Abstract:
A centrifugal clutch having an interlocking combination of shoes and stop pins includes compression biasing springs that are adjustable with tamper-free locking, using reversible spring biased wedges that smoothly transition between selected low-gain and high gain engagements within a desired narrow speed range. The clutch also includes a hub, a rotatable drum having a circularly cylindrical inside surface, and a plurality of shoes, each shoe including a rigid metallic shoe member and a lining. A stop pin is interposed between lock portions of each adjacent pair of the shoes for blocking the shoe members from moving radially outwardly into contact with the inside surface of the drum and for preventing separation of the shoes in the event of axial disengagement or structural failure of the drum during high-speed operation of the clutch. The compression biasing springs include a plurality of stacked spring washers assembled on the stop pins with respective adjustment nuts. A wedge member having oppositely inclined wedge surfaces slides in a slot of each shoe, for torque transmission in opposite rotational directions. The wedge surfaces can be asymmetrical for facilitating adjustment of a torque-speed characteristic of the clutch.

Description:
BACKGROUND 
     The present invention relates to centrifugal clutches such as are used in light vehicles, chain saws, pumps, wind mills, compressors, engine, electric motor, wind driven products and the like for variably coupling rotational power sources to driven loads. 
     Centrifugal clutches are disclosed, for example in U.S. Pat. Nos. 2,942,711 and 3,971,463 to the above-named inventor, which patents are incorporated herein by this reference. Typically, a plurality of radially movable shoes frictionally contact an inside surface of a drum that rotates concentrically with a hub to which the shoes are connected. Normally, the hub is fixed on a driving shaft or face plate that is coupled to an engine or power source, the drum being coupled to a driven load by means such as a belt, chain or direct coupled drive. Such clutches transmit little or no torque at low speeds, and progressively increased torque at higher speeds in proportion to centrifugal forces to which the shoes are subjected. In the above-identified patents, spring biasing elements are employed for maintaining the clutches in a disengaged condition at speeds below a predetermined threshold for permitting the engine or other power source to idle at no load. 
     Centrifugal clutches as previously implemented are subject to a number of problems and disadvantages. For example: 
     1. They are ineffective in that they fail to provide a desired torque-speed relationship to match the characteristics of the power source; 
     2. They are short-lived in that there is excessive slippage and consequent wear of the shoes, particularly when there is a large vibrational component or high inertia load in the torque; 
     3. They are unreliable in that springs used therein are subject to breakage, particularly in that they have fatigue inducing stress risers in hooks at opposite ends; 
     4. They are unsafe in that the shoes can fly apart in case there is structural failure of the drum; and 
     5. They are undesirably expensive to produce. 
     The clutch as disclosed in the &#39;711 patent can be assembled in a variety of ways to selectively provide correspondingly different torque-speed relations such as straight-centrifugal with selectable speeds of engagement; relatively quick-acting; or relatively smooth-acting. However, none of the torque-speed relations is entirely satisfactory, and there are numerous possibilities for incorrect assembly. The clutch as described in the &#39;463 patent provides driving member projections that are successively engageable with the shoes at increasing speed increments for progressively increased torque transmission. Again, none of the torque-speed relations is entirely satisfactory, particularly when it is desired to obtain initial engagement at a high percentage of the full-load speed of the clutch, and the clutch is excessively complex and expensive to produce. 
     It is also known to have direct metal-to-metal contact between the drum and metal shoe members of centrifugal clutches. However, the applications for such clutches are severely limited by a lower coefficient of friction, excessive wear, and galling. 
     U.S. Pat. No. 5,560,465 to the present inventor discloses a centrifugal clutch having an interlocking combination of shoes and compression biasing springs for preventing separation of the shoes in the event of axial disengagement or structural failure of the drum, and a spring biased wedge that smoothly transitions between low-gain and high gain orientations within a narrow speed range. Unfortunately, the clutch described in the &#39;465 patent is not entirely satisfactory in that the drum can become damaged by direct contact with metallic shoes in the event that lining material on the shoes is worn through. Also, desired gain-speed profiles typically desired in particular applications are difficult to obtain, particularly without requiring undesirably large inventories of variantly configured components. 
     Thus there is a need for a centrifugal clutch that provides desired torque-speed profiles that smoothly transition from no-load to full load in an adjustable narrow speed range, that is fail-safe against lining wear-through as well as against separation of the shoes in case of failure of the drum, and that is reliable, long-lasting, and inexpensive to produce. 
     SUMMARY 
     The present invention meets this need by providing a centrifugal clutch having an interlocking combination of shoes and stop pins, and that in its preferred implementations includes compression biasing springs that are adjustable with tamper-free locking, reversible spring biased wedges that smoothly transition between selected low-gain and high gain engagements within a desired narrow speed range. In one aspect of the invention, the clutch includes a rotatable drum having a circularly cylindrical inside surface; a plurality of shoes, each shoe including a rigid metallic shoe member and a lining having an external engagement surface for frictionally contacting the inside surface of the drum; means for coupling each shoe to a rotatable hub for rotation therewith when the hub rotates concentrically with the drum; means for permitting movement of each shoe radially inwardly and outwardly relative to the drum; each shoe having a radially inwardly projecting first lock portion and a radially outwardly projecting second lock portion circumferentially spaced from the first lock portion; and a stop pin interposed between the first and second lock portions of each adjacent pair of the shoes for blocking the shoe members from moving radially outwardly into contact with the inside surface of the drum and for preventing separation of the shoes in the event of axial disengagement or structural failure of the drum during high-speed operation of the clutch. 
