Abstract:
An electromagnetic clutch assembly having a solenoid activated ball ramp operator provides reduced power consumption and improved control. The electromagnetic clutch assembly includes a primary or pilot friction clutch pack and a secondary or main friction clutch pack. An annular solenoid coil cooperates with an annular operator or plunger. When the coil is energized, the annular plunger translates and compresses the primary or pilot friction clutch pack. Activation of the pilot clutch pack retards motion of one of the members of the ball ramp operator which, in turn, compresses a secondary or main friction clutch pack disposed across the input and output members.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates generally to electromagnetic clutches and more specifically to an electromagnetic clutch having a solenoid activated ball ramp operator. 
   Clutches which are activated or energized by electromagnetic coils are extraordinarily common components in rotary power transmission systems, both in stationary applications and in motor vehicles. Such electromagnetic clutches may be broadly characterized by whether they provide on-off energy transfer or modulating energy transfer. In the case of the former, dog clutches which may include auxiliary synchronizing devices are utilized whereas in the latter, friction clutch packs having a plurality of interleaved friction plates or discs are utilized. In either case, an electromagnetic operator which translates or compresses components of the clutch upon energization activates the clutch and upon deenergization deactivates or relaxes the clutch. 
   One of the design and operational characteristics of electromagnetic clutches which receives significant engineering attention is power consumption. It is desirable, especially in motor vehicles, to design and utilize a clutch having low power consumption. Low power consumption is desirable in and of itself but it also reduces the heat generated by the coil and lower power consumption can reduce the need for cooling the coil, can improve the service life of the coil and is therefore overall a desirable design goal. 
   A design which exhibits low power consumption is generally referred to as a cam or ball ramp actuated clutch. Here, a pair of opposed plates having caming members or opposed arcuate recesses which receive balls or roller bearings separate upon relative rotation caused by drag resulting from energization of the electromagnetic coil. Such separation compresses an adjacent friction clutch pack which transfers drive energy or torque across the friction clutch pack. Such a device is disclosed in conjunction with a transfer case in U.S. Pat. No. 4,989,686, co-owned by the Assignee herein. 
   SUMMARY OF THE INVENTION 
   An electromagnetic clutch assembly having a solenoid activated ball ramp operator provides reduced power consumption and improved control. The electromagnetic clutch assembly includes a primary or pilot friction clutch pack and a secondary or main friction clutch pack. An annular coil and housing cooperates with an annular operator or plunger. When the coil is energized, the annular plunger translates and compresses the primary or pilot friction clutch pack. Activation of the pilot clutch pack retards motion of one of the members of the ball ramp actuator which, in turn, compresses a secondary or main friction clutch pack disposed across the input and output members, thereby transferring torque. The friction clutch packs are preferably wet, i.e., are contained within a sealed housing containing clutch fluid. The clutch assembly of the present invention exhibits reduced power consumption and improved linearity of operation. 
   It is thus an object of the present invention to provide an electromagnetic clutch having a solenoid type operator. 
   It is a further object of the present invention to provide an electromagnetic clutch having a solenoid operator, a ball ramp actuator and primary and secondary friction clutch packs. 
   It is a further object of the present invention to provide an electromagnetic clutch having a solenoid operator and primary and secondary wet friction clutch packs. 
   It is a still further object of the present invention to provide an electromagnetic clutch having a solenoid type operator and ball ramp actuator which exhibits reduced power consumption and improved linearity of operation. 
   It is a still further object of the present invention to provide an electromagnetic clutch for use in transfer cases, rear axles and other motor vehicle drive line components. 
   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 view of a four-wheel drive motor vehicle power train including an electromagnetic clutch assembly having a solenoid type operator according to the present invention disposed adjacent a rear differential; 
       FIG. 2  is a full, sectional view of an electromagnetic clutch assembly having a solenoid type operator according to the present invention; 
       FIG. 3  is a flat pattern development of a ball ramp actuator of the electromagnetic clutch assembly according to the present invention taken along line  3 — 3  of  FIG. 2 ; 
       FIG. 4  a fragmentary, enlarged view of an electromagnetic clutch assembly having a solenoid type operator according to the present invention; 
       FIG. 5  is a fragmentary, sectional view of an electromagnetic clutch assembly having a solenoid type operator according to the present invention taken along line  5 — 5  of  FIG. 3 ; and 
       FIG. 6  is a side elevational view of an alternate embodiment of a cam actuator having oblique camming surfaces for an electromagnetic clutch assembly having a solenoid type operator according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a four-wheel vehicle drive train incorporating the present invention 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 transaxle  14 . The output of the transaxle  14  drives a beveled or spiral beveled gear set  16  which provides motive power to a primary or front drive line  20  comprising a front or primary propshaft  22 , a front or primary differential  24 , a pair of live front axles  26  and a respective pair of front or primary tire and wheel assemblies  28 . It should be appreciated that the front or primary differential  24  is conventional. 
