Abstract:
An apparatus and system are disclosed for automatic centrifugal engagement of a clutch incorporating a friction disk that expands when rotated above a threshold speed. The apparatus may include a clutch basket, a clutch pack capable of coupling the clutch basket with a center clutch, a pressure plate that may be easily adjusted to create a gap between the pressure plate and the clutch pack and an expanding friction disk assembly that may take the place of one or more friction disks. The assembly expands under centrifugal force to provide automatic engagement of the clutch. A mechanism is provided to manually override the automatic engagement of the clutch with a clutch lever and with improved operational performance of the clutch lever versus previous automatic clutch systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/040,081 entitled “AUTOMATIC CLUTCH EMPLOYING EXPANDING FRICTION DISK” and filed on Mar. 27, 2008 which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a clutch system of the friction type placed in a power transmission system. Typical clutch systems include a clutch input such as a clutch basket, a clutch output such as a center clutch, and one or more plates making up a clutch pack and disposed between the clutch input and clutch output. When the clutch pack is compressed, the clutch input and clutch output become rotationally coupled. More particularly, it is a device for automatically engaging or disengaging a clutch based upon engine speed. 
         [0003]    Most motorcycles incorporate a manual transmission coupled to the engine via a multi-plate clutch assembly. Typically, the multi-plate clutch is engaged/disengaged by the driver via a lever mounted on the handlebar. Although the lever operated clutch allows the driver to control the clutch engagement/disengagement, often times motorcycle drivers find the clutch lever difficult to operate smoothly. New riders have difficulty adjusting to smoothly engaging the clutch while operating the throttle to move the vehicle from a standing start. Experienced riders may need to partially disengage the clutch when traveling slowly to allow the engine to continue running without stalling. Motorcycle racers often have a difficult time controlling the engagement of the clutch and the application of the throttle to maximize acceleration. Off-road motorcycle racers often need to stop the rear wheel suddenly with the rear brake, causing the engine to stall if the clutch is not first disengaged. An automatic clutch can help overcome many of the problems associated with a manual clutch. 
         [0004]    Automatic clutches for motorcycles have existed for more than 30 years, primarily for entry-level motorcycles with low power. More recently, retrofit automatic clutches for high-performance motorcycles have become available. Automatic clutches for high-performance motorcycles have many advantages over manual clutches. Currently available retrofit automatic clutches have several disadvantages: 
         [0005]    Require replacement or changes to existing clutch parts. Most motorcycle clutch&#39;s pressure plates are spring loaded and attached to the center clutch. However, the centrifugal mechanism must spin with the clutch&#39;s outer basket which is coupled rotationally to the engine to provide clutch engagement force. Many existing retrofit automatic clutches require a modified clutch outer basket to bolt the new centrifugal pressure plate to. 
         [0006]    Clutch lever override not possible at high engine speeds. Existing retrofit automatic clutches use a rigid centrifugal engagement mechanism. To disengage the clutch, the entire force of the centrifugal engagement mechanism must be overcome. At higher engine speeds, the increased centrifugal force of the engagement mechanism becomes difficult or impossible to overcome. 
         [0007]    Clutch lever override has an inconsistent feel. Existing automatic clutches incorporate a clutch release mechanism that pushes against the centrifugal mechanism to release the clutch engagement. At low engine speeds, centrifugal force is low and the effort to release clutch engagement is low. At higher engine speeds, centrifugal force is high and the effort to release clutch engagement is high. Inconsistent clutch lever effort makes it difficult to effectively control manual override of the automatic clutch over different engine speeds. 
         [0008]    Too much centrifugal force at high engine speeds. The stock non-automatic, non-centrifugal pressure plate provides a fixed amount of pressure to the clutch disks. The stock pressure plate force is limited to ensure the clutch can slip if excessive force is sent through the driveline. Existing retrofit automatic clutches transfer all of the force generated by centrifugal engagement mechanism into the clutch disks. At higher engine speeds, the excess centrifugal force can prevent the clutch from slipping in the event an excessive force is sent through the driveline. The excessive force traveling through the clutch may cause a failure in the engine or transmission. 
         [0009]    Difficult to install automatic clutch. Most automatic clutches require many or even most of the OEM manual clutch components be replaced with the automatic clutch components. This requires time and cost to install the clutch. Also, it may be desirable for the motorcycle user to be able to switch back and forth between an automatic motorcycle clutch and a manual motorcycle clutch. A need exists for a clutch system that can easily be converted from automatic to manual operation and back again. 