     The compression springs can have openings therein, being located by the stop pins projecting therethrough. Preferably the compression springs each include a plurality of stacked spring washers for enhanced fatigue resistance. Preferably the stop pins have respective adjustment members assembled therewith for adjustably loading the compression springs. The adjustment members can be adjustment nuts having threaded engagement with corresponding stop pins, an end portion of each stop pin having non-circular axial engagement with a respective shoe member for preventing rotation of the stop pin during adjustment of the adjustment nut. 
     The means for coupling and the means for permitting can include a plurality of dogs formed on the hub; a generally radially oriented slot formed on each shoe; a wedge member slidably engaging the slot of each shoe and having a wedge surface for engagement by a corresponding one of the dogs; and a wedge spring for biasing the wedge member against the corresponding dog. Preferably each wedge member is formed with an oppositely inclined pair of the wedge surfaces, the clutch being operable with torque transmission in opposite rotational directions, each of the dogs engaging one of the oppositely inclined wedge surfaces in one direction of transmitted torque and the other of the wedge surfaces during torque transmission in an opposite direction. Also, the oppositely inclined wedge surfaces can be asymmetrical, a torque-speed characteristic of the clutch being adjustable by selectively reversing the wedge members in the respective slots. The respective slots can be inclined at an angle of approximately 10 degrees from being radially oriented with corresponding ones of the dogs. Preferably the clutch can further include a plurality of keeper members, each keeper member being pivotally located relative to the hub for limiting outward movement of a respective wedge member. 
     Preferably the clutch further includes a guide on each shoe for circumferentially engaging an adjacent shoe, thereby radially stabilizing the shoes relative to the drum. Each guide can be formed by a pair of facing parallel-spaced guide surfaces extending from proximate one of the contact extremities of the respective shoe, a tongue portion of each shoe extending from the other of the contact extremities into engagement with the guide surfaces of the adjacent shoe. The guide surfaces of each shoe can extend from the leading contact extremity, the tongue portion extending from the trailing contact extremity. Preferably the guides have locating contact with the adjacent shoes proximate opposite side extremities of the shoes for enhanced stabilization of the shoes against lateral tipping. 
     The inside surface can have a diameter of approximately 6 inches, the locking of the shoes by the lock portions of the shoe members preferably being effective at rotational speeds of at least 3000 RPM. The shoe members can each have an overall width, the lock portions of the shoe members each preferably extending solidly over the overall width for enhanced stability of the shoes. 
     The clutch of the present invention provides smooth shock-free load pick up without chatter or wear. The capacity increase from low torque to high torque is in the range of 500%. The combination including the stop pins allows non-fatiguing springs to be used, resulting in extended life. A fail-safe configuration prevents operation beyond approximately 95% of lining wear. The design of the clutch is notably versatile, resulting in significantly reduced tooling costs while serving a wide variety of applications. Also, the clutch is readily matched more precisely with particular power sources (which can be reciprocating diesel or gasoline engines, turbines, or electric motors) and loads than heretofore possible, resulting in longer life for both the power source and the driven mechanism (such as a compressor, pump, auger, or powered vehicle). The clutch, which incorporates the best features of low-torque, straight centrifugal, and high-torque configurations, is capable of transmitting more torque in a smaller, more economical package than existing clutches. 
     The clutch of the present invention starts with a softer torque characteristic than a conventional low-torque clutch, automatically converting with increased speed to a greater torque capacity than a conventional high-torque clutch. In a practical analogy to a car having a manual transmission, a variety of operating conditions are possible, including: 
     1. At start up and with the engine revved up the clutch pedal is gradually let out. A smooth get-away is accomplished, but at the expense of rapid wear and short clutch life. 
     2. At start-up and with the engine revved up the clutch pedal is abruptly let out, resulting in a jerky forward motion and/or stalling of the engine. 
     The first condition is analogous to a low-torque clutch and the second condition is analogous to a high-torque clutch. The clutch of the present invention automatically provides a short interval of low-torque operation for smooth starting, with a smooth but rapid change to a high-torque mode for achieving smooth starts without excessive wear. In summary, the clutch of the present invention exhibits a number of advantages from the perspective of a clutch manufacturer. For example: 
     1. It can be pre-set for optimum performance in each application; 
     2. There is increased customer satisfaction; 
     3. There are reduced warranty and liability claims; 
     4. Tooling and design staff requirements are reduced; 
     5. Both production and service inventory requirements are reduced; 
     6. New applications for the clutch can be serviced more quickly; and 
     7. Profitability extends to vast markets not now being served. 
     Many of the above advantages also apply to end-product manufacturers. In addition: 
     1. There is reduced shock to the overall system; 
     2. Operation is cooler, and with longer life; 
     3. Overall product performance can be fine-tuned, with separate speed-sensitive and torque-sensitive attributes; and 
     4. There is controllable over-load protection. 
    
