Abstract:
An electrohydraulic clutch includes a bi-directional electric motor, a hydraulic circuit and a multiple plate friction clutch pack. The bi-directional electric motor drives a ball screw through a gear reduction assembly. The ball screw output translates a master piston of the hydraulic circuit which in turn advances and retracts an annular sleeve piston disposed adjacent the friction clutch pack. Hence, actuation of the electric motor displaces hydraulic fluid and compresses or relaxes the friction clutch pack, thereby transferring or inhibiting torque.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates generally to an electrohydraulic clutch and more specifically to an electrohydraulic clutch having an electric motor, a hydraulic fluid circuit and a multiple plate friction clutch pack.  
         [0002]     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.  
         [0003]     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 thus 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.  
         [0004]     Another design consideration may be broadly characterized as control. It is desirable for motor vehicle drive line clutches to both engage smoothly and preferably imperceptibly and also modulate accurately in proportion to the control signal, that is, exhibit close correspondence between the magnitude of the electrical drive signal (representing the desired proportion of clutch engagement) and the actual clutch engagement.  
         [0005]     The present invention is directed to these design goals.  
       SUMMARY OF THE INVENTION  
       [0006]     An electrohydraulic clutch includes a bi-directionally rotatable electric motor, a hydraulic circuit and a multiple plate friction clutch pack. The electric motor drives a ball screw through a multiple gear speed reduction assembly. The ball screw output translates a master piston of the hydraulic circuit which in turn advances and retracts an annular slave piston disposed adjacent the friction clutch pack. Hence, actuation of the electric motor displaces hydraulic fluid and compresses or relaxes the friction clutch pack. An anti-back drive assembly disposed between the motor and gear reduction assembly includes a wrap spring disposed between two hubs and contained within a cylindrical aperture or housing.  
         [0007]     It is thus an object of the present invention to provide an electrohydraulically actuated friction clutch.  
         [0008]     It is a further object of the present invention to provide an electrohydraulic clutch including a multiple plate friction clutch assembly.  
         [0009]     It is a still further object of the present invention to provide an electrohydraulic clutch having an electric motor and anti-back drive assembly.  
         [0010]     It is a still further object of the present invention to provide an electrohydraulic clutch for use in transfer cases, rear axles and other motor vehicle drive line components.  
         [0011]     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  
       [0012]      FIG. 1  is a diagrammatic view of a four-wheel drive motor vehicle power train having an electrohydraulic clutch assembly according to the present invention utilized in conjunction with a rear differential;  
         [0013]      FIG. 2  is a full, sectional view of an electrohydraulic clutch assembly according to the present invention taken along line  2 - 2  of  FIG. 1 ; and  
         [0014]      FIG. 3  is a full, sectional view of an electrohydraulic clutch assembly according to the present invention taken along line  3 - 3  of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     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  such as an internal combustion gas or Diesel engine or hybrid power plant which is coupled to and directly drives a transaxle  14 . The output of the transaxle  14  drives a bevel or spiral bevel 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 assembly  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.  
         [0016]     The bevel or spiral bevel 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 .  
         [0017]     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 or adaptive 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.  
         [0018]     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.  
         [0019]     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 electrohydraulic clutch assembly  70  operably disposed before the secondary differential assembly  36 . Specifically, a first sensor such as a Hall effect or variable reluctance sensor  52  senses the rotational speed of the left primary (front) tire and wheel assembly  28  and provides an appropriate signal to the microprocessor  50 . Similarly, a second variable Hall effect or variable sensor  54  senses the rotational speed of the right primary (front) tire and wheel assembly  28  and provides a signal to the microprocessor  50 . A third Hall effect or variable reluctance sensor  56  senses the rotational speed of the left secondary (rear) tire and wheel assembly  40  and provides a signal to the microprocessor  50 . Finally, a fourth Hall effect or variable reluctance sensor  58  associated with the right 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 in the vehicle for anti-lock brake systems (ABS) or other traction control or stability systems. It should also 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 .  
         [0020]     The controller or microprocessor  50  may also receive information from other sensors regarding vehicle operating variables and conditions. For example, an engine speed sensor  62  may be utilized to provide a real time signal to the microprocessor  50  regarding the speed of the engine  12 . Additionally, a throttle position sensor  64  may be included to provide a real time signal to the microprocessor  50  regarding the degree or extent of activation of the accelerator pedal. Furthermore, a steering angle sensor  66  may be utilized to provide real time data to the microprocessor  50  regarding the angular position of the steering column, the lateral position of the steering rack or the angular position of the front tire and wheel assemblies  28 . The controller or microprocessor  50  includes software which receives and conditions the signals from the sensors  52 ,  54 ,  56  and  58  as well as the optional sensors  62 ,  64  and  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 and provides an output signal to the electrohydraulic clutch assembly  70 .  
