Abstract:
An electromagnetic clutch for motor vehicle drive line applications includes a friction clutch pack, a ball ramp actuator and an electromagnetic coil. The ball ramp actuator includes a pair of circular members having opposed pairs of arcuate recesses receiving load transferring members and a torsion spring which provides a restoring force to return the circular members to their center, disengaged positions. Improved clutch control, particularly during disengagment, is achieved by this ball ramp actuator configuration.

Description:
BACKGROUND  
         [0001]    The invention relates generally to electromagnetic clutches for use in motor vehicle drive line components such as transfer cases, transmissions and the like and more particularly to an electromagnetic clutch having a ball ramp actuator and a torsion spring operably disposed between components of the ball ramp actuator.  
           [0002]    Electromagnetic clutches utilizing ball ramp actuators are especially suited for use in motor vehicle driveline components. A typical ball ramp actuator electromagnetic clutch utilizes a pair of opposed circular plates defining opposed pairs of symmetrically ramped recesses. Adjacent the circular plates is a friction clutch pack having an input driven by the primary transfer case output shaft and a secondary output to the secondary driveline which is driven by the output of the friction clutch pack assembly. Retardation of motion of one of the circular plates causes axial separation thereof and such axial separation compresses the friction clutch pack and transfers torque from the input to the output.  
           [0003]    One of the foremost advantages of a ball ramp actuated electromagnetic clutch assembly is its relatively low power consumption. Since the electromagnetic coil must only retard one of the circular plates to commence engagement of the clutch, the clutch coil and its current draw may be significantly smaller than a clutch wherein the electrical energy is utilized as the sole and direct energy source which engages the clutch.  
           [0004]    While such clutch engagement can generally be accurately controlled by the controlled application of electrical energy to the electromagnetic coil, occasionally the deactivation of the clutch may be subject to mechanical and magnetic hysteresis. That is, clutch disengagement may not accurately track or correspond to decreasing clutch current. This may be the result of a failure by the clutch plates to modulatably disengage or may be the result of residual magnetism in the clutch components or other operating anomalies, which cause non-linear or non-proportional disengagement.  
           [0005]    The present invention is directed to improvements in the art of ball ramp clutch actuators for electromagnetic clutches which improve the proportional response of such clutches, particularly during deactivation.  
         SUMMARY  
         [0006]    An electromagnetic clutch for motor vehicle drive line applications includes a friction clutch pack, a ball ramp actuator and an electromagnetic coil. The ball ramp actuator includes a pair of circular members having opposed pairs of arcuate recesses receiving load transferring members and a torsion spring which provides a restoring force to return the circular members to their center, disengaged positions. Improved clutch control, particularly during disengagment, is achieved by this ball ramp actuator configuration.  
           [0007]    Thus it is an object to the present invention to provide an electromagnetic clutch having a ball ramp actuator providing improved modulating control.  
           [0008]    It is a further object to the present invention to provide an electromagnetic clutch having a ball ramp actuator including opposed circular members which are connected by torsion spring.  
           [0009]    It is a still further object to the present invention to provide an electromagnetic clutch having a ball ramp actuator including circular members with opposed recesses and which are biased to their center positions by a torsion spring.  
           [0010]    Further objects and advantages to the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like referenced numbers referred to the same component, element or feature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a diagrammatic, plan view of a four-wheel drive motor vehicle having a transfer case incorporating an electromagnetic clutch assembly according to the present invention;  
         [0012]    [0012]FIG. 2 is a side, elevational view in partial section of a transfer case incorporating an electromagnetic clutch assembly according to the present invention;  
         [0013]    [0013]FIG. 3 is an enlarged, fragmentary sectional view of a transfer case incorporating an electromagnetic clutch assembly according to the present invention;  
         [0014]    [0014]FIG. 4 is a flat pattern development of a section of one clutch ball and associated recesses incorporated in an electromagnetic clutch assembly according to the present invention, taken along line  4 - 4  of FIG. 3.  
         [0015]    [0015]FIG. 5 is an enlarged, perspective view with a portion broken away of components of an electromagnetic clutch assembly according to the present invention; 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    Referring now to FIG. 1, a four-wheel vehicle drive train is diagrammatically illustrated and designated by the reference number  10 . The four-wheel vehicle drive train  10  includes a prime mover  12  which is coupled to and directly drives a transmission  14 . The transmission  14  may either be an automatic or manual type. The output of the transmission  14  directly drives a transfer case assembly  16  which provides motive power to a primary or rear drive line  20  comprising a primary or rear prop shaft  22 , a primary or rear differential  24 , a pair of live primary or rear axles  26  and a respective pair of primary or rear tire and wheel assemblies  28 .  
