Abstract:
A kinematically simple centrifugal clutch to minimize frictional induced hysteresis. The centrifugal clutch includes a rotatable output driven member, two drive plates, and at least one clutch disc disposed adjacent to the drive plates. A drive hub is adapted to receive a rotational input such as that from an output shaft of a motor. Two weighted members are pivotally mounted to the drive hub. Two spring members are operative between the drive hub and the weighted member to apply a restoring force to the weighted members. Under the influence of centrifugal force generated by the rotation of the drive hub, the weighted members pivots against the resistance of the spring member so as to engage the drive plates and thereby urge the drive plates into engagement with the clutch disc such that the driven member rotates with the drive plates.

Description:
This application is a continuation-in-part of application Ser. No. 09/489,071, filed Jan 21, 2000, now U.S. Pat. No. 6,279,711. 
    
    
     FIELD OF THE INVENTION 
     This invention is directed to clutches, and more particularly to centrifugal clutches. 
     BACKGROUND OF THE INVENTION 
     Centrifugal clutches are well known in the art. In essence, these devices, sometimes known as frictional contact axial clutches, utilize mating frictional members to transfer torque from an input shaft to an output shaft. This is accomplished by harnessing the effects of centrifugal force upon pivoted weights to generate axial movement and ultimately axial thrust. This axial thrust is applied upon an output frictional member which, by interacting with an input shaft frictional member, effectively transmits the input shaft torque to the output or driven shaft. 
     In one such type of clutch, weights are attached to a support disc which is mounted for rotation with a rotating input shaft. The weights are mounted to pivot about an axis perpendicular to the rotational axis of a support disc. The weights are spring biased to a non-engaging position relative to a clutch plate. As the angular speed increases, the weights pivot as the centrifugal force of the pivoting weights overcome the force generated by the springs and engage the clutch plate. With increasing angular speed, the weights pivot more and the clutch plate engages a clutch disc which is splined to an output drum. The plurality of springs which are operative between the support disc and the clutch plate pull the clutch plate toward the support disc. Thus, as the angular speed decreases, the springs push against the pivoted weights to restore the weights to their non-engaging, i.e., non-pivoted, position. Consequently, the clutch plate disengages the clutch disc such that the output drum is not actively driven. 
     There are several disadvantages associated with the type of clutch described above. One particular problem associated with this centrifugal clutch is frictional induced hysteresis. Friction acting upon the springs and weights causes the clutch to engage at one speed yet disengage at another speed. Typically, the frictional induced hysteresis causes the clutch to engage at a higher speed but disengage at a lower speed. Preferably, the engagement and disengagement speeds are substantially equivalent to one another, allowing for smoother operation of the centrifugal clutch when used on motorized vehicles such as racing karts. Additionally, adjusting the springs to achieve different engagement speeds is difficult, imprecise, and cumbersome. 
     What is needed, therefore, is a centrifugal clutch which is kinematically simpler so as to minimize frictional induced hysteresis during its operation to provide a centrifugal clutch with substantially equivalent engagement and disengagement speeds. This clutch should also be less complicated, less expensive, and relatively lightweight. Finally, the springs such be designed to be relatively simple to adjust and replace in order to allow for efficient adjustment of the engagement speed. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a kinematically simple centrifugal clutch which minimizes frictional induced hysteresis. The centrifugal clutch also provides an efficient and precise manner to adjust the clutch&#39;s engagement speed. To that end and in accordance with the principles of the present invention, the centrifugal clutch includes a rotatable output driven member, at least one drive plate, and at least one clutch disc disposed adjacent to the drive plate. The clutch disc is mounted for rotation with the driven member. A drive hub is adapted to receive a rotational input such as that from an output shaft of a motor. The drive hub is mounted for rotation with the drive plate. At least one weighted member is pivotally mounted to the drive hub. The weighted member has at least two adjustment holes. At least one spring member is operative between the drive hub and the weighted member to bias the weighted member to a normally disengaged position relative to the drive plate. The spring member has one end selectively movable between the two adjustment holes. Various types of spring members, such as torsion springs or extension springs, could be utilized. Under the influence of centrifugal force generated by the rotation of the drive hub, the weighted member pivots against the resistance of the spring member so as to engage the drive plate and thereby urge the drive plate into engagement with the clutch disc to rotate the driven member in unison with the rotating drive hub. As the rotational speed of the drive hub decreases, the centrifugal force is overcome by the restoring force of the spring member and the weighted member pivots out of engagement with the drive plate, causing the drive plate to disengage the clutch disc. Consequently, the driven member is no longer actively driven by the clutch disc. The restoring force of the spring member can be adjusted by moving the end of the spring to a different adjustment hole in the weighted member. 