   The beveled or spiral beveled gear set  16  also provides motive power to a secondary or rear drive line  30  comprising a secondary propshaft  32  having appropriate universal joints  34 , a rear or secondary differential assembly  36 , a pair of live secondary or rear axles  38  and a respective pair of secondary or rear tire and wheel assemblies  40 . 
   The foregoing description relates to a vehicle wherein the primary drive line  20  is disposed at the front of the vehicle and, correspondingly, the secondary drive line  30  is disposed at the rear of the vehicle, such a vehicle commonly being referred to as a front wheel drive vehicle. The designations “primary” and “secondary” utilized herein refer to drive lines providing drive torque at all times and drive lines providing supplemental or intermittent torque, respectively. 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 with vehicles wherein the primary drive line  20  is disposed at the rear of the vehicle and the secondary drive line  30  and components within the secondary differential assembly  36  are disposed at the front of the vehicle. 
   Thus, the illustration of  FIG. 1 , wherein the primary drive line  20  is disposed at the front of the vehicle should be understood to be illustrative rather than limiting and that the components and the general arrangement of components illustrated is equally suitable and usable with a primary rear wheel drive vehicle. In such a vehicle, the primary differential  24  would replace the secondary differential assembly  36  at the rear of the vehicle and the secondary differential assembly  36  would be moved to the front of the vehicle to replace the primary differential  24 . 
   Associated with the vehicle drive train  10  is a controller or microprocessor  50  which receives signals from a plurality of sensors and provides a control, i.e., actuation, signal to an electromagnetic clutch assembly  70  operably disposed before the secondary differential assembly  36 . Specifically, a first sensor such as a variable reluctance or Hall effect sensor  52  senses the rotational speed of the right primary (front) tire and wheel assembly  28  and provides an appropriate signal to the microprocessor  50 . Similarly, a second variable reluctance or Hall effect sensor  54  senses the rotational speed of the left primary (front) tire and wheel assembly  28  and provides a signal to the microprocessor  50 . A third variable reluctance or Hall effect sensor  56  senses the rotational speed of the right secondary (rear) tire and wheel assembly  40  and provides a signal to the microprocessor  50 . Finally, a fourth variable reluctance or Hall effect sensor  58  associated with the left secondary (rear) tire and wheel assembly  40  senses its speed and provides a signal to the microprocessor  50 . It should be understood that the speed sensors  52 ,  54 ,  56  and  58  may be independent, i.e., dedicated, sensors or may be those sensors mounted on the vehicle for anti-lock brake systems (ABS) or other speed sensing and control equipment. It is also to be understood that an appropriate and conventional counting or tone wheel is associated with each of the speed sensors  52 ,  54 ,  56  and  58  although they are not illustrated in FIG.  1 . 
   The controller or microprocessor  50  may also receive information regarding the output speed of the transaxle  14 . A variable reluctance or Hall effect sensor  62  which is associated with a tone wheel  64  on the front or primary prop shaft  22  may be utilized. In the alternative, a variable reluctance or Hall effect sensor  66  associated with the secondary drive line  30  may be positioned adjacent a tone wheel  68  on the secondary output of the transaxle  14  driving the secondary differential assembly  36 . The controller or microprocessor  50  includes software which receives and conditions the signals from the sensors  52 ,  54 ,  56  and  58  as well as either the sensor  62  or the sensor  66 , determines corrective action to improve the stability of the vehicle, maintain control of the vehicle and/or correct or compensate for a skid or other anomalous operating condition of the vehicle and provides an output signal to the electromagnetic clutch assembly  70 . 
   Referring now to  FIGS. 1 and 2 , the electromagnetic clutch assembly  70  includes a cylindrical, bell-shaped housing  72  having a continuous flange or a plurality of ears or lugs  74  defining a plurality of through openings  76  which are adapted to receive, for example, threaded fasteners (not illustrated) which facilitate installation and disassembly of the clutch assembly  70  from the housing of the secondary differential assembly  36  illustrated in FIG.  1 . An O-ring seal  78  facilitates a fluid tight seal between the cylindrical housing  92  and the housing of the secondary differential assembly  36 . The cylindrical housing  72  receives and supports an anti-friction bearing such as a ball bearing assembly  80  which freely rotatably supports a bell-shaped input member  82 . 