         [0017]    The transfer case assembly  16  also selectively provides motive power to a secondary or front drive line  30  comprising a secondary or front prop shaft  32 , a secondary or front differential assembly  34 , a pair of live secondary or front axles  36  and a respective pair of secondary or front tire and wheel assemblies  38 . The front tire and wheel assemblies  38  may be directly coupled to a respective one of the pair of front axles  36  or, if desired, a pair of manually or remotely activateable locking hubs  42  may be operably disposed between the pair of front axles  36  and a respective one of the tire and wheel assemblies  38  to selectively connect same. Finally, both the primary drive line  20  and the secondary drive line  30  may include suitable and appropriately disposed universal joints  44  which function in conventional fashion to allow static and dynamic offsets and misalignments between the various shafts and components. A control console  46  which is preferably disposed within convenient reach of the vehicle operator includes a switch or a plurality of individual switches or push buttons  48  which facilitate selection of the operating mode of the transfer case assembly  16  as will be further described below.  
         [0018]    The foregoing and following description relates to a vehicle wherein the rear drive line  20  functions as the primary drive line, i.e., it is engaged and operates substantially all the time and, correspondingly, the front drive line  30  functions as the secondary drive line, i.e., it is engaged and operates only part-time or in a secondary or supplemental fashion, such a vehicle commonly being referred to as a rear wheel drive vehicle.  
         [0019]    These designations “primary” and “secondary” are utilized herein rather than “front” and “rear” inasmuch as the invention herein disclosed and claimed may be readily utilized in transmissions and transfer cases wherein the primary drive line  20  is disposed at the front of the vehicle and the secondary drive line  30  is disposed at the rear of the vehicle. Such designations “primary” and “secondary” thus broadly and properly characterize the function of the individual drive lines rather than their specific locations.  
         [0020]    Referring now to FIGS. 1 and 2, the transfer case assembly  16  incorporating the present invention includes a multiple piece, typically cast, metal housing assembly  50  having planar and circular sealing surfaces, openings for shafts and bearings and various recesses, shoulders, flanges, counterbores and the like to receive various components and assemblies of the transfer case assembly  16 . An input shaft  52  includes female or internal splines or gear teeth  54  or other suitable structure which drivingly couple an output of the transmission  14  illustrated in FIG. 1 to the input shaft  52 . The input shaft  52  is rotatably supported externally by an anti-friction bearing such as the ball bearing assembly  56  and internally by an anti-friction bearing such as the roller bearing assembly  58 . The roller bearing assembly  58  is disposed upon a reduced diameter portion of an output shaft  60 . An oil seal  62 , positioned between the input shaft  52  and the housing assembly  50 , provides an appropriate fluid tight seal therebetween. The opposite end of the output shaft  60  is supported by an anti-friction bearing such as a ball bearing assembly  64 . An end cap or seal  66  closes off the end of an axial passageway  68  in the output shaft  60 . A gerotor pump P will typically be utilized to provide a flow of lubricating and cooling fluid to the axial passageway  68  which is thence distributed through a plurality of radial ports in the output shaft  60  to the components of the transfer case assembly  16 .  
         [0021]    The transfer case assembly  16  also includes a two-speed planetary (epicyclic) gear assembly  70  disposed generally about the input shaft  52 . The planetary gear assembly  70  includes a sun gear collar  72  having a plurality of external gear teeth  74  and a plurality of internal splines or gear teeth  76 . The internal splines or gear teeth  76  are engaged by complementary external splines or gear teeth  78  formed on the input shaft  52 . Radially aligned with the sun gear  72  and its teeth  74  is a ring gear  80  having internal gear teeth  82 . The ring gear  80  is fixedly retained within the housing assembly  50  by any suitable retaining structure such as a projection or lip  84  formed in the housing assembly  50  and a cooperating snap ring  86 . A plurality of pinion gears  88  are rotatably received upon a like plurality of anti-friction bearings such as roller bearings  90  which, in turn, are supported and located by a like plurality of stub shafts  92 . The plurality of stub shafts  92  are mounted within and secured to a planet carrier  94 . The planet carrier  94  includes a plurality of internal or female splines or gear teeth  96 . The planetary gear assembly  70  is more fully described in co-owned U.S. Pat. No. 4,440,042 which is herein incorporated by reference.  
         [0022]    The planetary gear assembly  70  also includes a dog clutch or clutch collar  100  defining elongate internal splines or gear teeth  102 . The internal splines or gear teeth  102  of the clutch collar  100  are slidably received upon a complementary plurality of external splines or gear teeth  104  on the output shaft  60 . The clutch collar  100  thus rotates with the output shaft  60  but may translate bi-directionally therealong. The clutch collar  100  also includes external splines or gear teeth  106  on one end which are in all respects complementary to the internal splines or gear teeth  96  on the planet carrier  94 . The end of the clutch collar  100  opposite the gear teeth  96  defines a circumferentially and radially extending flange  108 .  
