Torque limiting coupling
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention is a continuation-in-part of copending U.S. Pat. application Ser. No. 447,471 filed Mar. 4, 1974, and now U.S. Pat. No. 3,942,338. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to torque limiting, or torque overload couplings for transmitting torque and motion from a drive shaft to a tubular member, which is capable of preventing the transmission of excessive torque when such excessive torque may be detrimental to related apparatus. 
     II. Description of the Prior Art 
     Although not exclusively limited to such applications, the torque overload coupling of the invention has particular applications as a coupling for driving a conveyor roller from a power or drive shaft. In a conveyor system, one or more rollers are generally power driven, while a plurality of supporting and guiding rollers, engaging the conveyor belt, are simply supported by appropriate support means for free rotation about their axis. in the event of a jam of the articles transported by the conveyor, damage may result to the articles on the conveyor belt, to the conveyor itself, or to the conveyor driving mechanism, if the conveyor belt is continuously power driven. It is, therefore, desirable that the conveyor be stopped immediately, either through manual controls, thus necessitating the presence of an operator constantly at the controls, or by means of some automatic device, in order to prevent damage to the articles on the conveyor, to the conveyor itself, or injury to personnel. Torque limiting slip clutches and other mechanisms may be used, but torque limiting clutches, automatic torque limiting devices and other automatic shut-off devices are rather complex and costly. 
     The present invention, by providing a simple torque limiting coupling for driving a conveyor roller from a power shaft, automatically prevents damage in the event of a jam or of an obstruction tending to slow down or stop the motion of the conveyor belt. The present invention also permits the provision of a plurality of drive rollers for a conveyor system capable of individually adapting their respective driving torque to load conditions and to power upsurges, and capable of accepting variations in driving speed from one power shaft to another. Furthermore, the present invention permits tailoring of the torque transmitting factor of the coupling as a function of the number of substantially identical intermediary coupling members, and as a function of their resiliency. The present invention provides a torque overload coupling utilizing rolling forces and, when operating in a disengaged mode, does not utilize slippage of friction surfaces, or engagement and disengagement of connecting elements, as is generally the case in prior art devices. 
     Since the number of coupling members, rather than the physical dimension of the coupling member as taught by my aforementioned patent, defines the torque transmitting characteristics of the device, the previously known adjustments to the coupling member are rendered unnecessary. Furthermore, the present invention provides a more even torque transmission from the drive member to the driven member thereby minimizing strain, and resultant wear on the system&#39;s bearings. 
     SUMMARY OF THE INVENTION 
     The many objects and advantages of the present invention are achieved by providing a simple structure consisting of a hollow or tubular driven member coaxially disposed for free rotation about a driving shaft disposed within the tubular member, and a plurality of resilient spherical members, disposed between the shaft and the inner bore surface of the tubular member, such that the spherical members act as resilient wedge elements transmitting the rotation from the shaft to the tubular member as long as the resistant torque applied to the tubular member does not exceed a predetermined limit. When the torque opposing the rotation of the tubular member exceeds such a limit, the tubular member ceases to rotate and the driving shaft continues to rotate, thus causing the spherical members to roll in the space between the shaft and the inner bore of the stalled tubular member. 
     The many objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of an example of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view, with portions broken away to show the internal construction, of an example of application of the invention to a drive roll such as a conveyor drive roll; 
     FIG. 2 is an end view thereof; 
     FIGS. 3 - 4 are each a transverse section thereof, at an enlarged scale, showing the operation of the torque overload coupling of the invention in a torque transmitting mode; and 
     FIGS. 5 - 6 are views similar to FIGS. 3 - 4 showing the coupling of the invention in a torque overload mode. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawing and more particularly to FIGS. 1 and 2 thereof, the invention is shown embodied in a torque overload coupling, or torque limiting coupling, for a roller such as a conveyor drive roller. The roller consitsts of a tubular member 10 supported at each end by a generally disc shaped end member, shown at 12, consisting, for example, of the outer race of an appropriate bearing 13 such as a roller, ball, needle, or plain bearing, the inner race 14 of which is supported by a shaft 16 disposed axially throughout the tubular member 10. In this manner, the tubular member 10 is free to rotate in any direction independently of the shaft 16. When used as part of a conveyor the roller may support and guide a conveyor belt (not shown), a plurality of such rollers being normally disposed and supported by a frame (not shown). 
     The tubular member 10 is fastened to the end bearing 13 by any conventional means such as having the outer race 15 of each bearing press-fitted on an end of the tubular member 10, or welded or cemented thereto, and the shaft 16 which, in the example illustrated is shown as being of polygonal shape in cross-section and more specifically as being hexagonal, is passed through the central bore 18 of the bearing inner race 14, appropriate conventional means such as a transversely extending pin or other means being used for holding the shaft 16 against longitudinal displacement relative to the inner race 14. 
