Abstract:
An electromagnetic clutch assembly for transfer cases and other motor vehicle powertrain applications includes a friction disc clutch pack and an electromagnetic operator that applies compressive force to the clutch pack. The electromagnetic operator includes an electromagnetic coil, a rotor partially surrounding the coil, an armature and a ball ramp operator assembly that provides compressive force when the coil is energized. The rotor and armature include aligned paths or tracks of arcuate slots separated by webs or spokes which create a sinuous magnetic flux path and improve performance. The number of slots and thus the number of spokes in the rotor and armature paths are unequal. This inequality greatly reduces pulsations caused by small fluctuations in clutch engagement associated with the repeated, simultaneous alignment and non-alignment of the spokes in a rotor and armature having equal numbers of spokes. This results in improved smoothness of operation and reduces noise and vibration.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to electromagnetic clutches for use in motor vehicle powertrain components and more specifically to an electromagnetic clutch having a rotor and armature with an unequal or asymmetric spoke patterns. 
     The popularity of sport utility vehicles which typically include selectable four-wheel drive systems and other vehicles such as minivans which offer similar part-time four-wheel drive systems have resulted in extensive development work on such systems and improved sophistication thereof. Whereas several years ago part-time, i.e., operator selectable, four-wheel drive systems were typically only utilized in pickup trucks and other vehicles actually likely to be used off-road and on rough terrain, four-wheel drive systems are now provided in vehicles which are outfitted comparably to near luxury vehicles and include as standard equipment such features as power windows, air conditioning, sunroofs, leather seats and sophisticated audio systems. Accompanying such up-market vehicles is an emphasis on smooth and quiet highway performance where such vehicles are most typically utilized. 
     Accordingly, extensive effort has been expended by purveyors in this field to provide equipment such as transfer cases which satisfy both the mechanical demands of the particular four-wheel drive system as well as the convenience and performance parameters demanded by purchasers of such vehicles. The present invention is directed to an electromagnetic clutch assembly which provides improved smooth and quiet operation. 
     SUMMARY OF THE INVENTION 
     An electromagnetic clutch assembly for transfer cases and other motor vehicle powertrain applications includes a friction disc clutch pack and an electromagnetic operator that applies compressive force to the clutch pack. The electromagnetic operator includes an electromagnetic coil, a rotor partially surrounding the coil, an armature and a ball ramp operator assembly that provides compressive force when the coil is energized. The rotor and armature include aligned paths or tracks of arcuate banana slots separated by webs or spokes which create a sinuous magnetic flux path and improve performance. The number of slots and thus the number of webs or spokes in the rotor and armature paths are unequal. This inequality greatly reduces pulsing associated with the repeated, simultaneous alignment and non-alignment of spokes in rotors and armatures with equal numbers of spokes and results in improved smoothness of operation and reduced noise and vibration. 
     It is thus an object of the present invention to provide an electromagnetic clutch assembly which provides improved smooth, non-pulsing operation. 
     It is a further object of the present invention to provide an electromagnetic clutch assembly having a rotor and armature with asymmetric banana slot and spoke patterns. 
     It is a still further object of the present invention to provide an electromagnetic clutch assembly wherein a sinuous flux path and a rotor and armature are created by banana slots with unequal or asymmetric spoke or web patterns. 
     It is a still further object of the present invention to provide an electromagnetic clutch assembly wherein a rotor and armature have banana slots and a spoke pattern wherein only a single pair of spokes in the rotor and armature are in alignment at one time. 
    
    
     Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers refer to the same component, element or feature. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic, plan view of a four-wheel drive motor vehicle powertrain having a transfer case which incorporates the present invention; 
     FIG. 2 is a full, sectional view of a transfer case having an electromagnetic clutch assembly incorporating the present invention; 
     FIG. 3 is a flat pattern development of a portion of the ball ramp operator taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is a front, elevational view of a rotor of an electromagnetic clutch assembly according to the present invention; 
     FIG. 5 is a front, elevational view of an armature of an electromagnetic clutch according to the present invention; 
     FIG. 6 is a fragmentary, sectional view of an armature of an electromagnetic clutch according to the present invention taken along line  6 — 6  of FIG. 5; 
     FIG. 7 is a fragmentary, sectional view of an armature of an electromagnetic clutch according to the present invention taken along line  7 — 7  of FIG. 5; and 
     FIG. 8 is a rear, elevational view of an armature and rotor of an electromagnetic clutch according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 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 . 
