Abstract:
A resettable torque limiter for installation between two rotary members, which can smoothly disengage upon application of predetermined torque acting between the members and smoothly reset upon decline of applied torque below the predetermined level. An undulating engagement surface formed on one member according to a function preferably an eighth power polynomial is engaged by a plurality of engagement elements on the other member which smoothly ride over lobes formed on the engagement surface when the torque limit is exceeded without causing any discontinuities in acceleration or jerk when the driving connection between the rotary members is interrupted until the applied torque declines below the preset limit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation in part of U.S. patent application Ser. No. 12/481,701 filed on Jun. 20, 2009 which claims the benefit of U.S. provisional application No. 61/060,288 filed on Jun. 10, 2008, the corresponding published application no. 2009-0305794-A1 is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention concerns automatically resetting torque limiting clutches, commonly referred to as limiters and more particularly automatically resetting torque limiters which can disconnect drive on overload. 
         [0003]    Torque limiters are designed to release a driving connection when an excessive load is experienced. Such limiters are typically used in the context of a drive for various types of machinery subject to jams and other overloads in order to not damage the driving motor or the machinery. In most instances, the release of the torque limiter is noticed soon after it occurs since the machinery ceases to operate, and the need to reset the torque limiter input becomes apparent. 
         [0004]    Automatic resetting torque limiting clutches have been in existence for many years, typically of a friction clutch or ball detent type. The friction type will release or slip at a preset overload torque value, and will reengage when the overload is removed. The disadvantage of this arrangement is that repeated heating of the torque limiter friction linings (as heat is generated by the slipping) causes the clutch capacity to fade, as the higher lining temperatures reduces the coefficient of friction, until the torque limiter slips continuously and destroys itself. 
         [0005]    Another long known torque limiter type is the ball-detent reset torque limiter, which uses spring forces to push balls into drill point cavities in the other member, with the geometry thereof establishing forces and angles to produce a retraction of said balls to create a release at a preset torque level. The torque limiter will reengage when the torque demand falls somewhere below the release torque. The disadvantage of this device is the sudden changes in the acceleration of the connected components, which produces stresses in the material which exceeds the elastic limits of the components when running, disengaged, or when reengaging, which in turn produces deformations which greatly reduce the torque limiter service life. 
         [0006]    There have also been developed further refinement of such torque limiters in which smoothly curved engagement surfaces defining the cavities in attempt to reduce the stresses occasioned by engagement and reengagement. 
         [0007]    In some torque limiter installations, relatively high speeds of the input is involved, commonly as high as motor speed, when there is no reduction gearing interposed between the motor and the input to the torque limiter lowering the rotational speed of the input. In those instances of high speed torque limiter release, the overrunning components would be subjected to severe shock loads as a result of being driven at these high rotational speeds, which can be on the order of 1800 rpm or greater. 
         [0008]    This could cause deformation of the mating torque limiter components, leading to failure or to changes in the set and reset torque levels due to deformations of the follower elements or engagement surfaces. 
         [0009]    These applications create severe demands in cases where access to the torque limiters is very difficult or impossible as when the machine is located in tight spaces such as in tunneling and mining operations. In these applications, a torque limiter will likely be operated in the disengaged mode for extended time periods, and complete failures or significant degrading of performance would be very undesirable and costly. 
         [0010]    A satisfactory torque limiter for such high speed limited access applications has not yet been developed. 
         [0011]    Those working in the torque limiter field and on resettable torque limiters of the type described have heretofore long recognized that the drive surfaces which act to interrupt the drive through the limiter by causing drive elements under excessively high loads to be moved to a released position should be smoothly curved to reduce shocks when engaging and disengaging these elements. See U.S. Pat. No. 2,501,648 for a discussion of these matters in the context of an automatically resetting torque limiter. 
         [0012]    In the field of cams design in a machine where there is a particular designed for output motion there has long been recognition that simply smoothly curving these surfaces is not sufficient to avoid shocks and early failures. 