         [0010]    Difficult to adjust automatic clutch. Existing automatic clutches require that the gap between the clutch plates and the centrifugal mechanism be maintained to precise tolerances. Adjusting this tolerance may require swapping in different thickness clutch plates or somehow shimming the centrifugal mechanism. Additionally, clutch lever override performance may be optimized by very precisely adjusting the gap between the centrifugal mechanism and clutch plates. A need exists for easily and precisely adjusting the gap between the centrifugal mechanism and the clutch plates. 
         [0011]    Adjusting the engine speed at which the clutch begins to engage and how quickly the clutch becomes fully engaged is important for proper operation of an automatic clutch. Existing automatic clutches require changing or shimming springs within the automatic clutch to adjust the engagement speed. Changing or shimming the engagement speed springs typically involves working with small fasteners within the confines of the engine. The operator may drop a small part in the engine requiring significant time and effort to retrieve the dropped piece. 
         [0012]    Other automatic clutch solutions exist but all of the prior art fails to address all of the needs described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is an isometric exploded view of a preferred embodiment of the invention; 
           [0014]      FIG. 2   a  is an isometric exploded top view of a preferred embodiment of an expanding friction disk assembly,  FIG. 2   b  is an isometric exploded bottom view of a preferred embodiment of an expanding friction disk assembly; 
           [0015]      FIG. 3   a  is a bottom isometric view of a preferred embodiment of a centrifugal wedge,  3   b  is a top isometric view of a preferred embodiment of a centrifugal wedge; 
           [0016]      FIG. 4   a  is a cross-section view of a preferred embodiment of an expanding friction disk assembly detailing the centrifugal wedge and ramps;  4   b  is a cross-section view of a preferred embodiment of an expanding friction disk assembly detailing a preferred embodiment of a spring biasing structure; 
           [0017]      FIG. 5   a  is an isometric back view of a preferred embodiment of an adjustable pressure plate assembly;  5   b  is an isometric top view of a preferred embodiment of an adjustable pressure plate assembly; 
           [0018]      FIG. 6   a  is cross-section view of a preferred embodiment of the invention in an automatically disengaged state;  6   b  is cross-section view of a preferred embodiment of the invention in an automatically partially engaged state,  6   c  is cross-section view of a preferred embodiment of the invention in an automatically fully engaged state; 
           [0019]      FIG. 7  is an isometric exploded view of a second embodiment of an expanding friction disk assembly; 
           [0020]      FIG. 8  is an isometric exploded view of a third embodiment of an expanding friction disk assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Reference throughout this specification to “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
         [0022]    Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of the embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
         [0023]    The present invention provides for a novel automatic clutch solution that maximizes the use of traditional components found in a multi-plate clutch typical of motorcycles and incorporates many new capabilities that allow for easier installation and maintenance of the clutch, improved manual override performance and the ability to quickly change between an automatic clutch and a manual clutch. 
         [0024]    To achieve many of the benefits described above, a self-contained, centrifugally actuated, expanding friction disk assembly is disclosed. The expanding friction disk assembly is shaped like and designed to replace one or more friction disks in a multi-plate clutch pack. The expanding friction disk is very simple and compact and is designed to stay together as a single piece during installation and maintenance. It can easily be swapped in or out of the multi-plate clutch pack as a complete assembly. When combined with a pressure plate that includes a means for adjusting its position axially away from or towards the multi-plate clutch pack, forms a system that meets the objectives outlined in the present invention. 
         [0025]      FIG. 1  is an isometric exploded view diagram illustrating an embodiment of a clutch apparatus (hereinafter “clutch”)  100  in accordance with the present invention. For clarity purposes, reference will be made to “outward” and “inward” directions. As used herein, the term “outward” refers to a direction pointing away from the clutch basket  104 . Arrow  102  illustrates this outward direction. The term “outward direction” may also refer to a radial direction pointing away from a longitudinal axis. As used herein “inward” refers to the opposite direction of “outward.” 
         [0026]    The clutch  100  is provided with, in one embodiment, a clutch basket  104  having an opening in the center for receiving the transmission input shaft (not shown). Typically the clutch basket  104  is configured with a bearing between the clutch basket  104  and the transmission shaft so that the clutch basket can rotate independently of the transmission shaft with minimal friction. Typically a center clutch thrust washer  106  is disposed between a center clutch  108  and the clutch basket  104 . A center clutch nut  110  secures the center clutch  108  against the center clutch thrust washer  106  which in turn is secured against a shoulder on the transmission input shaft (not shown). The center clutch is typically rotationally coupled to the transmission shaft via a suitable spline. 