     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 front axles  36  or, if desired, a pair of manually or remotely activatable locking hubs  42  may be operably disposed between the 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. 
     Mounted within the passenger cabin in a location proximate the driver of the motor vehicle is a control assembly  50  having a plurality of push buttons  52  which correspond to the various driver selectable operating modes of the transfer case assembly  16  such as high, neutral and low. Alternatively, the push buttons  52  may be replaced by a rotary switch or other analogous driver selectable input device. The control assembly  50  may include various electronic computation, logic and output devices as well as devices which receive signals and provide logic decisions based upon feedback or signals provided by components of the transfer case assembly  16 . 
     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. 
     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 characterizing the function of the individual drive lines rather than their specific locations. 
     Referring now to FIGS. 1 and 2, the transfer case assembly  16  incorporating to the present invention includes a multiple piece housing assembly  60  having a plurality of planar sealing surfaces, openings for shafts and bearings and various recesses, shoulders, counterbores and the like to receive various components or assemblies of the transfer case assembly  16 . An input shaft  62  includes female or internal splines or gear teeth  64  or other suitable coupling structures which drivingly couple the output of the transmission  14  illustrated in FIG. 1 to the input shaft  62 . The input shaft  62  is rotatably supported at one end by an anti-friction bearing such as the ball bearing assembly  66  and at its opposite end by an internal anti-friction bearing such as the roller bearing assembly  68 . The roller bearing assembly  68  is disposed upon a portion of a stepped output shaft  70 . A suitable oil seal  72 , positioned between the input shaft  62  and the housing assembly  60 , provides an appropriate fluid tight seal therebetween. The opposite end of the output shaft  70  is supported by an antifriction bearing such as the ball bearing assembly  74  and include a flange  76  which may be a portion of a universal joint  44  or may be secured to associated drive line components such as the primary prop shaft  22 . A suitable oil seal  78 , disposed between the flange  76  and the housing assembly  60  provides an appropriate fluid tight seal therebetween. 
     Referring now to FIGS. 2 and 3, the transfer case assembly  16  includes a two-speed planetary gear drive assembly  80  disposed about the input shaft  62 . The planetary gear drive assembly  80  includes a sun gear  82  having a plurality of female or internal splines or gear teeth  84  which engage a complementary plurality of male splines or gear teeth  86  on the input shaft  62 . The sun gear  82  is thus coupled to the input shaft  62  and rotates therewith. The sun gear  82  includes external or male gear teeth  88  about its periphery. Radially aligned with the sun gear  82  and its teeth  84  is a ring gear  90  having inwardly directed gear teeth  92 . The ring gear  90  is retained within the housing assembly  60  by a cooperating circumferential groove and snap ring assembly  94 . A plurality of pinion gears  96  are rotatably received upon a like plurality of stub shafts  98  which are mounted within and secured to a planet carrier  100 . The planet carrier  100  includes a plurality of female or internal splines or gear teeth  102  disposed generally adjacent the male splines or gear teeth  86  on the input shaft  62 . The planetary gear assembly  80  is more fully described in co-owned U.S. Pat. No. 4,440,042 which is herein incorporated by reference. 
     The planetary gear drive assembly  80  also include a dog clutch or clutch collar  104  defining female or internal splines or gear teeth  106  which are axially aligned with and, in all respects, complementary to the male splines or gear teeth  86  on the input shaft  62 . The clutch collar  104  and its internal splines or gear teeth  106  are slidably received upon a complementary plurality of male or external splines or gear teeth  108  on the stepped output shaft  70 . The clutch collar  104  thus rotates with the output shaft  70  but may translate bidirectionally along it. The clutch collar  104  also includes male or external splines or gear teeth  110  on one end which are in all respects complementary to the female splines or gear teeth  102  on the planet carrier  100 . 
