Abstract:
A limited slip differential assembly for a vehicle has a differential assembly drivingly engaged with a prime mover of the vehicle. The assembly has a differential mechanism disposed in a differential case and two opposite output shafts outwardly extending from the differential case. A torque coupling unit is provided for selectively restricting rotation between one of the output shafts and the differential case. The torque coupling unit has a friction clutch assembly disposed about one of the output shafts. The friction clutch assembly has a first portion drivingly engaged with one of the output shafts and a second portion drivingly engaged with the differential case. A ball and ramp assembly is disposed adjacent the friction clutch assembly for selectively frictionally loading the friction clutch assembly.

Description:
RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/931,001 filed on Jan. 24, 2014 which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     Limited slip differentials facilitate a reduction of or an elimination of a differential function present in a vehicle driveline. By engaging a clutch or otherwise drivingly engaging a side shaft of the vehicle with a remaining side shaft or a housing of the differential, the differential function can be reduced or eliminated. Such functionality may be used to increase traction in reduced friction environments or to facilitate a variety of operations which may be performed to increase control of the vehicle. 
     Typically, through engagement of a plate style clutch forming a portion of the limited slip differential, the differential function is reduced or eliminated. A customized differential case is required to accommodate the clutch. Such a customized differential case may greatly increase a cost of the vehicle incorporating the limited slip differential. Further, orientation of the differential, a plurality of bearings which support the differential, the clutch, and the side shafts may pose additional design considerations which increase a cost and a complexity of the limited slip differential. 
     The present invention relates to limited slip differentials for a vehicle, and more particularly to a design for a limited slip differential that increases manufacturability and decreases a cost of the limited slip differential compared to conventional designs. 
     SUMMARY 
     A limited slip differential assembly for a vehicle has a differential assembly drivingly engaged with a prime mover of the vehicle. A differential mechanism is disposed in a differential case and has two opposite output shafts outwardly extending from the differential case. The output shafts are drivingly engaged with wheels of the motor vehicle. 
     The assembly also has a torque coupling unit for selectively restricting rotation between one of the output shafts and the differential case. The torque coupling unit has a friction clutch assembly disposed about one of the output shafts. The friction clutch assembly comprises a first portion drivingly engaged with one of the output shafts and a second portion drivingly engaged with the differential case. 
     The torque coupling unit also has a ball and ramp assembly disposed adjacent the friction clutch assembly for selectively frictionally loading the friction clutch assembly. The ball and ramp assembly comprises at least a first portion and a second portion. The second portion is rotatable with respect to the first portion. 
     The torque coupling unit also has an actuator in driving engagement with one of the first portion and the second portion of the ball and ramp assembly to selectively cause a rotation therebetween. 
     The assembly also has a differential housing rotatably supporting the differential assembly and the friction clutch assembly therewithin. The differential housing includes a radially inwardly extending portion about one of the output shafts for receiving a ball and ramp assembly support bearing and an output shaft support bearing. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of one embodiment of an active limited slip differential. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts of the present invention. Hence, specific dimensions, directions, orientations or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless expressly stated otherwise. 
     As shown in the FIGURE, a differential  10  with a torque coupling unit  12  is provided. A pinion shaft  14  is provided and is connected to a source for rotational power. The pinion shaft  14  is mounted on bearings  16  for rotation by the power source. The power source may be an internal combustion engine (not shown), or the like. 
     The pinion shaft  14  has a pinion gear  18  located thereon that rotates with the shaft  14 . The pinion gear  18  is meshed with a ring gear  20 . The pinion gear  18  has a first set of teeth (not shown) and the ring gear  20  has a second set of teeth (not shown). The two sets of teeth are complimentary to one another and are meshed with one another to provide rotational drive from one set to the other set. 
     The ring gear  20  is connected to a differential case  22 . More particularly, the ring gear  20  may be integrally formed with the differential case  22 , welded to the differential case  22 , or it may be secured to the differential case  22  with a plurality of fasteners. It can be appreciated that the connection of the ring gear  20  between the differential case  22  and pinion gear  18  results in rotation of the differential case  22 . 
     The differential case  22  houses a set of differential pinion gears  24  rotatably supported on a spider shaft  26  secured to the differential case  22 . More particularly, the differential pinion gears  24  are located opposite one another on the spider shaft  26 . The FIGURE illustrates the use of two differential pinion gears  24 ; however, it is understood that the differential  10  may be configured for use with a greater number of differential pinion gears  24 . The differential pinion gears  24  engage a pair of opposite side gears. The side gears comprise first and second side gears  28 ,  30  adapted to rotate about an axis  32 . 
