Abstract:
A multi-spring system pressure plate member is provided for a spring biased clutch. The pressure plate member has a spring side and an opposed clutch side. The spring side of the pressure plate member includes spring confinement members for at least two clutch spring systems. The spring confinement members serve to hold spring members on said spring side in operative association with said pressure plate member. The spring confining members are provided to confine at least a diaphragm spring element and a compression coil spring element. The pressure plate member is configured to accept without modification either a diaphragm spring element or a set of compression coil spring elements. The clutch can be converted from one spring system to another by simply changing the spring elements.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/665,147, filed Mar. 25, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates in general to methods and devices for a clutch with multiple spring systems and, particularly, embodiments of the present invention relate to clutches wherein multiple spring systems with many common components are provided for clutch activation systems, and more particularly motorcycle clutches wherein one of a multiplicity of spring systems may be selected from common components to provide a preferred “feel” or other characteristic for the operation of a clutch.  
         [0004]     2. Description of the Prior Art  
         [0005]     Mechanical clutches are widely used in power transmissions systems between motors or engines and transmissions or other applications. Air, land, and sea vehicles, agricultural and grounds maintenance implements, construction equipment, stationary systems, and the like, all require the use of some type of clutch to disengage the engine from the application. Certain clutches, and particularly conventional motorcycle clutches, comprise a stack of friction discs and steel plates sandwiched between a generally axially fixed clutch hub and an axially moveable pressure plate. In the engaged position the friction disks and steel plates are forced together and rotate together so that power is transmitted through the clutch. In the disengaged position the disks and plates are slightly separated and do not rotate together, so power is not transmitted through the clutch.  
         [0006]     Conventional spring biased pressure plates are generally spring biased into the engaged mode by either compression springs or a diaphragm spring. Conventional pressure plates generally can not accommodate both spring systems either when used concurrently or alternatively. Clutch control members are typically actuated to disengage the pressure plate from the stack of friction discs and steel plates. Disengagement typically involves moving the pressure plate axially against the force of the spring bias. The disengagement force must be sufficient to overcome the force of the spring bias.  
         [0007]     The nature of the spring that serves to bias the pressure plate determines the “feel” or behavior of the clutch during engagement and disengagement. Compression springs resist disengaging the clutch with a force that increases progressively as the pressure plate moves towards full disengagement. The force is generally greatest in the fully disengaged configuration. A diaphragm spring (sometimes described as a bellville or disk spring) resists disengaging the pressure plate until the spring substantially flattens, at which point the force diminishes abruptly and substantially, so that little force is required to hold the clutch in the fully disengaged configuration. Upon engagement, the compression spring biased pressure plate systems tend to engage slowly and smoothly with some clutch slippage as the engagement phase of the cycle progresses. Diaphragm spring biased pressure plate systems tend to go very quickly from full disengagement to full engagement with little slippage.  
         [0008]     When a clutch is required, for example, to function between a high horsepower motor (for example, over  80  horsepower) and a transmission, the force of the required spring bias may become so great that the clutch controls require some augmentation, such as hydraulic assistance. Such augmented systems may be configured and adjusted to mask the inherent “feel” or behavior of a particular system, but at substantial cost in complexity, reliability, maintenance and initial price.  
         [0009]     Some clutch users, particularly motorcycle riders, from time to time wish to change the “feel” or behavior of the clutch. This had previously involved considerable expense, because the pressure plate, spring, and spring confinement components all needed to be changed. In general, it had previously been impossible to tailor the characteristics of a diaphragm spring clutch to a particular application, and it had not been possible to use a diaphragm spring clutch in certain high performance or high horsepower applications. Likewise, it had been impossible to similarly tailor a compression spring clutch to a particular application. Changing from a compression spring system to a diaphragm spring system had generally been prohibitively expensive because so many new parts had to be purchased. Also, such conversions were sometimes not available, or were of limited applicability. This inflexibility in clutch options tended to limit the usefulness and enjoyment of the motorcycles. Those concerned with these problems recognize the need for improvements.  