     Finally, the dog clutch or clutch collar  104  includes a pair of radially extending, spaced-apart flanges  114  on its end opposite the splines or gear teeth  110  which define a circumferential channel  116 . The channel  116  receives a complementarily configured semi-circular throat or yoke  118  of a shift fork  120 . The shift fork  120  includes a through passageway defining female or internal threads  122  which engage complementarily configured male or external threads  124  on a rotatable shift rail  126 . The shift rail  126  is received within suitable journal bearings or bushings  128  and is coupled to and driven by a rotary electric, pneumatic or hydraulic motor  130 . The rotary motor  130  is provided with energy through a line  132 . 
     The end of the shift fork  120  opposite the semi-circular yoke  118  includes a cam  136  having a recess and a first, taller or longer lug or projection and a second, shorter lug or projection having a height greater than, or at least distinct from, the recess. A three position sensor  140  having a roller or ball actuator  144  includes proximity or position sensors such as Hall effect sensors which provide outputs in a preferably multiple conductor cable  146  defining a first signal indicating that the shift fork  120  and associated clutch collar  104  is in the neutral position illustrated in FIG. 2 that the shift fork  120  has moved to the left from the position illustrated such that the actuator  144  engages and is translated by the taller lug or projection of the cam  136  and the sensor  140  provides a signal indicating that the clutch collar  104  is in a position which selects high gear or direct drive, effectively bypassing the planetary gear assembly  80 , or, conversely, that the shift fork  120  has moved to the right from the position illustrated in FIG. 2 such that the second, shorter lug or projection of the cam  136  has engaged and translated the actuator  144  so that the sensor  140  indicates that the shift fork  120  has translated to select the low speed output or speed range of the planetary drive assembly  80 . Such translation is achieved by selective bidirectional operation of the drive motor  130  which rotates the shift rail  126  and bidirectionally translates the shift fork  120  along the male threads  124  of the shift rail  126 . 
     The transfer case assembly  16  also includes an electromagnetically actuated disc pack type clutch assembly  150 . The clutch assembly  150  is disposed about the stepped output shaft  70  and includes a circular drive member  152  coupled to the output shaft  70  through a splined interconnection  154 . The circular drive member  152  includes a plurality of circumferentially spaced apart recesses  156  in the shape of an oblique section of a helical torus. Each of the recesses  156  receives one of a like plurality of load transferring balls  158 . 
     A circular driven member  162  is disposed adjacent the circular drive member  152  and includes a like plurality of opposed recesses  156  defining the same shape as the recesses  156  in the circular drive member  152 . The oblique side walls of the recesses  156  and  166  function as ramps or cams and cooperate with the balls  158  to drive the circular members  152  and  162  apart in response to relative rotation therebetween. It will be appreciated that the recesses  156  and  166  and the load transferring balls  158  may be replaced with other analogous mechanical elements which cause axial displacement of the circular members  152  and  162  in response to relative rotation therebetween. For example, tapered rollers disposed in complementarily configured conical helices may be utilized. 
     The circular driven member  162  extends radially outwardly and is secured to a soft iron rotor  170 . The rotor  170  is disposed in opposed relationship with an armature  174 . The armature  174  includes a plurality of gear teeth or splines  176  on its periphery. The rotor  170  is U-shaped and partially surrounds a housing  178  containing an electromagnetic coil  180 . A single or two conductor cable  182  provides electrical energy to the electromagnetic coil  180 , 
     Providing electrical energy to the electromagnetic coil  180  through the cable  182  causes magnetic attraction of the armature  174  to the rotor  170 . This magnetic attraction results in frictional contact of the armature  174  with the rotor  170 . When the output shaft  70  is turning at a different speed than the armature  174  which turns at the same rotational speed as a secondary output shaft  184 , this frictional contact results in drag torque being transferred from the output shaft  70 , through the circular drive member  152 , through the load transferring balls  158  and to the circular driven member  162 . The resulting drag torque causes the balls  158  to ride up the ramps of the recesses  156  and  166  and axially displaces the circular drive member  152 . Axial displacement of the circular drive member  152  translates an apply plate  186  axially toward a disc pack clutch assembly  188 . A compression spring  190  provides a restoring force which biases the circular drive member  152  toward the circular driven member  162  and returns the load transferring balls  158  to center positions in the circular recesses  156  and  166  to provide maximum clearance and minimum friction between the components of the electromagnetic clutch assembly  150  when it is energized. 
