Abstract:
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.

Description:
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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view taken through the controlled differential actuator in accordance with the present invention incorporated within a vehicle differential assembly. 
         FIG. 2  is a pictorial view partially cut away showing the carrier subassembly of the differential actuator shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a differential assembly in accordance with the present invention which is generally designated by reference number  10 . Power transfer to the differential assembly  10  through propeller shaft  12  coupled with drive pinion gear  16  through U-joint  14 . Differential case  18  encloses the internal components of differential assembly  10  and is a sealed enclosure which retains lubricating oil. Pinion bearing  20  supports the drive pinion  16  for rotation. Propeller shaft  12  is coupled typically through a gear transmission or other power transmission system which transmits mechanical power from the vehicle prime mover to differential assembly  10 . Within differential case  18  is disposed carrier assembly  22 . Carrier assembly  22  includes housing  24  which forms a radially protruding shoulder or flange  26  upon which ring gear  28  is fixed by screw fasteners. Rotation of drive pinion  16  causes rotation of ring gear  28  and carrier housing  24 . Carrier assembly  22  rotates about a vertical axis, as the components are positioned in  FIG. 1 . A pair of axle halfshafts  30  and  32  protrude from differential case  18  and engage laterally disposed road wheels through suitable drive connections, such as universal or constant velocity joints. In other applications, axle halfshafts  30  and  32  run through rigid axle tubes and do not incorporate flexible rotational connections between the differential and the road engaging wheels. Axle halfshafts  30  and  32  are supported for rotation on rolling element bearings  34  and  36 , respectively. Carrier shaft  38  is positioned within carrier housing  24  and supports a pair of bevel gears  40  and  42 . A pair of side gears  44  and  46  mesh with bevel gears  40  and  42  and are respectively attached to axle halfshafts  30  and  32 . The differential assembly  10  of this invention can be adapted for front, rear, or four-wheel drive applications. For example differential assembly  10  may 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 assembly  10  could be used to drive front and rear propeller shafts rather than axle half shafts (in which case elements  30  and  32  would 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 pinion  16  causes rotation of carrier housing  24  through its meshing engagement with the ring gear  28 . This rotational motion of carrier housing  24  causes bevel gears  40  and  42  to also rotate which are coupled to axle halfshafts  30  and  32  through their meshing engagement through side gears  44  and  46 . Such an open differential arrangement allows one of the axle halfshafts  30  or  32  to operate at a different speed than the other axle halfshaft. 
     In accordance with the present invention, differential actuator assembly  48  is employed which selectively couples axle halfshaft  32  with carrier housing  24 . Mechanically coupling these two elements together has the effect of causing both axle halfshafts  30  and  32  to 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 shaft  32  and carrier housing  24  allows a variable degree of mechanical engagement to occur between the axle halfshafts  30  and  32  from 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 to  FIGS. 1 and 2 , differential actuator assembly  48  is primarily disposed within carrier assembly housing  24  by end cap  50  which engages with the carrier housing at carrier flange  26 . Secured within differential case  18  is an electromagnetic solenoid coil  52  positioned within a flux concentrating annular housing  54 . Conductor leads  55  provide electrical energy to solenoid coil  52 . Surrounding portions of solenoid coil  52  and annular housing  54  is an annular solenoid armature member  56  which forms a small air gap between itself and housing  54 , and rotates with carrier assembly  22 . The application of electrical energy to solenoid coil  52  causes armature  56  to be attracted toward annular housing  54 . Tab  73  protrudes from housing  54  and fits into a pocket formed by differential case  18  to position the housing and prevent rotational movement of the housing. 
     Armature  56  is coupled with a plurality of stanchions or studs  57  which extend axially away from the armature through suitable axial passageways. Studs  57  are connected with apply plate  58 . A pilot or primary friction clutch pack  60  is disposed between an internal surface of end cap  50  and apply plate  58 . Energization of the solenoid coil  52  urges the armature  56  toward housing  54  which closes the air gap between these components and urges primary clutch apply plate  58  to compress the pilot or primary friction clutch pack  60 . 
