PATENT ABSTRACT
A spring clutch has an arbor, a spring, and a sleeve. The spring at least partially surrounds the arbor and has a first portion coupled to the arbor to confine relative rotation of the first portion and arbor about an axis. The sleeve at least partially surrounds the spring and cooperates with the spring. The cooperation is sufficient so that an initial relative rotation between the arbor and sleeve in a first direction about the axis tends to uncoil the spring and bias the spring into firmer engagement with the sleeve. The cooperation is sufficient such that relative rotation in an opposite direction tends not to uncoil the spring and maintains the clutch in a disengaged condition.

PATENT DESCRIPTION
U.S. GOVERNMENT RIGHTS  
       [0001] The invention was made with U.S. Government support under contract DAAH10-01-2-0032 awarded by the U.S. Army. The U.S. Government has certain rights in the invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    (1) Field of the Invention  
           [0003]    This invention relates to power transmission, and more particularly to spring clutches.  
           [0004]    (2) Description of the Related Art  
           [0005]    Overrunning spring clutches are a well developed art. Such clutches make use of the principle that a spring coil will expand if twisted one way about its axis and contract if twisted the other way. In an exemplary clutch, respective portions of a coil spring are positioned within respective sleeves. In a neutral condition, of the spring portion within each sleeve, an end portion is lightly frictional engaged to the sleeve and a remaining portion is slightly radially spaced from the sleeve. When the sleeves rotate relative to each other about their common axis, friction between the sleeves and the associated end portions will tend to twist the spring. If the relative rotation is in the direction which would tend to contract the spring, there will be slippage or overrunning. If the relative rotation is in the opposite direction, the normal forces between the end portions and sleeve will increase and the heretofore spaced portions will expand into frictional engagement with the sleeves thereby resisting the relative rotation. Accordingly, when such a clutch is used to drive an output from an input rotating (absolutely) in a first direction, the clutch permits the output to rotate faster than the input in the first direction. This permits the output to continue to rotate if the input slows or is stopped. Absolute rotation of the input (or both the input and output) in an opposite second direction may be prevented by additional internal or external mechanisms.  
           [0006]    U.S. Pat. No. 5,799,931 (the &#39;931 patent) discloses an exemplary such spring clutch. In that patent, the spring is formed into a coil by a machining a helical slot in a tubular form (e.g., as distinguished from winding a wire or somehow casting without machining a slot).  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    One aspect of the invention involves a clutch apparatus having an arbor, a spring, and a sleeve. The arbor has a first end, a second end, and an externally toothed portion. The spring at least partially surrounds the arbor and has an internally toothed portion intermeshed with the arbor externally toothed portion. The sleeve at least partially surrounds the spring and frictionally engages the spring. The engagement is sufficient so that an initial relative rotation between the arbor and sleeve in a first direction tends to uncoil the spring and bias the spring into firmer engagement with the sleeve. The engagement is sufficient that initial relative rotation between the arbor and sleeve in a second direction, opposite the first direction, tends not to uncoil the spring.  
           [0008]    In various implementations, a pinion gear may be unitarily formed with the sleeve. The spring may have a slot between interior and exterior surfaces and extending between first and second axial ends and having a nonconstant helix angle. The slot may extend longitudinally at the first axial end and nearly circumferentially at the second axial end.  
           [0009]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a longitudinal, partially sectional, view of a clutch according to principles of the invention.  
         [0011]    [0011]FIG. 2 is a view of a spring of the clutch of FIG. 1.  
         [0012]    [0012]FIG. 3 is a longitudinal sectional view of the spring of FIG. 2, taken along line  3 - 3 .  
         [0013]    [0013]FIG. 4 is a transverse sectional view of an arbor shaft, spring, and sleeve of the clutch of FIG. 1 in a disengaged condition.  
         [0014]    [0014]FIG. 5 is a transverse sectional view of an arbor shaft, spring, and sleeve of the clutch of FIG. 1 in an engaged condition.  
         [0015]    Like reference numbers and designations in the various drawings indicate like elements.  