     An important design consideration of the recesses  156  and  166  and the balls  158  is that the geometry of their design, the design of the compression spring  190  and the clearances in the clutch assembly  150  ensure that it is not self-engaging. The electromagnetic clutch assembly  150  must not self-engage but rather must be capable of modulated operation and torque transfer in direct response to a modulating input signal. 
     The disc pack clutch assembly  188  includes a plurality of interleaved friction plates or discs  192 A and  192 B. A first plurality of discs  192 A are coupled by interengaging splines  194  to a clutch hub  196  which is, in turn, coupled to the stepped output shaft  70  for rotation therewith. A second plurality of discs  192 B are coupled to an annular housing  198  by interengaging splines  202  for rotation therewith. The splines or gear teeth  176  on the armature  174  also engage the splines  202  on the annular housing  198 . 
     The annular housing  198  is disposed for free rotation about the output shaft  70  and is rotationally coupled to a chain drive sprocket  204  by a plurality of interengaging lugs and recesses  206 . The drive sprocket  204  is also freely rotatably disposed on the stepped output shaft  70 . A drive chain  208  is received upon the teeth of the chain drive sprocket  204  and engages and transfers rotational energy to a driven chain sprocket  212 . The driven chain sprocket  212  is coupled to the secondary output shaft  184  of the transfer case assembly  16  by interengaging splines  214 . 
     The transfer case assembly  16  also includes a first Hall effect sensor  220  which is disposed in proximate, sensing relationship with a plurality of teeth  222  on a tone wheel  224 . The tone wheel  224  is coupled to and rotates with the stepped output shaft  70 . A second Hall effect sensor  226  is disposed in proximate, sensing relationship with a plurality of teeth  228  of a tone wheel  230  disposed on the driven chain sprocket  212 . Preferably, the number of teeth  222  on the tone wheel  224  is identical to the number of teeth  228  on the tone wheel  230  of the driven sprocket  212  so that identical shaft speeds result in the same number of pulses per unit time from the Hall effect sensors  220  and  226 . This simplifies computations and improves the accuracy of all decisions based on such data. As to the actual number of teeth  222  on the tone wheel  224  and teeth  228  on the tone wheel  230 , it may vary from thirty to forty teeth or more or fewer depending upon rotational speeds and sensor construction. The use of thirty-five teeth on the tone wheels has provided good results with the Hall effect sensors  220  and  226 . 
     Referring now to FIG. 4, the rotor  170  of the electromagnetic disc pack type clutch assembly  150  includes a first, outer plurality or circle of through, arcuate banana slots  242  which are separated by webs or spokes  244  and a second, inner plurality or circle of through, arcuate banana slots  246  which are separated by webs or spokes  248 . As used herein, the term “banana slots” refers to a plurality of curved or arcuate slots which are arranged end-to-end to form a discontinuous circular path of open or through slots which resemble bananas. Preferably, the inner and outer pluralities of banana slots  242  and  246  include equal numbers of slots. Six of the outer banana slots  242  and six of the inner banana slots  246  have been found suitable. When six of the banana slots  242  and  246  are utilized, one of the pluralities of banana slots  242  or  246 , for example, the inner plurality of banana slots  246 , is circumferentially offset 30° from the outer plurality of banana slots  242 . In this way, the spokes or webs  244  and  248  are staggered or offset from one another. If the number of banana slots  242  and  246  is reduced to four, the webs or spokes  244  and  248  are then circumferentially offset between the inner and outer pluralities of banana slots  242  and  246  by 45°. Correspondingly, if the number of banana slots  242  and  246  is increased to eight, the webs or spokes  244  and  248  are preferably offset by 22.5°. It will be appreciated that other numbers of banana slots  242  and  246  and corresponding numbers of spokes or webs  244  and  248  will preferably be circumferentially offset correspondingly. 