    
     DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings, where: 
     FIG. 1 is a sectional end view of a centrifugal clutch according to the present invention; 
     FIG. 2 is a sectional side view of the clutch of FIG. 1; 
     FIG. 3 is a detail sectional view of a wedge member portion of the centrifugal clutch of FIG. 1; and 
     FIG. 4 is graph of clutch torque as a function of time for low torque, high torque and mode change configurations of the centrifugal clutch of FIG.  1 . 
    
    
     DESCRIPTION 
     The present invention is directed to a centrifugal clutch that is particularly safe, reliable, and easily configurable for smoothly transitioning from no-load to full load within a desired narrow speed range. In preferred implementations, the clutch also provides fail-safe operation wherein activation is suspended when lining wear is approximately 95% for protecting the drum from scoring. With reference to FIGS. 1 and 2 of the drawings, a centrifugal clutch  10  includes a drum  12  having a circularly cylindrical inside surface  14 , the drum  12  being concentrically rotatably mounted by a needle bearing  16  to a drive shaft  18 , the shaft  18  being configured for being supported and rotationally driven by conventional means (not shown), such as by being installed on an engine crank shaft. A hub  20  having tapered radially projecting dogs  22  is fixedly mounted to the shaft  18  such as by being pressed onto the shaft  18  over axially oriented serrations that are formed on the shaft, each dog  22  projecting into and engaging a respective wedge block  46  that is radially movable and circumferentially restrained relative to a corresponding shoe  24 . Each shoe  24  includes a rigid shoe member  26  and a lining  28  for frictionally engaging the drum  12  whereby the torque transmission capability of the clutch  10  increases with the speed of the shaft  18 . The lining  28  extends circumferentially on the shoe member  26  between a leading extremity  30 A and a trailing extremity  30 B of the lining  28 , the extremities  30 A and  30 B being generically referred to as lining extremities  30 . It will be understood that the shoe member  26  can be adapted for direct contact with the drum  12  in some applications, the lining  28  being omitted. 
     Each of the shoe members  26  is formed with a generally radially oriented drive slot  32  that is engaged by the corresponding wedge block  46 . In the exemplary configuration of the clutch  10  as shown, the drive slot  32  is located circumferentially closer to the leading extremity  30 A than the trailing extremity  30 B of the lining  28  for producing a soft-start response of the shoes  24 . The centrifugal force producing frictional engagement is proportional to the square of the rotational speed. It will be understood that locating the drive slot  32  midway between the extremities  30 A and  30 B provides a neutral response. Conversely, locating the drive slot  32  closer to the trailing extremity  30 B provides a more rapidly progressive torque characteristic (higher gain) torque profile of the clutch  10  as described in the above-referenced U.S. Pat. No. 2,942,711. As described below, it is contemplated that the clutch  10  can be selectively assembled for high and low gain profiles by reversing the orientation of shoes  24  relative to the direction of rotation. 
     As disclosed in U.S. Pat. No. 5,560,465, each shoe member  26  has an inwardly projecting first lock portion  34  and a circumferentially spaced outwardly projecting second lock portion  36 , with respective compression springs being interposed between corresponding lock portions  34  and  36  of adjacent shoes for biasing the shoes away from the drum  12 . According to the present invention, the biasing is provided by respective spring assemblies  38 , each spring assembly  38  including a calibrated stop pin  38 A that rigidly stops expansion of the shoe members  26  beyond a predetermined limit diameter D′ by directly abutting the lock portions  34  and  36  of adjacent shoes  24 . The lock portions  34  and  36  of adjacent shoes  24  interlock to prevent separation of the shoes  24  more than a predetermined amount beyond engagement with the drum  12  (the spring assemblies  38  having maximum compression in such condition). Thus, in case of fracture of the drum  12  or axial movement thereof away from the shoes  24 , the shoes  24  are advantageously retained in end-to-end engagement about the shaft  18 , rather than being thrown outwardly. This is a significant advantage in that the hub  20  is likely to be rotated at speeds significantly higher than those attained by the drum  12  in case of failure of the drum  12 . Also, in configurations of the clutch  10  normally having direct contact by the shoe members  26  against the drum inside surface  14 , the combination of the lock portions  34  and  36  with the stop pin  38 A provides an accurately determined condition of maximum allowed combined wear of the shoe members  26  and the drum inside surface  14 . 
     Preferably the limit diameter D′ is just slightly less than the drum diameter D for preventing metal-to-metal contact between the shoe members  26  and the drum  12  resulting from wear of the linings  28 . More particularly, the stop pins  38 A are preferably configured (calibrated) to prevent outward movement of the shoes beyond a point that would be reached when the linings  28  have worn through approximately 95 percent of their thickness. Each spring assembly  30  also includes a stack of spring washers  38 B through which the stop pin  38 A protrudes, and an adjustment nut  38 C that threadingly engages the corresponding stop pin  38 A for adjustably preloading the shoes  24  inwardly toward the hub  20 . It will be understood that while other forms of compression springs are contemplated, the implementation as spring washers is preferred, such being highly resistant to fracture failure as compared with highly loaded helical compression springs. Advantageously, the travel limitation is implemented without bottoming out the spring washers  38 B, resulting in spring washers  38 B being even less subject to fatigue failure than in the disclosed configurations of the &#39;465 patent. 