         [0013]    However, in the field of automatic resetting torque limiters, it has not heretofore been recognized that simply smoothly curving the engagement surfaces which are engaged by drive elements is not sufficient to eliminate noise and impact wear of the engagement surfaces and elements when continuously operating in the released condition at high speeds, i.e. around 1800 rpm or higher. In fact, despite a long felt need for such a torque limiter, no automatically reset torque limiter has been produced which can operate successfully under those conditions. 
         [0014]    It is the object of the present invention to provide an automatically resettable torque limiter which is capable of operating in its released mode at high speeds for extended periods without damaging the mechanism such as to result in a failure or in a progressive change in the release and resetting torque levels. 
         [0015]    It is a further object to provide such an automatically resetting torque limiter which Is not excessively noisy when running in its released condition at high speeds. 
       SUMMARY OF THE INVENTION 
       [0016]    The above recited objects and other objects which will be understood upon a reading of the following specification and claims are achieved by providing a particular engagement surface curved shape which does not create large dynamic loads or stresses in the components which exceed the elastic limit of the elements even when operated at high speeds at 1800 rpm or greater such as to minimize shocks and resultant wear and noise during operation. 
         [0017]    Such engagement surface curve is of a shape which does not produce discontinuities in the acceleration and jerk functions of the follow elements induced thereform at any point during overrunning operation. Such engagement surface curve shape is preferably defined by a polynomial of at least fifth power and preferably a seventh power and most preferably an eighth power polynomial with other parameters producing the desired torque drive release value and designed for the particular installation of the resettable torque limiter. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a pictorial view of a torque limiter according to the present invention with an outer rotary member shown in phantom lines to reveal interior details. 
           [0019]      FIG. 2  is an enlarged fragmentary side view of a portion of the torque limiter shown in  FIG. 1 . 
           [0020]      FIG. 3  is a pictorial view of the rotary member included in the torque limiter of  FIG. 1 . 
           [0021]      FIG. 3B  is a pictorial of a different embodiment of the rotary member shown in  FIG. 3A . 
           [0022]      FIG. 3C  is a pictorial view of another embodiment of the rotary member shown in  FIG. 3A . 
           [0023]      FIGS. 4A and 4B  are diagrams of the force relationship between an external engagement surface and follower roller follower in normal operation in the released condition. 
           [0024]      FIG. 5  is a diagram of a pair of engagement elements comprised of two rollers urged into contact with a two lobed engagement surface on a drive rotor with a plot in broken lines of the level of acceleration of the follower elements occurring as the rotor rotates through one complete rotation. 
           [0025]      FIG. 6  is a plot of the shape of each of three forms of an engagement surface. 
           [0026]      FIGS. 7A-7C  are respective plots of displacement, velocity, accelerations, and jerk produced by the three engagement surface shapes shown in  FIG. 6 . 
           [0027]      FIGS. 8A ,  8 B,  8 C and  8 D are plots of the displacement, velocity, acceleration and jerk of engagement elements produced by an eighth power polynomial defined engagement surface when the torque limiter engagement elements are traversed over the engagement surface. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims. 
         [0029]    A torque limiter  10  which is automatically resettable is shown in  FIG. 1 . 
         [0030]    The automatically resetting torque limiter  10  includes two relatively rotary members  12 ,  14 , one of the members  12  or  14  drivingly connected to a source of power  13  such as an electric motor, the other of the members  12  or  14  drivingly connected to driven machinery  15 . One member  12  is formed with an engagement surface  16 , which extends circumferentially about the axis of rotation of the member  12 . The engagement surface  16  in this embodiment undulates to form one or more peak undulations or lobes  16 A, the distance from the axis of rotation to points on the engagement surface  16  varying about the outer perimeter of the member  12  in the region of the lobes  16 . 