         [0010]     These and other difficulties of the prior art have been overcome according to the present invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or completely solved by currently available clutches. Thus, it is an overall object of the present invention to effectively resolve at least the problems and shortcomings identified herein. In particular, it is an object of the present invention to provide a clutch spring kit that includes all of the components required to assemble either a compression coil spring or a diaphragm spring biased pressure plate clutch using many common components. Embodiments of the present invention are particularly suitable for use in motorcycles, other land sea and air vehicles, agricultural and maintenance implements, stationary applications, and the like.  
         [0012]     A preferred embodiment of the present invention comprises a clutch spring kit that includes a pressure plate member with spring confining members that are adapted to confine a spring member such as, for example, a diaphragm spring and/or one or more compression coil springs, or the like, in operative position with the pressure plate member. Suitable spring confining members where the spring member comprises a diaphragm spring include, for example, at least one peripheral flange element on the pressure plate member. Suitable spring confining members where the spring member comprises a compression coiled spring include, for example, at least one spring pocket or boss on the pressure plate member. A spring retaining member is provided to retain the spring member in spring biasing association with the pressure plate member. Typically, the spring retaining member includes at least an anchor element that anchors the spring retaining member in a relatively fixed axial position relative to the pressure plate member.  
         [0013]     The pressure plate member in the kit has a spring side and a clutch side. The pressure plate member is configured so as to accommodate, alternatively or concurrently, a variety of spring members of different types. A peripheral flange element, for example, is adapted to confine one or more diaphragm springs in spring biasing association with the spring side of the pressure plate member. Generally, the peripheral flange element comprises one or more generally annular flanges, or the like, which may or may not be continuous. Such flanges may be in the form of a continuous rim, or a generally circular array of posts or arcuate segments positioned to engage the periphery of a diaphragm spring and confine it in operative position with the spring side of a pressure plate member. One or more spring confining members in the nature of a spring pocket, spring boss, or the like is also provided on the spring side of the pressure plate member. The spring pocket or boss is typically provided to confine the proximal generally cylindrical end of a compression coil spring in a desired location on a pressure plate member.  
         [0014]     The function of the anchor element may be served, for example, by providing several bolt elements, clip elements, or the like. Such bolt elements may include at least a shoulder bolt, and may additionally include appropriate washers, spacers, or the like. These anchor elements serve at least two functions. They project axially and slidably through the pressure plate from the spring side to the opposed clutch side. Rotational and lateral movement of a pressure plate member may thus be substantially prevented by the presence of such elements. The shoulder bolts, clips, or the like are usually anchored, for example, to an axially fixed structure on the clutch side of the pressure plate, such as, for example, a clutch hub, or the like, and serve to retain the spring member in a fixed axial position relative to this axially fixed structure. The spring member is generally held between the axially stationary anchor elements and the axially moveable pressure plate member so that axial movement of the pressure plate relative to the axially fixed anchor elements serves to compress or relax the spring member. Typically, it is necessary to compress the spring member in order to release the clutch. Thus, axial movement of the pressure plate member engages or releases the clutch. Typically the axially fixed structure and the spring member and the pressure plate member rotate together about the longitudinal axis of the clutch system.  
         [0015]     The kit also preferably includes at least two different types of spring members. A kit may include, for example, both one or more diaphragm spring elements and a set of compression coil spring elements. The shoulder bolts elements are preferably adapted to extending axially within the compression coil springs with the distal end of the spring trapped under the head of the bolt. This aids in holding or retaining the coiled springs in a spring biasing relationship with the pressure plate member, and, together with a spring confining member, in confining the coil spring in operative position on the spring side of the pressure plate member. The shoulder bolts also serve to retain (generally by the underside of the bolt head bearing on a retainer ring element) the diaphragm spring elements in spring biasing relationship with the pressure plate member. Thus, the pressure plate member and the shoulder bolt elements are preferably used interchangeably in the kit with both compression coil spring elements and diaphragm spring elements. This minimizes the number of components in the kit and simplifies its installation.  
         [0016]     All that is needed to convert from one spring system to the other according to the present invention is the spring elements themselves and a retainer ring element for use with the diaphragm spring elements. The pressure plate member is especially configured so that it is common to all spring systems. The anchor element is preferably common to all spring systems.  
