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Description  |
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BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to automatically engageable and disengageable
differential inhibiting devices and more particularly relates to limited
slip differential devices utilizing friction disc clutch arrangements and
a single sensor for sensing the acceleration of a monitored gear, or
related member, and a control for engaging the disc clutch when the
acceleration of the monitored gear exceeds a predetermined maximum value.
More particularly, this invention relates to an automatically engageable
and disengageable limited slip differential for a drive axle which
includes a friction disc clutch which will be automatically engaged or
disengaged dependent upon ring gear acceleration.
2. Description of the Prior Art
The advantages of a differential inhibiting device such as a limited
differential arrangement of the interaxle or final drive of a vehicle are
well known. A conventional vehicle differential allows a difference in
angular velocity between two driving wheels while turning corners;
however, if one of the driving wheels encounters poor traction on a
slippery surface, such will tend to cause the wheel to spin and thus limit
total driving torque to twice that of the spinning wheel causing the
opposite wheel to remain stationary. This operating condition, generally
referred to as a "spin out", is encountered when a vehicle has one of its
driving wheels bearing on an icy or slippery spot on the roadway, while
the other wheels are contacting and/or bearing on a surface having a
greater coefficient of friction. Under such circumstances, a relatively
low wheel torque will cause the wheel which bears on the slippery surface
to spin and a torque equal in magnitude to this low torque will be all the
torque available to the wheel on the drive pavement and/or surface of
greater coefficient of friction, which will more than likely be
insufficient to move the vehicle. Well known means are often provided for
automatically reducing or eliminating the normal action of the axle
differential.
In conventional types of limited slip differentials employing friction
clutch discs the friction discs are generally continuously loaded by
compression springs and/or similar conventional force loading apparatus.
However, these previous friction clutch devices are usually continuously,
operatively engaged, even though the majority of the time such a device is
not required. Accordingly, such friction clutch devices are continuously
subjected to extreme and detrimental wear which results in frequent repair
and replacement thereof. Moreover, these prior art spring biased friction
clutches are generally disposed about one or both of the output axle
shafts and are operatively positioned in the throat section of the axle
housing. Structurally locating the spring biased friction clutch in this
manner generally requires an extremely large number of friction discs and
a corresponding increase in the spring rate and/or load to effectively
achieve reasonable clutching action.
An improved limited slip differential device is described in U.S. Pat. No.
3,448,635, issued Mar. 27, 1967, assigned to the assignee of this
application, and hereby incorporated by reference. While the limited slip
differential described in U.S. Pat. No. 3,448,635 provided improved
mechanical characteristics, the device did require manual actuation by the
vehicle operator.
Other differential devices, such as illustrated in U.S. Pat. No. 3,138,970,
utilized automatic means to positively lock a differential, or to apply a
friction clutch in a limited slip differential device. These devices have
not been totally satisfactory as at least two sensors, one for each axle
shaft, are usually required. Providing two sensors introduces additional
costs, additional circuitry and the like. Further, such devices usually
sensed axle rotation and were normally located at the outer ends of the
axle housing and were thus more subject to the jolting often experienced
by a vehicle axle. U.S. Pat. Nos. 3,473,120; 6,683,219; 3,732,752;
3,845,671 and 3,871,249 are representative of the prior art devices. Also,
many of these prior art devices were less than totally satisfactory as
they were speed sensitive devices which might lock up the differential, or
engage the limited slip clutch discs, during a high speed turn when both
wheels were rolling substantially without slippage and full differential
action was desired to prevent undue tire wear, excessive stress to the
differential mechanism and the like.
SUMMARY OF THE INVENTION
In accordance with the present invention, many of the drawbacks of the
prior art have been overcome to the extent that a limited slip, or lock
up, differential mechanism is provided which requires only a single sensor
to sense undesirable spin out to activate the limited slip friction clutch
or positive lock up mechanism. The single sensor provides a first signal
proportional to angular velocity of a monitored member, preferably the
driven ring gear. The first signal is converted into a second signal
proportional to acceleration of the monitored member. Applicant has found
that ring gear acceleration exceeding predetermined values is an accurate
indication of undesired spin out and thus a single sensor may be utilized
to provide an indicator of a spin out to activate the differential
inhibiting device. That is, under given conditions, acceleration of
various gears, such as the ring gear, is not expected to exceed a given
value in the absence of a spin out condition.