         [0023]    The clutch collar  100  is capable of three positions and operational modes. In the lower portion of FIG. 2, the clutch collar  100  is illustrated in its leftmost or direct drive position. Direct drive is achieved when the internal splines or gear teeth  102  of the clutch collar  100  engage the external splines or gear teeth  78  on the input shaft  52  thereby directly coupling the input shaft  52  to the output shaft  60  and providing direct or high gear drive therebetween.  
         [0024]    When the clutch collar  100  is moved to the right from the position illustrated in the lower portion of FIG. 2, to the position illustrated in the upper portion of FIG. 2, the speed reduction achieved by the planetary gear assembly  70  is engaged through engagement of the external splines or gear teeth  106  on the clutch collar  100  with the internal splines or gear teeth  96  on the planet carrier  94 . So engaged, the planetary gear assembly  70  is active and provides a speed reduction, typically in the range of from 3:1 to 4:1 between the input shaft  52  and the output shaft  60 . Between these two positions is a neutral position. In the center, neutral position both the input shaft  52  and the planet carrier  94  are disconnected from the output shaft  60  and no power is transmitted therebetween.  
         [0025]    Referring to FIG. 2, the position of the clutch collar  100  is commanded by an electric shift control assembly  160 . The shift control assembly  160  includes an electric drive motor  164  which drives a gear train (not illustrated) disposed in a housing  166 . The housing supports an output shaft  168 . The output shaft  168  is coupled by interengaging splines (not illustrated) to a rotatable shift rail  180 . An oil seal  184  provides a suitable fluid-tight seal between the shift rail  180  and the housing assembly  50 .  
         [0026]    The rotatable shift rail  180  extends across the housing assembly  50  and its opposite end is seated within a suitable counterbore  186  formed in the housing assembly  50 . A pair of spaced-apart radially disposed stanchions or stub shafts  190  are securely seated within radial passageways  192  in the shift rail  180 . The stub shafts  190  include enlarged heads which retain freely rotatable cam followers or rollers  194  thereon. Disposed between the spaced-apart stanchions or stub shafts  190  and the rollers  194  is a shift fork assembly  200 . The shift fork assembly  200  includes a body  202  having a through passageway  204  which is sized to just freely rotatably receive the shift rail  180 . At each end of the shift fork body  202  is a helical cam  206 . The helical cams  206  are correspondingly disposed such that the axial distance from corresponding points on their surfaces is just slightly less than the distance between the inside surfaces of the rollers  194 . An axially extending face or shoulder  208  which represents a discontinuity in the helical cams  206  acts as a positive stop in one direction of rotation as it engages one of the rollers  196 .  
         [0027]    The shift fork assembly  200  also includes an obliquely extending web  210  which terminates in a discontinuous, semi-circular channel or groove  214 . The semi-circular channel or groove  214  receives and engages the flange  108  of the clutch collar  100 . Such engagement inhibits rotation of the shift fork assembly  200 . Accordingly, as the shift rail  180  and the cam followers or rollers  194  rotate, the shift fork assembly  200  and specifically the clutch collar  100  translate axially and bi-directionally. Such translation effects selective engagement of the clutch collar  100  and selection of high gear, neutral or low gear as previously explained.  
         [0028]    Referring now to FIGS. 2, 3,  4  and  5  the transfer case assembly  16  also includes an electromagnetically actuated disc pack type clutch assembly  220 . The clutch assembly  220  is disposed about the output shaft  60  and includes a circular drive member  222  coupled to the output shaft  60  through, for example, a splined interconnection. The circular drive member  222  includes a plurality of circumferentially spaced-apart recesses  226  in the shape of an oblique section of a helical torus. Each of the recesses  226  receives one of a like plurality of load transferring balls  228 . The circular drive member also includes a radially offset, axially extending first rectangular slot  230 .  
         [0029]    A circular driven member  232  is disposed adjacent the circular drive member  222  and includes a like plurality of opposed recesses  234  defining the same shape as the recesses  226 . The oblique side walls of the recesses  226  and  234  function as ramps or cams and cooperate with the balls  228  to drive the circular members  222  and  232  apart in response to relative rotation therebetween. It will be appreciated that the recesses  226  and  234  and the load transferring balls  228  may be replaced with other analogous mechanical elements which cause axial displacement of the circular members  222  and  232  in response to relative rotation therebetween. For example, tapered rollers disposed in complementarily configured conical helices may be utilized.  
