Patent Abstract:
A differential having a driving member; an input member; first and second rotatable output members; differential gearing operable between the input member and the first and second output members, for transmitting rotation from the input member to the first and second output members and providing for differential rotation of the first and second output members relative to one another; an engaging device operable to establish a driving connection between the driving member and the input member; an inhibiting device operable to inhibit relative rotation between the first and second output members; and an actuating device for causing the operation of the engaging device and the inhibiting device.

Full Description:
TECHNICAL FIELD 
     This invention relates to a differential gear device for use in a motor vehicle. More particularly, it relates to two aspects of the differential gear—the control of input torque to the device and the control of output torque from the device. 
     BACKGROUND OF THE INVENTION 
     Differential gear devices, commonly referred to as differentials, are well known devices used in motor vehicle drive lines. A differential gear is designed to distribute torque from an input element to two output elements while permitting the two output elements to rotate at different speeds under certain conditions. The output elements may be connected to two wheels alongside one another at opposite sides of, a vehicle, in which case the wheels are required to rotate at different speeds when the vehicle is cornering. The differential may alternatively be an inter-axle differential in a multiple-wheel-drive system, in which case the wheels on the two axles may be required to rotate at different speeds from one another again, for example, when cornering. 
     In two-wheel drive vehicles, it may be desirable to have the facility to switch to a four-wheel drive system if the vehicle needs more traction. It is known to provide an auxiliary drive to the second axle of the vehicle, in the form of an electric motor drive, to provide this four-wheel drive system. The engagement of such a drive may be achieved by a controlled clutch which engages the auxiliary drive to the second axle. 
     It is also known to provide a differential with a means for inhibiting the differential action thereof. The differential action may be inhibited to the extent that it is completely locked, i.e., no relative rotation is possible between its two outputs. This is carried out to overcome problems with uneven traction surfaces such as where a wheel or wheels driven by one of the differential outputs is on a slippery surface and the wheel or wheels driven by the other of the outputs is on a surface which is not slippery. Under these conditions, a differential without any means for inhibiting or locking its differential action directs only a small torque to both wheels, limited by that torque transmitted by the slipping wheel thus potentially immobilizing the vehicle. 
     On existing systems which incorporate both an auxiliary drive to the second axle and a differential inhibiting means, the method of engaging these two systems would be achieved by two separate clutch systems. This method is very expensive to install, and also the engagement times for the systems are not fast enough to react to emergency situations such as when the driver is no longer in control of the motor vehicle. 
     SUMMARY OF THE INVENTION 
     The present invention provides an auxiliary drive allowing a two-wheel drive vehicle to be temporarily converted into a four-wheel drive vehicle, while also inhibiting the differential action of the differential on the auxiliary drive to cope with the aforementioned problems on uneven traction surfaces. 
     According to one embodiment of the invention, a differential is provided comprising: a driving member; an input member; first and second rotatable output members; differential gearing operable between the input member and the first and second output members, for transmitting rotation from the input member to the first and second output members and providing for differential rotation of the first and second output members relative to one another; an engaging device operable to establish a driving connection between the driving member and the input member; an inhibiting device operable to inhibit relative rotation between the first and second output members; and an actuating device for causing the operation of the engaging device and the inhibiting device. 
     In another embodiment, the actuating device is adapted to cause sequential operation of the engaging device and the inhibiting device. 
     In a further embodiment, the engaging device is operated to establish the driving connection between the driving member and the input member prior to the inhibiting device being operated ultimately to lock the first and second rotatable output members so that there is no relative motion therebetween. 
     The engaging device and the inhibiting device may be contained within the input member. The inhibiting device may be a first clutch mechanism such as a multi-plate clutch pack. The engaging device may be a second clutch mechanism such as a multi-plate clutch pack. 
     In a further embodiment, the actuating device comprises an electric rotational actuator such as an electric motor, and an actuator member operable on the engaging device. 
     In order for the engaging device to be operated prior to the inhibiting device, the operation of the inhibiting device by the actuating device may be effected through a first spring, exerting a force which has to be overcome before the inhibiting device is operated. In one example, the engaging device is operated by the exertion of a force thereon by the actuator, which force is reacted against the first spring. 
     In another embodiment, the force to operate the engaging device is exerted through a second spring. The inhibiting device is operated by the exertion of a force thereon by the engaging device through the first spring and against the input member. 
     When the operation of the actuator member is partially reversed, the first spring may return the inhibiting device to its original position, thereby allowing the first and second output members to rotate relative to one another. 
     When the operation of the actuator member is completely reversed to its original position, the second spring may return the engaging device to its original condition, thereby disconnecting the driving connection between the driving member and the input member. 