Accordingly, it is an object of the present invention to provide a new and
improved automatic limited slip, or lock up, differential mechanism.
A further object of the present invention is to provide an automatically
engageable and disengageable limited slip, or lock up, differential
mechanism which requires only a single sensor.
Another object of the present invention is to provide an automatically
engaged and disengaged limited slip, or lock up, differential mechanism
which is responsive to ring gear acceleration.
These and other objects and advantages of the present invention will become
apparent from a reading of the Description of the Preferred Embodiment
taken together with Drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of one embodiment of the differential
mechanism of the present invention;
FIG. 2 is a detailed sectional view taken along line 2--2 of FIG. 1;
FIG. 3 is a sectional view taken along line 3--3 of FIG. 1;
FIG. 4 is a cross-sectional view of an alternate embodiment of the present
invention as utilized in connection with a locking type differential for a
tandem axle mechanism.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Certain terminology will be used in the following description for
convenience in reference only and will not be limited. The words
"upwardly", "downwardly", "rightwardly", and "outwardly" will refer to the
directions toward and away from, respectively, the geometric center of the
device and designated parts thereof. Said terminology will include the
words specifically mentioned, derivatives thereof, and words of similar
import.
The present invention will be described in the embodiment of a limited slip
differential mechanism similar to U.S. Pat. No. 3,448,635. In general, the
described embodiment consists of providing a differential friction clutch
assembly having a plurality of friction clutch discs splined to a power
input member for rotation therewith cooperating with complementary
alternate interleaved friction clutch discs splined to an elongated
portion of a selectively actuable clutch means that is adapted to be
engaged and disengaged to a differential output member. A plurality of
springs are radially positioned about the geometric axis of the friction
clutch discs for applying a predetermined force to the interleaved
friction discs. An automatically actuable control means is provided for
operatively connecting or disconnecting the friction clutch assembly, such
that the biased friction discs may be operatively connected, respectively,
to the differential cage and one of the differential output members to
effectively restrain and/or retard relative rotation of the differential
output members until one differential output member encounters a
predetermined loss in resistance to rotation and tends to rotate relative
to the other resulting in an unsafe load on the rotating axle shaft. And,
should operating conditions warrant, the automatic control for the
friction clutch assembly may be selectively disconnected by the operator
and the differential will function as a conventional differential to
equally divide the input torque between the output members thereof.
Thus, when the friction clutch assembly is engaged to the differential cage
or one of the differential output members and a vehicle is moved in a
normal straightahead path, wherein substantially no differential action is
required, the friction clutch assembly and the differential rotate
simultaneously as an integral unit and there is substantially no relative
rotation between the side gears of the differential or between the
friction clutch discs of the clutch assembly. Conversely, when one driven
output shaft encounters less resistance to rotation and attempts to rotate
relative to the other driven output shaft, this inclination is restrained
by the frictional forces developed between the clutch discs and driving
torque is continually made available to the other driven output shaft. It
is readily apparent the magnitude of this effect depends on the total
spring force of the compression springs employed which apply the force to
the friction clutch discs and the characteristics of material employed to
form the individual friction discs. The force on the friction clutch discs
is accomplished by compressing the springs, from a free unloaded height to
a specified and/or predetermined load height wherein the springs may apply
a specific predetermined load to the friction clutch assembly.
In the present invention no axial displacement or movement of the
differential side gears relative to the differential pinion gears occurs
and the proper pitch line of rolling contact between the respective side
gears and the pinion gears will remain substantially constantly uniform,
thereby substantially eliminating the following disadvantages:
(1) The incident of fractured or broken gear teeth due to shock loading is
substantially eliminated, such fractures being the result of increased
backlash due to improper variation of the pitch line contact between the
differential side and pinion gears; and
(2) Misalignment of the gear teeth which causes stress concentrations in
localized areas and progressive fatigue resulting in excessive gear teeth
wear and eventually broken gear teeth.
While the preferred embodiment of the present invention is illustrated in
connection with a limited slip differential, it is understood the
invention is equally applicable to lock up type differentials, such as the
type illustrated in U.S. Pat. No. 3,388,760, assigned to the assignee of
this invention and hereby incorporated by reference. It is also understood
that while the preferred embodiment of the present invention is
illustrated in connection with a drive axle differential, the present
invention is equally applicable to tandem axle interaxle differentials and
to transfer case differentials.