     In one embodiment, the drive hub also includes at least two adjustment holes associated with the spring. As such, the other end of the spring member is selectively movable between the adjustment holes in the drive hub. Consequently, the restoring force of the spring member can be adjusted by moving the respective ends of the spring between the adjustments holes in the weighted member and the drive hub. 
     In another embodiment, the centrifugal clutch includes at least two weighted members. Instead of each weighted member having at least to adjustment holes, each weighted member includes at least two adjustment slots. The centrifugal clutch further includes a garter spring that is coupled between one of the two adjustment slots of each of the weighted members to bias each weighted member to a disengaged position. The bias force exerted by the garter spring can be adjusted by moving the garter spring to a different adjustment slot in each of the weighted members. 
     Although the centrifugal clutch of the present invention may be used in applications requiring a clutch mechanism intermediate to a drive motor and a drive wheel, it is specifically adapted for application in motorized carts, especially racing karts. 
     The invention also contemplates a method for adjusting the engagement speed of a centrifugal clutch. During the operation of the centrifugal clutch, the clutch engages at a given rotational speed as determined by several factors one of which is the spring constant of the spring members. All things being equal, the engagement speed increases proportionally with the spring constant. Therefore, the engagement speed can be adjusted by replacing the existing spring members with spring members having different a spring constant. Under this method, the engagement speed is altered by changing the spring members as opposed to providing the clutch with adjustment holes in the weighted member, the drive, or both. 
     Various additional advantages, objects and features of the invention will become more readily apparent to those of ordinary skill in the art upon consideration of the following detailed description of the presently preferred embodiments taken in conjunction with the accompanying drawings. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a bottom plan view of a vehicle including a centrifugal clutch according to the principles of the present invention; 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of the clutch of FIG. 1; 
     FIG. 3 is a view similar to FIG. 2 illustrating the clutch engaged; 
     FIG. 4 is a cross-sectional view taken along line  4 — 4  of FIG. 3; 
     FIG. 5 is an exploded perspective of the clutch of FIGS. 2-4; 
     FIG. 6A is perspective view showing an alternate spring arrangement; 
     FIG. 6B is a partial cross-sectional view showing another spring arrangement; 
     FIG. 6C is partial cross-sectional view showing still another spring arrangement; and 
     FIG. 7 is an exploded partial perspective view of another embodiment of the clutch of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, a vehicle  10 , such as a motorized cart, includes a motor  12  connected to a centrifugal clutch  14  of the present invention for transmitting rotation via a drive chain  16  to a drive shaft  18  for driving wheels  20 . 
     Referring now to FIGS. 2-5, the centrifugal clutch  14  of the present invention is shown in more detail. More particularly, the centrifugal clutch  14  comprises a drum or housing  30  to which is attached an output sprocket  32  for accepting the chain  16  (FIG.  1 ). A drive shaft  34  includes a necked down portion  36  which extends into the centrifugal clutch  14 . Bearing  38  having an inner race  40  supported by the necked down portion  36  allows the drive shaft  34  to rotate independently of the drum  30 . Retaining rings  42 ,  44  cooperate respectively with washers  46 ,  48  to secure the inner race  40  within bearing  38 . The sprocket includes peripheral members  52 ,  54  having grooves  56 ,  58 , respectively. The peripheral members  52 ,  54  are fitted respectively into slots  60 ,  62  so that the sprocket  32  and the drum  30  rotate in unison. Retaining ring  64  inserted in grooves  56 ,  58  cooperates with lip  70  to secure sprocket  32  to drum  30 . 
