TECHNICAL FIELD 
     The present invention relates generally to the field of clutches and, more particularly, to an improved externally adjustable clutch for preventing the transmission of excessive torque in, for example, hoist systems. 
     BACKGROUND ART 
     Clutches are well known in the art and are generally used to transmit force between two rotating shafts. One of the shafts is typically attached to a motor, sometimes referred to as the driving member, and the other shaft provides output power for work to be done, often referred to as the driven member. The clutch connects the two shafts across a slip joint so that they can be either engaged so that they spin at the same speed, or decoupled and disengaged so they spin at different speeds. 
     German Patent Publication DE102004054890 B3 is directed to a clutch that is adjusted by tightening a compression nut positioned outside of a spring on the threaded end of a rod that is connected at the other end to a pressure disk which is pressured to increase friction against a movable friction plate. 
     German Patent Publication DE19927847 C1 is directed to a slip clutch that is adjusted by tightening a compression nut positioned outside of a spring on one end of a threaded pull rod that is connected at the other end to a pressure disk which is pressured to increase friction between clutch discs. 
     U.S. Pat. No. 5,853,165 is directed to a clutch in which the preload of disks is provided by a nut acting on a helical spring and screwed onto a corresponding thread on a stub shaft that projects beyond the clutch disk and a second spring. 
     German Patent Publication DE4408578 A1 is directed to a slip clutch which can be adjusted without opening the casing. A shaft carries an external thread onto which a nut is screwed. A compression spring acts between the nut and a bearing such that the spring bias force on friction rings can be adjusted. 
     United Kingdom Patent Application GB2321504 A is directed to a slip clutch in which the amount of friction force is varied by rotation of an adjustment member on a threaded shaft portion. 
     U.S. Pat. No. 5,533,712 is directed to a slip clutch having an elastic member and an adjustment member. 
     DE19757500 is directed to a clutch with an adjustment mechanism comprising a compression spring in a cap having a threaded ring that is in threaded engagement with a threaded portion of a shaft. 
     U.S. Pat. No. 1,807,210 is directed to a friction coupling and generally discloses a key gear having a hub, follower ring, spring and cylindrical shell. 
     U.S. Pat. No. 2,953,911 is directed to a drive coupling and discloses a driven plate with radial grooves, hub, driving plate, pressure plate and clutch springs. 
     U.S. Pat. No. 7,591,357 is directed to a crank shaft torque modulator and discloses a driven hub, clutch spring, carrier disk, thrust washer, crank shaft pulley and mounting hub. 
     BRIEF SUMMARY OF THE INVENTION 
     With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiments, merely for purposes of illustration and not by way of limitation, the present invention provides an adjustable clutch assembly ( 15 ) comprising a base portion ( 18 ); a clutch housing cap ( 19 ); a first rotary member ( 20 ) having a first torque transfer surface ( 23 ); the first rotary member configured to rotate about a clutch axis ( 22 ) and to rotationally couple to a first shaft; a second rotary member ( 21 ) having a second torque transfer surface ( 24 ) opposing said first torque transfer surface of said first rotary member at a slip joint ( 25 ); the second rotary member configured to rotate about the clutch axis and to rotationally couple to a second shaft; a spring element ( 26 ) disposed axially between the clutch housing cap and the first rotary member and configured to bias the opposed first and second torque transfer surfaces axially towards each other; a gear drive ( 28 ) comprising a wheel ( 29 ) configured to rotate about the clutch axis and a shaft ( 30 ) in engagement with the wheel and configured to rotate about an adjustment axis ( 31 ) to thereby selectively rotate the wheel about the clutch axis; the wheel in engagement with the clutch housing cap; the shaft having a first portion ( 32 ) to which an adjustment torque may be applied; a locking element ( 34 ) constraining rotational movement of the clutch housing cap relative to the base portion; and the gear drive, the base portion, the locking element and the clutch housing cap configured such that selective rotation of the wheel about the clutch axis causes axial movement of the clutch housing cap along the clutch axis relative to the base portion or the second rotary member; whereby the bias of the spring may be adjusted by rotation of the shaft. 
     The first rotary member may be a driving member and the second rotary member may be a driven member. The first torque transfer surface may comprise a friction liner layer ( 35 ). The spring element may comprise a cylindrical compression spring oriented about the clutch axis. 