Referring in greater detail to the drawings, wherein like numerals are used
in the case of similar parts throughout the several views of the drawings,
reference numeral 10 of FIG. 1 discloses an axle differential mechanism. A
differential carrier 12 is fastened to the axle housing 14 by a peripheral
flange portion 16 utilizing conventional means such as bolts 18. The
differential carrier 12 is provided with opening 20 within which is
disposed a bearing assembly 22 comprising outer race 24, bearings 26, and
innerrace 28 which rotatably supports input pinion 30 connected by adapter
flange 32 to a drive shaft (not shown) emanating from a conventional prime
mover having a conventional clutch and change speed transmission
mechanism.
A ring gear 34 in gear meshing relationship with output pinion 30 is
connected to and rotatably supported on a housing or cup-form member 36 by
a plurality of conventional fasteners, as bolts 38. The cup-form member 36
has an outwardly extending tubular or hub portion 40 which is rotatably
supported in the differential carrier 12 by a plurality of anti-friction
bearing means 42. The housing or cup-form member 36 and attached ring gear
34 together define a hollow substantially circular internal cavity or
chamber 44. The differential case 36 comprises a two-piece case having a
first case portion 48 and a second case portion 50 with the first case
portion 48 being provided with a radially extending flange portion 52
having the ring gear 34 fixedly attached thereto by fasteners 38. A pair
of differential bevel or side gears 54 and 56 are splined or otherwise
structurally disposed on complementary splines or structure of
differential output shafts 58 and 60 respectively, and have a plurality of
teeth meshing with opposed complementary teeth on at least one pinion gear
62 which is rotatably mounted on differential pinion gear shaft 64
centrally positioned within differential case 36 and structurally secured
thereto for rotation therewith. A plurality of fasteners 66, disposed in a
plurality of complementary bores 68 secure the two halves 48 and 50 of the
case 36 together. A friction clutch assembly including a biased multiple
friction clutch pack 70 is positioned within the cavity 44. A selected
number of friction discs 72 are provided with tab extensions 73 (see FIG.
2) having circular portions 76 in surrounding relationship to bolt means
38 which are secured to the ring gear 34. Thus, friction discs 72 are
maintained in position by bolt means 38 and drivingly rotated with ring
gear 34. An equal number of complementary friction discs 74 are splined to
the intermediate clutch gear 88 and are interdigitated or interleaved with
friction discs 72.
A hub portion 80 of side gear 54 having gear teeth 82 extends into cavity
44, automatically actuable clutch member 85 includes an annular sliding
clutch member 86 having gear teeth 87 to selectively engage or disengage
gear teeth 82 of side gear 54. Intermediate clutch gear member 88 and
slide clutch member 86 have complementary gear teeth 90 which are in
cooperating engagement when the sliding clutch member 86 is moved to the
right (as in FIG. 1) and are disengaged when the sliding clutch member 86
is moved to the left. Thus, rightward movement of sliding clutch member 86
(as seen in FIG. 1) effects a gear meshing and torque transfer engaging
relationship between gear teeth 90 through gear teeth 82 and 87 for
simultaneous rotation of side gear 54, clutch member 86, clutch gear
member 88 and the associated friction discs 74. Whereas, upon leftward
movement as illustrated in broken or dashed lines in FIG. 1, sliding
clutch member 86 disengages teeth 82 and 87 permitting relative rotation
between gear 54 and sliding clutch member 86. The shift mechanism 85 can
be actuated by conventional air, hydraulic, electric or air/electric shift
control systems of the type which are utilized in effecting shifting of
two-speed axle arrangements. It is readily apparent that sliding clutch
member 86 and intermediate clutch gear element 88 may be constructed as an
integral unit instead of two separate parts, for example, 86 and 88 as
illustrated in FIG. 1.