         [0030]    The circular driven member  232  includes a counterbore or reentrant region  236  which receives a coiled torsion spring  238 . The configuration of the torsion spring  238  is preferably of rectangular cross-section and it defines a plurality of turns or coils terminating in a pair of axially extending lugs or tangs  242 . One of the tangs  242  is received within a first rectangular slot  230  formed in the circular drive member  222  and the other tang  242  is received within a second rectangular slot  244  formed in the circular driven member  234 .  
         [0031]    The circular driven member  232  extends radially outwardly and is secured to a soft iron rotor  252 . An armature  254  is disposed adjacent the face of the rotor  252 . The rotor  252  surrounds an electromagnetic coil  256  on three sides.  
         [0032]    The electromagnetic coil  256  is provided with electrical energy preferably from a pulse width modulation (PWM) control through an electrical conductor  258 . The pulse width modulation scheme increases or decreases the average current to the electromagnetic coil  256  of the electromagnetically actuated disc pack type clutch assembly  220 , as will be more fully described below, by increasing or decreasing the on time (duty cycle) of a drive signal. It will be appreciated that other modulating control techniques may be utilized to achieve engagement and disengagement of the electromagnetic disc pack type clutch assembly  220 .  
         [0033]    Providing electrical energy to the electromagnetic coil  256  causes magnetic attraction of the armature  254  with the rotor  252 . This magnetic attraction results in frictional contact of the armature  254  to the rotor  252 . When the output shaft  60  is turning at a different speed than the armature  254  this frictional contact results in a frictional torque being transferred from the output shaft  60 , through the circular drive member  222 , through the load transferring balls  228  and to the circular driven member  232 . The resulting frictional torque causes the balls  228  to ride up the ramps of the recesses  226  and  234 , causing axial displacement of the circular drive member  222 . Axial displacement of the circular drive member  222  translates an apply plate  262  axially toward a disc pack clutch assembly  270 . A compression spring  272  which may comprise a stack of Belleville washers provides a restoring force which biases the circular drive member  222  toward the circular driven member  232 .  
         [0034]    An important design consideration of the recesses  226  and  234  and the balls  228  is that the geometry of their design and the design of the compression spring  272  and the clearances in the disc pack assembly  270  ensure that the electromagnetic clutch assembly  220  is not self-locking. The electromagnetic clutch assembly  220  must not self-engage but rather must be capable of controlled, proportional engagement and torque transfer in direct response to the modulating control input.  
         [0035]    The torsion spring  238  provides a defined, repeatable restoring torque to the first and second circular members  222  and  234  which improves operation, especially when the current to the electromagnetic coil  256  is being reduced, compression of the disc pack clutch assembly  270  is being relaxed and torque transferred through the disc pack clutch assembly is diminishing. The restoring torque provided by the torsion spring  238  dampens the apply response of the disc pack clutch assembly  220  and reduces hysteresis during release thereof. The spring rate of the torsion spring  238  will be determined both experimentally and empirically to provide optimum operation and smooth and controlled, i. e., corresponding to the modulating current supplied to the electromagnetic coil  256 , clutch engagement and disengagement.  
         [0036]    Referring now to FIGS. 2 and 3, the disc pack clutch assembly  270  includes a first plurality of smaller friction plates or discs  274 . The first plurality of discs  274  are coupled by interengaging splines to a clutch hub  276  which is coupled to the output shaft  60  for rotation therewith. A second plurality of larger friction plates or discs  278  are coupled to an annular housing  280  by interengaging splines for rotation therewith and are interleaved with the first plurality of friction discs  274 .  
         [0037]    The annular housing  280  is disposed concentrically about the output shaft  60  and is coupled to a chain drive sprocket  282  by a plurality of interengaging splines or lugs and recesses  284 . The chain drive sprocket  282  is freely rotatably disposed on the output shaft  60  and is supported by a journal or needle bearing assembly  286 . When the clutch assembly  220  is engaged, it transfers energy from the output shaft  60  to the chain drive sprocket  282 . A drive chain  288  is received upon the chain drive sprocket  282  and engages and transfers rotational energy to a driven chain sprocket  290 . The driven chain sprocket  290  is coupled to a front (secondary) output shaft  292  of the transfer case assembly  16  by interengaging splines  294 .  
         [0038]    The foregoing disclosure is the best mode devised by the inventors for practicing this invention. It is apparent, however, that apparatus incorporating modifications and variations will be obvious to one skilled in the art of shift control mechanisms. Inasmuch as the foregoing disclosure presents the best mode contemplated by the inventors for carrying out the invention and is intended to enable any person skilled in the pertinent art to practice this invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.