     The differential case  22  is mounted for rotation within a differential housing  34  (partially shown). The differential case  22  is mounted on bearings  36  to support its rotation within the differential housing  34 . 
     A first and a second side gear shaft  38 ,  40  are depicted in the FIGURE extending from the first and second side gears  28 ,  30 , respectively. The first side gear shaft  38  has a first end portion  42  and a second end portion  44 . The second side gear shaft  40  has a first end portion  45  and a second end portion  46 . 
     The first end portion  42  of the first side gear shaft  38  has a set of splines (not shown) on an exterior surface (not shown) that fit within a central aperture (not shown) of the first side gear  28 . The central aperture is defined by complimentary, internal splines (not shown). The first side gear shaft  38  thus turns with the first side gear  28 . The first side gear shaft  38  is mounted for rotation within the differential housing  34  on bearings  46 . Bearings  46  may be plain or roller bearings. 
     The second end portion  44  of the first side gear shaft  38  is drivingly engaged with a wheel assembly (not shown) in any conventional manner, such as through a constant velocity joint (not shown) or a wheel flange. 
     The first end portion  45  of the second side gear shaft  40  has a set of splines (not shown) on an exterior surface (not shown) that fit within a central aperture (not shown) of the second side gear  30 . The central aperture is defined by complimentary, internal splines (not shown). The second side gear shaft  40  thus turns with the second side gear  30 . The second side gear shaft  40  is mounted for rotation within the differential housing  34  on bearings  47 . Bearings  47  may be plain or roller bearings. 
     The second end portion  46  of the second side gear shaft  40  is drivingly engaged with a wheel assembly (not shown) in any conventional manner, such as through a constant velocity joint (not shown) or a wheel flange. 
     The differential case  22  is connected to a clutch can  48 . The clutch can  48  and the differential case  22  may be integrally formed and unitary with one another or they may be separately formed. If separately formed, the differential case  22  may be connected to the clutch can  48  by mechanical fasteners, splines, or the like. As shown in the FIGURE, bearings  36  support rotation of the differential case  22  within the housing  34 . 
     In the depicted embodiment, the clutch can  48  is a cylindrically shaped object. The clutch can  48  carries a first plurality of clutch plates  52 ; the clutch plates  52  are radially inwardly extending and are secured to an inner surface  54  of the clutch can  48 . The clutch plates  52  are fixed for rotation with the clutch can  48  through a plurality of splines (not shown), but are permitted to move axially along the inner surface  54 . 
     The clutch plates  52  are interleaved with a second plurality of clutch plates  56  located on an inner clutch hub  58  to form a clutch pack  60 . The inner clutch hub  58  is a cylindrical shaped member having a radially inwardly extending portion. The inner clutch hub  58  is at least partially located within the clutch can  48 . More particularly, the clutch can  48  is at least partially concentric with the inner clutch hub  58 . The inner clutch hub  58  has a splined inner surface  62  and a splined outer surface  64 . The inner clutch hub  58  is drivingly engaged with the first side gear shaft  38  through the splined inner surface  62  and a plurality of corresponding splines formed on the first side gear shaft  38 . 
     The second plurality of clutch plates  56  are fixed for rotation with the inner clutch hub  58  through a plurality of splines (not shown), but they are permitted to move axially along the splined outer surface  64  of the inner clutch hub  58 . 
     A clutch actuator assembly  66  is located adjacent the clutch pack  60 . The clutch actuator assembly  66  comprises an actuator  68 , a gear set  70 , and a ball and ramp assembly  72 . A bearing  73  supports rotation of the ball and ramp assembly  72  within the housing  34  when the ball and ramp assembly  72  is placed in an engaged position. 
     The actuator  68  may be such as a reversible electric motor as it is compact and easily controllable. It will be appreciated that any other appropriate type of actuator may be used, such as hydraulic or pneumatic, and these are within the scope of the invention. 
     The actuator  68  drives the gear set  70 , which is a reduction set of gears. As shown in the FIGURE, a first gear  74  of the actuator  68  drives a second gear  76 . The second gear  76  drives a third gear  78 . The gears  74 ,  76 ,  78  achieve the desired and torque speed reduction between the motor and the third gear  78 . Other gear numbers and orientations are possible other than as shown to result in different speeds and torques. 