     The spring washers  38 B are located by being assembled onto respective ones of the stop pins  38 A, the pins  38 A in turn being located by opposite ends thereof having pivotable socket-engagement with respective transversely circularly cylindrical depressions  39  that are formed in lock portions  34  and  36  of the shoe members  26 . Also, opposite ends of the stop pins  38 A have transversely oriented cylindrical engagement surfaces  40  that pivotally abut the depressions  39  as shown in FIG. 1 when the shoes  24  are displaced sufficiently outwardly. (Although the engagement surfaces  40  are shown circularly cylindrical, other profiles are contemplated, such as beveled. Cylindrical means having a surface that is generated by a line that moves parallel to a fixed line.) Additionally, the stop pins  38 A can have a rectangular (typically square) cross-section to facilitate forming the engagement surfaces  40  on opposite ends thereof in parallel alignment as well as for preventing rotation of the stop pins  38 A during adjustment of the nuts  38 C. Thus the outward travel limitation of the shoe members  26  is accurately dictated by the stop pins  38 A independently of both the number and thickness of the spring washers  38 B and the adjustment of the nut  38 C, as long as the nut  38 C does not directly contact a shoe member  26 . As also shown in FIG. 2, The adjustment nut  38 C has a plurality of radially extending cavities  38 D formed therein for engagement by a suitable adjustment tool  38 E, which can be a length of commonly available drill rod. 
     As shown in FIG. 2, a pair of disk-shaped retainer members  41  are rigidly fastened on opposite sides of the hub  20  by a plurality of fasteners  42 , the shoes  24  being slidably confined between the retainer members  41 . The fasteners  42  are shown as threaded fasteners in the drawings, it being understood that rivet fasteners are alternatively contemplated. The retainer members  41  also serve to exclude contamination from the inside of the clutch  10  and for further purposes described herein. The retainer members  41  have radially extending clearance slots  43  formed therein for accessing the adjustment cavities  38 D and operation of the adjustment tool  38 E, the slots  43  being sufficiently large (and the cavities  38 D being sufficiently closely spaced) for permitting continuous adjustment of the nuts  38 C. Once a desired adjustment is achieved as described below, the adjustment can be locked by installing a lock pin (not shown) in an exposed one of the cavities  42  of each of the nuts  38 C (at the location  38 E depicted by broken lines in FIG.  2 ), each nut  38 C being lockable in one of a plurality of angular positions per revolution depending on the number of the cavities  42  therein. In the exemplary configuration of FIG. 2, the shoes  24  have a width W that is approximately 2.5 inches in the illustrated exemplary configuration of the clutch  10 , one of the spring assemblies  38  being laterally centrally located in space between adjacent shoe members  26 , the nut  38 C having eight of the cavities  38 D for permitting the continuous adjustment with the tool  38 E operating in space between adjacent ones of the shoe members  26 . Alternatively, the springs  38  can be laterally spaced in groups of two between each adjacent pair of shoes  24 , although adjustment of inboard ones of the spring assemblies  38  requires disassembly of the drum  12  from the shaft  18 . 
     As further shown in the drawings, the dogs  22  of the hub  20  are smoothly rounded between oppositely sloping ramp surfaces  21 . A wedge block  46  is interposed within the drive slot  32  between each shoe member  26  and the corresponding dog  22 , the wedge block  46  having sliding engagement with the slot  32  and being biased inwardly toward the hub  20  by at least one wedge spring  47  for producing a variable gain torque characteristic of the clutch  10 . More particularly, the wedge block  46  has at least one sloping wedge surface  48  that can contact the dog  22 . The wedge surface  48  is inclined at an angle  0  from a leading surface  49  of the wedge block  46 , a normal direction of rotation of the hub  20  relative to the drum  12  being counter-clockwise as indicated by the curved arrow in FIG.  1 . In the exemplary and preferred configuration shown in the drawings, the wedge block  46  has an oppositely inclined pair of the wedge surfaces, individually designated  48 H and  48 L, that project on opposite sides of the dog  22 . 
     The wedge spring  47  is located within a spring cavity  50  of the block  46 , being compressively loaded between the bottom of the cavity and a bail or keeper  51  that is supported by opposite ends thereof having pivotal engagement with the retainer members  41 . Significantly, the keepers  51  enable the inward loading of the wedge blocks  46  by the springs  47  without producing an outward reaction against the shoes  24  as previously disclosed in the above-referenced &#39;465 patent. Each of the drive slots  32  has a leading surface  52  and a trailing surface  54  with reference to the previously indicated curved arrow in FIG. 1, the surfaces  52  and  56  of the slot  32  being parallel. The shoes  24  can move outwardly into contact with the drum  12  while the wedge blocks  46  remain fully inwardly advanced relative to the hub  20 , the wedge block  46  slidingly contacting the leading surface  52  of the drive slot  32 . This condition of the clutch  10  is referred to as a threshold gain mode in the above referenced &#39;465 patent. The threshold gain mode continues until the speed increases to a first intermediate speed at which the wedge blocks  46  begin to move outwardly relative to the hub  20  in response to centrifugal force on the wedge block  46  in combination with a radial torque load component that may be applied to the wedge block  46  by the dog  22  in response to torque loading of the clutch  10 . In the threshold gain mode, the shoes  24  are driven outwardly only by centrifugal force acting on the shoes  24 , and the spring assemblies  38 , to the extent that these forces overcome the inward bias of the spring assemblies  38 . 
     As shown in FIG. 3, the wedge blocks  46  have inwardly extending slots  56  formed therein, the keepers  51  entering the slots  56  as the wedge blocks  46  move outwardly (when centrifugal force in combination with the radial torque load component exceeds initial loading of the wedge springs  47 ). The slots  56  extend partway into the respective cavities  50  that are formed in the wedge blocks  46  for receiving the wedge springs  47 . 
     As discussed above, the spring assemblies  38  bias the shoes  24  inwardly, away from the drum  12  for achieving a desired torque-speed characteristic, being selected and/or adjusted for providing the desired threshold speed of engagement, such as approximately 1800 RPM when the drum diameter D is on the order of 6 inches. The torque capacity increases with the centrifugal component of frictional engagement increasing proportional to the square of the rotational speed of the hub  20  as discussed in the above-referenced &#39;465 patent. 
     From the first intermediate speed to a second intermediate speed, the clutch  10  operates in a first gain mode wherein the wedge blocks  46  are displaced partly outwardly within the drive slot  32  by the combination of centrifugal force and the radial component of the torque reaction imparted by the dogs  22 . In the first gain mode, the shoes  24  continue to be driven outwardly by centrifugal force as offset by the loading of the spring assemblies  38  as in the threshold mode described above, until the keepers  51  reach the bottoms of the slot  56 . As used herein, the threshold gain mode and the first gain mode are collectively referred to as a low gain mode. 