     
    
     DETAILED DESCRIPTION  
       [0016]    [0016]FIG. 1 shows a spring clutch  20  having a housing  22  with a central longitudinal axis  500 . The clutch input housing  22  is itself mounted within a main housing  23  (e.g., a main gearbox housing). The clutch receives a driving torque about the axis  500  from an external source (e.g., an engine (not shown)) through an input drive flange  24 . The clutch may transmit a first sense or direction of such torque to an external load (e.g., a helicopter rotor system (not shown)) through an output pinion gear  26 . The clutch advantageously does not transmit substantial torque of an opposite sense. Accordingly, input rotation in a first direction will be transmitted as output rotation, although the output pinion gear may rotate faster in that direction in an overrunning condition. Opposite input rotation (if permitted) will not be so transmitted to the output pinion gear.  
         [0017]    In the illustrated embodiment, the input drive flange  24  drives an arbor shaft  28  via a diaphragm coupling  30 . Specifically, the flange is secured to one end of the coupling while the other end is secured to an outer collar  32 . The outer collar  32  surrounds and engages an upstream or input end portion of an inner collar  34  via interfitting teeth. The inner collar  34  surrounds a portion of the arbor shaft  28  and is similarly engaged thereto via interfittting teeth. At the upstream end, an outer bearing sleeve  40  is mounted within an opening in the housing  22 . A ball bearing system  42  is radially positioned between the outer bearing sleeve  40  and the inner collar  34 . Near an upstream end  44  of the arbor shaft, a nut  46  is threaded onto the shaft. The nut  46  bears against a spacer  48  which, in turn, bears against an upstream flange of the outer collar  32 . A downstream rim of the outer collar  32  bears against a spacer  50  which, in turn, bears against the upstream rim of the inner race of the ball bearing system  42 . The downstream rim of that race, in turn, bears against an upstream-facing external shoulder of the inner collar  34 . A downstream-facing internal shoulder of the inner collar  34  bears against an upstream-facing external shoulder of the arbor shaft so that the foregoing series of components is compressively sandwiched between the nut and arbor shaft. The outer race of the ball bearing system  42  is captured between an upstream-facing internal shoulder of the outer bearing sleeve  40  and a retainer  52 .  
         [0018]    The exemplary pinion gear  26  extends radially outward from a sleeve  60  unitarily formed therewith. An upstream portion of the sleeve is rotatably mounted to the housing  22  via an upstream roller bearing system  62  and a ball bearing system  64  immediately downstream thereof. The bearing systems  62  and  64  are captured between a downstream-facing internal shoulder of the housing  22  and retainers  66  secured to the downstream rim of the housing  22 .  
         [0019]    A downstream portion of the sleeve  60  is clear of the housing  22  but rotatably mounted to the main housing  23  via a roller bearing system  70  captured between an upstream-facing internal shoulder of the main housing and retainers  72 .  
         [0020]    A portion of the arbor shaft  28  adjacent its downstream end  80  lies concentrically within the sleeve  60 . In the exemplary embodiment, this includes a tubular section  82  of relatively enlarged internal and external diameter, but similar wall thickness to a main central portion of the arbor shaft. A downstream part  83  of the tubular section  82  surrounds an upstream portion of a sleeve bearing  84 , a downstream portion of which is mounted to a downstream end of the sleeve  60 . The interior surface  85  of the downstream part  83  and exterior surface  86  of the upstream part of the sleeve  84  are in sliding contact, lubricated through passageways  87  in the sleeve bearing upstream portion. The lubricant may be introduced through a jet  88  concentrically within the sleeve bearing and having lateral outlet ports. The exemplary embodiment of FIG. 1 shows numerous additional lubrication features which are not separately discussed.  
         [0021]    The downstream part  83  of the section  82  is externally toothed while an upstream part is smooth. The externally toothed portion is enmeshed with an internally toothed proximal section  90  of a spring  92 . The circumferential exterior surface  94  of the spring  92  is in close facing spaced or contacting proximity to an interior surface  96  of the sleeve  60  as is further described below.  
