Abstract:
A harmonic drive apparatus (also known as a strain wave gear) is constructed with a circular, flexible spline disc having a slightly conical configuration. The flexible saline disc has opposite convex and concave surfaces with a plurality of radial, flexible gear teeth arranged circumferentially on the convex surface. The flexible spline disc enables the harmonic drive apparatus to provide a thinner or more compact packing when needed for unique space requirements of a speed reduction transmission.

Description:
FIELD 
     The present disclosure relates generally to a harmonic drive apparatus, also known as a strain wave gear. In particular, the present disclosure relates to a harmonic drive apparatus having a flexible spline with a conical configuration. 
     BACKGROUND 
     Harmonic drive apparatus are generally cylindrical speed reduction transmissions that can drastically reduce the speed of an input shaft transmitted to an output shaft with little or no backlash. Harmonic drive apparatus can be contained in small housings and are used extensively in applications where size and backlash are critical, such as in robotics. Harmonic drive apparatus were first introduced around 1957. 
     The typical construction of a harmonic drive apparatus comprises a cylindrical flexible spline inside a cylindrical fixed spline. A plurality of flexible, parallel gear teeth are provided on a cylindrical exterior surface of the flexible spline. A further plurality of parallel, rigid gear teeth are provided on a cylindrical interior surface of the fixed spline. The fixed spline gear teeth project inwardly toward the flexible spline gear teeth. The number of fixed spline gear teeth is larger than the number of flexible spline gear teeth. 
     An output shaft of the harmonic drive apparatus is connected to the flexible spline. 
     An input shaft of the harmonic drive apparatus is connected to a wave generator having an elliptical exterior surface. The wave generator elliptical exterior surface is positioned inside the flexible spline. The elliptical surface is positioned in a same plane as the flexible spline gear teeth and the fixed spline gear teeth. The elliptical surface engages an interior surface of the flexible spline at two 180 degrees spaced locations on the elliptical surface and pushes the flexible spline teeth into engagement with the fixed spline teeth. On rotation of the input shaft and rotation of the elliptical surface, the gear teeth on the flexible spline 180 degrees apart are pushed into engagement with the gear teeth of the fixed spline and the engagement between the gear teeth causes rotation of the flexible spline and the output shaft. 
     Even though harmonic drive apparatus are typically smaller than other gear reduction transmissions, there may be situations where even more compact, thinner packaging would be desirable. Thus, there is a need for an alternative packaging envelope of a harmonic drive apparatus where thinness or compactness is desirable. 
     SUMMARY 
     The harmonic drive apparatus (also known as a strain wave gear) of this disclosure includes a conical or flat flexible spline disc. The flexible spline disc enables the apparatus to provide an even thinner or even more compact packaging when needed for unique space requirements of a speed reduction transmission. 
     The harmonic drive apparatus includes an input shaft having a center axis that defines mutually perpendicular axial and radial directions, and an output shaft that is coaxial with the input shaft. 
     The apparatus also includes a wave generator disc that has axially opposing first and second circular surfaces that are coaxial with the center axis. The wave generator disc first surface is connected to the input shaft, whereby rotation of the input shaft rotates the wave generator disc. The wave generator disc second surface is a saddle shaped wave surface. The wave surface rotates around the center axis and has a sequential wave pattern of a crest that projects axially from the wave generator disc, then a trough that is axially recessed into the wave generator disc, then another crest that projects axially from the wave generator disc and then another trough that is axially recessed into the wave generator disc. The pair of crests project axially from the wave generator disc on radially opposite sides of the center axis and the pair of troughs are recessed axially into the wave generator disc on radially opposite sides of the center axis. 
     A plurality of roller bearings are arranged circumferentially around the center axis. The plurality of roller bearings are configured in rolling engagement with the wave generator disc wave surface. 
     A conical or flat, flexible spline disc is positioned on the axially opposite side of the roller bearings from the wave generator disc wave surface. The flexible spline disc has axially opposing first and second circular surfaces that are coaxial with the center axis. When the flexible spline disc has a conical configuration, the flexible spline first surface is generally concave and the flexible spline second surface is generally convex. The flexible spline first surface engages in rolling contact with the plurality of roller bearings. The flexible spline second surface is connected to the output shaft. The flexible spline second surface also has a plurality of radial, flexible gear teeth that are arranged around the center axis on the flexible spline second surface. 
     A rigid, fixed spline disc is positioned axially adjacent the flexible spline. The fixed spline has axially opposing first and second surfaces that are coaxial with the center axis. The fixed spline first surface has a plurality of radial, rigid gear teeth arranged circumferentially around the center axis on the first surface. The plurality of rigid gear teeth axially oppose the plurality of radial, flexible gear teeth on the flexible spline. 
     The pair of crests on the wave generator disc push a first and a second group of the flexible gear teeth on the flexible spline disc axially into engagement with a first and a second group of the rigid gear teeth on the fixed spline disc. The pair of troughs on the wave generator disc allow a third and a fourth group of the flexible gear teeth on the flexible spline disc to move axially away from and out of engagement with a third and a fourth group of rigid gear teeth on the fixed spline disc. 
