Abstract:
A torque-limiting accessory drive assembly comprising a pulley to transfer torque to an accessory drive system, a torque limiting device adapted to limit the maximum amount of transferred torque, and a one-way clutch assembly adapted to both selectively transfer torque and selectively interrupt torque transfer at predetermined times to the vehicle accessory drive.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to an accessory drive assembly and, more specifically, to a torque-limiting accessory drive assembly for the accessory drive system of an engine of a motor vehicle. 
     DESCRIPTION OF THE RELATED ART 
     In automotive applications, engine torque and speed are translated from a prime mover, such as an internal combustion engine, to one or more wheels through the transmission in accordance with the tractive power demand of the vehicle. In addition to supplying power to move the automotive vehicle, the engine drives a variety of accessories, such as; the power steering pump; air conditioning compressor; cooling fan; water pump; alternator; and any other engine driven accessory. These various accessories are usually operatively belt driven via one or more pulley systems deriving their power source from an engine power take-off, typically a crankshaft pulley. In conventional accessory drive systems engine power is constantly supplied by the crankshaft pulley to the belts. Some of the driven accessories are fitted with clutch assemblies to lessen the mechanical load and drain on engine power when they are not needed (e.g., air conditioning compressor and cooling fan). However, clutches have yet to be employed at the power take-off from the engine that drives such accessories. 
     As certain drivetrain technologies have developed, there has become a need in the art to selectively interrupt power to the driven accessories as well as selectively initiate or return power to the driven accessories. One example of such a situation involves the use of automated manual transmissions in automotive applications. These types of transmissions automate the moving of shift forks and clutches so that the operator is not required to manually disengage the engine from the transmission by depressing a clutch pedal nor by moving a shift lever to change gears. As in more conventional transmissions, during a transmission upshift, engine speed is governed by throttle position while transmission speed tends to remain essentially constant. This is due to the fact that the transmission remains engaged to the driving members and wheels, and these parts will initially sustain transmission speed then, being subject to frictional and drag forces, will slowly decrease transmission speed accordingly, the longer the transmission is decoupled from the engine. Once the next gear is selected, the transmission and engine speeds must be synchronized. The gear change upward will cause the transmission to have a lower input speed requirement. As the clutch mechanism is re-engaged the engine is commanded to a lower throttle setting to match the required input speed to the transmission. This approach relies on the slow or unpredictable loss of engine speed by either mechanical factors or a driver input to the throttle setting and while this is generally accepted in the related art, it is inefficient. When the engine and transmission speeds are relatively the same, the clutch is engaged, recoupling the engine to the transmission and delivering torque to the drivetrain. The process works but has room for improvement. 
     Additionally, automated manual transmissions suffer from the problem of noticeable torque interrupt during shifting, unlike that which occurs in a fully automated transmission or which can be compensated for in a fully manual transmission. This phenomenon adversely affects the “feel” of the vehicle during shifting and is undesirable from an operator&#39;s viewpoint. Beyond the feel of a transmission shift, other adverse effects of a delayed and hard torque transfer can damage or reduce the efficiency of the vehicle&#39;s accessory components. More specifically, when conventional accessory drive systems are employed excessive mechanical forces and stresses are often transferred to the accessories through the belt drive. This is due to the fact that, in conventional systems, the accessories are constantly connected and constantly driven. In such systems commonly employed in the related art, when the engine speed increases or decreases sharply, a quick change in torque is applied to the accessory components. This spikes, or sharply peaks, the load on the various accessories. These rapid load shifts can cause excessive or rapid wear in accessory bearings and the belt materials as well. Therefore, there is a need in the art for a device that can limit the torque transfer from the engine to the accessories. 
     Alternatively, it is often desirable to maximize power at vehicle launch. The engine driven accessories noted above have the undesirable effect of draining power that could otherwise be delivered to the drivetrain in this operating mode. Thus, there remains a need in the art for a device that selectively interrupts power to the driven accessories as well as selectively initiates or returns power to the driven accessories in an automotive application. 