Slidably receivable in cavity 44 (FIG. 1) is friction clutch assembly 70
including alternate interleaved friction discs or plates 72 and 74
interconnected to ring gear 34 by bolts 38 and intermediate clutch element
88 respectively. An axially slidable pressure plate 94 is positioned on
one side (left) of the friction disc pack while the differential housing
defines the other (right) side of the friction disc pack 70. Spring means
102 are resiliently compressed between surface 104 of differential casing
36 and pressure plate 94. In the embodiment shown in FIG. 1, a plurality
of springs 102 are radially disposed in spaced relationship (See FIG. 3)
about pressure plate 94 for exerting an inwardly, substantially constant
force of a predetermined magnitude against plate 94, whereby a
substantially uniform force is exerted against the surface of friction
discs 72 and 74. Pressure plate 94 may be restrained from rotating
relative to discs 72 and 74 by splines 106 which interfit or mesh with
complementary splines on intermediate clutch element 88 or may be free to
rotate relative to the discs.
Intermediate clutch element 88 is precluded from substantial axial movement
by the confining limits of the pressure plate 94 and the differential
housing.
Mounted in the differential carrier 12 is a single sensor 200 which may be
threadably received in the carrier 12 as at 202 or may be carried by the
carrier in any other suitable manner. The sensor may be electro magnetic,
photoelectric or the like non-contacting sensors. Alternately, the sensor
may be the input of a tach generator or the like. The sensor is located
near a rotor 204 attached to the ring gear 34 for rotation therewith. The
rotor 204 may be grooved, slotted, convoluted or of any other form to
allow the sensor 200 to sense the rotation of the ring gear.
An alternate placement of the sensor is illustrated at 206 wherein the
sensor is positioned to sense rotation of the ring gear by sensing
rotation of the ring gear teeth.
The sensor 200 provides a first signal proportional to the rotational speed
of the gear. The first signal is preferably an electrical signal having a
frequency or voltage of a magnitude proportional to, or representative of,
the rotational speed of the gear. This first signal is transmitted to the
control 208 which converts the rotational speed first signal to a second
signal of a magnitude proportional to acceleration of the ring gear and
then compares the second signal to a predetermined maximum reference
signal. Circuits which will differentiate the variable frequency or
variable voltage first signal with respect to time to provide a second
signal proportional to, or representative of, ring gear acceleration are
well known in the art and an example thereof may be seen by reference to
U.S. Pat. No. 3,966,267, assigned to the assignee of this invention and
hereby incorporated by reference. The magnitude of the predetermined
maximum reference signal is substantially equal to the magnitude of the
second signal when acceleration of the ring gear is a predetermined
maximum acceleration. The predetermined maximum acceleration is typically
the maximum ring gear acceleration expected in the absence of a "spin out"
condition. In the preferred embodiment, the reference signal is a fixed
self-contained signal and the monitoring of system parameters other than
acceleration of the monitored gear is not required. If the second signal
exceeds the reference signal, i.e., the acceleration of the ring gear
exceeds a predetermined maximum acceleration, the controller will cause
the actuator 210 to shift the lever 85 whereby the sliding clutch member
will be engaged. The actuator 210 may, of course, comprise a solenoid, air
motor, fluid actuator, or the like. The control 208 may, of course, be of
any commercially available type such as fluidic, electrical analog,
electrical digital, or the like.
OPERATION
Although the operation of the limited slip differential device 10 embodying
the invention has been disclosed somewhat above, the same will now be
briefly described to ensure a full understanding of the invention. When
the sliding clutch member 86 has been shifted to its rightmost position
(see FIG. 1) by shifting means 85, the gear teeth 87 of the sliding clutch
member 86 cooperatively engage the corresponding gear teeth 82 of the
differential side gear member 54. Likewise, the corresponding gear teeth
90 of the sliding clutch member 86 and the intermediate clutch gear member
88 also are in cooperative engagement when the sliding clutch member 86 is
shifted to its rightmost position, thereby effecting locking of the side
gear member 54 to the ring gear member 34 through the before-mentioned
cooperating gear teeth sets 82 and 87 and 90 through the friction disc
plates 72 and 74, which locking arrangement precludes relative rotation of
the side gear ring gears. The "locking" of the side gear 54 in effect also
locks side gear 56 against relative rotation to the ring gear member 34
because the differential pinion gears 62 are then precluded from spinning
about their mounting shafts 64. With the differential lock arrangement so
engaged, the respective axle shafts 58 and 60 are rotated at the same
speeds and transfer the same amount of torque to their driving wheels.