         [0017]     The retainer ring element that is commonly associated with a diaphragm spring system may also serve as a spacer member where different thicknesses or combinations of diaphragm spring elements are used. Different thicknesses of spacers may be provided, if desired. Two or more diaphragm springs of different thicknesses or diameters (particularly outside diameters) may be provided. The diaphragm springs may have the same or different spring characteristics. Providing a plurality of diaphragm spring members with different spring characteristics permits the feel and character of the clutch to be finely adjusted to achieve exactly the desired feel and response. Adjustment is accomplished by adding or removing spring members to achieve the desired spring characteristics. In general, the length of the shoulder portions of the shoulder bolts is selected so that they will accommodate all possible configurations of spring member systems that can be assembled from the components in a kit. Spacers of appropriate thicknesses are provided to accommodate the variations in the axial lengths of the spring packs that result from the different spring configurations.  
         [0018]     The coiled compression spring elements are typically (although not necessarily) confined by spring confining elements (for example, recessed spring pockets or upwardly projecting spring sockets, bosses, or the like) on the spring side of the pressure plate. The spring retention elements are preferably arrayed in a regular pattern and spaced from the peripheral flange of the pressure plate so that the coiled compression spring retention elements do not interfere with the operation of the diaphragm spring elements, and the peripheral flange does not interfere with the operation of the coiled compression spring elements. It is generally not beneficial to use more than one spring member system at the same time, although they can be used in combination, if desired. In general, specially shortened coil compression springs are required for use with a retainer ring, so the interchangeability of such shortened springs between spring systems would be limited unless spacers or washers were also used with the shortened coiled springs in the embodiment where they serve as the only spring member. This adds to the complexity of the kit and its use. One of the advantages of the present invention is that the kit can be successfully installed or changed from one spring system to another without a great deal of instruction or experience. Sufficient flexibility in the behavior of the clutch can generally be achieved without resorting to using both systems concurrently.  
         [0019]     In a preferred embodiment of a clutch spring kit according to the present invention, the kit includes at least a pressure plate member that can accommodate both coiled compression spring elements and diaphragm spring elements, several substantially identical shoulder bolt or clip elements, and at least one and preferably two or more spring member systems (for example, a plurality of substantially identical coiled compression spring elements, and/or one or more diaphragm spring elements with a retainer ring element). Once a basic clutch spring kit with one spring member system has been obtained, a second different spring member system may be obtained at a later time, when desired. The two spring member systems can then be used interchangeably.  
         [0020]     Other objects, advantages, and novel features of the present invention will become more fully apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, or may be learned by the practice of the invention as set forth herein.  
         [0021]     To acquaint persons skilled in the pertinent arts most closely related to the present invention, a preferred embodiment of a clutch system that illustrates a best mode now contemplated for putting the invention into practice is described herein by, and with reference to, the annexed drawings that form a part of the specification. The exemplary clutch assembly is described in detail without attempting to show all of the various forms and modifications in which the invention might be embodied. As such, the embodiments shown and described herein are illustrative, and as will become apparent to those skilled in the arts, can be modified in numerous ways within the scope and spirit of the invention, the invention being measured by the appended claims and not by the details of the specification or drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The present invention provides its benefits across a broad spectrum of clutch applications. While the description which follows hereinafter is meant to be representative of a number of such applications, it is not exhaustive. As those skilled in the art will recognize, the basic apparatus taught herein can be readily adapted to many power transmission systems. This specification and the claims appended hereto should be accorded a breadth in keeping with the scope and spirit of the invention being disclosed despite what might appear to be limiting language imposed by the requirements of referring to the specific examples disclosed.  
         [0023]     Referring particularly to the drawings for the purposes of illustrating the invention and its presently understood best mode only and not limitation:  
         [0024]      FIG. 1  is a diagrammatic plan view of a preferred embodiment of a multi-spring system pressure plate according to the present invention.  
         [0025]      FIG. 2  is a cross-sectional view taken along line  2 - 2  in  FIG. 1  including two compression coil springs mounted on shoulder bolts as in a compression coil spring biased configuration.  
         [0026]      FIG. 3  is a cross-sectional view similar to  FIG. 2  showing a further embodiment adapted to accommodate two or more diaphragm springs with different outside diameters, and two shoulder bolts as in a diaphragm spring biased configuration.  