     At speeds exceeding the second intermediate speed, the wedge block  46  is fully retracted outwardly within the drive slot  32  against the keepers  51 , the clutch  10  operating in a second or high gain mode wherein the shoes  24  are driven outwardly by centrifugal force on the shoes  24  and the spring assemblies  38  and in addition, a radial component of force between the wedge blocks  46  and the leading surface  52  of the drive slot  32  that results from pressure against the wedge blocks  46  from the dog  22  in transmitting torque from the drive shaft  18 , and offset by the inward bias by the spring assemblies  38 . In contrast with the above-referenced &#39;465 patent, the centrifugal force on the shoes  24  does not include the mass component of the wedge blocks  46  as in the &#39;465 patent since the wedge blocks  46  are prevented from reaching the outer ends of the drive slots  32  by the keepers  51 . 
     As also shown in FIG. 1, the first lock portion  34  of each shoe member  26  is formed proximate the trailing extremity  30 B of the respective lining  28 , the second lock portion  36  being spaced beyond leading extremity  30 A of the lining  28 . Thus the trailing extremity  30 B extends circumferentially a significantly a greater distance from the drive slot  32  than does the leading extremity  30 A for stabilizing the shoes  24 . As further shown in FIG. 1, a preferred configuration of the clutch  10  has adjacent ones of the shoes  24  in close sliding engagement for enhancing dynamic stability of the combination, avoiding chattering, and reducing wear. Accordingly, each shoe member  26  has a guide tongue  62  extending from the first lock portion  34  and slidably engaging a guide slot  64  of the adjacent shoe member  26  as described in the above-referenced &#39;465 patent. 
     The clutch  10  of the present invention thus provides a particularly advantageous combination of soft start and high gain that provides a high capacity in a relatively small package. The automatic shifting between the low and high gain modes is bidirectional in the sense that the clutch  10  also provides protection against excessive overloading of the power source in that the low gain mode is entered almost immediately upon the occurrence of a blockage of a pump, for example, and although the clutch  10  might eventually overheat, the more expensive pump and engine are protected by the clutch  10  from damage that might otherwise occur. In addition, the bidirectionality extends to operating the clutch in reverse (opposite to the direction of the curved arrow in FIG. 1) in that the wedge blocks  46  have the oppositely inclined wedge surfaces  48 . It will be understood that the additional wedge surface  48  can be inclined appropriately for achieving a desired torque-speed characteristic when operating with reverse rotation. Alternatively, the wedge surfaces  48  can be inclined differently relative to the wedge blocks  46  for facilitating adjustment of the torque-speed relation in a normal direction of rotation by selectively reversing the orientation of the wedge blocks  46  in the drive slots  32 . 
     In a further improvement of the clutch  10 , the drive slots  32  are inclined at an angle φ from being radially oriented relative to the dogs  22  when the wedge blocks  46  are fully inwardly advanced against the dogs  22 , the angle φ being approximately 10 degrees. 
     A series of tests were performed during development of the clutch  10  in order to verify and refine the operation thereof. In order to facilitating of the results, embodiments of the clutch  10  modified to be locked in low torque and high torque modes were tested in addition to the inventive mode changing configuration. In the low torque case, the wedge blocks  46  were fixedly joined to the hub  20 ; in the high torque mode, the wedge blocks  46  were fixedly advanced fully outwardly in the drive slots  32 . In one test wherein the angle θ was approximately 37.5 degrees, the clutch  10  was operated in a set up driven by an engine of approximately 20 horsepower and accelerating a load. In this preliminary test, the clutch having the mode changing configuration was observed to exhibit the mode change from low gain to high gain modes at only about 2.5 seconds into the run and having a capacity of 18.34 foot pounds at the end of the run as compared with 19.08 foot pounds that was obtained with the counterpart high torque clutch. In subsequent tests, the angle θ was decreased by approximately 12 degrees to approximately 25.5 degrees. 
     Data for these tests, listed below in Table I, was obtained under computer control, whereby a series of engine RPM, dynamometer RPM, torque. As before, three clutches were tested: a low-torque clutch, a high-torque, and the mode-change clutch. All three clutches had identical drums, shoes, spring assemblies, and spring settings. The low-torque and High-torque clutches were tested using a conventional solid driving hub and were assembled respectively in low and high torque configurations. The mode-change clutch included the driving hub  20  described above with the wedge blocks  46  and wedge springs  47 . Each clutch was tested with the same dynamometer load for approximately the same interval of time, that is 30 to 31 seconds. The engine speed was gradually brought up to approximately 3500 RPM and gradually decreased. The circled numbers in the Key columns are selectively located for approximately coincident time intervals in the respective tests. The data for those nine time intervals is summarized together with the corresponding times in Table II, below, and plotted in FIG.  4 . With the angle θ being 25.5 degrees, a marked improvement in the results occurred. For example, the change from low to high gain modes did not occur until approximately 5 seconds into the run advantageously corresponding to lighter engine loading during acceleration of loads in practical applications. Also, the torque capacity of the clutch increased to 25.11 foot pounds, which was more than the 24.47 that attained with the high torque clutch at the high end of the power curve. Interestingly, the mode change clutch capacity was still climbing at the high end, whereas the high torque clutch&#39;s capacity had long since begun to fall off. Of particular advantage is that the majority of slippage of the mode change clutch of the present invention occurs at the low end of the power curve (a low energy state), and as a result there is minimal heating, a factor which extends clutch life. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Sample Computer/Dyno Readouts 
               