         [0022]    [0022]FIGS. 2 and 3 show further details of the spring  92 . The spring extends from a proximal axial end  100  to a distal axial end  102 . A slot  104  extends between these ends to form the spring as a coil having proximal and distal coil ends  106  and  108 , respectively. The exemplary slot  104  has a nearly longitudinal portion  110  along the proximal section  90  transitioning to a helical portion  112 . The helical portion has a helix angle θ which for purpose of reference is identified as the acute angle between the helix and the longitudinal direction. In the exemplary embodiment, the angle is nonconstant, progressively decreasing from the proximal section  90 . At a distal portion of the spring, the rate of decrease may nearly cease or nearly cease so that the distal portion is of approximately constant helix angle. In the exemplary embodiment, in a relaxed condition the spring outer circumferential surface  94  is close to cylindrical, being essentially cylindrical along a major portion of its length and flaring out slightly at a tip or teaser portion  120 . In a relaxed condition, the tip portion  120  is in frictional contact with the sleeve interior surface  96 . The wall thickness of the spring is thinned along this distal portion for lightness of frictional engagement between the spring tip portion and sleeve. The spring may be manufactured by known techniques for or by techniques to be developed. The general form of the spring may tend to resemble half of a spring such as that identified in the &#39;931 patent with the addition of the internally toothed proximal section.  
         [0023]    When the arbor is initially rotated relative to the sleeve, there is frictional engagement between the tip portion  120  and the sleeve interior surface  96 . If this initial rotation is in the direction wherein the friction would tend to contract the spring, the rotation may continue with substantial slippage between the tip portion  120  and sleeve interior surface  96 . If this initial relative rotation is in the opposite, expanding, direction, the forces associated with expansion will increase the normal force between the tip portion  120  and the sleeve interior surface  96  and expand the heretofore noncontacting spaced-apart portions of the surface  94  into contacting frictional engagement with the surface  96 . This enhanced friction resists further relative rotation.  
         [0024]    The role of the spring and arbor shaft teeth is now addressed. FIG. 4 shows engagement of the teeth of the spring and arbor shaft in a disengaged neutral or overrunning condition. In this condition, the slot portion  110  is relatively closed and the spring exterior circumferential surface  94  is spaced apart from the sleeve interior surface  96 . In this disengaged condition, the teeth are intermeshed but in generally non-contacting, non-load-bearing relation. For example, the resilience of the spring tending to close the gap  110  will bring the gap-facing surfaces of adjacent spring teeth into contact with the opposite faces of arbor shaft teeth. Moving circumferentially away from the gap, the two groups of teeth will tend to be non-contacting. The torque transmission associated with expansion of the spring in the engaged condition causes a camming interaction between the teeth  121  and  122  of the spring and arbor, expanding the slot  110  and radially expanding the spring surface  94  into engagement with the sleeve surface  96 . In the exemplary embodiment, this expansion sequentially brings more pairs of spring and arbor teeth into engagement with each other. Advantageously, the teeth pitch and other dimensions (such as the initial radial gap between surfaces  94  and  96 ) are selected so that a maximally engaged condition is achieved when all arbor teeth are bearing against adjacent spring teeth (FIG. 5).  
         [0025]    The expansion of the slot portion  110  advantageously reduces stress concentrations which would otherwise be present, for example, if the slot terminated at the section  90 . Thus, although a more rigid nonmoving mounting between the spring and arbor is possible, it is advantageous that the mounting have sufficient play to permit the spring expansion along the section  90 . Accordingly, in the exemplary embodiment the cooperation of the teeth constrains relative movement to a fraction of the tooth pitch appropriate to permit the expansion. In the exemplary embodiment, near the distal axial end  102  the spring&#39;s coil extends nearly circumferentially (e.g. typically well under 10° off circumferential). Near the proximal axial end  100  the slot extends longitudinally (e.g. well within 15° of longitudinal and effective to permit the slot expansion). If the teeth were helically or otherwise formed, the slot would/could be otherwise formed to match.  
         [0026]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, in various embodiments or uses, the input may be through the sleeve rather than the arbor. Also, the invention may be applied to various spring and clutch configurations both known and yet developed. Details of any particular application (e.g., the environment in which the clutch is used) may influence the structure of such implementation. Accordingly, other embodiments are within the scope of the following claims.