Abstract:
Continuously variable speed drive (CVT) have improved performance due to use of two groups of flyweight cams, each group being mounted on separate rotational members of a centrifugal clutch. One set of flyweight cams provides force to close the sheaves through the entire movement of the shift, whereas the other set of flyweight cams provides force only through an initial portion of the shift and then signs off. This provides a two stage shift behavior for the CVT that enhances power transfer to the driven pulley.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from U.S. Provisional Patent Application Ser. No. 61/034,730, filed Mar. 7, 2008, and entitled “Dual Stage Clutch”, which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to clutch mechanisms. More particularly, the present invention relates to a clutch mechanism that provides a variable diameter for driving of an endless belt and which is adapted for use in connection with a high performance engine. 
       BACKGROUND INFORMATION 
       [0003]    One class of conventional continuously variable transmissions (or “CVT&#39;s”) has two tapered-faced pulleys interconnected with a belt of essentially fixed length. The sheaves of each pulley are able, under control, to move axially. The pulley on one shaft is connected to the crankshaft of the engine. The system including the pulley, and its ancillary parts, that is connected to the engine is called the driving, driver, or primary clutch. The other pulley is connected through a linkage to the drive train of a vehicle. This other pulley and its related parts, is called the driven or secondary clutch. Of necessity, when the sheaves of either pulley are close together, the associated belt must be located at a relatively large radius (distant from the axis of rotation of the pulley) and when the sheaves of a pulley are far apart, the associated belt must be located at a relatively small radius (close to the axis of rotation of the pulley). 
         [0004]    Typically, because of the essentially fixed length of the belt, when the sheaves of one pulley are far apart, then the sheaves of the other pulley must be close together. Larger shift ratios, characteristic of slower vehicle speeds, occur when the sheaves of the primary pulley are far apart and the sheaves of the secondary pulley are close together. With this configuration, the rotational speed of the primary pulley is greater than the rotational speed of the secondary pulley. Smaller shift ratios, characteristic of higher vehicle speed, occur when the sheaves of the primary pulley are close together and the sheaves of the secondary pulley are far apart. With this configuration, the rotational speed of the primary pulley is less than the rotational speed of the secondary pulley. 
         [0005]    Ordinarily, the primary clutch has a compression spring, or the like, tending to push the sheaves apart such that, at rest, the sheaves of the primary pulley have opened to allow the belt to lie close to the pulley&#39;s rotational axis, effecting a large shift ratio. Such a belt position at rest results in the engine having a desirable minimal load at the start of driving. The force produced by this spring increases as the sheaves of the primary pulley get closer together (lower shift ratios) and further compress the spring. Other parts of the primary clutch include a set of pivoting flyweights on the primary clutch pushing on a roller, or the like, linked such that the sheave spacing, and thus shift ratio, is responsive to speed and torque needs of the secondary clutch. 
         [0006]    In the known CVT systems, the net result of the spring and flyweights of the primary clutch provide some beneficial results. Specifically, these CVT systems yield a primary pulley belt side force that is sufficient to allow the engine to start and promptly get up to approximately a rotational speed where the engine can deliver maximum power to its shaft. These CVT systems also yield a belt side force that increases with increasing vehicle speed (decreasing shift ratio) to a peak. Another benefit of these CVT systems is that they provide a belt side force that decreases with increasing vehicle speed. 
         [0007]    An undesirable result of these CVT systems is a tendency to lose power because of belt slippage due to insufficient belt side force while the vehicle is accelerating to near maximum speed. A desirable result of the just described belt side force is a tendency for the system to increase the shift ratio (deliver more torque) when the vehicle slows down. 
         [0008]    The typical role of the engine is to start, to accelerate promptly to a high rotational speed where the engine can deliver approximately its maximum power, and to remain at that high speed delivering approximately a constant amount of power. Power, in this context, is the product of torque and rotational velocity (expressed as P=τ×ω, where P is power, τ is torque, and ω is rotation velocity). The role of the CVT is to apportion the power delivered by the engine into a torque and speed portion depending on the vehicle&#39;s speed. When the vehicle is moving slowly, the CVT has a high shift ratio, and the torque factor is relatively large. When the vehicle is moving rapidly, the CVT has a smaller shift ratio, and the torque factor is smaller. 