Under certain road conditions as when one driving wheel would encounter a
slippery or low friction surface, while the other driving wheel has a
heavy load or torque requirement thereon, the relative loads or torque
requirements on the respective axles is sufficient to overcome the
frictional forces within the friction disc clutch pack (provided by spring
biasing means 102) and effects an "unlocking" of the differential
permitting relative rotation of the axle shafts and side gears. This
before-described "unlocking" of the friction disc clutch pack is only
attained upon a predetermined load or torque requirements on the
respective drive wheels and under normal conditions would not occur.
However, in the interest of preventing any overstressing of the axle
shafts or the differential gears it is necessary to provide such a safety
release of the limiting differential arrangement of the friction disc
clutch pack. When the shifting means 85 has shifted the sliding clutch
member 86 to its leftmost position (opposite that position seen in FIG. 1)
the sliding clutch member 86 will have been removed from its cooperative
engagement with the corresponding gear teeth of the side gear member 54
which will no longer be in "locked" engagement with the ring gear 34 as
described hereinabove. In the "unlocked" or disengaged condition, the
differential will function normally dividing the propelling effort of the
input pinion 30 equally between the two driving wheels of the vehicle.
FIG. 4 illustrates an alternate embodiment of the present invention in
which a single sensor provides a first signal proportional to angular
velocity of a monitored gear which is converted into a second signal
proportional to acceleration of the monitored gear to control the
actuation of a differential lock up for the interaxle axle differential of
a tandem axle mechanism. Tandem axle mechanisms utilizing positive locking
type differentials are well known and a representative version may be seen
by reference to U.S. Pat No. 3,388,760.
Briefly, the tandem axle mechanism 300 typically comprises a pair of
driving axles (not shown) each of which receive power from a power divider
302 which includes an interaxle differential 304. A conventional lock up
mechanism 306 will render the interaxle differential inoperative when
actuated by shifting sliding clutch member 308 to the right to engage side
gear 312. Sliding clutch member 308 is splined to the input shaft 310 as
thus will rotationally lock the interaxle differential side gear 312 to
the input shaft 310 to lock up the interaxle differential. Of course,
other members such as the other side gear or the differential case may be
positively engaged to the input shaft or to one another to effect a lock
up of the interaxle differential 304.
A sensor 320 is mounted in the tandem axle assembly, preferably adjacent
the monitored gear or monitored input member, such as the front rear drive
axle ring gear 322 or the interaxle differential case to sense the
rotational speed thereof. The sensor provides a first signal, proportional
to rotational speed of the monitored gear or input member, to a control
member 324. An alternate embodiment is illustrated in phantom lines
wherein the control receives signals from a sensor 320' which is mounted
to sense velocity of rear rear drive axle ring gear 322'. The control
member 324 converts the first signal into a second signal proportional to
monitored acceleration. The second signal is then compared to a reference
signal corresponding to maximum monitored acceleration that is expected in
the absense of a spin out condition. If the second signal exceeds the
reference signal, the control will cause the actuator 326 to shift the
sliding collar 308 to the right. Of course, when the acceleration sensed
falls below the reference acceleration, the control will cause the
actuator to shift collar 308 to the left to unlock the interaxle
differential.
In both controls, 208 illustrated in FIG. 1 or 326 illustrated in FIG. 4, a
time delay mechanism or the like may be utilized to prevent undesirable
operating characteristics, such as cycles of rapid engagement and
disengagement of the differential inhibiting clutch. Such time delay
features are well known in prior art controls.
The reference signal may, of course, be permanently set, adjustable for
various types of vehicles, adjustable by the operator to compensate for
expected operating conditions and/or may automatically vary with vehicle
speed or the like.
It should be readily apparent the embodiments of the present invention as
described hereinabove function in a substantially similar manner and
certain modifications, changes, and adaptations may be made in the
disclosed structures and it is hereby intended to cover all such
modifications, changes, adaptations and constructions which fall within
the scope of the appended claims. For example, the limited slip
differential arrangement of the embodiments disclosed and shown in the
present invention, particularly the clutching arrangement with its
friction discs, is adaptable to a two-speed axle notwithstanding the fact
that the embodiments disclosed herein are all single speed axle
constructions.
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