     Turning now to FIGS. 5,  6  and  7 , the armature  174  is illustrated and includes a single path or circle of banana slots  252  having webs or spokes  254 . Cooperation between the banana slots  242 ,  246  and  252  provides a sinuous magnetic flux path, thereby increasing the attraction between the rotor  170  and the armature  174  for a given magnetic flux level and increasing clutch engagement and torque throughput for a given current flow, thereby reducing current consumption and energy dissipation in the electromagnetic coil  180 . Preferably, there are five banana slots  252  and five webs or spokes  254  although this number may be adjusted up or down in conjunction with the number of banana slots  242  and  246  in the rotor  170 . In accordance with the present invention, the number of banana slots  252  is not equal to the number of banana slots  242  or  246  in the rotor  170  but is preferably a number one integer larger or smaller than the number of banana slots  242  and  246  in each circle or path of the rotor  170 . That is, where N equals the number of banana slots  242  in the outer circle as well as the number of banana slots  246  in the inner circle, N±1 represents a suitable number of banana slots  252  in the armature  174  although other combinations of unequal numbers of slots  242 ,  246  and  252  may be utilized. Alternatively, the number 1 in the expression may be replaced by an odd integer such as 3 or 5. 
     As illustrated in FIG. 8, the radial location of the banana slots  252  in the armature  174  is preferably equally spaced between the two circles or paths of the banana slots  242  and  246  in the rotor  170 . As illustrated in FIG. 6, extending across each of the spokes  254 , and aligned with the banana slots  252 , is a semicircular recess or channel  256 . Aligned with each of the webs or spokes  254  is a radially extending recess or channel  258  which is illustrated in FIG.  7 . Thus, at each of the webs or spokes  254 , the armature  174  is cut away such that the face or surface of the web or spoke  254  aligned with the banana slots  252  is recessed or spaced from the face of the armature  174  and the adjacent face of the rotor  170 . 
     Turning now to FIG. 8, the utilization of a first plurality of staggered banana slots  242  and  246  and webs or spokes  244  and  248  on the rotor  170  and an unequal plurality of banana slots  252  and webs or spokes  254  in the armature  174  is graphically presented. The rotor  170  and the armature  174  are positioned such that at the top of FIG. 8, a web or spoke  258  of the armature  174  is aligned with the web or spoke  244  associated with the first or outer plurality of banana slots  242  in the rotor  170 . Inspection of the remainder of the rotor  170  and armature  174  reveals that at no other location around the rotor  170  and armature  174  are any other webs or spokes  244 ,  248  or  254  aligned. If, for example, the inner and outer pluralities of banana slots  242  and  246  were not staggered and an equal number of banana slots  242 ,  246  and  252  were formed in both paths of the rotor  170  and the single path of the armature  174 , there would be six simultaneous locations of alignment of the webs or spokes  244 ,  248  and  254 . Configuration of the rotor  170  and the armature  174  according to the present invention greatly reduces pulsations of the electromagnetic disc pack clutch assembly  150  and reduces low frequency noise and vibration associated with its operation. 
     While the foregoing description of the electromagnetic disc pack type clutch assembly  150  has been in conjunction with a transfer case assembly  16 , it will be appreciated that the electromagnetic clutch assembly  150  is suitable for application in other motor vehicle driveline components, specifically, in a twin clutch rear axle such as disclosed in co-owned U.S. Pat. No. 5,884,738 granted Mar. 23, 1999 which is incorporated herein by reference for use with a front wheel drive vehicle and in a clutch mounted at an individual wheel location for selectively coupling an axle to a tire and wheel assembly. 
     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 electromagnetic clutches. 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.