         [0027]      FIG. 4  is a diagrammatic plan view of a diaphragm spring that is adapted to be assembled together with the pressure plate shown diagrammatically in  FIG. 1 .  
         [0028]      FIG. 5  is a cross-sectional view taken along line  5 - 5  in  FIG. 4 .  
         [0029]      FIG. 6  is a cross-sectional view similar to  FIG. 5  showing a thicker diaphragm spring that is adapted to be used interchangeably or in combination with the embodiment of diaphragm spring depicted in  FIG. 5 .  
         [0030]      FIG. 7  is a diagrammatic plan view of a retainer plate or ring that is particularly adapted for use with a variety of diaphragm spring configurations.  
         [0031]      FIG. 8  is a cross-sectional view taken along line  8 - 8  in  FIG. 7 .  
         [0032]      FIG. 9  is a cross-sectional view similar to  FIG. 8  showing a thicker retainer plate that is adapted to be used interchangeably with the embodiment of  FIG. 8 .  
         [0033]      FIG. 10  is a diagrammatic exploded view of an embodiment wherein the multi-spring system pressure plate of  FIG. 1  is deployed in a dual diaphragm spring embodiment, and wherein the axial excursion of the pressure plate between the clutch engaged and clutch disengaged positions is illustrated.  
         [0034]      FIG. 11  is an exploded diagrammatic view of an embodiment wherein the multi-system pressure plate of  FIG. 1  is deployed in a compression coil spring embodiment.  
         [0035]      FIG. 12  is a diagrammatic exploded view of an embodiment wherein the multi-system pressure plate of  FIG. 1  is deployed in a single diaphragm spring embodiment.  
         [0036]      FIG. 13  is a diagrammatic fragmentary view of a compression coil spring embodiment wherein the proximal end of the coil spring element is confined on the spring side of a pressure plate member by a boss received within the coil spring element.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]     Referring now to the drawings wherein like reference numerals designate identical or corresponding parts throughout the several views. It is to be understood that the drawings are diagrammatic and schematic representations of various embodiments of the invention, and are not to be construed as limiting the invention in any way. The use of words and phrases herein with reference to specific embodiments is not intended to limit the meanings of such words and phrases to those specific embodiments. Words and phrases herein are intended to have their ordinary meanings, unless a specific definition is set forth at length herein.  
         [0038]     Referring particularly to the drawings, there is illustrated generally at  10  a pressure plate member for a multi-spring system clutch. Pressure plate member  10  has a clutch side  12  and a spring side  14 . Pressure plate member  10  is suited for use as an after market replacement for a conventional clutch pressure plate, or as a part of a custom designed clutch. Spring side  14  is adapted to being engaged by a spring member, which may be in the form of coiled compression springs of which  42  ( FIGS. 2 and 11 ) is typical, or one or more diaphragm springs of which  44  and  46  are typical ( FIGS. 4, 5 ,  10 , and  12 ). The spring member is held in spring biasing association with pressure plate member  10  by shoulder bolt elements, of which  36  and  38  ( FIG. 3 ) are typical, on the spring side  14  of pressure plate member  10 .  
         [0039]     In those embodiments where the spring member is a compression coil spring,  42  or  43 , there are spring confining elements on the spring side  14  in the form of spring pockets of which  26  is typical, or annular bosses of which  27  ( FIG. 13 ) is typical. Spring pockets  26  are formed as a generally circular bore that terminates in an annular face  22 . A proximal end of a coiled compression spring is prevented from moving laterally of the pressure plate member  10  because it is surrounded by the cylindrical bore. The proximal end of the coiled compression spring is prevented from passing through the pressure plate member by the annular face  22 . Annular boss  27  projects outwardly from spring side  14  and is received within the inside of generally cylindrical coiled spring  43 . The diameter of the coil spring is increased to accommodate the boss  27 . This expands the diameter of the spring to the point where a washer  29  is required to trap the distal end of the spring  43  under the head of bolt  38 . The bore  20  extends entirely through the boss  27  and from spring side  14  to clutch side  12  at a constant diameter.  