         [0009]    It is known to use a clutch having a plurality of flyweights pivotally mounted on a single rotatable member, with the flyweights arranged to move radially outward with increasing rotational velocity of the shaft. For additional details, refer to U.S. Pat. No. 3,727,478 to Erickson. 
         [0010]    U.S. Pat. No. 5,529,544 to Berto is another example of a system having a plurality of flyweights pivotally mounted on a single rotatable member. Specifically, this patent teaches a clutch having speed responsive means consisting of two sets of flyweights having different size, shape and/or weights. One of the first set of flyweights and one of the second set of flyweights are positioned side-by-side and have the same axis of rotation. The first and second sets of flyweights operate simultaneously by exerting an initial displacement force against the moveable sheave. Then, at a predetermined rotational speed of the drive clutch or predetermined position of the second set of flyweights, the flyweights of the second set are prevented from exerting force on or further displacing the moveable sheave. Thus, for rotational speeds greater than the preselected rotational speed, the first set of flyweights act alone in displacing the moveable sheave. 
         [0011]    while the clutch disclosed in Berto yields some benefits, it still suffers from significant disadvantages. For example, due to the side-by-side arrangement of the first and second set of flyweights, the mass of the flyweights is limited due to space constraints and structural integrity constraints. Because of the limited mass of the flyweights, this arrangement cannot provide sufficient force necessary to perform the function of today&#39;s high performance engines. 
       SUMMARY OF THE INVENTION 
       [0012]    In general terms, the present invention provides an improved centrifugal clutch the structure of which permits its use with engines having different performance characteristics, and enables simple modification resulting in a desired performance relationship with the selected engine and different operating environments. 
         [0013]    One advantage of the present invention centers on two sets of flyweights or cam arms, each set consisting of a plurality of flyweights that are mounted on a separate rotatable member of the clutch such that each set of flyweights has a different axis of rotation. The flyweights each have an engageable cam surface that relates engine revolutions-per-minute to the input/output revolution ratio to effect a simulated gear shifting transmission characteristic. 
         [0014]    Another advantage of the present invention is directed to flyweights made of different materials and weights to obtain a desired centrifugal action, and the mounting of flyweights having predetermined centers of mass to obtain the greatest mechanical advantage at the optimum time during the shifting process and variable ratio of the transmission. One of the sets of the flyweights is mounted in a manner to create a supplemental dual force to help eliminate belt slippage during high load use. 
         [0015]    It is still another advantage of the present invention to provide an improved centrifugal clutch that imparts increased force at inertial shift. 
         [0016]    It is yet another advantage of the present invention to provide an improved centrifugal clutch that affords increased performance for high performance engines, including lower RPM higher horsepower engines, due to the application of more force than by prior clutch mechanism. 
         [0017]    Embodiments of the present invention use flyweights on two distinct rotatable members of the clutch that lie in two different parallel planes. The utilization of flyweights on two separate members creates massive adjustable force in the first ¼ to ¾ of the shift then signs off or stops engagement letting the flyweights on only one of the members to complete the shifting of the sheaves. This is a significant improvement over prior clutches that use only flyweights on one rotating member. 