         [0027]    The clutch basket  104  is typically coupled rotationally to a power input source such as an engine and the center clutch  108  is typically coupled rotationally to an output such as a transmission. In another embodiment of the invention, the clutch basket  104  is coupled rotationally to an output and the center clutch  108  is coupled rotationally to a power input. 
         [0028]    A clutch pack  128  is disposed between the clutch basket  104  and the center clutch  108 . The clutch pack  128  consists of friction plates coupled rotationally to the clutch basket  104  and drive plates interleaved with the friction plates; the drive plates being coupled rotationally with the center clutch  108 . When an axial force compresses the clutch pack, friction couples the clutch pack rotationally allowing the clutch basket  104  to turn with the center clutch  108 . Consequently when there is no axial force compressing the clutch pack  128 , the friction plates are free to rotate independently of the drive plates, allowing the clutch basket  104  to rotate independently of the center clutch  108 . In another embodiment of the invention, the clutch pack  128  consists of drive plates coupled rotationally to the clutch basket  104  and friction plates coupled rotationally to center clutch  108 . 
         [0029]    The configuration of the clutch basket  104 , center clutch thrust washer  106 , center clutch  108 , and center clutch nut  110 , clutch pack  128 , and the engine and transmission is typical of most modern motorcycle clutches and this structure is commonly employed in many types of power transmission devices. 
         [0030]    Next in the outward direction is the expanding friction disk assembly  114 . The expanding friction disk assembly&#39;s profile is designed to match that of the friction disk in the clutch pack to be rotationally coupled to the clutch basket  104 . In another embodiment of the invention the expanding friction disk assembly&#39;s profile is designed to match that of the drive plate in the clutch pack to be rotationally coupled to the center clutch  108 . In another embodiment of the invention, the expanding friction disk assembly  114  is disposed inwards of the clutch pack  128 . In another embodiment of the invention, the expanding friction disk assembly  114  is disposed within the clutch pack  128 . 
         [0031]    Next in the outward direction is a clutch throw-out  130  and an adjustable pressure plate  135 . Springs  136  are disposed into counter-bored holes formed in the pressure plate  135 . Bolts  138  having a large diameter head or washer capture the outward edge of the spring, pass through the springs  136  and through the counter-bore of the holes in the pressure plate and are threaded into the center clutch  108 . In this configuration, the pressure plate  135  is secured and biased in the inwards direction towards the expanding friction disk assembly  114  by the springs  136 . 
         [0032]      FIGS. 2   a ,  2   b ,  3   a ,  3   b ,  4   a  and  4   b  detail the structure of a preferred embodiment of an expanding friction disk assembly  114 . In a preferred embodiment, the expanding friction disk  114  consists of a top plate  250 , anti-friction pads  240 , centrifugal wedges  230 , a bottom plate  220 , biasing springs  210  and fasteners  200 . 
         [0033]    The centrifugal wedges  230  are shaped substantially like a segment of the friction disk profile to maximize the volume and therefore the mass and centrifugal effect of the centrifugal wedge  230 . The centrifugal wedges  230  are formed with a series of ramps  234  on the inward side facing the bottom plate  220 . The bottom plate  220  is formed with matching ramps  222  such that when the centrifugal wedge  230  slides radially outward against the ramps  222  of the bottom plate  220 , the centrifugal wedge  230  is pushed axially upward away from the bottom plate  220 . The bottom plate  220  is also formed with a vertical wall  223  that limits the outward radial movement of the centrifugal wedge  230 . The vertical wall  223  and ramps  222  are configured to provide a precise amount of expansion of the expanding friction disk assembly  114 . In one embodiment, the expansion is limited to about 0.050″. In another embodiment, the expansion is limited to about 0.080″. 
         [0034]    The centrifugal wedge  230  is also formed with an inward pocket  236 . The inward pocket  236  is formed to engage the inward stop  224  formed in the bottom plate  220  such that when the centrifugal wedge  230  slides radially inwards, the inward pocket  236  engages the inward stop  224 , preventing the centrifugal wedge  230  from traveling further inward. The inward stop  224  prevents the centrifugal wedges  230  from falling out of the expanding friction disk assembly  114 . 