     The clutch housing cap may comprise a spring seat ( 36 ) axially retaining a first end of the spring and the assembly may further comprise a spring cup ( 38 ) receiving a second end of the spring. The assembly may further comprise a bearing ( 39 ) disposed axially between the spring cup and the first rotary member and disposed radially between the first rotary member and the clutch housing cap. The assembly may further comprise a cylindrical bushing ( 40 ) disposed radially between the bearing and the clutch housing cap. 
     The wheel may comprise an outer cylindrical surface having outwardly extending circumferentially spaced teeth ( 41 ) and the shaft may comprise an outer cylindrical surface having a threaded portion ( 42 ), and the engagement of the wheel with the shaft may comprise the threaded portion of the shaft rotationally engaging the teeth of the wheel. The first portion of the shaft may comprise a recess ( 33 ) configured to receive a torqueing tool. The clutch housing cap may comprise an outer cylindrical surface having a threaded portion ( 43 ) and the wheel may comprise an inner cylindrical surface having a threaded portion ( 44 ), and the engagement of the wheel with the clutch housing cap may comprise the threaded portion of the inner cylindrical surface of the wheel rotationally engaging the threaded portion of the outer cylindrical surface of the clutch housing cap. 
     The locking element may comprise an annular locking plate ( 34 ) fixed to the base portion and engaging an outer surface of the clutch housing cap. The annular locking plate may comprise multiple protrusions ( 45   a - c ) extending radially inwards towards the clutch axis and the outer surface of the clutch housing cap may comprise multiple axially extending grooves ( 46   a - c ), wherein the protrusions may slide axially relative to the clutch axis in the grooves and are constrained from rotational movement about the clutch axis by the grooves, whereby the clutch housing cap is axially movable relative to the base portion and rotationally constrained relative to the base portion. The annular locking plate may be fixed to the base portion via a plurality of shaft elements ( 48   a - c ). The wheel may be rotationally supported by a plurality of bearings ( 49   a - c ) and the bearings may be supported between the annular locking plate and the base portion on the respective shaft elements. The shaft elements may comprise turn studs and the assembly may further comprise multiple spring elements ( 50   a - c ) disposed between the annular locking plate and the respective bearings on the respective turn studs and configured to bias the locking plate away from the base portion. 
     The assembly may further comprise a hoist housing ( 16 ) fixed to the base portion and a second bearing ( 51 ) disposed radially between the second rotary member and the hoist housing. The second rotary member may comprise an outer journal ( 52 ) for receiving the second bearing. 
     The first rotary member ( 20 ) and the spring element ( 26 ) may be disposed between the base portion ( 18 ) and the clutch housing cap ( 19 ); the clutch housing cap may have an inner surface ( 55 ) and the first rotary member may have a surface ( 53 ) opposing the inner surface of the clutch housing cap; the spring element may act between the inner surface of the clutch housing cap and the surface of the first rotary member opposing the inner surface of the clutch housing cap; and the clutch housing cap and the first rotary member may be configured such that axial movement of the clutch housing cap relative to the first rotary member adjusts the bias of the spring element. The gear drive may be disposed outside of the clutch housing cap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of an embodiment of the improved clutch. 
         FIG. 2  is a front partial view of the clutch shown in  FIG. 1   
         FIG. 3  is a vertical cross-sectional view of the clutch shown in  FIG. 1 , taken generally on line A-A of  FIG. 1 . 
         FIG. 4  is a vertical cross-sectional view of the clutch shown in  FIG. 1 , taken generally on line B-B of  FIG. 1 . 
         FIG. 5  is a vertical cross-sectional view of the clutch shown in  FIG. 1 , taken generally on line C-C of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., crosshatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate. 
     Referring now to the drawings and, more particularly, to  FIGS. 1-3  thereof, an improved clutch assembly is provided, an embodiment of which is generally indicated at  15 . Assembly  15  generally includes base plate  18 , fixed to hoist housing  16 , clutch housing cap  19 , worm drive  28  acting between base plate  18  and clutch housing cap  19 , driving hub  20 , driven hub  21 , and spring  26  acting between clutch housing cap  19  and driving hub  20 . Clutch assembly  15  is configured and arranged such that internal spring  26  and the spring force of clutch  15  can be regulated or adjusted externally via worm drive  28 , thereby eliminating the need to remove any covers or components. 