         [0018]    These and other features and advantages of this invention will become more apparent to those skilled in the art from the detailed description of a preferred embodiment. The drawings that accompany the detailed description are described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a perspective view of a dual stage primary clutch according to an embodiment of the present invention; 
           [0020]      FIG. 2  shows a sectional view of a dual stage primary clutch according to an embodiment of the present invention, with the clutch in a first position of engagement at the start of shifting; 
           [0021]      FIG. 3  shows a sectional view of a dual stage primary clutch according to an embodiment of the present invention, with the clutch in a second position at a transition point between a first stage of shifting and a second stage of shifting; 
           [0022]      FIG. 4  shows a sectional view of a dual stage primary clutch according to an embodiment of the invention, with the clutch in a third position at the end of shifting; 
           [0023]      FIG. 5  shows a perspective view of a spider assembly for use in an embodiment of the present invention; and 
           [0024]      FIG. 6  shows a perspective view of a moveable sheave assembly for use in an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The disclosure is directed to an improved centrifugal clutch the structure of which permits its use with engines having different performance characteristics, and enables simple adjustable modification resulting in a desired performance relationship with the selected engine and different operating environments. One feature centers on the inclusion of a plurality of flyweights, each having an engageable cam surface that relates engine revolutions-per-minute to the input/output revolution ratio to effect a simulated gear shifting transmission characteristic. Other features are directed to flyweights made of different materials and weights to obtain the proper centrifugal action, and the mounting of flyweights having predetermined centers of mass to obtain the greatest mechanical advantage at the optimum time during the shifting process and variable ratio of the transmission, as is well known. Still another feature includes flyweights mounted in a manner to create a supplemental dual force to help eliminate belt slippage during high load use by providing a non-linear characteristic of the force used to compress the spring in the clutch. 
         [0026]    The improved centrifugal clutch of the present invention is intended for use in applications utilizing high performance engines with continuously variable transmission belt driving systems. These CVT systems are known in the art to be used in many different types of vehicles, such as snowmobiles, golf carts, go-karts, all terrain vehicles, riding lawn mowers, electric-powered cars, and the like. These systems are also known to be used on many different types of machines used in the manufacturing and service industries. The present invention can also be utilized in connection with other types of vehicles or machines. 
         [0027]    Two-stroke and four-stroke engines that commonly use variable speed transmission belt driving systems operate more efficiently in a known power region where the engine delivers optimum power. The lower gear ratio of these typical variable speed transmissions occurs where the transmission belt is near the bottom of the sheaves of the driving clutch and near the outer edges of the sheaves of the driven clutch. In the low gear ratio period, the CVT is generally inefficient. The high gear ratio occurs where the belt is near the outer edges of the driving clutch and near the bottom of the sheaves of the driven clutch. The high and low gear ratios are known and can vary as will be appreciated by one of ordinary skill in the art. 
         [0028]    The present invention relates to a driving clutch  10  for a continuously variable belt driving system. As is known, the system is connected between a horizontal shaft that is in communication with an engine. As discussed in more detail below, the driving clutch  10  is mounted on the engine drive shaft to cause rotation thereof. The system also includes a driven clutch mounted on a driven shaft that is rotatably mounted to the vehicle such that it is parallel to the drive shaft. An endless drive belt connects the driving clutch  10  to the driven clutch to effectuate rotation thereof, as is well known in the art. 
         [0029]    With reference to the Figures, in accordance with one embodiment, the driving clutch  10  includes a fixed sheave  12  and a moveable sheave  14  The driving clutch  10  is mounted in any of a variety of known suitable methods such that the fixed sheave  12  is located inboard (closer to the engine) of the movable sheave  14 . The fixed sheave  12  is secured to a post  16  at a bottom end  18  thereof. The moveable sheave  14  is disposed on the post  16  such that it is axially moveable toward and away from the fixed sheave  12 , as discussed in more detail below. The sheaves each have tapered faces as is known in the art. The post  16  has a lower portion  20  having a first diameter (d 1 ) and a middle portion  22  having a second diameter (d 2 ) that is smaller than the first diameter (d 1 ). The post  16  also includes an upper portion  24  with a third diameter (d 3 ) that is smaller than the second diameter (d 2 ). The bottom end  18  of the post  16  engages the drive shaft to effectuate rotation of the post  16  and the sheaves  12 ,  14 . The post  16  can obviously have a variety of different shapes and configurations. 
         [0030]    The moveable sheave  14  includes a plurality of primary rollers  26  located thereon. The primary rollers  26  are disposed radially about the circumference of the moveable sheave  14 . In one preferred embodiment, each primary roller  26  is secured between a pair of adjacent tower portions  28  that extend upwardly from a base portion  30  of the moveable sheave  14 . When secured in place, each primary roller  26  spans an opening  32  formed between adjacent tower portions  28 . This configuration provides structural support for the primary rollers  26  and minimizes failure when a flyweight is disposed thereon, as discussed in more detail below. In accordance with one embodiment, the moveable sheave  14  has four primary rollers  26  equally spaced about its periphery. However, it will be understood that the number of primary rollers and attached flyweights can vary as required. 