     SUMMARY OF THE INVENTION AND ADVANTAGES 
     The present invention overcomes the disadvantages and drawbacks in the related art as a torque-limiting accessory drive assembly including a pulley adapted to transfer torque from the power take-off of an internal combustion engine to an accessory drive system, and a torque limiting device operatively connected to the pulley. The torque limiting device is adapted to limit the maximum amount of torque transferred from the power take-off of an internal combustion engine to the pulley. The assembly also includes a one-way clutch assembly operatively interposed between the engine power take-off and the torque limiting device. The one-way clutch assembly acts to selectively transfer torque from the power take-off to the torque limiting device thereby providing motive force to the pulley and vehicle accessory drive, and acts to selectively interrupt torque transfer at predetermined times. 
     The torque-limiting accessory drive assembly of the present invention therefore overcomes the disadvantages of conventional designs in two ways. First, the torquelimiting accessory drive assembly of the present invention is selectable in its engagement and disengagement, thereby allowing controlled interruption of the engine torque supplied to the accessories. Second, the multi-plate friction clutch provides for selective adjustment of the maximum torque transfer capacity. Furthermore, it does this without the need for supplying hydraulic power, which is the typical mechanism used to apply multi-disc friction clutches known in the art. The wrapped spring type, one-way clutch of the present invention can operate at high speeds over a wide range of temperatures. In addition, by using the one-way, spring clutch technology, the torque-limiting accessory drive assembly of the present invention is small and lightweight, having long life with an efficiency that is virtually unchanging. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top schematic view of an automotive drive train illustrating the general location of the torque-limiting accessory drive assembly of the present invention in a rear wheel drive motor vehicle application; 
     FIG. 2 is a cross-section view of a the torque-limiting accessory drive assembly of the present invention; 
     FIG. 2A is a cross-section, cut-away view of a disengagement sleeve of the torque-limiting accessory drive assembly of the present invention; 
     FIG. 3 is an oblique, partial view of a one-way clutch assembly employed in the torque-limiting accessory drive assembly of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A motor vehicle having a rear-wheel drive system and incorporating the present invention is schematically illustrated in FIG.  1  and generally indicated at  10 . The vehicle  10  includes a prime mover, such as an internal combustion engine  12 , which drives a conventional transmission  14 . Those having ordinary skill in the art will appreciate from the description that follows that the transmission  14  may be either a manual transmission with a clutch or an automatic transmission. The vehicle  10  further includes a pair of aligned, rear drive axles  16 . The outer ends of the axles  16  are coupled to rear drive tire and wheel assemblies  18  and their opposite ends are coupled to and driven by a rear drive differential  20 . The rear differential  20 , in turn, is driven by a rear drive or prop shaft  22  which is, in turn, driven by an output shaft  24  of the transmission  14 . Figure also depicts a set of front tire and wheel assemblies  26 , which are coupled to various suspension and steering mechanisms (not shown) as required. It will be appreciated that the schematically illustrated vehicle of FIG. 1 is not complete, is shown for general reference, and is not part of the present invention. 
     In reference to the present invention, FIG. 1 shows a front-end accessory drive (FEAD) pulley  40  of a torque-limiting accessory drive assembly of the present invention, generally indicated at  30 . The pulley  40  is rotatably disposed on the engine power take-off  32 , shown in relative relation to an accessory, generally represented at  34 . The torque-limiting accessory drive assembly  30  of the present invention is shown in FIG. 1 in use with an otherwise typical flexible belt-driven accessory system of a motor vehicle. By way of example, accessory  34  is shown operatively connected by belt  36  to the torque-limiting accessory drive assembly  30 . 
     As shown in FIG. 2, a sleeve bushing, or bearing  42 , is disposed between the FEAD pulley  40  and the engine power take-off  32  so that the FEAD pulley  40  may freely rotate relative to the engine power take-off  32 . A drive belt engagement area is shown at  41 , which is disposed about the outer circumference of the FEAD pulley  40  to engage with an accessory drive belt. By way of a non-limiting example, the drive belt engagement area  41  is illustrated in FIG. 2 as a serrated cross-section to represent usage with a flat “serpentine” type drive belt. It should be appreciated however, that the present invention may be used with any type of known operative drive connection. 
