Planetary gear transmission systems or components – Differential planetary gearing – Bevel gear differential
Reexamination Certificate
1999-08-12
2001-07-17
Marmor, Charles A (Department: 3681)
Planetary gear transmission systems or components
Differential planetary gearing
Bevel gear differential
C192S066200
Reexamination Certificate
active
06261202
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to differentials, and more particularly, to traction enhancing differentials having cone clutch elements.
2. Description of the Related Art
Differentials are well known in the prior art and 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. Such a device essentially distributes the torque provided by the input shaft between the output shafts. However, the necessity for a differential which limits the differential rotation between the output shafts to provide traction on slippery surfaces is well known.
The completely open differential, i.e., a differential without clutches or springs, 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 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. 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 include use of a frictional clutch mechanism having a frusto-conical engagement structure and a bias mechanism, usually spring loaded, to apply an initial preload between the side gears and the differential casing. By using a frictional clutch with an initial preload 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 engage the frictional clutch and develop additional torque.
The initial preload initiates the development of side gear separating forces which provide further braking action between the side gears and the differential casing. In general, gear separating forces are forces induced, due to the angle of contact or “pressure angle”, on any set of meshing gears by the application of torque to the gears and which tend to separate the gears. 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. When one wheel is on a surface having a low coefficient of friction, the initial preload creates some contact and frictional engagement between the differential casing and the clutch mechanism disposed between the side gears and the differential casing to allow the engine to provide torque to the wheel having traction. This initial torque transfer induces gear separating forces on the side gears which tend to separate the side gears to further frictionally engage the clutch mechanism with the casing. The increased frictional engagement of the clutch allows more torque to be developed between the casing and the clutch element, thus further increasing the side gear separating forces and limiting the slippage between the side gears and the differential casing.
It is well known in the art to use frusto-conical clutch elements providing, on the outside surfaces thereof, a spiral structure which winds helically about the clutch element from its base to its tip, the tip comprising the annular edge resulting when the right circular cone is truncated at a plane parallel to its base, producing a frustum. The spiral structure provides a clutch engagement surface which frictionally engages an adjacent frusto-conical clutch interior surface of the differential casing. Generally, such cones are provided with a double helical structure, each helix beginning and ending at radially opposite points of the outside surface of the clutch element. These frusto-conical clutch elements are made of generally ferrous material and are produced using powdered metal or machined casting methods.
FIG. 1
illustrates one embodiment of prior art limited slip differential
10
having helical cone clutch elements. Differential
10
comprises casing
20
, which includes casing parts
22
and
24
which are assembled via threaded joint
26
. Casing part
22
includes radial flange
28
, to which a ring gear (not shown) is attached by, for example, threaded fasteners (not shown). Torque output from a vehicle transmission applied to the ring gear causes differential casing
20
to rotate about axis
29
. Casing parts
22
and
24
are provided with hollowed hub portions
30
and
32
, respectively, through which extend output shafts or axles
34
and
36
along longitudinal axis
29
. Fixed for rotation with the end of each axle
34
and
36
in the interior of casing
20
are bevel side gears
38
and
40
, respectively. In the embodiment shown in
FIG. 1
, each side gear
38
and
40
is fixed for rotation with cone clutch element
42
and
44
, respectively, having the above mentioned double helical structure about their outside surfaces. Cone clutch elements
42
and
44
are usually identical and do not necessarily provide helical structures which spiral outwardly from the center of case
20
along axis
29
as mirror images of one another. Notably, other embodiments of limited slip differentials may provide only one cone clutch member.
Intermeshed with the teeth of side gears
38
,
40
are pinion gears
46
,
48
. The pinion gears rotate about cross shaft
50
which extends therethrough and is attached to casing
20
to rotate therewith. Thus pinion gears
46
,
48
rotate about cross shaft
50
and revolve about axis
29
with casing
20
. Cross shaft
50
is usually retained to casing
20
with a fastener such as bolt
52
. Disposed between the facing surfaces of bevel side gears
38
,
40
is some form of preload mechanism
54
. In the shown embodiment preload mechanism
54
comprises a plurality of compression springs
56
and bearing plates
58
,
60
. Bearing plates
58
and
60
bear on the facing surfaces of bevel side gears
38
and
40
, respectively, urging them apart under the influence of springs
56
. This separating force is imparted through the side gears to the cone clutch elements
42
,
44
, urging their outside frusto-conical surfaces into relatively light frictional engagement with mating frusto-conical clutch seat surfaces
62
,
64
of the interior of casing part
22
. When the wheels (not shown) attached to axles
34
,
36
have equal traction, input torque to casing flange
28
is distributed approximately equally therebetween, transmitted from casing
20
to cross pin
50
, to pinion gears
46
,
48
, to side gears
38
,
40
and then to axles
34
,
36
, which generally rotate at the same speed as casing
20
. Under this condition, little appreciable torque is transmitted directly from casing
20
to side gears
38
,
40
and axles
34
,
36
through cone clutch elements
42
,
44
because the frictional engagement between clutch seat surfaces
62
,
64
and cone clutch elements
42
,
44
is generally rather light and minor clutch slippage is allowed when turning. However, as one of the wheels attached to axles
34
,
36
loses traction, the two axles and the cone clutch elements fixed to rotate therewith begin to rotate at different speeds relative to each other and to rotating casing
20
. Under this condition, separation forces acting between pinion gears
46
,
48
and side gears
38
,
40
, plus the spring preload forces, in conjunction with the sliding relative motion between clutch elements
42
,
44
and seat surfaces
62
,
64
, cause frictional torque transfer between cone clutch elements
42
,
44
and casing surfaces
62
,
64
, braking the axle rotating faster than casing
20
and transferring tor
Forrest James L.
Leeper Robert
Auburn Gear, Inc.
Baker & Daniels
Marmor Charles A
Parekh Ankur
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