a transaxle assembly. Therefore, it is to be understood that the term “axle assembly” encompasses both conventional (rear wheel drive) axle assemblies as well as transaxle assemblies. Differential assembly
12
comprises electromagnet
14
, ferrous rotatable casing
16
constructed of joined first and second casing parts
16
a
and
16
b
, respectively, and providing inner cavity
18
, which is defined by the interior surface of the circumferential wall portion of first casing part
16
a
and end wall portions
20
,
22
of first and second casing parts
16
a
,
16
b
, respectively. Casing part
16
a
may be a machined iron or steel casting; casing part
16
b
may also be such a casting, or a ferrous, sintered powdered metal part. Disposed within cavity
18
are side gears
24
,
26
and pinion gears
28
,
30
. The teeth of the side gears and pinion gears are intermeshed, as shown. Pinion gears
28
,
30
are rotatably disposed upon cylindrical steel cross pin
32
, which extends along axis
34
. The ends of cross pin
32
are received in holes
36
,
38
diametrically located in the circumferential wall of casing part
16
a.
Axles
40
,
42
are received through hubs
44
,
46
, respectively formed in casing end wall portions
20
,
22
, along common axis of rotation
48
, which intersects and is perpendicular to axis
34
. Axles
40
,
42
are respectively provided with splined portions
50
,
52
, which are received in splines
54
,
56
of side gears
24
,
26
, thereby rotatably fixing the side gears to the axles. The axl
Forrest James L.
Leeper Robert
Scott James P.
Auburn Gear, Inc.
Baker & Daniels
Wright Dirk
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