         [0040]     In those embodiments where the spring member comprises one or more diaphragm springs, the spring retention elements generally take the form of a peripheral flange element  16 . The peripheral flange element serves to keep the diaphragm springs centered with respect to longitudinal axis  82  ( FIG. 10 ). The peripheral flange element may include at least one annular boss  18  ( FIGS. 2, 10 , and  12 ), and may include two or more such annular bosses  32  and  34  ( FIG. 3 ). Each of the annular bosses  18 ,  32  and  34  defines an upstanding rim that serves to receive the outer annular edge  54  of a diaphragm spring ( FIGS. 4, 5 ,  6 ,  10 , and  12 ). The annular bosses are generally in the form of a cylindrical wall, and serve to prevent the diaphragm springs from moving laterally, much as the cylindrical wall of spring pocket  26  and boss  27  prevents the proximal end of a coiled compression spring from moving laterally.  
         [0041]     The diaphragm springs are in the nature of disk or Bellville springs. The various diaphragm springs typically have different properties. The diaphragm springs may be used one at a time ( FIG. 12 ), or they may be stacked with two or more springs in one spring pack ( FIG. 10 ). The spring characteristics are thus controlled to provide certain desired characteristics in the operation of the clutch. The properties of the diaphragm springs may be adjusted as desired by using different thicknesses (compare, for example, spring members  44  and  46  in  FIGS. 5 and 6 ) or different outside diameters (for example, bosses  32  and  34 ,  FIG. 3  are adapted to accommodate diaphragm springs of different outside diameters). There is generally a practical limit to the thickness of a diaphragm spring. Such spring members are generally formed by stamping or other sheet metal working operations. The formed spring members are then heat treated. In general, such forming and treating operations can not be performed, or are at least impractical to perform reliably on sheet metal that is thicker than approximately  0 . 125  inches thick. Diaphragm springs with a thickness of less than approximately  0 . 040  inches are generally not used in clutch applications.  
         [0042]     The shoulder bolt elements  36  and  38  engage a structure that is generally axially fixed relative to the clutch plate member  10 , and they serve at least two main purposes. They serve as guides for the pressure plate member  10  as it moves axially along axis  82  ( FIG. 10 ). The shoulder bolt elements also confine the spring member in spring biasing association with the pressure plate member  10 . The spring member is positioned between the shoulder bolt elements and the spring side of the pressure plate. The shoulder bolt elements retain the spring member in a generally axially fixed position with respect to the pressure plate member  10 .  
         [0043]     Axial movement of the pressure plate member  10  along longitudinal axis  82  ( FIG. 10 ) serves to either compress or relax the spring member depending on the direction of the axial movement. The spring member is generally serves to bias the pressure plate member in an axial direction that the clutch side of the pressure plate member faces. The pressure plate member  10  is thus spring biased into a configuration where the clutch is engaged. Moving the pressure plate member axially against the force of the spring member to disengage the clutch at least initially increases the spring force. Where the spring member is a diaphragm spring, the spring force generally drops off abruptly when the spring is flattened, and little force is required to hold the clutch in the disengaged configuration. As the disengaging force is released the force of the diaphragm spring generally increases abruptly. Where the spring member is a coiled compression spring, the spring force generally increases as the pressure plate member is moved axially in the direction that the spring side of the pressure plate member faces, and it requires considerable force to hold the clutch in the released configuration. The compression spring is compressed during the release phase of the release-engage cycle of the clutch operation. During the engagement part of the clutch release-engage cycle, the coiled compression spring expands and the spring force generally decreases somewhat in a generally uniform manner.  
         [0044]     The limits of the axial travel of the pressure plate member between full clutch engagement and full clutch disengagement are illustrated in  FIG. 10 . On the clutch release phase of the clutch release-engage cycle the pressure plate member  10  moves axially through distance  78  to the fully released configuration. A shoulder bolt is anchored to axially fixed structure  84  so that the cylindrical body  40  of a shoulder bolt slides through bore  20  ( FIGS. 1, 2 ,  3 , and  13 ) with the internal cylindrical wall  24  of bore  20  in close but sliding engagement with the external cylindrical wall of cylindrical body  40 . The position of axially fixed structure  80  ( FIG. 10 ) indicates in phantom lines the axial position of the pressure plate member relative to the fixed structure when the clutch is fully engaged. For purposes of ease of illustration, the limits of travel of the pressure plate member are illustrated by showing the relative positions of the fixed structures  80  and  84  and the pressure plate member at the fully engaged and fully released positions. It is emphasized that the structures are typically axially fixed relative to the pressure plate member and to one another. It is the pressure plate member that moves through distance  78  between the position shown with reference to the structure  80  and the position shown with reference to structure  84 . The structures  80  and  84  generally do not move relative to one another. The clutch is generally fully engaged in the configuration illustrated by the position of structure  80 , and fully disengaged in the configuration illustrated by the position of structure  84 . The distance  78  for a typical motorcycle clutch is generally in the nature of approximately 0.080 to 0.160 inches, although greater or smaller axial distances may be employed in certain clutches.  