         [0035]    In a preferred embodiment, the centrifugal wedge  230  includes pockets  232  to accept an anti-friction material  240  such as Teflon. In this configuration, the anti-friction material rubs against the top plate  250  providing for easier inward and outward radial movement of the centrifugal wedge  230 . In another embodiment of the centrifugal wedge  230 , no pocket for anti-friction material is provided. In another embodiment an anti-friction coating over the surface of centrifugal wedge  230  is incorporated. In another embodiment a ball bearing or needle bearing is disposed between the centrifugal wedge  230  and the top plate  250  and or bottom plate  220  to reduce friction. 
         [0036]    As can be best seen in  FIG. 4   b , in a preferred embodiment, the bottom plate  220  is formed with counter bored holes  226 . The counter-bored holes  226  are configured to receive the internally threaded posts  251  of the top plate  250 . A tight fit between the counter-bored holes  226  and the internally threaded posts  251  limit rotational movement between the top plate  250  and the bottom plate  220 . Biasing springs  210  are disposed within the counter bored holes  226 . Screws  200  capture the inward end of the biasing springs  210 , the counter bored holes  226  of the bottom plate  220  capture the outward end of the biasing springs  210 . When the screws  200  are threaded into the internally threaded posts  251  of the top plate  250 , the biasing springs  210  bias the top plate  250  towards the bottom plate, capturing the centrifugal wedges  230  and anti-friction material  240 . 
         [0037]    The axial biasing force of the biasing springs  210  through the top plate  250  towards the bottom plate  220  forces the centrifugal wedges  230  down the ramps  222  and radially inwardly up against the inward stop  224  of the bottom plate  220 . As the expanding friction disk assembly  114  is rotated around its central vertical axis defined by the outward direction  102 , centrifugal forces urge the centrifugal wedges  230  in the radial outward direction against the ramps  222  and the biasing force of the biasing springs  210 . When sufficient rotational speed is achieved, the centrifugal force of the centrifugal wedges  230  exceeds the force of the biasing springs  210  and the centrifugal wedges slide out radially against the ramps  222 . As the centrifugal wedges slide out radially, the ramps push the wedge vertically away from the bottom plate  220  and force the top plate  250  away from the bottom plate  220  resulting in the entire friction disk assembly  114  expanding. 
         [0038]    Typically a biasing spring force is selected that allows the clutch to begin to engage just above engine idle speed. In one embodiment, the spring force selected is about 35 pounds to achieve an engagement speed of approximately 1500 RPMs. In another embodiment a spring force of about 50 pounds is selected to achieve an engagement speed of approximately 2000 RPMs. Selection of an appropriate biasing spring force has many different factors. The primary factors include mass and radius of action of the centrifugal wedges  230 , angle of the ramps  222  between the centrifugal wedges  230  and bottom plate  220 , gearing between the engine and the clutch, and engine idle speed. 
         [0039]      FIGS. 5   a  and  5   b  disclose the structure of a preferred embodiment of an adjustable pressure plate assembly  135 . As a whole, the adjustable pressure plate assembly  135  is largely of typical construction for a motorcycle clutch. It is formed with counter-bored pockets  560 , a hole in the center of the assembly to which a clutch throw-out  130  is journaled and incorporates a surface to axially engage and provide a compressive force to a clutch pack  128 . However the adjustable pressure plate assembly also incorporates in a preferred embodiment a threaded pressure plate adjuster  510  that is not of typical construction for a clutch. The threaded pressure plate adjuster  510  is formed on the inward side with a shoulder  512 , a throw-out surface  516  and oil flow slots  514 . The threaded pressure plate adjuster  510  is formed on the outward side with spanner-wrench holes  518  and a tick mark  519 . 
         [0040]    A preferred embodiment of the pressure plate  500  incorporates a centrally threaded opening to receive the threaded pressure plate adjuster  510 . Around the periphery of the centrally threaded opening are clocking marks  550 . The clock marks are useful for tracking the position of the adjustable pressure plate assembly as described more completely below. The pressure plate also incorporates slots  542  and threaded holes  540  to receive tapered set screws  541 . When the tapered set screws  541  are threaded into the threaded holes  540 , the taper of the tapered set screws  541  creates an expansion force allowing the web of material between the tapered set screws  541  and the slots  542  to expand into the threads of the pressure plate adjuster  510  securing it against rotation. 