             
          
           
               
                 Low-Torque-Clutch 
                 High-Torque-Clutch 
                 Mode-Change-Clutch 
               
             
          
           
               
                 KEY 
                   
                 KEY 
                   
                 KEY 
                   
               
               
                   
               
             
          
           
               
                 {circle around (1)} 
                 443 
                 3056 
                 2.73 
                 {circle around (1)} 
                 810 
                 3304 
                 5.00 
                 {circle around (1)} 
                 218 
                 300 
                 1.13 
               
               
                   
                 467 
                 3085 
                 2.88 
                   
                 910 
                 3287 
                 5.79 
                   
                 233 
                 3108 
                 1.18 
               
               
                   
                 490 
                 3115 
                 3.05 
                   
                 988 
                 3297 
                 6.50 
                   
                 322 
                 3122 
                 1.65 
               
               
                 {circle around (2)} 
                 506 
                 3126 
                 3.17 
                 {circle around (2)} 
                 1060 
                 3299 
                 7.06 
                 {circle around (2)} 
                 496 
                 3125 
                 2.60 
               
               
                   
                 517 
                 3131 
                 3.21 
                   
                 1132 
                 3298 
                 7.75 
                   
                 707 
                 3137 
                 3.94 
               
               
                   
                 533 
                 3145 
                 3.41 
                   
                 1203 
                 3297 
                 8.48 
                   
                 852 
                 3135 
                 5.00 
               
               
                   
                 566 
                 3174 
                 3.62 
                   
                 1268 
                 3300 
                 9.05 
                   
                 948 
                 3147 
                 5.75 
               
               
                   
                 614 
                 3196 
                 3.98 
                   
                   
                   
                   
                   
                 1019 
                 3149 
                 6.35 
               
               
                 {circle around (3)} 
                 662 
                 3204 
                 4.38 
                 {circle around (3)} 
                 1332 
                 3305 
                 9.72 
                 {circle around (3)} 
                 1091 
                 3157 
                 6.98 
               
               
                   
                 720 
                 3222 
                 4.87 
                   
                 1446 
                 3300 
                 10.88 
                   
                 1122 
                 3156 
                 7.23 
               
               
                   
                 764 
                 3238 
                 5.22 
                   
                 1505 
                 3305 
                 11.55 
                   
                 1150 
                 3159 
                 7.46 
               
               
                   