     Primary clutch pack  60  includes a first plurality of larger diameter clutch discs or plates  62  with a male or exterior splines which engage complimentary female splines formed by the inner surface of end cap  50 . The clutch plates  62  thus rotate with carrier housing  24 . Interleaved within the clutch plates  62  is a second plurality of smaller diameter friction clutch discs or plates  64  which engage with complimentary splines of a circular clutch hub  66 . Both the first and second pluralities of clutch plates  64  and  66  preferably 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 housing  24 . Clutch hub  66  is freely rotatably disposed upon the output hub  68 . The circular clutch hub  66  includes a plurality, preferably three, ramped recesses disposed in a circular pattern about the axis of the output hub  68 . The recesses each define an oblique section of a helical torus. Disposed within each of the recesses is a load transferring member, such as ball  70 . Output hub  68  is disposed facing clutch hub  66  and includes a similar plurality of complimentary sizing arranged recesses. Balls  70  are received and trapped within the pairs of opposing recesses between clutch hub  66  and output hub  68 . Relative rotation between clutch hub  66  and output hub  68  causes balls  70  to move within their recesses which causes clutch hub  66  and output hub  68  to become separated, and therefore these elements comprise a relative rotation actuator. 
     The clutch hub  66 , output hub  68 , and balls  70  together cooperate to form a relative rotation actuator and more specifically a ball-ramp actuator. These devices function to convert relative rotation between components (hubs  68  and  66 ) to axial motion (separation of the hubs  66  and  68 ). 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 pack  60  is a main or secondary friction clutch pack assembly  72 . The secondary clutch pack  72  includes a first plurality of larger diameter friction clutch discs or plates  74  having external splines which are received within complimentary female splines of the inside diameter of carrier housing  24 . Interweaved within the first plurality of friction clutch plates  74  is a second plurality of smaller diameter friction clutch discs or plates  76  having internal or female splines which engage complimentary configured male splines of axle hub  78 . Both sets of the friction plates  74  and  76  preferably include friction clutch paper or material at their interfaces which functions optimally when disposed and wetted by clutch fluid. Axle hub  78  includes internal splines which mesh with the splined end of axle shaft  32 . Axle shaft  32  thus has an end spline not only to engage with axle hub  78 , but also to have splined engagement with side gear  46 . As best shown in  FIG. 1 , differential actuator assembly  48  is packaged in a compact manner within the diameter of ring gear  28  and the differential actuator assembly is contained within differential case  18 . 
     In operation, application of electrical energy to solenoid coil  52  draws armature  56  toward the annular housing  54 , compressing the primary clutch pack  60  and creating drag which tends to rotate the clutch hub  66  relative to output hub  68 , causing the load transferring balls  70  to ride up within their recesses and thus driving the clutch hub  66  and output hub  68  apart. The output hub  68  acts as an apply plate such that axial motion compresses secondary clutch pack  72 . Applying secondary clutch pack  72  has the effect of coupling carrier housing  24  to axle half shaft  32 . As mentioned previously, this has the effect of restricting relative rotation between the axle halfshafts  30  and  32 , thus providing a degree of frictional coupling between the axle halfshafts. The geometric characteristics of the recesses having ball bearings  70  disposed therein is designed such that the clutches are not self actuating. In other words, the controlled application of coupling torque between the axle halfshafts  30  and  32  can be provided by modulating the electric current applied to solenoid coil  52 . 
     It should be noted that the driving input for compressing main or secondary clutch pack  72  only occurs when there is a relative speed difference between axle halfshaft  32  and carrier housing  24 . It is this difference in relative speed which causes relative rotation to occur between clutch hub  66  and output hub  68 . This relative rotation can occur in either rotational direction sense. In some operating conditions, axle halfshaft  32  would be rotating faster than the carrier housing, and in other applications, slower. In either case, the relative speed difference occurs between hubs  66  and  68  which enables balls  70  to actuate the secondary clutch pack  72 . Additional details of the internal components of differential actuator  48  is 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. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.