     As shown in  FIGS. 2-5 , clutch housing cap  19  is a generally bell-shaped structure orientated about axis x-x, also referred to as clutch axis  22 , and having cylindrical opening  68  at its top. With reference  FIG. 4 , housing cap  19  is generally bounded by inwardly-facing vertical cylindrical surface  54 , downwardly-facing horizontal annular surface  55 , inwardly-facing vertical cylindrical surface  56 , downwardly-facing horizontal annular surface  57 , inwardly-facing vertical cylindrical surface  58 , downwardly-facing horizontal annular surface  59 , inwardly and downwardly-facing rounded frusto-conical surface  60 , inwardly-facing vertical cylindrical surface  61 , downwardly-facing horizontal annular surface  62 , outwardly-facing vertical cylindrical surface  63 , outwardly and upwardly-facing frusto-conical surface  64 , outwardly and upwardly-facing second frusta-conical surface  65 , and upwardly-facing horizontal annular surface  66 , joined at its inner marginal end to the upper marginal end of surface  54 . 
     Surface  54  defines opening  68  in the top of cap housing  19 . Surfaces  55  and  56  define annular spring seat  36 . The bottom portion  43  of outwardly-facing vertical cylindrical surface  63  is threaded. Housing cap  19  contains concentric spring cup  38 , bearing  39 , bushing  40 , spring  26  and driving hub  20 . 
     As shown, spring cup  38  is a generally ring-shaped annular structure orientated about clutch axis  22  and has an inwardly and upwardly facing annular seat  69  for receiving one end of spring  26  and an outer cylindrical surface. Bearing  39  is a generally ring-shaped annular structure orientated about clutch axis  22  and has an outer cylindrical surface and an inner cylindrical surface. Bushing  40  is a generally elongated ring-shaped annular structure orientated about clutch axis  22  and has an outer cylindrical surface and an inner cylindrical surface. 
     In this embodiment, spring  26  is a cylindrical compression spring oriented about clutch axis  22 . As shown, spring  26  is radially retained about clutch axis  22  by seat  36  in housing cap  19  at its top end and seat  69  in spring cup  38  at its bottom end. Spring  26  is compressed axially directly between seat  36  in housing cap  19  and seat  69  in spring cup  38 . 
     The outer cylindrical surface of bushing  40  is affixed to inner surface  58  of housing cap  19 . As shown, both bearing  39  and spring cup  38  are radially retained about clutch axis  22  by bushing  40  and are in axial sliding engagement with bushing  40  such that they can move axially along clutch axis  22  as a function of the compression force of spring  26 . Thus, bushing  40  extends between the outer cylindrical surfaces of spring cup  38  and bearing  39  and the inner cylindrical surface  58  of housing cap  19 . Bearing  39  is sandwiched axially between spring cup  38  and annular seat  53  of driving hub  20 . 
     As shown, driving hub  20  is generally a ring-shaped annular structure orientated about clutch axis  22 . The inner cylindrical surface of driving hub  20  is splined and forms a bore configured to receive the corresponding splined end of a motor shaft for rotational engagement therewith. Thus, when a motor shaft extends through opening  68  in housing cap  19  and engages the splined bore of driving hub  20 , rotation of the motor shaft about clutch axis  22  causes corresponding rotation of driving hub  20  about clutch axis  22 . 
     As shown, driven hub  21  is generally a cylindrical annular structure oriented about clutch axis  22 . Driven hub  21  also has an inner cylindrical surface that is splined and forms a bore configured to receive the corresponding splined end of a drive shaft for rotational engagement. Thus, when the drive shaft engages the splined bore of driven hub  21 , rotation of driven hub  21  about clutch axis  22  causes corresponding rotation of that drive shaft about clutch axis  22 . 
     As shown, in this embodiment driving hub  20  includes a conventional annular non-metallic composite friction liner  35  bonded to driving hub  20  as shown. Friction liner  35  provides a desired contact area between the opposed torque transfer surfaces  23  and  24  of driving hub  20  and driven hub  21 , respectively. While in this embodiment liner  35  is bonded to driving hub  20 , alternatively it may be free floating or bonded to driven hub  21 . Friction liner  35  may also be contoured to control the size, shape and location of the contact area and resulting torque at slip joint  25  between torque transfer surface  23  of driving hub  20  and torque transfer surface  24  of driven hub  21 . For example, friction liner  35  may be tapered. 