                 815 
                 3246 
                 5.63 
                   
                   
                   
                   
                   
                 1175 
                 3166 
                 7.70 
               
               
                   
                 875 
                 3223 
                 6.14 
                   
                 1661 
                 3300 
                 13.28 
                   
                 1193 
                 3167 
                 7.85 
               
               
                   
                 912 
                 3260 
                 6.47 
                   
                 1701 
                 3301 
                 13.67 
                   
                 1203 
                 3167 
                 7.94 
               
               
                 {circle around (4)} 
                 955 
                 3251 
                 6.82 
                 {circle around (4)} 
                 1735 
                 3302 
                 14.16 
                 {circle around (4)} 
                 1212 
                 3166 
                 8.03 
               
               
                 {circle around (5)} 
                 1779 
                 3320 
                 15.93 
                 {circle around (5)} 
                 2243 
                 3302 
                 20.43 
                 {circle around (5)} 
                 1347 
                 3256 
                 9.23 
               
               
                   
                 1785 
                 3329 
                 15.97 
                   
                 2257 
                 3302 
                 20.50 
                   
                 1347 
                 3260 
                 9.19 
               
               
                   
                 1786 
                 3332 
                 15.94 
                   
                 2269 
                 3301 
                 20.66 
                   
                 1349 
                 3259 
                 9.23 
               
               
                   
                 1804 
                 3322 
                 16.18 
                   
                 2281 
                 3308 
                 20.89 
                   
                 1353 
                 3261 
                 9.27 
               
               
                 {circle around (6)} 
                 2142 
                 3383 
                 20.43 
                 {circle around (6)} 
                 2605 
                 3288 
                 24.76 
                 {circle around (6)} 
                 2189 
                 3253 
                 17.98 
               
               
                   
                 2166 
                 3385 
                 20.73 
                   
                 2600 
                 3286 
                 24.65 
                   
                 2153 
                 3252 
                 18.57 
               
               
                   
                 2186 
                 3390 
                 20.98 
                   
                 2594 
                 3284 
                 24.50 
                   
                 2179 
                 3238 
                 18.97 
               
               
                   
                 2211 
                 3390 
                 21.30 
                   
                 2596 
                 3283 
                 24.49 
                   
                 2196 
                 3237 
                 19.17 
               
               
                   
                   
                   
                   
                   
                 2591 
                 3280 
                 24.53 
                   
                 2223 
                 3243 
                 19.54 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 2253 
                 2141 
                 19.95 
               
               
                   
                 2459 
                 3409 
                 23.47 
                   
                 2639 
                 3273 
                 24.34 
               
               
                 {circle around (7)} 
                 2461 
                 3399 
                 23.48 
                 {circle around (7)} 
                 2644 
                 3281 
                 24.48 
                 {circle around (7)} 
                 2534 
                 3214 
                 23.74 
               
               
                   
                 2467 
                 3401 
                 23.51 
                   
                 2654 
                 3290 
                 24.50 
                   
                 2529 
                 3214 
                 23.62 
               
               
                   
                 2469 
                 3402 
                 23.52 
                   
                 2644 
                 3282 
                 24.36 
                   
                 2515 
                 3216 
                 23.35 
               
               
                   
                   
                   
                   
                   
                 2652 
                 3271 
                 24.53 
                   
                 2519 
                 3221 
                 23.53 
               
               
                   
                 2530 
                 3408 
                 23.75 
                   
                 2661 
                 3271 
                 24.70 
                   
                 2537 
                 3220 
                 23.71 
               
               
                   
                 2534 
                 3408 
                 23.79 
                   
               
               
                 {circle around (8)} 
                 2536 
                 3407 
                 23.85 
                 {circle around (8)} 
                 2684 
                 3281 
                 24.63 
                 {circle around (8)} 
                 2635 
                 3209 
                 24.91 
               
               
                   
                 2539 
                 3406 
                 23.77 
                   
                 2679 
                 3277 
                 24.45 
                   
                 2607 
                 3210 
                 24.36 
               
               
                   
                 2542 
                 3406 
                 23.84 
               
               
                   
                 2556 
                 3407 
                 23.60 
               
               
                 {circle around (9)} 
                 2554 
                 3408 
                 23.47 
                 {circle around (9)} 
                 2698 
                 3281 
                 24.47 
                 {circle around (9)} 
                 2655 
                 3209 
                 25.12 
               
               
                   
                 2557 
                 3407 
                 23.53 
                   
                 2705 
                 3276 
                 24.73 
                   
                 2662 
                 3206 
                 25.11 
               
               
                   
                 2561 
                 3407 
                 23.54 
                   
                 2710 
                 3282 
                 24.65 
                   
                 2648 
                 3206 
                 24.86 
               
               
                   
                 2566 
                 3407 
                 23.59 
                   
                 2698 
                 3283 
                 24.43 
                   
                 2640 
                 3205 
                 24.74 
               
               
                   
                 2572 
                 3407 
                 23.59 
                   
                 2704 
                 3276 
                 24.56 
                   
                 2644 
                 3203 
                 24.90 
               
               
                   