     A housing encloses the wave generator disc, the plurality of roller bearings, the flexible spline disc and the fixed spline disc. The input shaft and the output shaft extend from axially opposite ends of the housing. 
     On rotation of the input shaft, the wave surface on the wave generator disc is rotated around the center axis and the pair of crests axially push the flexible spline and the first and second groups of gear teeth on the flexible spline into engagement with the first and second groups of rigid gear teeth on the fixed spline. The engaging groups of teeth rotate around the center axis in response to rotation of the input shaft and produce rotation of the flexible spline and the output shaft at a substantially reduced rate of rotation. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in other embodiments, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representation of a perspective view of the disassembled component parts of the harmonic drive apparatus from the left of the apparatus. 
         FIG. 2  is a representation of a perspective view of the component parts of the apparatus from the right of the apparatus. 
         FIG. 3  is a representation of a perspective view similar to that of  FIG. 1 , but showing enlarged views of selected drive components of the apparatus. 
         FIG. 4  is a representation of a cross-section view of the flexible spline of the apparatus. 
         FIG. 5  is a representation of a perspective view of a right side housing piece shown in  FIG. 1 . 
         FIG. 6  is a representation of a perspective view of a left side housing piece shown in  FIG. 1 . 
         FIG. 7  is a representation of a cross-section perspective view of the apparatus. 
         FIG. 8  is a representation of a cross-section view of a further embodiment of the flexible spline of the apparatus. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show representations of the component parts of the harmonic drive apparatus  10  (also known as a strain wave gear) of this disclosure with the component parts being disassembled to better show the relative relationship of the parts. The parts are arranged coaxially along a center axis  12  of the apparatus in the sequential positions represented in  FIGS. 1 and 2 . The parts of the apparatus  10  can be constructed of any materials typically employed in constructing the component parts of a speed reduction transmission. 
     The apparatus  10  includes a first housing piece  14  having a circular end wall  16  and a cylindrical side wall  18  that extends axially from the outer periphery of the end wall  16 . A small circular opening  22  is provided through the housing end wall  16 . The opening  22  is coaxial with the center axis  12 . The cylindrical side wall  18  surrounds a larger opening  24  into the interior of the first housing piece  14 . A conventional cylindrical bearing assembly  26 , for example a cylindrical roller bearing is press fit into the end wall opening  22 . A conventional thrust bearing, for example a single roll thrust roller bearing  28  is inserted through the housing larger opening  24  and is positioned against the interior surface of the end wall  16 . 
     An input shaft  32  of the apparatus extends through the center of the thrust roller bearing  28  and the cylindrical bearing  26 . The cylindrical bearing  26  mounts the input shaft  32  for rotation relative to the apparatus  10  about the center axis  12 . 
     A wave generator disc  34  is positioned in the first housing piece  14 . The wave generator disc  34  has axially opposing first  36  and second  38  circular surfaces that are coaxial with the center axis  12 . The wave generator disc first surface  36  is positioned in rolling engagement against the thrust roller bearing  28  as represented in  FIG. 7 . As represented in  FIG. 2 , the input shaft  32  is connected to the wave generator disc first circular surface  36 , whereby rotation of the input shaft rotates the wave generator disc  34  in the first housing piece  14 . Referring to  FIGS. 1 and 3 , the wave generator disc second circular surface  38  has the configuration of a saddle shaped wave surface. The wave surface circles around the center axis  12  and has a sequential wave pattern of a first crest  42  that projects axially from the wave generator disc  34 , then a trough  44  that is axially recessed into the wave generator disc  34 , then another crest  46  that projects axially from the wave generator disc  34  and then another trough  48  that is axially recessed into the wave generator disc  34 . As represented in  FIGS. 1 and 3 , the pair of crests  42 ,  46  project axially from the wave generator disc  34  on radially opposite sides of the center axis  12  and the pair of troughs  44 ,  48  are recessed axially into the wave generator disc  34  on radially opposite sides of the center axis  12 . Also represented in  FIGS. 1 and 3  is a circular bearing groove  52  formed in the wave generator disc second circular surface  38  adjacent the outer periphery of the second circular surface  38 . 
     As represented in  FIGS. 1, 3 and 7 , a plurality of bearings  54  are positioned in the circular bearing groove  52  in the wave generator disc second circular surface  38 . In the representation of the apparatus in  FIGS. 1, 3 and 7 , the plurality of bearings  54  are separate spherical ball bearings. It should be understood that other equivalent types of bearings and bearing assemblies could be substituted for the ball bearings  54 . 
