Controlled differential actuator

A controlled differential actuator particularly adapted for differential applications for motor vehicle powertrains. The differential operator incorporates an electromagnetically applied first or primary clutch coupled to a second multi-disc clutch pack through a ball ramp operator. Energization of the solenoid coil selectively applies the main clutch pack which is coupled to a differential carrier and one of the axle half shafts connected with the differential assembly. Modulation of current applied to the solenoid coil allows a selective frictional coupling between the differential axle half shafts which provides desirable traction features for the associated motor vehicle.

FIELD OF THE INVENTION

This invention relates to a motor vehicle powertrain differential assembly, and particularly to one that provides controlled actuation to selectively modulate coupling between axle halfshafts of a motor vehicle axle assembly. The assembly couples the halfshafts to the differential housing resulting in differential control.

BACKGROUND OF THE INVENTION

Motor vehicle powertrains typically incorporate a differential assembly which couples mechanical input power from a propeller shaft or other input member to drive a pair of vehicle wheels through axle halfshafts. Differentials allow differences in rotational speed to occur between the left and right-hand side driven axle halfshafts. The most basic design of differentials are known as open differentials and provide constant torque between the two axle halfshafts and do not operate to control the relative rotational speed between the axle shafts. A well-known disadvantage of open differentials occurs when one of the driven wheels engages the road surface with a low coefficient of friction (μ) with the other having a higher μ. In such operating conditions, the low tractive effort developed at the low p contact surface prevents significant toque from being developed on either axle. Since the torque between the two axles shafts is relatively constant for an open differential, little tractive effort can be developed to pull the vehicle from its position in the above described operating condition. Similar disadvantages occur in dynamic conditions when operating, especially in low μor so-called split μdriving conditions.

The above limitations of open differentials are well-known and numerous design approaches have been applied to address such shortcomings. One approach is known as a mechanical locking differential. These systems typically use mechanical or hydraulic actuators to couple the two axle halfshafts together such that they rotate at nearly constant speed. Thus in that operating condition, the two axles are not mutually torque limited. A mechanically based locking differential typically uses a clutch pack or interlocking components which lock the two axles together when a speed difference between the axles is detected or the operator commands that function. Other systems incorporate fluid couplings between the axles which provide a degree of mechanical coupling. Locking differentials which mechanically interlock the two axles together do not permit modulation of the coupling between the axles. Instead, the axles are either locked to rotate together or operate in the open condition. Other systems use electric motors or hydraulic pumps to actuate a coupling system across the differential. Electro-mechanical actuators are also used in some designs.

This invention is related to a differential actuator which enables a highly controlled coupling to occur between axle half shafts of a driven axle through a solenoid applied primary clutch. The differential assembly of this invention can be adapted for front, rear, or four-wheel drive applications. For example the differential assembly may be applied to a front wheel drive transaxle or an all wheel drive center differential.

SUMMARY OF THE INVENTION

In accordance with the present invention, an electromagnetic clutch assembly having a solenoid actuator ball ramp operator is used to selectively couple and decouple a pair of axle shafts. The electromagnetic clutch assembly includes a primary or pilot friction clutch pack controlled by the primary clutch coupled with a secondary or main friction clutch pack. An annular solenoid coil and housing cooperate with an annular armature or plunger. When the coil is energized, the plunger translates and compresses the primary friction clutch pack. Activation of the primary clutch pack retards motion of one of the members of ball ramp actuator which in turn compresses the secondary friction clutch pack which couples the differential carrier with one of the axle halfshafts, resulting in differential control. This coupling also has the effect of directly coupling the two axle shafts. The electromagnetic clutch assembly provides a high degree of controlled coupling between axle shafts, with low power consumption necessary for controlling the actuation.

Additional benefits and advantages of the invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates a differential assembly in accordance with the present invention which is generally designated by reference number10. Power transfer to the differential assembly10through propeller shaft12coupled with drive pinion gear16through U-joint14. Differential case18encloses the internal components of differential assembly10and is a sealed enclosure which retains lubricating oil. Pinion bearing20supports the drive pinion16for rotation. Propeller shaft12is coupled typically through a gear transmission or other power transmission system which transmits mechanical power from the vehicle prime mover to differential assembly10. Within differential case18is disposed carrier assembly22. Carrier assembly22includes housing24which forms a radially protruding shoulder or flange26upon which ring gear28is fixed by screw fasteners. Rotation of drive pinion16causes rotation of ring gear28and carrier housing24. Carrier assembly22rotates about a vertical axis, as the components are positioned inFIG. 1. A pair of axle halfshafts30and32protrude from differential case18and engage laterally disposed road wheels through suitable drive connections, such as universal or constant velocity joints. In other applications, axle halfshafts30and32run through rigid axle tubes and do not incorporate flexible rotational connections between the differential and the road engaging wheels. Axle halfshafts30and32are supported for rotation on rolling element bearings34and36, respectively. Carrier shaft38is positioned within carrier housing24and supports a pair of bevel gears40and42. A pair of side gears44and46mesh with bevel gears40and42and are respectively attached to axle halfshafts30and32. The differential assembly10of this invention can be adapted for front, rear, or four-wheel drive applications. For example differential assembly10may be applied to a front wheel drive transaxle or an all-wheel drive center differential. In a front wheel drive application axles halfshafts would drive the front wheels of the vehicle, or rear wheels in the case of a rear wheel drive vehicle. In the case of a four wheel drive vehicle, differential assembly10could be used to drive front and rear propeller shafts rather than axle half shafts (in which case elements30and32would be the propeller shafts rather than axle halfshafts).