     As shown, driving hub  20  includes annular ledge, journal or seat  53  in which bearing  39  is positioned. The inwardly-facing vertical cylindrical surface of bearing  39  is configured to bear against the outer cylindrical surface of seat  53  of driving hub  20  and the outer cylindrical surface of bearing  39  is configured to bear against the inner cylindrical surface of bushing  40 . Thus, bearing  39  allows for rotation of driving hub  20  about clutch axis  22  relative to housing cap  19  and base plate  18 . 
     As shown, bearing  51  is generally a ring-shaped annular structure orientated about clutch axis  22 . As shown, bearing  51  is radially retained about clutch axis  22  by an inner cylindrical surface of hoist housing  16 , which is fixed relative to base plate  18 . Driven hub  21  includes annular ledge, journal or seat  52  in which bearing  51  is positioned. The inwardly-facing vertical cylindrical surface of bearing  51  is configured to bear against the outer cylindrical surface of seat  52  of driven hub  21  and the outer cylindrical surface of bearing  51  is configured to bear against an inner cylindrical surface of hoist housing  16 . Thus, bearing  51  allows for rotation of driven hub  21  about clutch axis  22  relative to hoist housing  16  and base plate  18  while holding driven hub  21  in proper radial and axial alignment. 
     As shown, spring  26  bears on one side against surface  55  of housing cap  19  and on the other side against the opposing surface of seat  69  of spring cup  38 . In operation, spring  26  presses axially against spring cup  38 , causing spring cup  38  to in turn press axially against bearing  39 , which in turn causes bearing  39  to press axially against driving hub  20 . This in turn causes driving hub  20  and friction liner  35  to press axially against driven hub  21 . This encourages driven hub  21  to rotate together with driving hub  20  due to contact friction at slip joint  25 . However, when the driving torque exceeds the friction torque, driven hub  21  will slip relative to driving hub  20 , resulting in the drive shaft no longer rotating at the same speed as the motor shaft. 
     In general, worm drive  28  comprises a conventional type worm  30  in threaded engagement with a conventional type worm gear  29 . As shown in  FIG. 1 , worm shaft  30  is a generally cylindrical structure orientated about axis  31  and configured to rotate about axis  31 . Shaft  30  is fixed axially such that it does not move along axis  31 . Shaft  30  includes adjustment portion  32  and threaded portion  42 . Adjustment portion  32  is located externally of housing cap  19  and at one end includes recess  33 , which is configured to receive a tool for selectively applying an adjustment torque about axis  31  to shaft  30 . In this embodiment, recess  33  is hexagonal and is configured to receive a hex wrench, but it is contemplated that other mechanisms or configurations may be used by which a torque is selectively applied to rotate shaft  30  about axis  31 . Shaft  30  includes a second medial portion  42  having a shallow spiral thread that engages worm gear  29  in a non-intersecting, perpendicular axis configuration. 
     Worm gear  29  is a ring-shaped annular structure oriented about clutch axis  22  and operatively configured to rotate about clutch axis  22 . As shown, worm gear  29  is configured to rotate about clutch axis  22  on outer bearings  49   a - c  and includes an outer center cylindrical surface having multiple circumferential spaced outwardly protruding teeth  41  orientated substantially parallel to clutch axis  22 . Bearings  49   a - c  have upper and lower cylindrical portions that rotationally bear on the outer cylindrical edges of worm gear  29  with the center of the outer cylindrical surface of worm gear  29  having wheel teeth  41 . 
     Worm gear  29  also includes inner threaded portion  44  on its inner cylindrical surface, with portion  44  in engagement with threaded portion  43  of outer cylindrical surface  63  of housing cap  19 . The relative threads of inner cylindrical portion  44  of worm gear  29  and opposed outer cylindrical portion  43  of housing cap  19  spiral about clutch axis  22  and such that relative rotation of worm gear  29  about clutch axis  22  causes axial movement of housing cap  19  along clutch axis  22 . 
     Locking ring or plate  34  is generally a ring-shaped annular structure orientated around clutch axis  22  and positioned externally or radially outside of housing cap  19  relative to clutch axis  22 . Locking plate  34  is fixed to base plate  18  by three circumferentially spaced turn studs  48   a - c  that rotationally support bearings  49   a - c , respectively. Bearings  49   a - c  keep worm gear  29  concentric with driving hub  20 , driven hub  21  and housing cap  19  on clutch axis  22 . Wave springs  50   a - c  are positioned on turn studs  48   a - c  between the bottom of plate  34  and bearings  49   a - c , respectively, and bias locking plate  34  away from base plate  18 . E-clips  70   a - c  secure locking plate  34  to the ends of turn studs  48   a - c , respectively. 