     A flexible spline disc  56  is positioned on the axially opposite side of the ball bearings  54  from the wave generator disc second circular surface  38 . The flexible spline disc  56  has axially opposed first  58  and second  62  circular surfaces that are coaxial with the center axis  12 . In the embodiment of the apparatus  10  represented in the drawing figures, the flexible spline disc  56  has a conical configuration. Referring to the cross section of the flexible spline disc  56  represented in  FIG. 4 , the conical configuration gives the flexible spline disc first circular surface  58  a general concave configuration and gives the flexible spline disc second circular surface  62  a general convex configuration. In an alternate embodiment of the flexible spline disc  56  the disc is flat and the axially opposed first  58  and second  62  circular surfaces of the disc are flat.  FIG. 8  shows a representation of the flat embodiment of the flexible spline disc  56 ′. In  FIG. 8  the same reference numbers of  FIG. 4  are used, with the reference numbers being followed by a prime (′). As represented in  FIG. 7 , the flexible spline disc first circular surface  58  engages in rolling contact with the plurality of ball bearings  54 . The flexible spline disc second circular surface  62  is connected to an output shaft  64  of the apparatus  10 . The flexible spline disc  56  and the output shaft  64  are coaxial with the center axis  12 . The flexible spline disc second circular surface  62  also has a plurality of radial, flexible gear teeth  66  that are arranged circumferentially on the second circular surface  62  around the center axis  12  and adjacent the periphery of the disc. An annular lip  68  (shown in  FIGS. 1 and 2 ) can be provided on the output shaft  64  to enable a bearing (to be described) to provide minimal thrust pushing the flexible spline disc  56  against the wave generator disc  34 . 
     A rigid, fixed spline disc  72  is positioned axially adjacent the flexible spline disc  56 . The fixed spline disc  72  has axially opposed first  74  and second  76  circular surfaces that are coaxial with the center axis  12 . The fixed spline disc first circular surface  74  has a plurality of radial, rigid gear teeth  78  arranged circumferentially around the center axis  12  on the first circular surface  74  adjacent the periphery of the first circular surface. The plurality of rigid gear teeth  78  axially oppose the plurality of flexible gear teeth  66  on the flexible spline disc  56 . The fixed spline disc  72  has a center hole  82  through the disc that is coaxial with the center axis  12 . With the fixed spline disc  72  positioned axially adjacent the flexible spline disc  56 , the output shaft  64  extends coaxially through the fixed spline disc  72  center hole  82 . 
     A second housing piece  84  is secured to the fixed spline disc second circular surface  62  and the cylindrical side wall  18  of the first housing piece  14 . As represented in drawing  FIGS. 1, 2 and 6 , the second housing piece  84  is circular with opposing first  86  and second  88  surfaces that are coaxial with the center axis  12 . The second housing piece  84  also has a center opening  92  that is circular and coaxial with the center axis  12 . The second housing piece  84  center opening  92  is dimensioned to receive a second cylindrical bearing assembly  94  that is press fit in the opening  92 . The second housing piece  84  first surface  86  is secured to the rigid, fixed spline disc  72 , thereby fixing the fixed spline disc  72  stationary relative to the apparatus  10 . A peripheral portion of the second housing piece  84  first surface  86  is also secured to the edge of the first housing piece  14  side wall  18  by a plurality of threaded fastener assemblies  96  as represented in  FIG. 7 . The output shaft  64  extends axially from the flexible spline disc  56 , through the fixed spline disc  72  center hole  82  and the second cylindrical bearing assembly  94 . Thus, the housing assembled from the first housing piece  14  and the second housing piece  84  encloses the thrust roller bearing  28 , the wave generator disc  34 , the plurality of ball bearings  54 , the flexible spline disc  56  and the rigid, fixed spline disc  72  in an interior of the housing. The input shaft  32  and the output shaft  64  extend axially from opposite ends of the housing  98 . 
     On rotation of the input shaft  32 , the wave surface or second circular surface  38  of the wave generator disc  34  is rotated around the center axis  12  and the pair of crests  42 ,  46  axially push first and second groups of the ball bearings  54  on radially opposite sides of the center axis  12  into engagement with the flexible spline disc  56  first circular surface  58 . Through the intermediary of the first and second groups of ball bearings  54 , the pair of crests  42 ,  46  push the flexible spline disc  56  and first and second groups of radial, flexible gear teeth  66  on the flexible spline disc  56  on radially opposite sides of the center axis  12  into engagement with first and second groups of the radial, rigid gear teeth  78  on the rigid, fixed spline disc  72  on radially opposite sides of the center axis  12 . The engaging groups of gear teeth on radially opposite sides of the center axis  12  rotate around the center axis  12  in response to rotation of the input shaft  32  around the center axis and produce rotation of the flexible spline disc  56  and the output shaft  64  at a substantially reduced rate of rotation. 
     The reduction ratio of the harmonic drive apparatus  10  is calculated in the same manner as a conventional harmonic drive apparatus. The reduction ratio is equal to the number of radial, flexible gear teeth  66  minus the number of radial, rigid gear teeth  78 , divided by the number of radial, flexible gear teeth  66 . 
     As various modifications could be made in the construction of the apparatus and its method of operation herein described and illustrated without departing from the scope of the disclosure, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present disclosure should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.