   The input member  82  preferably includes an input stub shaft  84  having male splines  86 . Other positive drive means such as keyways, hexagonal flats and the like may also be defined by the input stub shaft  84 . The male splines  86  may be engaged by complementary female splines  88  formed within a flange  90 . The flange  90  may be a portion of a universal joint  34  (illustrated in  FIG. 1 ) or other component of the secondary prop shaft  32 . The flange  90  will typically include through apertures  92  which facilitate such connection. The flange  90  is preferably retained on the input stub shaft  84  by a nut  94  received on male threads  96  on the input stub shaft  84 . A second anti-friction bearing such as a ball bearing assembly  98  freely rotatably supports a cylindrical output hub  100 . The end of the output hub  100  opposite the ball bearing assembly  98  is supported by an anti-friction bearing such as a roller bearing  102 . The interior cylindrical wall of the output hub  100  defines a plurality of female splines or gear teeth  104 . Adjacent and outboard of both of the ball bearing assemblies  80  and  98  are oil seals  106  which provide fluid tight seals between adjacent components of the clutch assembly  70 . 
   Secured to the cylindrical, bell-shaped housing  72  is an electromagnetic or solenoid coil  110  which is positioned and protected within a flux concentrating annular housing  112 . The annular housing  112  includes an oblique or frusto-conical surface  114 . The annular housing  112  is secured to the cylindrical housing  72  by a plurality of threaded studs, fasteners  116  or similar devices, one of which is illustrated in  FIG. 2. A  single or multiple conductor cable  118  provides electrical energy to the solenoid coil  110 . Surrounding portions of the solenoid coil  110  and the annular housing  112  is an annular solenoid operator member or plunger  120  having a complementary oblique or frusto-conical surface  122  which aligns with and is adjacent to the frusto-conical surface  114  on the annular coil housing  112 . 
   As illustrated in  FIGS. 4 and 5 , the solenoid plunger  120  receives a plurality, preferably three, stanchions or studs  124  which extend axially and in parallel away from the solenoid plunger  120  and through axial passageways  126  in a radially and circumferentially extending portion of the input member  82 . If three stanchions or studs  124  are utilized, the angle α between them will be 120°. If four stanchions or studs  124  are utilized, the angle α will be 90°. Stated more generally, the stanchions or studs  124 , however many are utilized, should be equally angularly spaced about the plunger  120 . The stanchions or studs  124  are preferably threaded as illustrated and a suitable thread locking compound may be utilized. Alternatively, the stanchions or studs  124  may be an interference fit within blind bores in the solenoid plunger  120  or they may be secured thereto by radially oriented retaining pins (not illustrated) extending through the wall of the plunger  120  and the studs  124  or, for example, by welding. 
   The stanchions or studs  124  each include a circumferential groove  128  which receives an O-ring seal  132  which proves a fluid tight seal between the stanchion or stud  124  and the axial passageway  126  of the input member  82 . The ends of the stanchions or studs  124  opposite the solenoid plunger  120  are also preferably threaded and received within and secured to an annular first pressure or apply plate  136 . A pilot or primary friction clutch pack  140  is disposed between the radially and circumferentially extending portion of the input member  82  and the first apply plate  136 . It will be appreciated that energization of the solenoid coil  110  urges the solenoid plunger  120  toward the left as illustrated in  FIGS. 2 and 4 . As it closes the air gap between the frusto-conical surfaces  114  and  122 , the first apply plate  136  compresses the pilot or primary friction clutch pack  140 . 
   Referring now to  FIGS. 2 and 3 , the pilot or primary friction clutch pack  140  includes a first plurality of larger diameter clutch discs or plates  142  with male or exterior splines  144  which engage complementary female splines  146  on the inner surface of the input member  82 . The first plurality of larger diameter friction clutch plates  142  thus rotate with the input member  82 . Interleaved with the first plurality of larger diameter clutch plates  142  is a second plurality of smaller diameter friction clutch discs or plates  148  which have internal female splines  150  which engage complementarily configured to male splines  152  on a circular clutch hub  154 . Both the first and second pluralities of clutch plates  142  and  148  preferably include suitable friction clutch paper or material which functions optimally when disposed in and wetted by clutch fluid. The clutch hub  154  is freely rotatably disposed upon the output hub  100  and a anti-friction roller bearing assembly  156  disposed between the circular clutch hub  154  and an adjacent radially and circumferentially extending surface of the input hub  82  ensures such free rotation. 