     The first spring can have a higher stiffness than the second spring, thereby allowing the engaging device to establish a driving connection between the driving member and the input member with enough force to transmit torque from the driving member to the input member, without engaging the inhibiting device. 
     To allow smooth and sequential engagement of the engaging device and the inhibiting device, the actuating device may be a ball ramp actuator. 
     In another aspect of the invention, the drive to the driving member is an electric motor. 
     Other advantages and features of the invention will also become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention. 
     In the drawings: 
     FIG. 1 is a cross-section through a differential in accordance with one embodiment of the invention; 
     FIG. 2 is an end face view of the actuator member in accordance with one embodiment of the invention; and 
     FIG. 3 is a schematic view of a vehicle including a differential in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 there is shown a differential comprising a torque input member in the form of a spur gear  10  connected to a sleeve  12 . The spur gear  10  and the sleeve  12  are supported in a housing (not shown) by bearings  14 ,  16  disposed either side of the spur gear  10  and fixed to the sleeve  12 . The bearings  14 ,  16  provide for the rotation of the spur gear  10  and the sleeve  12  about an axis  28 . 
     An input member in the form of a differential carrier  18  is arranged in line with the spur gear  10  and the sleeve  12  and supported, for rotation about the axis  28 , in the housing by bearings  24 ,  26  respectively on a spigot  20  at one end of the carrier  18  and a bearing seating  22  on the outside of the carrier  18 . 
     Two bevel side gears  30 ,  32  are supported in the carrier  18  for relative rotation therewithin, and mesh with bevel differential gears  34 ,  36  rotatably carried by a transverse pin or shaft  38  held in the carrier  18 . The interior of the side gear  30  is splined at  40  for torque-transmitting connection with a first output member in the form of an output shaft  42 . Likewise, the side gear  32  is also splined at  44 , for torque-transmitting connection with a second output member in the form of an output shaft  46 . The output shaft  42  extends through the sleeve  12  and is supported within the sleeve  12  by bushes  47   a,    47   b.  The output shaft  46  extends through the spigot  20  of the carrier  18 . 
     Within a portion  48  of the carrier  18  there is an annular chamber indicated generally at  50 , with an axially facing annular end wall  52 . Adjacent to the wall  52  is an inhibiting device in the form of a clutch pack  54 . The clutch pack  54  comprises a plurality of annular plates  56  rotationally fast with the side gear  30  and a plurality of annular plates  58  respectively interposed between adjacent plates  56  and rotationally fast with the carrier  18 . The plates  54 ,  56  are axially moveable and are able to be urged together into frictional engagement with one another. Other mechanisms for rotationally locking the carrier  18  and side gear  30  are also contemplated for the inhibiting member. 
     Adjacent to the clutch pack  54  is an engaging device in the form of a further clutch pack  60  with a plurality of annular plates  62  rotationally fast with the sleeve  12  and a plurality of annular plates  64  respectively interposed between the plates  62  and rotationally fast with the carrier  18 , these plates being frictionally engageable with one another. Other apparatus for rotationally locking the carrier  18  and the sleeve  12  are also contemplated for the engaging mechanism. 
     Inbetween the dutch packs  54  and  60  is an annular engaging member  66 . which is urged away from the wall  52  by a spring  68  in the form of a Beileville spring. The spring  68  reacts against circlip  70  contained within a circumferential recess on the sleeve  12 . Adjacent to the outermost end of the clutch pack  60  is a further annular engaging member  72 , which reacts against a second spring  74 , whose inner diameter abuts a shoulder on the sleeve  12 . The spring  74  is also shown as a Belleville spring. The second spring  74  has a lower stiffness than the first spring  68 . 
     Neighboring the bearing  16  is a thrust bearing  83 , which abuts a circlip  85  engaging a recess on the sleeve  12  between the bearing  16  and the bearing  83 . Axially facing, and in contact with, the bearing  83  is an actuator member in the form of a ball ramp actuator  75 . The actuator  75  comprises an annular disc  76 , an axial view of which is shown in FIG. 2, and six balls  80 . The disc  76  has six recesses  78 , which are curvilinear and spiral outwardly from the center of the disc  76 . The depth of each recess  78  decreases as it spirals away from the center of the disc. 
     Adjacent to the disc  76  is a further disc  81  with spiral recesses which face the recesses  78  and spiral in the opposite direction thereto. The recesses  78  have a cross-section conforming to the cross-section of the balls  80 . Each ball  80  is held in a facing pair of recesses  78  in the disc  76  and the disc  81 . Abutting the disc  81  is a further thrust bearing  82  which also abuts the engaging member  72 . Disc  81  is held against rotational movement by any one of several known mechanisms. 
     The disc  76  has a toothed profile around its circumference, which provides for rotational connection of the disc  76  to an electric actuator motor  86 , through reduction gears  84 . 