     The third gear  78  is in driving engagement with an actuating ring  80 . More preferably, the actuating ring  80  has a set of teeth  81  on an outer radial surface that engages with the teeth on the third gear  78 . The teeth  81  of the actuating ring  80  are circumferentially extending from a peripheral edge of the actuating ring  80 . The teeth  81  of the actuating ring  80  may cover the full circumference of the actuating ring  80  or a portion of the circumference. The rotation of the third gear  78  drives the teeth  81  of the actuating ring  80 , thus rotating the actuating ring  80 . 
     The actuating ring  80  is part of the ball and ramp assembly  72 . The ball and ramp assembly  72  also comprises a pressure plate  82  and a plurality of balls  84  between the pressure plate  82  and the actuating ring  80 . 
     The pressure plate  82  resists an axial force applied thereto, causing the actuating ring  80  to apply a force to a first thrust bearing  85  located adjacent thereto. The force applied to the first thrust bearing  85  is used to load the clutch pack  60 . The pressure plate  82  is non-rotatably mounted within the housing  34 . 
     An annular radial surface  86  of the pressure plate  82  facing the actuating ring  80  is formed with a set of circumferentially extending grooves (not shown) of varying axial depth. The grooves in the pressure plate  82  face complementary grooves (not shown) on an opposite annular surface  88  of the actuating ring  80 , whose depth varies in the opposite circumferential sense. 
     A corresponding number of the balls  84  are disposed between the pressure plate  82  and the actuating ring  80 , one in each pair of the facing grooves. It is understood that the balls  84  may also be rollers which function in a similar manner. 
     Alternatively, a cam disc actuator (not shown) including cooperative cam surfaces provided on opposite sides of an actuating ring and a pressure collar may be used. It is also appreciated that other types of actuators may be used. 
     It will be further appreciated that when the actuator  68  moves the actuating ring  80  angularly relative to the pressure plate  82 , the actuating ring  80  moves axially and causes the actuating ring  80  to frictionally load the clutch pack  60 . The axial movement of the actuating ring  80  is transmitted to the clutch pack  60  through the first thrust bearing  85 . The first thrust bearing  85  is provided between the actuating ring  80  and the clutch pack  60  to allow for relative rotation and to reduce the friction between the actuating ring  80  and the clutch pack  60 . A second thrust bearing  90  is disposed between the clutch can  48  and the differential housing  34  to allow for relative rotation and to reduce the friction between the clutch can  48  and the differential housing  34 . 
     Wave springs (not shown) may be positioned between the first plurality of clutch plates  52  and the second plurality of clutch plates  56  to ensure the clutch plates  52 ,  56  are equally spaced in order to minimize the viscous drag torque between the clutch plates  52 ,  56 . The wave springs also produce a preload to ensure seating of the bearing  73 , the thrust bearings  85 ,  90 , and the ball and ramp assembly  72 . 
     The compression of the clutch plates  52 ,  56  causes the clutch plates  52 ,  56  to rotate together. The connection of the clutch can  48  to the inner clutch hub  58  causes the differential case  22  to be in driving engagement with the first side gear shaft  38 . Further, when the differential case  22  is in driving engagement with the first side gear shaft  38 , the differential case  22  is also in driving engagement with the second side gear shaft  40  through the differential pinion gears  24 . Depending on an engagement level of the ball and ramp assembly  72 , a differential function of the differential  10  is reduced or eliminated. 
     The actuator  68 , and thus the torque coupling unit  12 , is controlled by an electronic control unit (not shown). The control is carried out by judging vehicle running conditions according to at least one vehicle parameter, including but not limited to, wheel speeds. The differential  10  is thus provided with a limited slip function, which allows torque to be directed to the wheel assembly having a greater amount of traction. When the actuator  68  is not actuated, the differential  10  operates in an open mode without limited slip. Further, the torque coupling unit  12  may be engaged to reduce a slipping of at least one of the wheel assemblies during a vehicle start operation. The torque coupling unit  12  may also be engaged during a vehicle acceleration operation to damp the vehicle against undesirable yaw disturbances. Still yet, the torque coupling unit  12  may also be engaged during a vehicle corning operation to transfer torque to a wheel assembly having an inner position to correct for an undesirable over steer condition. 
     The differential  10  with the torque coupling unit  12  has several advantages over limited slip differentials known in the art. The differential case  22  may be a standard style open differential case when the differential case  22  is formed separate from the clutch can  48 . When the differential case  22  is formed separate from the clutch can  48 , the only modification required of the differential case  22  is to enable driving engagement with the clutch can  48 , such as through mechanical fasteners, splines, or the like. Accordingly, a position and type of each of the bearings  36  also remains the same as with the standard style open differential case, reducing a cost of the differential  10  with the torque coupling unit  12 . 
     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments, however, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its scope or spirit.