The above description of components are common to conventional open differentials. Operating in an open differential mode, rotation of drive pinion16causes rotation of carrier housing24through its meshing engagement with the ring gear28. This rotational motion of carrier housing24causes bevel gears40and42to also rotate which are coupled to axle halfshafts30and32through their meshing engagement through side gears44and46. Such an open differential arrangement allows one of the axle halfshafts30or32to operate at a different speed than the other axle halfshaft.

In accordance with the present invention, differential actuator assembly48is employed which selectively couples axle halfshaft32with carrier housing24. Mechanically coupling these two elements together has the effect of causing both axle halfshafts30and32to rotate at the same speed or approach the same speed. As will be explained in more detail below, however, limited frictional coupling between axle half shaft32and carrier housing24allows a variable degree of mechanical engagement to occur between the axle halfshafts30and32from a range in which the differential operates in an open differential mode to a fully coupled or “locked” condition. A modulation of the coupling across the axles provides improved drivability and traction performance in many operating conditions.

Now referring toFIGS. 1 and 2, differential actuator assembly48is primarily disposed within carrier assembly housing24by end cap50which engages with the carrier housing at carrier flange26. Secured within differential case18is an electromagnetic solenoid coil52positioned within a flux concentrating annular housing54. Conductor leads55provide electrical energy to solenoid coil52. Surrounding portions of solenoid coil52and annular housing54is an annular solenoid armature member56which forms a small air gap between itself and housing54, and rotates with carrier assembly22. The application of electrical energy to solenoid coil52causes armature56to be attracted toward annular housing54. Tab73protrudes from housing54and fits into a pocket formed by differential case18to position the housing and prevent rotational movement of the housing.

Armature56is coupled with a plurality of stanchions or studs57which extend axially away from the armature through suitable axial passageways. Studs57are connected with apply plate58. A pilot or primary friction clutch pack60is disposed between an internal surface of end cap50and apply plate58. Energization of the solenoid coil52urges the armature56toward housing54which closes the air gap between these components and urges primary clutch apply plate58to compress the pilot or primary friction clutch pack60.

Primary clutch pack60includes a first plurality of larger diameter clutch discs or plates62with a male or exterior splines which engage complimentary female splines formed by the inner surface of end cap50. The clutch plates62thus rotate with carrier housing24. Interleaved within the clutch plates62is a second plurality of smaller diameter friction clutch discs or plates64which engage with complimentary splines of a circular clutch hub66. Both the first and second pluralities of clutch plates64and66preferably include suitable friction clutch paper or material at their interfaces which functions optimally when disposed in and wetted by a clutch fluid bath within carrier housing24. Clutch hub66is freely rotatably disposed upon the output hub68. The circular clutch hub66includes a plurality, preferably three, ramped recesses disposed in a circular pattern about the axis of the output hub68. The recesses each define an oblique section of a helical torus. Disposed within each of the recesses is a load transferring member, such as ball70. Output hub68is disposed facing clutch hub66and includes a similar plurality of complimentary sizing arranged recesses. Balls70are received and trapped within the pairs of opposing recesses between clutch hub66and output hub68. Relative rotation between clutch hub66and output hub68causes balls70to move within their recesses which causes clutch hub66and output hub68to become separated, and therefore these elements comprise a relative rotation actuator.

The clutch hub66, output hub68, and balls70together cooperate to form a relative rotation actuator and more specifically a ball-ramp actuator. These devices function to convert relative rotation between components (hubs68and66) to axial motion (separation of the hubs66and68). It is within the scope of this invention to replace the ball-ramp operator described herein with other types of relative rotation actuators.

Disposed adjacent primary clutch pack60is a main or secondary friction clutch pack assembly72. The secondary clutch pack72includes a first plurality of larger diameter friction clutch discs or plates74having external splines which are received within complimentary female splines of the inside diameter of carrier housing24. Interweaved within the first plurality of friction clutch plates74is a second plurality of smaller diameter friction clutch discs or plates76having internal or female splines which engage complimentary configured male splines of axle hub78. Both sets of the friction plates74and76preferably include friction clutch paper or material at their interfaces which functions optimally when disposed and wetted by clutch fluid. Axle hub78includes internal splines which mesh with the splined end of axle shaft32. Axle shaft32thus has an end spline not only to engage with axle hub78, but also to have splined engagement with side gear46. As best shown inFIG. 1, differential actuator assembly48is packaged in a compact manner within the diameter of ring gear28and the differential actuator assembly is contained within differential case18.

In operation, application of electrical energy to solenoid coil52draws armature56toward the annular housing54, compressing the primary clutch pack60and creating drag which tends to rotate the clutch hub66relative to output hub68, causing the load transferring balls70to ride up within their recesses and thus driving the clutch hub66and output hub68apart. The output hub68acts as an apply plate such that axial motion compresses secondary clutch pack72. Applying secondary clutch pack72has the effect of coupling carrier housing24to axle half shaft32. As mentioned previously, this has the effect of restricting relative rotation between the axle halfshafts30and32, thus providing a degree of frictional coupling between the axle halfshafts. The geometric characteristics of the recesses having ball bearings70disposed therein is designed such that the clutches are not self actuating. In other words, the controlled application of coupling torque between the axle halfshafts30and32can be provided by modulating the electric current applied to solenoid coil52.

It should be noted that the driving input for compressing main or secondary clutch pack72only occurs when there is a relative speed difference between axle halfshaft32and carrier housing24. It is this difference in relative speed which causes relative rotation to occur between clutch hub66and output hub68. This relative rotation can occur in either rotational direction sense. In some operating conditions, axle halfshaft32would be rotating faster than the carrier housing, and in other applications, slower. In either case, the relative speed difference occurs between hubs66and68which enables balls70to actuate the secondary clutch pack72. Additional details of the internal components of differential actuator48is provided by U.S. Pat. No. 6,905,008 which is co-owned by the Assignee of this invention, which is hereby incorporated by reference.