     The torque-limiting accessory drive assembly  30  also includes a center hub  44  that is splined at  45  to the engine power take-off  32 . An intermediate race  46  is rotatably supported on a shoulder  48  of the FEAD pulley  40 . A sleeve bushing  29  is disposed on shoulder  48  between the intermediate race  46  and the FEAD pulley  40  to allow them separate rotational movement. Together, the center hub  44  and intermediate race  46  define coextensive annular surfaces, as represented in FIG. 2 by  52  and  54 , respectively. The innermost end of the intermediate race  46 , adjacent to the inner diameter of the center hub  44 , includes a stepped portion upon which a sleeve bushing  56  is disposed to allow relative rotational movement between the intermediate race  46  and center hub  44 . 
     A torque-limiting device, generally indicated at  60 , is defined between the intermediate race  46  and the FEAD pulley  40 . As shown, the preferred embodiment of the torque-limiting device is a multi-plate friction clutch. To this end, the inner diameter of the intermediate race  46  includes a splined portion  47  on which a plurality of separator plates  62  is supported for axial movement. Similarly, the FEAD pulley  40  includes a splined portion  49  on which is supported a plurality of friction plates  64  that are interleaved between the various separator plates  62 . A Belleville spring  66  is disposed against the separator plates  62 , as a rotary ring, and applies constant pressure to the separator plates  62  such that the friction clutch  60  translates a constant torque between the intermediate race  46  and the FEAD pulley  40 . This friction clutch assembly  60  provides a constant friction connection between the intermediate race  46  and the FEAD pulley  40  and serves to set a maximum value for torque transfer. 
     A self-energizing, contracting spring, one-way clutch, is generally shown at  70 . FIG. 3 depicts the one-way clutch  70  in cutaway view for illustrative detail. The one-way clutch  70  utilizes a wound spring member  74  that operatively disposed about the coextensive annular surfaces  52  and  54  of center hub  44  and intermediate race  46 . The spring  74  is wrapped about both the center hub  44  and the intermediate race  46  in interference fit. The interconnection between the center hub  44  and the intermediate race  46  through the wound spring  74  functions to transfer torque between the hub and the intermediate race. 
     Specifically, this occurs as the center hub  44  and the intermediate race  46  are normally coupled together through the interference fit of the spring  74 . Any relative rotation by the center hub  44  (drive member) in the direction of the spring&#39;s windings (as shown at  80 ) causes an increase in frictional drag and a winding of the spring  74  to occur. This winding of the spring  74  causes it to grip the both coextensive annular surfaces  52  and  54  of center hub  44  and intermediate race  46  tighter. As the center hub  44  rotates and the grip of spring  74  increases, torque is thereby transferred through the spring  74  to rotate the intermediate race  46  in the same direction as the center hub  44  (shown at  82 , in FIG. 3) ultimately rotating the FEAD pulley  40  and driving the vehicle accessories. 
     In regard to the one-way operation of clutch  70 , if either the center hub  44  (as the input, or driving member), or the intermediate race  46  (as the output, or driven member) were to turn in the opposite direction (i.e., opposite to the rotational directions shown as  80  and  82 ), the spring  74  would slip, allowing the shafts to freewheel. This is due to the nature of a wound spring, such that if one end is held stationary (in this case by the interference fit), rotation in the direction of the windings tightens the spring coils and rotation in the opposite direction opens, or loosens, the spring coils. In other words, in the normally understood operation of a self-energizing one-way spring clutch, rotation by either the driving, or the driven member in the opposite direction to the spring&#39;s windings would slightly unwind the spring and allow freewheeling movement of the shafts. However, in the case of the present invention, neither the engine power take-off  32 , and thereby the center hub  44 , nor the accessories, through the intermediate race  46 , counter-rotate. Therefore, additional physical structure is necessary to disengage the mechanism. 
     To facilitate an unwinding of spring  74  and a release the intermediate race  46  from the center hub  44 , a disengagement sleeve  72  is placed over the spring  74 . The disengagement sleeve  72  has an inner diameter slightly larger than the spring outer diameter to allow for the winding and unwinding action of the spring  74 . As best seen in FIG. 2A, the disengagement sleeve  72  has an extension tab  76  with a blunt or squared face  78  that extends inward from the inside diameter of the sleeve  72  to meet and oppose the squared inner end  71  of the spring  74  along the inner most end of the center hub  44 . The disengagement sleeve  72  also has a plurality of release notches  77  defined on its outer diameter. A torque interruption assembly, generally shown at  90 , is secured to a stationary portion of the vehicle by its base plate  96  and includes a solenoid  94  and a disengagement arm  92  that is operatively connected to the solenoid  94 . When the solenoid  94  is activated, the disengagement arm  92  operatively engages one of the plurality of the release notches  77  of the disengagement sleeve  72 . 