         [0031]    The other rotary member  14  mounts one or more engagement elements, here comprised of rocker arm roller follower assemblies  18  including rolling engagement elements comprising rollers  20  spring urged into engagement with the engagement surface  16  with a radially inwardly directed force which increases as the engagement elements or rollers  20  move up a lobe  16 A. 
         [0032]    Rocker arm mounted follower rollers  20  are preferred over slidable contact elements such as shown in U.S. Pat. No. 2,501,648 as the rollers and the rocker arm mount therefore minimizes sliding contact and thereby reduces friction and the resultant heat buildup therefrom as the limiter continuously operates in a released mode for sustained periods. 
         [0033]    As long as the torque level transmitted between the members  12 ,  14  is below a predetermined release torque, the spring force prevents the engagement rollers  20  from completely ascending the peaks defining undulations or lobes  16 A since the spring force resisting movement of the rollers increases as the engagement rollers  20  move out radially in moving up the lobes  16 A until the applied torque can no longer generate sufficient force to further displace the engagement rollers  20 , and relative rotation is prevented so that the driving relationship between the members is maintained. 
         [0034]    The rotary driving connection is between the engagement surface  16  and the engagement elements comprise of the roller assemblies  18 , and there is thus only slight relative rotation therebetween as the rollers  20  partially ascend the lobes and the driving relationship between members  12  and  14  is maintained. The radially directed spring force will prevent movement of the engagement rollers  20  all the way up the lobe  16 A, due to the increasing resistance preventing relative rotation until the torque reaches the preset torque limit. 
         [0035]    The reaction force between the engagement rollers  20  and the engagement surface  16  produces a tangential component capable of transmitting a torque as long as the members  12 ,  14  do not rotate relative to each other. This relative rotation is prevented as long as the torque level generates a radial or axial component not sufficiently high to be able to move the follower elements  20  completely past the peak undulations or lobes  16 A. That is, the torque must be high enough to develop a force component able to overcome the increasing force generated by the springs and force the engagement rollers  20  to move a sufficient radial distance in a direction away from the engagement surface  16  to clear the lobes  16 A against the resistance of the component of the reaction to spring force acting on the engagement rollers  20  in opposition to the applied torque. 
         [0036]    Once that preset torque limit is exceeded, the engagement rollers  20  will overcome the spring force and completely ascend and move past the respective lobes  16 A of the engagement surface  16 , and relative rotation between the members  12 ,  14  will commence and continue as long as the applied torque remains at or above that level. If the torque level declines below that predetermined level, drive is automatically re-established between the members  12 ,  14  as the engagement rollers  20  can no longer completely ascend the lobes  16 A due to the resistance created by the spring forces. 
         [0037]    The moving parts may be submerged in an oil bath, the oil held outward by centrifugal force, and heat from churning the oil when the torque limiter  10  is continuously running in a released mode is thereby dissipated to the air. 
         [0038]    The rocker arm-roller assemblies  18  and lobes  16 A may be variously configured and mounted as described in the published patent application referenced above. 
         [0039]    The engagement surface  16  can be varied to accommodate additional roller assemblies as required to produce the required release torque level, with one lobe for each roller assembly  18 . The engagement surface shape can also be varied to produce high torque attack, i.e., resistance to radial or axial movement of the follower rollers  20  can be made to increase rapidly when ascending the lobes  16 A and a lower rate of torque decline when descending past the lobes  16 A. 
         [0040]    The engagement surface  16  can also be on the exterior perimeter of the rotary drive member  12  with the engagement element rollers  20  moving radially outwardly against inwardly directed spring forces to release as shown in  FIG. 1 , or a surface can be formed on an internal surface, with the rollers  20  spring urged to move radially outwardly as described in U.S. 2009-0305794 A1. 
         [0041]    The engagement surface can also be formed on an axial face of a drive member with the rollers  20  moved axially as described in U.S. 2009-0305794 A1. 