     The centrifugal clutch  14  further includes a clutch disc  80  which includes tabs  82 . The clutch disc  80  is interleaved between two drive plates  84 ,  86 . Screws  88  are inserted through holes  90  of drive plate  84  to secure drive plate  84  to drive hub  92 . Spacers  94  (FIGS. 2,  3 ) fixedly space drive plate  84  away from drive hub  92 . Drive plate  86  is rotated relative to drive plate  84  such that the screws  88  do not pass through holes  95  of drive plate  86  but through open areas  96 . Consequently, drive plate  86  rotates in unison with drive plate  84  and drive hub  92 , but is able to translate axial relative to drive plate  84  and drive hub  92  along spacers  94 . It can be appreciated that additional clutch discs  80  could be used in the centrifugal clutch  14 . For each additional clutch disc  80  added, an additional drive plate  86  is added with the additional clutch disc  80  interleaved therebetween. Additional clutch discs  80  may be required when a relatively large amount of torque is to be transmitted from the motor  12  to the wheels  20 . 
     Drive hub  92  includes a plurality of attachment posts or spring towers  100  equally spaced about the drive hub  92 . Torsion springs  102  are concentrically mounted on each spring tower  100 . Weighted members or levers  104  are pivotally attached to drive hub  92  with dowel pins  106 . Each weighted member  104  includes a plurality of spaced-apart adjustment holes  107  located along one edge of the weighted member  104 . Torsion springs  102  have first and second ends  108 ,  110 . The first end  108  of torsion spring  102  is placed within one of the several adjustment holes  107  in weighted member  104  (FIGS.  2  and  3 ). The second end  110  of torsion spring  102  is placed within one of the several adjustment holes  114  in drive hub  92  (FIG.  4 ). The first and second ends  108 ,  110  may be selectively placed in any of the adjustment holes  107 ,  114  in order to alter the restoring force that the torsion spring  102  exerts on the weighted member  104 . Although the centrifugal clutch  10  has been described with adjustment holes  107 ,  114  in both the weighted member  104  and the drive hub  92 , the centrifugal clutch  10  may contain adjustment holes  107  in only the weighted members  104  or, alternatively, weighted members  114  only in drive hub  92 . The drive hub  92  further includes a keyway  116 . Keyway  116  cooperates with key  118  and keyway  120  of necked down portion  36  such that the drive hub  92  rotates in unison with drive shaft  34 . 
     A cover  126  concentrically abuts drum  30  to enclose the operational structure of the centrifugal clutch  14 , protecting it from foreign debris which may damage or destroy it. A spacer  128  is positioned between the interior of the cover  126  and the drive hub  92 . The cover  126  is secured to the end of the drive shaft  34  with bolt  130  and washer  132 . The bolt  130  is threaded into a threaded opening  133  in the necked down portion  36  of drive shaft  34 . 
     The operation of the centrifugal clutch  14  is best illustrated with respect to FIGS. 2 and 3. In FIG. 2, drive shaft  34  is not rotating. Accordingly, each weighted member  104  is held by torsion spring  102  in a non-pivoted position. As such, the drive plate  86  is still free to move axially along the spacers  94 . As the drive shaft  34  begins to rotate, the drive hub  92  and drive plates  84  and  86  rotate as well. However, because drive plate  84  is spaced away from drive hub  92  and drive plate  86  is free to move along spacers  94 , the clutch disc  80  does not rotate in unison with the drive plates  84 ,  86 . Consequently, the drum  30  and the sprocket  32  do not turn, and hence the vehicle  10  does not move. 
     As the rotational speed of the drive shaft  34  increases and as illustrated in FIG. 3, the centrifugal force experienced by the weighted members  104  cause the weighted members  104  to pivot about dowel pins  106 . Corner section  134  of weighted member  104  engages drive plate  86  and urges drive plate  86  into engagement with clutch disc  80 . When the rotational speed of drive shaft  34  and drive hub  92  reach a predetermined speed, commonly referred to as the engagement speed, the weighted members  104  pivot sufficiently such that the drive plates  84 ,  86  fully engage clutch disc  80 . Accordingly, the clutch disc rotates drum  30  and sprocket  32  and the vehicle  10  is propelled forward. As the rotational speed decreases the centrifugal force is reduced and the torsion springs  102  act to reduce the engagement force the weighted members  104  exert on the drive plates  84 ,  86 . At a predetermined speed, commonly referred to as the disengagement speed, the weighted members  104  disengage drive plates  84 ,  86  such that the clutch disc  80  no longer rotates in unison with drive shaft  34  and drive hub  92 . Consequently, the vehicle is no longer actively propelled forward. As mentioned above, first and second ends  108 ,  110  of torsion spring  102  can be repositioned in adjustment holes  107 ,  114  so that the restoring force can be changed. As the restoring force is reduced, the engagement speed is correspondingly reduced. That is, with reduced restoring force acting on the weighted members  104 , less rotational speed is required to engage the clutch disc  80  and propel the vehicle  10  forward. 