     During the normally engaged mode of operation, the engine power take-off  32  turns and the rotation of attached center hub  44  tightens the spring  74  causing a transfer of torque from the center hub  44  to the intermediate race  46 , thereby driving the FEAD pulley  40  and the vehicle&#39;s accessories. As such, the spring  74  is constantly rotating along with the center hub  44  and intermediate race  46 . The disengagement sleeve  72  also rotates with the spring  74  by virtue of having the face  78  of its inwardly directed extension tab  76  engaged on the end  71  of the spring  74 . 
     When it is desirable to disengage the vehicle accessories, the torque-limiting accessory drive assembly  30  operates in the following manner. The solenoid  94  of the torque interruption assembly  90  is actuated to move the locking arm  92  from its normally retracted position (as shown in FIG. 2A) to engage one of the plurality of notches  77  on the rotating disengagement sleeve  72 . As the locking arm  92  engages one of the notches  77 , the disengagement sleeve&#39;s rotation is stopped and the spring  74 , being held from its inner end  71  by the blunt, squared face  78  of extension tab  76 , also stops rotating. This stopping and holding action placed on the spring  74  causes it to unwind and expand slightly; the center hub  44  is thereby released from the spring  74  and it continues to rotate, no longer transferring torque to the intermediate race  46 . Without the torque transfer from the center hub  44 , the intermediate race  46 , and thereby the FEAD pulley  40  with its connected accessories will freewheel, stopping quickly due to mechanical forces. In this manner, the parasitic mechanical power drain to the engine from the driven accessories is removed. 
     Conversely, as soon as the locking arm  92  is retracted from the notches  77  of the disengagement sleeve  72 , the spring  74  will rewind to its normal state, which reestablishes the interference fit and subsequently reengages the spring  74  to the center hub  44  for torque transfer through the clutch assembly  30 . Therefore, during shifting of the transmission, at launch, or any other pre-selected operative disengagement mode, torque transfer may be interrupted to the FEAD pulley  40  by activating the disengagement solenoid  94 , and may be then reestablished by deactivating the solenoid. In this way, torque transfer is selectively interruptible to all the accessories driven by the engine through the FEAD pulley  40 . 
     Additionally, in the engaged, operative mode, the engine torque that is transferred from the engine power take-off  32 , to the center hub  44 , through the spring  74 , to the intermediate race  46  first reaches the torque-limiting device  60  before being transferred to the FEAD pulley  40 . The constant application of pressure on the torque-limiting device  60  by the Belleville spring  66  further translates the applied torque from the intermediate race  46  to the FEAD pulley  40  thereby driving the associated accessories. Thus, any desired maximum predetermined torque transfer capacity can by set within the clutch assembly  30  by the selection of a specific spring force to be applied by the Belleville spring  66 . This is the torque transfer-limiting feature of the present invention. If a torque value greater than the holding pressure supplied by the Belleville spring  66  is applied across the spring  74  of the one-way clutch  70  to the intermediate hub  46 , the torque-limiting device  60  will simply slip thereby passing the designed maximum, but not the excessive, torque to the FEAD pulley  40  and accessories. This operative limitation of the torque supplied to the accessories prevents spikes, high levels, or rapid changes of torque from reaching the vehicle&#39;s accessories. 
     Therefore, the torque-limiting accessory drive assembly of the present invention provides two functions. First, the torque-limiting device  60  of the present invention has the advantage in that it provides a selective, predetermined maximum torque transfer through the drive assembly  30  by allowing design adjustment through the choice of a specific spring force capacity of the supplied Belleville spring  66 . It does this without the need for supplying hydraulic power, which is the typical mechanism used to engage multi-disc friction clutches that are known in the art. Second, the torque-limiting accessory drive assembly of the present invention is operatively selectable in its disengagement and engagement, thereby allowing controlled interruption of the engine torque that is transferred to the accessories.