         [0021]     Referring now to  FIG. 2 , the electrohydraulic clutch assembly  70  includes a preferably metal housing  72  having various bores, ports, slots, faces, passageways and the like which receive the various components thereof. A first end plate  74  is especially formed to receive various shafts, fits tightly on one end face of the housing  72  and is secured thereby a plurality of fasteners (not illustrated). A second end plate  76  is secured to the other end face of the housing  72  by a plurality of fasteners  78 . Disposed within a suitably sized region of the housing  72  is a bi-directional, fractional horsepower electric motor  80 . The electric motor  80  includes an output shaft  82  which is supported upon suitable bearings  84  and includes a drive hub  86  having a diametric vane. A driven pinion gear  88  which is freely rotatably disposed on the output shaft  82  includes two-axially extending lugs  90 . The lugs  90  engage opposite sides or faces of the vaned drive hub  86  thus allowing limited (approximately 1500 to 1600) angular relative rotation between the vaned drive hub  86  and the pinion gear  88 . A wrap spring  92  is wrapped about and extends between the vaned drive hub  86  and the lugs  90  and the pinion gear  88 .  
         [0022]     The wrap spring  92  is received within a relatively closely fitting cylindrical aperture or passageway  94  which may be formed in the housing  72  or may be a bore or passageway in a stationary collar or similar component. The wrap spring  92 , the associated drive hub  86  and the pinion gear  88  cooperate to accommodate bi-directional drive of the pinion gear  88  by the motor  80  as the lugs  90  engage and thus achieve direct drive of the pinion gear  88  by the vaned drive hub  86 . However, when electrical power to the electric motor  80  is terminated, and forces attempt to back drive the electric motor  80 , the wrap spring  92  is unwound by rotation of the pinion gear  88 . As the wrap spring  92  is unwound and expands, it engages the surface or wall of the aperture or passageway  94  thus inhibiting further reverse rotation of the pinion gear  88 .  
         [0023]     The pinion gear  88  is in constant mesh with a first spur gear  96 . The first spur gear  96  is supported upon a first shaft  98  and is coupled to or integrally formed with a smaller diameter second pinion gear  100  which is in constant mesh with a second spur gear  102 . The second spur gear  102  is likewise rotatably supported upon a second stub shaft  104 . The second spur gear  102  is coupled to or preferably integrally formed with a third pinion gear  106 . The third pinion gear  106  is in constant mesh with and drives a third spur gear  108  which is secured to a drive shaft  110 .  
         [0024]     The drive shaft  110  is preferably supported by a pair of anti-friction bearings such as roller bearing assemblies  112 . The drive shaft  110  includes a ball screw portion  114 . Between the drive shaft  110  and the ball screw portion  114  are mounted a plurality of Belleville springs or washers  116  that function as a resilient stop. Disposed about the ball screw portion  114  is a recirculating ball nut  122 . The recirculating ball nut  122  includes a plurality of balls or roller bearings  124  which recirculate about the complementarily configured grooves in the ball screw  114  and thus provide a low friction interconnection between the ball screw  114  and the nut  122 . As the shaft  110  bi-directionally rotates in response to bi-directional rotation of the output shaft  84  of the electric motor  80 , the recirculating ball nut  122  translates to the left and right. The ball screw  114  and the recirculating ball nut  122  thus function as a rotary to linear motion transducer.  
         [0025]     The recirculating ball nut  122  is coupled to a master piston  130  which translates axially within an elongate cylinder  132  which also contains the lead screw portion  114 . The master piston  130  includes a pair of O-ring seals  134  which are received within suitably configured circumferential grooves  136  near each end of the piston  130 . The master piston  130  is shown in  FIG. 2  in its fully advanced or extended position. As the master piston  130  is retracted by rotation of the ball screw  114 , it passes a port  138  which communicates with a fluid reservoir  140 . The fluid reservoir  140  is preferably maintained substantially full of a hydraulic fluid  142  such that the cylinder  132  may be fully filled with hydraulic fluid when the piston  130  is retracted. A flexible seal  144  accommodates changes in volume of the hydraulic fluid  142  and a metal plate or cap  146  secures the flexible seal  144  and maintains a fluid tight seal thereabout. The cylinder  130  narrows to a first fluid passageway  150  which provides for communication and flow of the hydraulic fluid  142  to the driven components of the electrohydraulic clutch assembly  70 .  
         [0026]     Turning now to  FIG. 3 , the electrohydraulic clutch assembly  70  includes an input shaft  170  preferably including a set of external or male splines or gear teeth  172  and a smaller diameter threaded region  174 . The male or external splines or gear teeth  172  are engaged by complementarily configured female splines or gear teeth  176  formed on the interior of a cylindrical region  178  of a flange  180 . The flange  180  preferably includes a plurality of through apertures  182  which may receive threaded fasteners or other components (not illustrated) associated with a drive component to the electrohydraulic clutch assembly  70  such as a universal joint  34 , illustrated in  FIG. 1 . A retaining nut  184  as well as one or more flat washers  186  may be utilized to positively retain the flange  180  upon the input shaft  170 . A tapered roller bearing assembly  188  rotatably supports the input shaft  170  within the housing  72  of the electrohydraulic clutch assembly  70 .  