         [0042]    In the torque limiter  10  shown in  FIGS. 1-3 , one rotary drive member  12  comprises a rotor having a peripherally extending external engagement surface  16  as described above, and the rocker arm-roller assemblies  18  each include an engagement element comprised of a roller  20  mounted on one end of a rocker arm  22  formed by a pair of rocker arm plates  22 - 1 ,  22 - 2  pivotally mounted on the other rotary driven member  14  with pivot pin assemblies  29 . 
         [0043]    The other rotary member  14  is formed in an annular shape which encloses the rotary member  12 . The other end of each of the pivoted rocker arms  22  mounts a cross pin  24  which acts to compress a pair of springs  26  disposed in spring seat cavities  27  formed in the member  14 . The rocker the arms  22  pivot up as the engagement rollers  20  are moved radially outwardly in ascending the lobes  16 A but are unable to completely pass over the lobes  16 A until the transmitted torque exceeds a predetermined level. Other arrangements are possible as described in published patent application U.S. No.2009-0305794 A1. 
         [0044]    As described above, the displacement of the engagement rollers produced by the curve of the engagement surface  16  must produce smooth accelerations of the rollers  20  when ascending the lobes  16 A, in order to avoid shocks when the torque limiter  10  is running released or when resetting. 
         [0045]      FIGS. 4A and 4B  plot the forces generated in a driving condition transmitting torque below the preset limit ( FIG. 4A ) and high torque exceeding the preset limit ( FIG. 4B ). As torque is applied to the member  12 , each engagement roller element begins to roll up a lobe  16 A, increasingly compressing the spring through the rocker arm  22 . The force of the spring (F S ) keeping the engagement element roller  20  in constant contact with the engagement surface  16  (through the rocker arm) causes a reaction force (F N ) normal to the cam surface. A component of (F N ) acting perpendicular to a radial line to the point of contact is shown as (F T ). The magnitude of (F T ) multiplied by the radial distance to the point of contact is the torque transmitted by the follower. The magnitude of (F T ) increases as the follower rollers rolls further up the lobes  16 A due to the increased spring compression combined with the increased pressure angle between (F N ) and the radial line to the point of contact. As the magnitude of (F T ) increases and the radial distance between (F T ) and the axis of rotation increases, the transmitted torque increases until it equals the input torque. When a torque overload situation occurs and the engagement roller  20  rolls up and over the lobe  16 A, the torque transmitted drops until the engagement element encounters the next lobe  16 A on the engagement surface  16  in a continuously repeating cycle. 
         [0046]      FIG. 6  shows a plot of three typical smooth overlapping curves, displacement versus rotational angle, a simple harmonic curve A (broken line), a cycloidal curve B (solid line), and a seventh power polynomial curve C (partially broken line). 
         [0047]    It should be noted that these are all smooth curves. However,  FIG. 7A  which shows plot of the first three derivatives of displacement for the simple harmonic function corresponding to the curve reveals that the plots of acceleration A and jerk J evidence discontinuities which theoretically represent infinite values of acceleration. Corresponding accelerations and jerk would impose destructive loadings and excessive noise, and leading to progressively variable performance and perhaps early failure of a torque limiter using such a shape for an engagement surface. This is despite the gradual appearance of the gentle curved shape of an engagement surface according to a simple harmonic function. 
         [0048]      FIG. 7B  shows plots of the three derivatives of the cycloidal function plot, revealing such discontinuities in the jerk plot, which have it also been determined to create unacceptable loadings and noise. 
         [0049]      FIG. 7C  shows the plots of the derivatives of seventh power polynomial, revealing that no such discontinuities occur, and this shape would allow a torque limiter to operate continuously in a released mode at relatively high speed, i.e., on the order of 1500-1800 rpm, without damage or changing release and reset torque levels. 