         [0031]    Each primary roller  26  has a primary flyweight  34  rotatably coupled thereto. Each primary flyweight  34  has a head portion  60  through which a respective primary roller  26  passes and a body portion  62  extending from the head portion  60 . The body portion  62  includes a cam surface  80 . Each of the primary rollers  26  defines an axis of rotation for the attached primary flyweight  34  and the axis of rotation for each of the primary flyweights  34  lies in the same horizontal plane. This horizontal plane is oriented perpendicular to an axis of rotation of the post  16  and engine drive shaft. Each primary flyweight  34  is positioned on a respective primary roller  26  such that it can pivot within an opening  32  between adjacent towers  28  as the clutch  10  rotates. The plurality of primary flyweights  34  assist in moving the moveable sheave  14  in increasing amounts in response to increasing rotational speed of an associated drive shaft, as will be discussed in more detail below. 
         [0032]    Each of the tower portions  28  also includes a secondary roller  36  secured thereto. The secondary rollers  36  are secured to an individual tower portion  28  to ensure their structure stability and integrity. In a preferred embodiment, there are four secondary rollers  36  uniformly spaced around the periphery of the moveable sheave  14 . However, more or less secondary rollers  36  may be included as desired. 
         [0033]    In accordance with a preferred embodiment, the driving clutch  10  also includes a spider assembly  40 . The spider assembly  40  is a separate component from the moveable sheave  14  and includes a plurality of spokes  42 . Each of the spokes  42  extends radially outwardly from a center portion  44 . Each of the spokes  42  is received within one of the openings  32  between the adjacent towers portions  28  such that the spider assembly  40  is retained in place as the clutch  10  and thus the primary sheave  12  and the moveable sheave  14  rotate. Each of the spokes  42  of the spider assembly  40  includes a spider roller  46  disposed in an underside thereof. As each spider roller  46  spans an opening  32 , it can engage an or contact a respective cam surface  80  of one of the flyweights  34  as it pivots. 
         [0034]    The spider assembly  40  also includes a plurality of secondary flyweights  48  that extend outwardly from the center portion  44 . The secondary flyweights  48  are positioned on the spider assembly  40  between the spokes  42  and are intended to contact a respective one of the secondary rollers  36 . The secondary flyweights  48  have a head portion  64  that is pivotally secured to the spider assembly  40  and a body portion  66  that extends outwardly therefrom. The body portion  66  of the secondary flyweight  48  also includes a cam surface  82  that engages the secondary roller  36 . The spider assembly  40  is positioned such that the secondary flyweights  48  pass through an opening in a respective tower portion  28  with the cam surface  82  engaging an upper side of the secondary roller  36 . 
         [0035]    The secondary flyweights  48  also assist in urging the moveable sheave  14  in increasing amounts in response to increasing rotational speed of the drive shaft. The secondary flyweights  48  are located on a separate member (the spider assembly  40 ) from the primary flyweights  34  (the moveable sheave  14 ). The secondary flyweights  48  also have a respective axis of rotation that all lie in the same horizontal plane. The plane in which the axis of rotation of the secondary flyweights  48  lie is parallel to the plane of the axis of rotation of the primary flyweights  34 . Thus, the primary flyweights  34  lie generally in a first plane and the secondary flyweights  48  lie in a second plane, which planes are parallel to one another and lie perpendicular to the axis of rotation of the clutch  10 . A cover  50  is attached to the tower portions  28  and engage the post  16  for axial movement. 