     It can be appreciated that the torsion springs  102  could be replaced by any suitable resilient member adapted to apply a tension, compression, or torsion force between the weighted members  104  and the drive hub  92  to bias the weighted members  104  to a non-engaging position. In addition to the torsion spring  102 , the resilient member may be an extension spring, a garter spring, or a leaf spring, for example. The resilient member may also be an elastic band, such as a rubber band. 
     With specific reference to FIG. 6A, an extension spring  136  with first and second ends  138 ,  140  is connected between the weighted member  104  and an attachment lug  141 . More specifically, first end  138  of extension spring  136  is removably inserted into one of a plurality of spaced-apart adjustment holes  107  in the weighted member  104 , and second end  140  is removably inserted into one of the adjustment holes  142  in attachment lug  141 . The restoring force of extension spring  136  may be altered by selectively changing which adjustment holes  107 ,  142  the first and second ends  138 ,  140  are placed. With reference to FIG. 6B, a torsion spring  144  with first and second ends  146 , 148  is mounted concentrically about dowel pin  106 . The first end  146  of the torsion spring  144  is fixedly held by lug  150  extending from drive hub  92 . The second end  148  is removably inserted into one of the adjustment holes  107  in weighted member  104 . The torsion spring  144  exerts a restoring force on weighted member  104  as it pivots under the influence of centrifugal force. The amount of restoring force can be changed by moving the second end  140  of torsion spring  144  to a different adjustment hole  107  in weighted member  104 . 
     A slightly different spring arrangement is illustrated in FIG.  6 C. In this arrangement, the weighted members  104  include a plurality of spaced-apart, outwardly-facing slots  152 . A garter spring  154  is placed into one of the plurality of slots  152  in each of the six weighted members  104 . Preferably, the garter spring  154  is placed into concentrically aligned slots  152  so that the garter spring  154  exerts an equal restoring force to each of the weighted members  104 . In this particular embodiment, the garter spring  154  is indirectly operative between the drive hub  92  and the weighted members  104 . That is to say, the garter spring  154  relies on the interaction between individual weighted members  104  and drive hub  92  to produce a restoring force. In addition to moving the garter springs  154  between slots  152  to alter the restoring force, garter springs  154  with different spring constants could be utilized to change the restoring force and thus, the engagement speed. 
     Generally, the restoring force of a spring is equal to the product of the spring constant multiplied by the change in length of the spring, usually referenced from its unstretched position. In each of the embodiments described above, the restoring force was altered by changing the length component of the restoring force equation. That is, by changing which adjustment holes the ends of the springs engage, the length of the spring can be altered, thereby changing the restoring force the spring exerts on the weighted member. The restoring force could also be altered by altering the spring constant of the spring. 
     The spring constant of a given spring is not readily changed. Consequently, to change the restoring force without using the adjustment holes described above, springs having one spring constant must be replaced with other springs having a different spring constant. As illustrated in FIG. 7, the drive hub  92  and the weighted members  104  include only one spring retention hole  158 ,  160 , respectively. As such, to change the restoring force, the torsion springs  102  described in connection with FIGS. 25 5 , is replaced with torsion springs  162  having a different spring constant. As illustrated in FIG. 7, the torsion spring  162  is not as tightly wound as torsion spring  102  illustrated in FIG.  5 . All things being equal, the spring constant of torsion spring  162  is less than that of torsion spring  102 . Accordingly, the restoring force for torsion spring  162  will be less than for torsion spring  102 . 
     While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in considerable detail in order to describe the best mode of practicing the invention, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications within the spirit and scope of the invention will readily appear to those skilled in the art. The invention itself should only be defined by the appended claims, wherein