         [0027]     The electrohydraulic clutch assembly  70  also includes a multiple plate friction clutch pack assembly  190 . Driving the friction clutch pack assembly  190  are a plurality of male or external splines or teeth  192  disposed on the input shaft  170  which engage complementarily configured female spines  194  on a first plurality of smaller diameter friction clutch plates or discs  196 . The first plurality of friction clutch plates or discs  196  are interleaved with a second plurality of larger diameter friction clutch plates or discs  198 . The friction clutch plates or discs  196  and  198  include suitable clutch paper or friction material in accordance with conventional practice. Each of the second plurality of larger diameter friction clutch plates or discs  198  includes male or external splines  202  which engage and drive complementarily configured female or internal splines  204  formed on the interior of a cylindrical portion  206  of an output shaft  210 . The output shaft  210  is rotationally isolated from and stabilized within a portion of the input shaft  170  by a roller bearing assembly  212 . A thrust bearing assembly  214  is also disposed between the input shaft  170  and the output shaft  210  which is further supported by a tapered roller bearing assembly  216 . Suitable oil seals  218  prevent the ingress of foreign matter and maintain a fluid tight seal between the housing  72 , the input shaft  170  and the output shaft  210 .  
         [0028]     The output shaft  210  preferably includes internal or female splines or gear teeth  222  which are complementary to and engage suitably configured male splines or gear teeth (not illustrated) disposed within the rear differential assembly  36  which receive torque from the electrohydraulic clutch assembly  70 .  
         [0029]     The first fluid passageway  150  illustrated in  FIG. 2  communicates with a cylinder  228  which receives an annular slave piston  230 . A first outer O-ring  232  and a second inner O-ring  234  disposed within suitable circular grooves provide a fluid tight seal against the side walls of the annular slave piston  230 . A register pin  238  seats within a complementarily configured blind aperture  242  in the annular slave piston  230  and inhibits rotation of the annular piston  230  within the cylinder  228 . The annular piston  230  engages a thrust bearing  244  which permits relative rotation between the annular piston  230  and a circular apply plate  246 . The circular apply plate  246  transfers axial motion and force generated by the piston  230  to the friction clutch pack assembly  190 . The apply plate  246  includes female or internal splines  248  which are complementary to and engage the male splines  192  on the input shaft  170 . Thus, the apply plate  246  rotates with the input shaft  170 .  
         [0030]     A second fluid passageway  252  provides communication between the cylinder  228  and a fluid pressure sensor or transducer  254 . The pressure fluid sensor or transducer  254  is preferably a piezoelectric device which provides a signal in a single or multiple conductor cable  256  to the microprocessor  50  regarding the real time hydraulic fluid pressure within the cylinder  228 . Electrical energy is provided to the electric motor  80  through a single or multiple conductor cable  258  illustrated in  FIGS. 1 and 2 .  
         [0031]     The operation of the electrohydraulic clutch assembly  70  will now be described with reference to all the drawing figures. As noted, signals are preferably provided by the wheel speed sensors,  52 ,  54 ,  56  and  58  and the other sensors  62 ,  64  and  66  to the microprocessor  50 . The microprocessor  50  provides a signal in the cable  258  to the electric motor  80  commanding it to rotate in one of two directions to increase or decrease the pressure of the hydraulic fluid  142  and thus the torque transferred through the friction clutch pack assembly  190 . If the command from the microprocessor  50  is to increase torque throughput, the electric motor  80  rotates in a direction to advance the recirculating ball nut  122  and advance the master piston  130  within the elongate cylinder  132 . As the master piston  130  translates, hydraulic fluid  142  is transferred, its pressure increases and the annular slave piston  230  translates, compressing the friction clutch pack assembly  190 . A command from the microprocessor  50  to reduce torque transferred through the friction clutch pack assembly  190  results in the opposite action.  
         [0032]     As noted above, the wrap spring  92  inhibits back driving of the electric motor  80  by the hydraulic pressure exerted on the piston  130  and the lead screw portion  114 . This is achieved, as also noted above, by the expansion of the wrap spring  92  and grounding or contact with the surface of the cylindrical aperture or passageway  94  as it is driven in a direction which both unwinds it and corresponds to retraction of the piston  130 . The prevention of back driving and thus the maintenance of a given pressure of the hydraulic fluid  142  and corresponding torque delivery through the friction clutch pack assembly  190  allows the electric motor  80  to be de-energized after it has achieved a desired position and fluid pressure thereby conserving electrical power. In this regard, it should also be noted that the pressure transducer  254  provides information to the microprocessor  50  regarding the current, actual pressure of the hydraulic fluid  142  which corresponds to a level of torque throughput. Such information may be utilized by the microprocessor  50  to adjust, in real time, the electrical energy delivered to the electric motor  80  to achieve a desired torque throughput.  
         [0033]     Finally, it should be noted that the design of the housing  72  as well as the arrangement of components provides a passive oiling or lubrication system to the various components within the electrohydraulic clutch assembly  70 . Thus, not only is the need for specific lubricating means such as a pump avoided but the assembly exhibits improved durability and service life.  
         [0034]     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 electrohydraulic 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.