         [0036]    Referring to  FIG. 2 , a dual stage primary clutch  10  is shown in section in a first position of engagement at the start of shifting. In this position, the fixed sheave  12  and the moveable sheave  14  are spaced apart their maximum distance. In this position, the engine is operating in a low gear and the belt is positioned near the inner portions  52 ,  54  of the sheaves  12 ,  14 . When the clutch  10  is operating at shift start, the cam surface  80  of the primary flyweights  34  engage or contact a respective spider roller  46 . Also, the primary flyweights  34  are positioned such that the body portion  62  extends inwardly and is oriented generally horizontally. In this position, the secondary or supplemental flyweights  48  engage a moveable secondary roller  36  disposed on the moveable sheave  14 . As shown, the supplemental flyweights  48  are angled slightly upwardly. Both the primary flyweights  34  and the secondary flyweights  48  are shown at the beginning of their pivot at the beginning of the shift. It will be appreciated by one of ordinary skill in the art that each of the primary flyweights  34  and the secondary flyweights  48  operate together such that a description of the structure, operation, and position of one individual flyweight applies to the remainder of flyweights in that set. A spring biases the fixed sheave  12  and the moveable sheave  14  away from one another. 
         [0037]    Referring to  FIG. 3 , the dual stage primary clutch  10  is shown in a second position at a transition point between a first stage of shifting and a second stage of shifting. In this position, the primary flyweights  34  are shown as being about 75% of the way through their pivot movement. As the primary flyweights  34  pivot such that the body portion  62  and cam surface  80  moves upwardly in contact with spider roller  46 , the cam surfaces  82  on the body portions  48  remain in contact with a respective moveable roller  36  moving downwards, and together urge the moveable sheave  14  downward against the force of the spring. As shown, the secondary flyweights  48  have at this point moved through their entire pivot movement such that they extend outwardly in a generally horizontal direction. While the completion of this stage is illustrated at 75% rotation of the primary flyweights  34 , the clutch  10  can be configured to occur anywhere between 25%-75% rotation of the primary flyweight. 
         [0038]    As the moveable sheave  14  travels toward the fixed sheave  12 , the spider assembly  40  moves further apart from the moveable sheave  14 . This is due to the pivoting of the primary flyweights  34  and their engagement with the spider rollers  46 . Additionally, at this speed of rotation, the centrifugal force has caused the secondary flyweights  48  to rotate about their pivot bar  90 . Up to this point (rotational speed), the secondary flyweights  48  apply an additional downward force on the moveable sheave  14 . As can be seen, at this position, the secondary flyweights arms  48  have pivoted throughout their full range and cannot pivot any further. The secondary flyweights  48  are no longer in contact with the secondary roller  36  and thus applies no additional downward force. Thus, as the rotational velocity of the shaft increases, the primary cam arm  34  continues to pivot through the remainder of the shift action of the clutch  10  and is the only flyweight applying any force to the moveable sheave  14 . 
         [0039]    Referring now to  FIG. 4 , the dual stage primary clutch  10  is illustrated in a final position. In this position, the primary flyweights  34  have pivoted through their full range of travel. As shown, the cam surfaces  80  of the primary flyweights  34  are still in contact with and apply a downward force on the primary rollers  46 . Also, the secondary flyweights  48  are in the same position as they were in  FIG. 3  and are no longer in contact with the secondary roller  36 . 
         [0040]    Referring to  FIG. 5 , a spider assembly  40  of the clutch  10  is shown separate from the rest of the clutch  10 . In this view, the parts of the spider assembly  40  can easily be seen, including the center portion  44  of the spider assembly  40 , the secondary flyweights  48 , and the spider rollers  46 . The cam surfaces  82  of the secondary flyweights  48  engage the secondary rollers  36  mounted on the moveable sheave assembly  14  (see  FIG. 4 ). The spider rollers  46  engage the primary flyweights  34  mounted on the moveable sheave assembly  14 (see  FIG. 4 ). 
         [0041]    Referring to  FIG. 6 , the moveable sheave  14  is shown detached from the rest of the clutch  10 . In this view, the parts of the moveable sheave  14  can easily be seen, including the base portion  30 , the primary flyweights  34 , the primary rollers  26 , and the secondary rollers  36 . The cam surfaces  80  of the primary flyweights  34  engage the spider rollers  46  mounted on the spider assembly  40 . The secondary rollers  36  engage the secondary flyweights  48  mounted on the spider assembly  40 . 
         [0042]    The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.