     As shown in FIG. 1 and at FIGS. 3 - 6, a plurality of spherical members 20 are somewhat randomly disposed in the space 22 formed between the inner bore 24 of the tubular member 10 and the peripheral surface of the shaft 16 extending throughout the tubular member. The spherical members 20 have an outside diameter larger than the widest distance separating the peripheral surface of the shaft 16 and the inner bore surface 24 of the tubular member 10. Such widest distance corresponds to the distance separating the center of a flat face 26 of the shaft 16 to the inner bore surface 24 of the tubular member 10 along a radius of the shaft and tubular member assembly. The spherical members 20 are made of a resilient elastic material, preferably an elastomer such as rubber or a resilient plastic such as polyester, polyethylene, polypropylene, polystyrene, and the like. The spherical members 20 may also be hollow as shown at 20&#39; (FIG. 5) or, alternatively and preferably, as shown, are of solid construction. 
     The shaft 16 is normally power driven in rotation, for example, in the direction of the arrow 28, by conventional means such as an electrical gear motor or the like, not shown, and normally drives the tubular member 10 in rotation in the same direction, as shown FIG. 3, as long as the torque required for driving the tubular member 10 does not exceed a predetermined torque. The drive between the shaft 16 and the tubular member 10 is effected by means of the spherical members 20 being placed in compression between a flat face 26 of the shaft 16 and the inner bore surface 24 of the tubular member 10, the members 20 acting as resilient wedges the shaft flat face 26 and the inner bore surface 24 of the tubular member 10. The greater the load applied to the member 10 opposing the rotation of the tubular member, the greater the members 20 are compressed and rollingly displaced in a counter-clockwise direction, relative to the showing of the drawing, thus rolling closer to an edge 30 between two consecutive flat faces 26 of the shaft 16, as illustrated at FIG. 4. 
     Under those conditions of very small load, or alternatively, of increased load not exceeding the driving torque limit of the coupling of the invention, the shaft 16 and the tubular member 10 are rotating in unison. However, as soon as the load on the tubular member 10 becomes excessive, as such will be the case in the event that a jam occurs in a conveyor system transporting articles, or in the event that for any reason the tubular member 10 is prevented from rotating, the spherical members 20 are caused to roll over and beyond the edge 30 between consecutive faces 26 of the shaft 16, thus permitting the shaft 16 to continue rotating without driving the tubular member 10, as long as the load on the tubular member remains excessive. 
     Under a completely stalled condition, that is under a condition preventing the tubular member 10 from rotating, the shaft 16 continues to rotate by being driven by its driving mechanism, not shown, and the spherical members 20 roll in the space 22 between the peripheral surface of the shaft 16 and the inner bore 24 of the tubular member 10, the members 20 rotating in the direction of the arrows 32 at FIGS. 5 and 6. 
     In the course of its rolling action between the peripheral surface of the shaft 16 and the inner bore surface 24 of the tubular member, the resilient members 20 pass through successive phases of maximum radial compression and minimum radial compression while rolling respectively over the edges 30 and the center portion of the flat faces 26 on the shaft 16, as shown respectively at FIG. 5 and FIG. 6. 
     As soon as the tubular member 10 is again free to rotate, the resilient wedging action of the spherical members 20 are reestablished and the tubular member 10 is again coupled to the driving shaft 16, thus automatically reestablishing the drive of the tubular member. 
     Although the tubular member 10 has been illustrated with a cylindrical inner bore 24, it will be obvious to those skilled in the art that the shaft 16 may be made with a cylindrical periphery and the tubular member 10 may be made with a polygonal inner bore cross section. Also it would be possible to make both the inner periphery of the tubular member 10 and the outer surface of the shaft 10 cylindrical. The member 20 would not couple in the manner described above, but would still act as a torque limiting coupling. 
     The torque transmitted by the coupling of the invention may be controlled to a desired limit by way of the number and the resiliency characteristics of the spherical members 20. For a given diameter of the members 20 and made of a given material the limit of the torque transmitted by the coupling is increased as the number of spherical members 20 between the shaft 16 and tubular member 10 is increased. Thus the torque transmitted can be conveniently varied by simply increasing or decreasing the number of spherical members as desired. By using spherical members 20 which are hollow, the transmitted torque limit is furthermore a function of the wall thickness of the members 20, for a given material, the number and diameter of the spherical members remaining constant. It should also be apparent that the members 20 may be randomly distributed within the tubular member 10 thereby effecting a more even torque transmission from the shaft 16 to the tubular member 10 than has been previously known thus minimizing strain, and resulting wear, on the bearings 13. 
     It will also be appreciated by those skilled in the art that the torque limiting coupling of the invention has many applications other than for conveyor roller drive.