         [0041]    In a preferred embodiment, the adjustable pressure plate assembly  135  also incorporates a replaceable friction plate  520  having friction pads  522 . The replaceable friction disk  520  incorporates a tooth profile  530  to rotationally couple the replaceable friction disk  520  with the pressure plate  500 . To keep the replaceable friction disk  520  with the adjustable pressure plate assembly  135 , a t-slot  534  extends into the tooth profile  530  of the pressure plate  500  to receive a small belleville washer  532 . When the belleville washer  532  is pushed into place, the edge of the belleville washer  532  prevents the replaceable friction disk  520  from separating from the adjustable pressure plate assembly  135 . 
         [0042]      FIGS. 6   a ,  6   b  and  6   c  provide a cross-sectional view of a preferred embodiment of the clutch assembly  100  in different states of operation. The transmission shaft, engine and clutch throw-out rod are not shown but are intended to be of typical construction.  FIG. 6   a  shows the expanding friction disk  114  in its collapsed state. In this figure it is apparent that the threaded pressure plate adjuster  510  has been turned inwards so that the shoulder  512  has engaged the upward surface of the center clutch nut  110 . By continuing to turn the threaded pressure plate adjuster  510  inward once the shoulder  512  has made contact with the center clutch nut  110 , the adjustable pressure plate assembly  135  is lifted in the upward direction away from the expanding friction disk assembly creating an installed gap  610  between the adjustable pressure plate assembly  135  and the expanding friction disk assembly  114 . In another embodiment, any threaded structure that limits the axial distance a pressure plate is biased towards a clutch pack is utilized. 
         [0043]    The installed gap  610  allows the clutch assembly  100  to remain disengaged until sufficient engine speed is reached and the expanding friction disk assembly  114  expands axially through the space of the installed gap  610 .  FIG. 6   b  shows the expanding friction in a partially expanded state. In  FIG. 6   b  it is apparent that the installed gap  610  is no longer presents as the expanding friction disk assembly has expanded axially through the installed gap and engaged the adjustable pressure plate assembly  135 . Initially, the centrifugal wedges  230  of the expanding friction disk assembly  114  must only overcome the biasing springs to begin to expand. However, once the expanding friction disk assembly axially engages the adjustable pressure plate assembly, it must generate enough axial force to overcome the pressure plate springs  136  to expand further. 
         [0044]      FIG. 6   c  shows the expanding friction disk assembly  114  in its fully expanded state. When the centrifugal wedges  230  of the expanding friction disk assembly  114  have generated enough force to overcome the pressure plate springs  136 , the centrifugal wedges  230  reach a vertical wall  223  in the bottom plate  220  restricting further expansion of the expanding friction disk assembly  114 . 
         [0045]    When the installed gap  610  is set properly with the threaded pressure plate adjuster  510  and the expanding friction disk assembly  114  is in its fully expanded state, the entire adjustable pressure plate assembly  135  is lifted off of the center clutch nut  110 . As can be seen in  FIG. 6   c  a threaded adjuster gap  620  is formed between the shoulder  512  of threaded pressure plate adjuster  510  and the center clutch nut  110 . As the adjustable pressure plate assembly  135  is lifted a small amount, there is also a small increase in the throw-out gap  600 . If a threaded adjuster gap  620  is not formed when the expanding friction disk assembly  114  is in its fully expanded state, then some or all of the force of the pressure plate springs  136  is being carried by the shoulder  512  of the threaded pressure plate adjuster  510  and not being transferred into the expanding friction disk assembly  114  and the clutch pack  128 . This can cause the clutch  100  to not transfer all of the rotational energy between the input and the output and lead to premature clutch pack  128  wear. 
         [0046]    When the installed gap  610  is too big, the expanding friction disk assembly  114  will run out of axial travel and will be unable to lift the adjustable pressure plate assembly  135  sufficiently to create a threaded adjuster gap  620 . Alternatively, if the installed gap  610  is too small, the adjustable pressure plate assembly  135  will be lifted a larger amount creating a larger throw-out gap  600 . A larger throw-out gap  600  will change the point at which the clutch lever will begin to disengage the clutch; a generally undesirable trait for the operator. 
         [0047]    There are many parts that affect the axial distance relationship that is necessary to create the optimal installed gap  610  measurement. Because the axial thickness of these parts may vary by a greater amount than can be tolerated to provide an optimal installed gap  610 , the adjustable pressure plate assembly  135  is a necessary and important component in realizing the full benefit of the present invention. The adjustable pressure plate assembly  135  may also be incorporated into other centrifugal clutch systems to improve those clutch systems. 