   The circular clutch hub  154  includes a plurality, preferably three, ramped recesses  158  disposed in a circular pattern about the axis of the output hub  100 . The recesses  158  each define an oblique section of a helical torus. Disposed within each of the recesses  158  is a load transferring member such a ball bearing  160  or similar component which rolls along the ramps defined by the recesses  158 . A larger diameter circular member  162  is disposed in opposed relationship with the circular clutch hub  154  and includes a like plurality of complementarily sized and arranged recesses  164 . The load transferring balls  160  are received and trapped within the pairs of opposed recesses  158  and  164 , the ends of the recesses  158  and  164  being curved and much steeper in slope than the central regions of the recesses  158  and  164 . 
   It will be appreciated that the recesses  158  and  164  and the load transferring balls  160  may be replaced with other analogous mechanical elements which cause axial displacement of the circular clutch hub  154  and second circular member  162  in response to relative rotation therebetween. For example, tapered rollers disposed in complementarily configured conical helices or complementary oblique face cams with or without intermediate mechanical elements such as rollers may be utilized. 
   As illustrated in  FIG. 6 , the camming operator may alternately include opposed face cams  166 A and  166 B disposed on a circular clutch hub  154 ′ and a larger diameter circular member  162 ′. In a manner similar to the action of the recesses  158  and  164  and load transferring balls  160 , upon relative rotation of the circular clutch hub  154 ′ and the circular member  162 ′, the face cams  166 A and  166 B axially separate the clutch hub  154 ′ and the circular member  162 ′. 
   An important design consideration of the recesses  158  and  164  and the load transferring balls  160  and the face cams  166 A and  166 B is that the geometry, the overall design and the clearances of the clutch assembly  70  ensure that it is not self-engaging. The electromagnetic clutch assembly  70  must not self-engage but rather must be capable of modulating clamping of the friction clutch packs in direct, proportional response to the input signal provided by the controller or microprocessor  50 . 
   Referring again to  FIG. 2 , the larger circular member  162  includes female or internal splines  168  which are engaged by male or external splines or gear teeth  172  on the output hub  100  such that the larger diameter circular member  162  rotates with the output hub  100 . 
   Disposed adjacent the larger circular member  162  is a main or secondary friction clutch pack assembly  180 . The main or secondary friction clutch pack assembly  180  includes a first plurality of larger diameter friction clutch discs or plate  182  having external or male splines  184  which are received within the female splines  146  on the input member  82 . Interleaved with the first plurality of friction clutch plates  182  is a second plurality of smaller diameter friction clutch discs or plates  186  having internal or female splines  188  which are engaged by complementarily configured male splines  192  on the output hub  100 . Both the friction plates  182  and  186  preferably include friction clutch paper or material which functions optimally when disposed in and wetted by clutch fluid. A circular collar  196  provides a reaction surface for the main or secondary friction clutch pack  180  and extends between the inner surface of the input member  82  and the ball bearing assembly  98  and the oil seal  106 . The collar  196  is maintained in position by a cooperating retaining washer  198  and a snap ring  202 . An O-ring  204  disposed within a circumferential channel  206  provides a suitable seal between the circular collar  196  and the input member  82 . 
   It will thus be appreciated that the interior region of the clutch assembly defined generally by the input member  82  and the circular collar  196  is sealed by the O-rings  106 ,  132  and  204  thereby facilitating the retention of clutch fluid  208  therein and the use of wet friction clutch pack  140  and  180  and the use of compatible friction materials on the friction plates  142 ,  148 ,  182  and  186 . 
   In operation, the application of electrical energy to the solenoid coil  110  draws the annular plunger  120  toward the annular housing  112 , compressing the pilot friction clutch pack  140  and creating drag which tends to rotate the first circular hub  154  relative to the second circular member  162 , causing the load transferring balls  160  to ride up the recesses  158  and  162  and driving the circular clutch hub  154  and the circular member  162  apart. The circular member  162  acts as an apply plate and such axial motion compresses the main or secondary friction clutch pack  180  and transfers torque between the input member  82  and the output hub  100 . The magnetic attraction between the solenoid housing  112  and the plunger and especially the oblique air gap between the frusto-conical surfaces  114  and  122  has been found to provide improved control and linearity of operation. 
   The foregoing disclosure presents the invention in conjunction with the rear differential of a motor vehicle having a primary front wheel drive powertrain. Nonetheless, it should be appreciated that the electromagnetic clutch having a solenoid type operator according to the present invention has wide application in mechanical power distribution systems requiring accurate and repeatable modulating clutch operation. Hence, the electromagnetic clutch may be utilized in motor vehicle driveline components such as transfer cases and the like. 
   The foregoing disclosure is the best mode devised by the inventors for practicing this invention. It is apparent however, that devices incorporating modifications and variations will be obvious to one skilled in the art of electromagnetic clutch assemblies. 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.