         [0050]      FIG. 8A  is a plot of displacement versus angular position, according to the eighth power polynomial equation: 
         [0000]        S=   h [56(θ/β) 5 −140(θ/β) 6 +120(θ/β) 7 −35(θ/β) 8 ]
 
         [0051]    The precise shape of curvature of the engagement surface is critical for a resettable torque limiter which is operated at relatively high speeds when in a released condition. While it has long been assumed by torque limiter designers that providing a smooth curvature of the engagement surface should suffice, resettable torque limiters have heretofore never been successfully able to operate at high speed in a released mode for extended periods. 
         [0052]    The reason is that simply providing a smooth curvature is not sufficient as there are dynamic effects which are not apparent. Referring to  FIG. 5 , the engagement surface  30 A of a rotary drive member  30  driven by an input shaft  32  has two oppositely located lobes or undulations  34  extending out from a constant diameter peripheral engagement surface  36 . 
         [0053]    A pair of rocker arms  38  pivotally mounted at  40  supported respective engagement elements comprised of rollers  42  urged into engagement with the engagement surface  36  by associated springs  44 . 
         [0054]    The path traced by broken line L- 1  represents the displacement of the center line of the rollers  42  as the member  30  rotates, reflecting the smooth curvature of the engagement surface  36 . 
         [0055]    However, the radial acceleration of the rollers  42 , represented by broken line L- 2  shows that acceleration of the rollers  42  sharply varies as the rollers  42  are displaced by the engagement surface  30 A of the drive member  30 . 
         [0056]    Smooth appearing curves can produce dynamic loading by the theoretical development of infinite or near infinite accelerations and/or jerk at transition points on the engagement surface curvature. 
         [0057]    A curvature which avoid such discontinuities power polynomials at least 5 th  power, including the seventh power, but most preferred is the eighth power  FIG. 8A  shows a plot of such an eighth power polynomial curve. 
         [0058]    The points 1-7 correspond to:
       1. Transition from Dwell to Rise   2. Maximum Acceleration   3. Maximum Negative Acceleration   4. Maximum Displacement   5. Maximum Negative Acceleration   6. Maximum Acceleration   7. Transition from Rise to Dwell       
 
         [0066]    These are repeated for travel over each lobe. 
         [0067]      FIG. 8B  is a plot of the first derivative (velocity) of the function of  FIG. 8A . 
         [0000]        dS/dθ=V=   h /β[280(θ/β) 4 −840(θ/β) 5 +840(θ/β) 7 ]
 
         [0000]    Where β is the total angle of rise. 
         [0068]    Points 1-7 correspond to:
       1. Increase in Vertical Velocity   2. Rise-Maximum Increasing Velocity Slope   3. Rise Maximum Decreasing Velocity Slope   4. Zero Velocity   5. Fall-Maximum Increasing Velocity Slope   6. Fall Maximum Decreasing Velocity Slope   7. Back to Dwell Rest       
 
         [0076]      FIG. 8C  is a plot of the second derivative (acceleration) of the function of  FIG. 8A : 
         [0000]        dV/dθ=A=h/β   2 [1120(θ/β) 3 −4200(θ/β) 4 +5040(θ/β) 5 −1960(θ/β) 6 ]
 
         [0000]    It can be seen that while there are large changes in accelerations, there are no discontinuities or infinite values of accelerations. 
         [0077]      FIG. 8D  is a plot of the third derivative (jerk) of the function of  FIG. 8A : 
         [0000]        dA/dθ=J=h/β   3 [3360(θ/β) 2 −16800(θ/β) 3 +25200(θ/β) 4 −11760(θ/β) 5 ]
 
         [0000]    It can be seen that again no discontinuities or very large or infinite values of jerk occur. 
         [0078]    Thus, an engagement surface formed in conformity to a plot of an eight power polynomial function above will provide a torque limiter which can operate continuously in the released condition at high motor speeds without overstressing the parts leading to noise, and changing release torque levels, and complete failures. 
         [0079]    Each particular application with a desired torque release set point and particulars as to the geometry of the various parts will reflect the actual dimensions of the curve of the engagement surface.