     As shown, base plate  18  is generally a ring-shaped annular structure orientated around clutch axis  22 . Base plate  18  is bolted to hoist housing  16  with bolts  71   a - d . Thus, base plate  18  and hoist housing  16  are fixed relative to each other. Driven hub  21  and base plate  18  are also axially fixed relative to each other. 
     As shown in  FIGS. 1 and 2 , locking plate  34  has three circumferentially spaced radially inwardly-extending protrusions  45   a - c  on its inner cylindrical surface. Outer surface  63  of housing cap  19  includes three axially-extending grooves  46   a - c  that correspond to and receive protrusions  45   a - c , respectively, of locking plate  34 . Protrusions  45   a - c  are configured with grooves  46   a - c  such that protrusions  45   a - c  may slide axially in grooves  46   a - c  with axial movement of housing cap  19  relative to clutch axis  22 . However, protrusions  45   a - c  fit within grooves  46   a - c , respectively, so as to constrain rotational movement of housing cap  19  about clutch axis  22 . Thus, clutch housing cap  19  is axially moveable along clutch axis  22  relative to base plate  18  and hoist housing  16  and is rotationally constrained relative to base plate  18  and hoist housing  16 . Due to locking plate  34 , housing cap  19  is restrained from rotational movement relative to base plate  18 . 
     As shown, in operation, rotation of shaft  30  about axis  31 , which can be performed externally with a hex wrench inserted into hex recess  33  in the end of shaft  30 , causes rotation of worm gear  29 . Because of the threaded connection between threaded portion  44  of the inner cylindrical surface of worm gear  29  and the overlapping threaded portion  43  of outer cylindrical surface  63  of housing cap  19 , rotation of worm gear  29  about clutch axis  22  causes axial movement of housing cap  19  relative to base plate  18 . Such movement in the clockwise direction about clutch axis  22  increases the pressure of spring  26  against driving hub  20 , and such movement in the counter-clockwise direction decreases the spring pressure and bias of spring  26  against driving hub  20 . This allows for the spring bias of spring  26  to be adjusted externally as desired. Thus, rotation of worm shaft  30  externally to cap housing  19  in the clockwise direction increases the spring pressure within cap housing  19 , and rotation of worm shaft  30  externally in the counter-clockwise direction decreases the spring pressure within cap housing  19 . Thus, for example, if over time either spring  26  loses its elasticity or if any of the liners or hub interfaces of clutch  15  are worn away, worm shaft  30  may be adjusted externally to maintain the desired bias of spring  26  internally to cap housing  19 . This adjustment may be done without having to removed housing  19  and/or housing  16 . In addition, worm gear  29  does not loosen easily and therefore there is no need for a separate mechanism to lock the spring adjustment of clutch  15 . 
     While a single driving hub  20  and driven hub  21  are shown and described, multiple pressure plates and friction hubs may be used to adjust torque transfer as desired. Furthermore, spring  26  may be separated from clutch housing cap  19  and driving hub  20  by more or fewer elements than spring cup  38  and bearing  39  shown and described. In such cases, spring  26  may still be disposed axially between clutch housing cap  19  and driving hub  20  as it may still exert a spring force on driving hub  20  and driven hub  21 , and that spring force may be adjustable by the axial movement of housing cap  19  relative to driven hub  21 . 
     The present invention contemplates that many changes and modifications may be made. Therefore, while the presently-preferred form of the externally adjustable clutch assembly has been shown and described, and several modifications and alternatives discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the scope of the invention, as defined and differentiated by the following claims.
An adjustable clutch assembly comprising a base portion, a clutch housing cap, a first rotary member having a first torque transfer surface rotatable about a clutch axis and a second rotary member having a second torque transfer surface opposing said first torque transfer surface, a spring element configured to bias the opposed first and second torque transfer surfaces towards each other, a gear drive comprising a wheel engaging the clutch housing cap and a shaft engaging the wheel, a locking element constraining rotational movement of the clutch housing cap relative to the base portion, the gear drive, base portion, locking element and clutch housing cap configured such that selective rotation of the wheel about the clutch axis causes axial movement of the clutch housing cap along the clutch axis relative to the base portion, whereby the bias of the spring may be adjusted.