     When the disc  76  is rotated by the motor  86 , the balls  80  move generally radially outwardly along their respective facing pairs of recesses  78 . As the recesses  78  decrease in depth, the disc  81  is displaced axially in the direction of the axis  28  towards the carrier  18 . The thrust bearings  82 ,  83  allow for the carrier  18  and the sleeve  12  to rotate about the axis  28  whilst the actuator member  75  is stationary. 
     Although the actuator assembly  75  has been shown as a ball-ramp actuator, other mechanisms could also be used to provide axial thrust in response to rotational movement. For example, a cammed disc arrangement or cam-follower arrangement could alternatively be used. 
     Referring to FIG. 3 of the drawings, there is schematically shown an embodiment of the invention applied to a two-wheel drive vehicle as an electrical four-wheel drive add-on or “hang-on” system. The vehicle is diagrammatically indicated at  100 , and has front wheels  101 ,  102  driven from an engine/gearbox unit  103  by half shafts  104 ,  105 . It has rear wheels  106 ,  107  connected by respective half shafts  108 ,  109  to the output shafts  42  and  46  of the differential in accordance with the invention, indicated at  110 . The spur gear  10  of the differential  110  is connected through appropriate gearing to an electric drive motor  111 , which thus applies torque to the sleeve  12 . 
     In conditions where two-wheel drive of the front wheels  101 ,  102  does not give sufficient traction, it may be desirable to drive the rear wheels  106 ,  107  of the vehicle, by the electric motor  111 . This is carried out by engaging the clutch pack  60 , to drivingly connect the sleeve  12  and the differential carrier  18 . This engagement is carried out by the motor  86  rotating the annular disc  76  about the axis  28 , so that the balls  80  move generally radially outwardly along their respective pair of facing recesses  78 , which produces an axial displacement of the disc  81 . The disc  81  axially displaces the engaging member  72 , against the action of the relatively light Belleville return spring  74 , into contact with the outermost annular plate  62  or  64 . This causes the annular plates  62 ,  64  to frictionally engage with one another, as the plate  62  or  64  closest to the engaging member  66  is restricted from moving axially by the engaging member  66  and the relatively heavy Belleville return spring  68 . As the plates  62 ,  64  frictionally engage they progressively inhibit the relative rotation of the carrier  18  and the sleeve  12  until there is no relative rotation between the carrier  18  and the sleeve  12 . Hence, this engagement of the clutch pack  60  provides a rotationally fast connection between the sleeve  12  and the carrier  18 , thereby providing for driving both output shafts  42  and  46  by the electric motor  86 , connected to spur gear  10 , while also providing for differential rotation between the two shafts  42 ,  46 . 
     However, if the rear wheels of the vehicle are on uneven traction surfaces relative to one another (i.e. if one wheel has more grip than the other wheel), it may be useful to inhibit the differential action so that a higher torque is applied through one output shaft  42  or  46  directing towards the wheel having the most grip. By rotating the disc  76  further in the same direction as the previous step, the disc  76  causes the balls  80  to rotate further around their respective pair of facing recesses  78 , which further axially displaces the disc  81  along the axis  28 . This further displacement of the disc  81  overcomes the reaction of the heavy Belleville return spring  68  and hence causes the annular plates  54 ,  56  to frictionally engage with one another as the plate  54  or  56  closest to the wall  52  is restricted from moving axially by the wall  52 . 
     As the plates  54 ,  56  frictionally engage they progressively inhibit relative rotation between the carrier  18  and side bevel gear  30  until the carrier  18 , side gear  30  and the side gear  32  rotate together. Therefore the engagement of the clutch pack  54  provides, eventually, depending on the relative grip of the left and right wheel with the road, a rotationally fast connection between the sleeve  12  and the bevel gear  30 , thereby redistributing the torque so that a high torque is applied to the wheel on the gripping surface. 
     Once both the wheels are on even traction surfaces relative to one another, the actuator motor  86  can be reversed slightly, thereby allowing the spring  68  to move the disc  81  in the opposite direction to its original position and hence disengage the clutch pack  54 . This will allow for differential rotation of the output shafts  42 ,  46 . If it is also decided that four-wheel drive is not needed, the actuator motor  86  can be reversed further still, thereby allowing the spring  74  to move the disc  81  back to its original position and hence disengage the clutch pack  60 , thereby disengaging the driving connection between the sleeve  12  and the carrier  18 . 
     While the invention has been described in connection with one or more embodiments, it should be understood that the invention is not limited to those embodiments. Thus, the invention covers all alternatives, modifications, and equivalents as may be included in the spirit and scope of the appended claims.

Technology Classification (CPC): 8