         [0048]    As described previously, the adjustable pressure plate assembly  135  includes clocking marks  550  on the pressure plate  500  and a tick mark  519  on the threaded pressure plate adjuster  510 . To set a proper installed gap  610 , the operator turns the threaded pressure plate adjuster inwards until significant resistance is felt indicating that the shoulder  512  of the threaded pressure plate adjuster  510  has engaged the outward surface of the center clutch nut  110 . Using the tick mark  519  on the threaded pressure plate adjuster  510  and the clock marks  550  on the pressure plate  500 , the operator can turn the adjuster inwards the specified number of clock tick marks to achieve the optimal installed gap  610 . In one embodiment, the optimal installed gap  610  is achieved by turning the threaded pressure plate adjuster  510  one complete turn plus three clock tick marks. In another embodiment, the optimal installed gap  610  is achieved by turning the threaded pressure plate adjuster  510  one complete turn. Determining the amount of rotation of the threaded pressure plate adjuster  510  necessary for an optimal installed gap  610  is based upon many different factors. Some of the primary factors include the thread pitch of the threaded pressure plate adjuster  510 , amount of axial travel available in the expanding friction disk assembly  114  and flex in the adjustable pressure plate assembly  135 . 
         [0049]      FIG. 7  discloses a second embodiment of an expanding friction disk assembly  114  utilizing balls for generating centrifugal expansion force. The second embodiment utilizes screws  700 , biasing springs  710 , a bottom plate  720 , centrifugal balls  730 , a top plate  750  and grooved ramps  752 . The second embodiment disclosed utilizes centrifugal balls  730  in place of centrifugal wedges but is otherwise similar to the preferred embodiment in other relevant structure. 
         [0050]      FIG. 8  discloses a third embodiment of an expanding friction disk assembly  114  utilizing extension springs  810  for biasing the top plate  850  towards the bottom plate  820 . The second embodiment utilizes a top plate  850 , top plate spring hooks  851 , top plate slots  852 , extension springs  810 , anti-friction pads  840 , centrifugal wedges  830 , a bottom plate  820 , bottom plate spring hooks  821 , and bottom plate rotational tab stops  824 . When assembled, the bottom plate rotational tab stops  824  pass through the top plate slots  852  and limit the rotational movement between the top plate  850  and bottom plate  820 . The extension springs  810  hook the top plate spring hooks  851  and the bottom plate spring hooks  821  keeping the assembly together axially and providing a biasing force against the centrifugal force of the centrifugal wedges  830  when the assembly is spinning. The third embodiment disclosed utilizes extension springs  810  for a biasing force and rotational tab stops to limit relative rotation between the bottom plate and top plate but is otherwise similar to the preferred embodiment in other relevant structure. 
         [0051]    In a preferred embodiment, the bottom plate  220  of the expanding friction disk assembly  114  is made of aluminum and has friction pads  228  bonded to the inward side and the top plate  250  is formed from steel and has no friction material bonded to it. In this preferred embodiment, the adjustable pressure plate assembly  135  has attached a replaceable friction plate  520 . In another embodiment the top plate  250  has friction pads bonded to it and the adjustable pressure plate assembly  135  has no friction pads bonded to it. In another embodiment the bottom plate  220  has no friction material bonded to it. 
         [0052]    In another embodiment of the clutch  100 , the expanding friction disk assembly  114  is replaced with a static friction disk having the same profile but slightly increased thickness compared to the expanding friction disk assembly  114 . The static friction disk is configured so as not to expand under centrifugal force. When the static friction disk is inserted in place of the expanding friction disk assembly  114  and the threaded pressure plate adjuster  510  is turned outward so as not to create an installed gap, the clutch  100  is transformed from an automatically engaging clutch to a manually controlled clutch. In another embodiment of the clutch  100 , the expanding friction disk assembly  114  is replaced with a standard thickness friction disk and the adjustable pressure plate assembly  135  is replaced with a manual pressure plate assembly. The manual pressure plate assembly is configured for the lower height of the clutch pack and without a threaded pressure plate adjuster configured to create an installed gap. These alternate embodiments show that another beneficial characteristic of the present invention is the ability to work with standard, non-centrifugal clutch parts and to be quickly and easily converted to and from an automatic centrifugal clutch to a standard manually controlled clutch. 
         [0053]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.