                 2577 
                 3405 
                 23.62 
                   
                 2700 
                 3282 
                 24.47 
                   
                 2654 
                 3202 
                 24.84 
               
               
                   
                 2584 
                 3407 
                 23.65 
                   
                 2700 
                 3276 
                 24.40 
                   
                 2656 
                 3201 
                 25.05 
               
               
                   
                 2588 
                 3407 
                 23.71 
                   
                 2700 
                 3276 
                 24.49 
                   
                 2653 
                 3202 
                 24.87 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Time-Correlated Subset of Test Data 
               
             
          
           
               
                   
                   
                 AUTOMATIC LOW TO HIGH 
                   
               
               
                 LOW TORQUE CLUTCH DESIGN 
                 HIGH TORQUE CLUTCH DESIGN 
                 MODE CHANGE CLUTCH DESIGN 
               
             
          
           
               
                   
                   
                 DY- 
                   
                   
                   
                   
                   
                 DY- 
                   
                   
                   
                   
                   
                   
                 DY- 
                   
                   
                   
               
               
                 TIME 
                 ENG. 
                 NO. 
                 % 
                 TORQ. 
                   
                 TIME 
                 ENG. 
                 NO. 
                 % 
                 TORQ. 
                   
                 TIME 
                 ENG. 
                 NO. 
                 % 
                 TORQ. 
                   
                   
               
               
                 SEC. 
                 RPM 
                 RPM 
                 SLIP 
                 Ft Lbs 
                 LINE 
                 SEC. 
                 RPM 
                 RPM 
                 SLIP 
                 Ft Lbs 
                 LINE 
                 SEC. 
                 RPM 
                 RPM 
                 SLIP 
                 Ft Lbs 
                 LINE 
                 KEY 
               
               
                   
               
               
                 1.05 
                 3056 
                 443 
                 86% 
                 2.73 
                 9 
                 1.12 
                 3304 
                 810 
                 75% 
                 5.00 
                 10 
                 1.10 
                 3090 
                 218 
                 93% 
                 1.13 
                 10 
                 {circle around (1)} 
               
               
                 1.40 
                 3126 
                 506 
                 83% 
                 3.17 
                 12 
                 1.47 
                 3299 
                 1060 
                 68% 
                 7.06 
                 13 
                 1.43 
                 3125 
                 496 
                 84% 
                 2.60 
                 13 
                 {circle around (2)} 
               
               
                 1.99 
                 3204 
                 662 
                 79% 
                 4.38 
                 17 
                 1.90 
                 3305 
                 1332 
                 60% 
                 9.72 
                 11 
                 1.98 
                 3157 
                 1.91 
                 65% 
                 6.98 
                 18 
                 {circle around (3)} 
               
               
                 2.69 
                 3271 
                 955 
                 71% 
                 6.82 
                 23 
                 2.69 
                 3302 
                 1735 
                 47% 
                 14.16 
                 24 
                 2.64 
                 3166 
                 1212 
                 62% 
                 8.03 
                 24 
                 {circle around (4)} 
               
               
                 4.91 
                 3320 
                 1779 
                 46% 
                 15.93 
                 43 
                 4.93 
                 3302 
                 2243 
                 32% 
                 20.43 
                 44 
                 4.95 
                 3256 
                 1347 
                 59% 
                 9.23 
                 45 
                 {circle around (5)} 
               
               
                 7.49 
                 3383 
                 2142 
                 37% 
                 20.43 
                 64 
                 7.50 
                 3288 
                 2605 
                 21% 
                 24.76 
                 67 
                 7.59 
                 3253 
                 2109 
                 35% 
                 17.98 
                 69 
                 {circle around (6)} 
               
               
                 11.35 
                 3399 
                 2461 
                 28% 
                 3248 
                 97 
                 11.31 
                 3281 
                 2644 
                 19% 
                 24.48 
                 101 
                 11.33 
                 3214 
                 2534 
                 21% 
                 32.74 
                 103 
                 {circle around (7)} 
               
               
                 13.81 
                 3407 
                 2536 
                 26% 
                 23.85 
                 118 
                 13.89 
                 3281 
                 2684 
                 18% 
                 24.63 
                 124 
                 13.86 
                 3209 
                 2635 
                 18% 
                 24.91 
                 126 
                 {circle around (8)} 
               
               
                 15.56 
                 3408 
                 2554 
                 25% 
                 23.47 
                 133 
                 15.57 
                 3281 
                 2698 
                 18% 
                 24.47 
                 139 
                 15.51 
                 3209 
                 2655 
                 17% 
                 25.12 
                 141 
                 {circle around (9)} 
               
               
                   
               
             
          
         
       
     
     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, the linings  28  can be omitted as discussed above. Also, the angle θ can be selected for greater or lesser energizing of the clutch  10 . The diameter D can be selected according to the application aver a wide range such as from 1.125 inch up to more than 6 feet. Operating speeds can range from less than 500 RPM to over 10,000 RPM. Therefore, the spirit and scope of the appended claims should not necessarily be limited to the description of the preferred versions contained herein.