         [0045]     Typically, pressure plate members according to the present invention include a central hub  28 . In general, in previous proposed clutches a clutch actuation mechanism (not shown) was actuatingly attached to the pressure plate so as to provide the operator with a way to move the pressure plate member through the release-engagement cycle. Numerous clutch actuation mechanisms had been previously proposed. For example, it had been previously proposed to provide a threaded axial bore or the like centrally of a conventional pressure plate, and to threadably attach a clutch activation mechanism to the threaded bore. Various other attachment mechanisms had also been proposed. The thusly attached clutch activation mechanism then either pushed or pulled the conventional pressure plate in an axial direction to initiate the clutch release-engagement cycle. The present invention is not limited to any particular clutch activation mechanism. By way of example only, a threaded axial bore  30  is illustrated extending axially through hub  28  and concentric with longitudinal axis  82 . The purpose of threaded axial bore  30  is to provide a location for the attachment of a clutch activation mechanism. Other arrangements for the mounting of a clutch activation mechanism may be provide, if desired.  
         [0046]     The diaphragm spring members generally have a convex side  52 , a radially outer edge  54 , and a radially inner edge  56 . The inner diameter of the spring member is generally large enough that the inner edge  56  clears the hub  28  as the spring member flexes. See, for example,  FIGS. 4, 10  and  12 . The inner edge  56  may, if desired, be broken or interrupted as at  58  to permit the passage of shoulder bolt elements. The inner edge may also be broken or interrupted as at  60  to, for example, reduce the force required to flex the spring member, or the like.  
         [0047]     Diaphragm spring members are preferably mounted in operative position on the spring side of the pressure plate member by their peripheral edges rather than being mounted on a central hub such as shown at  28 . The retention of the spring member on the spring side of a pressure plate member is generally accomplished by anchor elements. Where bolts serve as the anchor members for diaphragm springs, a retainer ring such as that shown at  48  ( FIGS. 7 and 10 ) is preferably used to spread the load uniformly around the diaphragm spring(s). The width of the retainer ring between inner annular edge  66  and outer annular edge  64 , and the thickness of body  62  are such that when bolts are inserted through holes  68 ,  70 ,  72 , and  74 , the undersides of the bolt heads (of which  39  is typical) engage body  62  and cause retainer ring  48  to bear firmly and uniformly against the surface of the diaphragm spring. As indicated by comparison of retainer rings  48  and  50  ( FIG. 9 ), the retainer rings may be provided in different thicknesses. Two or more retainer rings may be used in one spring pack, if desired. This accommodates the use of different thickness of diaphragm springs or spring packs with two or more springs ( FIG. 10 ) without the need for different length bolts. Generally, the retainer ring does not extend for the full width of the diaphragm spring. See, for example,  FIGS. 10 and 12 . The inner diameter of a retainer ring is generally such that it clears the hub  28  in all possible operational configurations.  
         [0048]     It will be appreciated that pressure plate members may be devised with the capability to used with multiple spring systems other than those illustrated herein. Accordingly, the present invention should not be construed as limited solely to the embodiments illustrated here.  
         [0049]     Clutch components such as springs, anchor members, and pressure plates, are typically constructed from steel or other metals, because of the substantial loads and high temperature conditions encountered in normal operation. However, any material or combination of materials, compatible with the functions and operation of a clutch are contemplated as being within the scope of the present invention.  
         [0050]     What has been described are preferred embodiments in which modifications and changes may be made without departing from the spirit and scope of the accompanying claims. Many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.