Abstract:
An eccentric planetary traction drive transmission which includes at least two planetary rollers, sun roller member, and a carrier member. One of the planetary rollers is flexible and is positioned between and in contact with an outer ring member and the sun roller member. Rotation of either the outer ring member or the sun roller member wedges the flexible planetary roller within a convergent wedge gap which squeezes the flexible planetary roller between the outer ring member and the sun roll member. Friction between the flexible planetary roller, the sun roller member, and the outer ring member transmits rotational motion and torque between the outer ring member and the sun roller member. The other at least one supporting planetary roller is a supporting roller which supports the sun roller member and the carrier member. A plurality of bearings supports the sun roller member within the outer ring member and the at least one supporting planetary roller.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to U.S. Provisional Patent Application 60/354,010 filed Jan. 31, 2002, and U.S. Provisional Patent Application No. 60/734,746 filed Apr. 23, 2002, from which priority to both applications is claimed. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates in general to a planetary traction drive transmission, and, more particularly, to a planetary traction drive transmission with at least one flexible roller having an adaptive self-loading mechanism. 
   2. Description of Related Art 
   Traction drives use frictional force to transmit torque and power. Because the power is transmitted between two smooth surfaces, often through a thin layer of lubricant, a traction drive possesses unique characteristics that are not readily attainable by gear drives. These characteristics include quietness, high-efficiency, high rotational accuracy, and zero-backlash. 
   Generating adequate normal force at the contact is essential for traction drives. Various loading mechanisms have been proposed. These mechanisms have lead to a host of designs. A common practice is to use tapered surfaces along the axial direction. By moving these surfaces axially, a radial displacement and thus normal force are generated. Examples of such designs are disclosed in U.S Pat. Nos. 3,475,993 and 3,375,739. 
   Since the envelopes of the tapered surfaces in most designs do not necessarily converge to a common point, this results in a so-called spin motion at contacting surfaces. The spin motion not only offsets the high-efficiency otherwise provided by the traction drive, but also causes component wear and high break away torque. 
   Recently, a design of zero-spin planetary traction drive has been proposed by Ai as disclosed in the U.S. Pat. No. 6,095,940. This design employs the on-apex concept similar to that of tapered roller bearings. Two rows of planetary rollers are used to balance the internal axial force on the planetary rollers. Although this design offers torque actuated loading mechanism and greater torque capability, it is somewhat complex in construction. 
   The cylindrical planetary traction drive is also able to achieve zero-spin motion. However, generating sufficient normal force at the contacts has been a challenge. Designs proposed in the past have offered various means to pre-load the drive either by mechanically deforming the outer rings or by thermal assembling the drive. The pre-load generated by such means, in general, can not be adjusted during operation. For partial load application, traction drives are unnecessarily overloaded. This has negative impacts on transmission efficiency and service life. 
   Perhaps the simplest means to generate torque responsive load is using eccentric planetary drives as was disclosed by Dieterich U.S. Pat. No. 1,093,922 in 1914. Over the years, various improvements have been proposed. See for example, U.S. Pat. Nos. 3,945,270, 4,481,842, 4,555,963, and foreign patent numbers JP10-311398, EP 0,856,462 A2. They all have multiple planetary rigid rollers, and each planetary roller requires a supporting shaft. 
   While friction actuated loading mechanisms have been disclosed in prior art where one of the planetary rollers was entrained by friction force into the wedged-space created by eccentric raceways, the loading roller was rigid and thus the entrainment angle was kept constant. This was based on the notion that the maximum traction coefficient was constant during the operation. The entrainment angle was determined based on a specified friction coefficient, which was assumed to be not a function of the contact load. Such a loading mechanism was either over conservative or inadequate when the maximum available traction coefficient changes with contact load. 
   For eccentric planetary drives, the planetary rollers have different sizes. In most of the prior art designs, this has led to different contact stresses at the contact between the sun roller and planets. 
   In some designs proposed in prior art, the drive had more than one loading roller that is moveable. This could cause a change in eccentricity between input and out put shafts as the loading roller adjust their positions. 
   Therefore, it is desirable to provide a simple and improved design that allows for an adaptive, torque responsive loading mechanism where the entrainment angle changes as load varies to match the variation in the maximum available traction coefficient, a design that offers balanced contact stresses for all planet rollers and that reduces or eliminates the change in eccentricity as loading roller adjusting its position. 
   SUMMARY OF THE INVENTION 
   This patent application relates to planetary traction drives in general and specifically to an eccentric, cylindrical planetary traction drive having at least one flexible planetary roller capable of providing adaptive self-loading mechanism to facilitate the transfer of rotational motion and torque. The current invention also provides a traction drive that allows for an adaptive torque responsive loading and also provides a traction drive with well-balanced contact stresses at the most critical contacts. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view of the present invention. 
       FIG. 2  is a left side sectional view of the present invention. 
       FIG. 3  is an oblique sectional view of the present invention. 
       FIG. 4  is an exploded view of the present invention. 
       FIG. 5  is a graphical representation of the mathematical relationship between various components of the present invention. 
       FIG. 6  is a graphical representation of the normal forces between the components of the present invention. 
       FIG. 7  is a graph showing the relationship between the contact load and the geometry/traction coefficient of the present invention. 
       FIG. 8  is an exploded perspective view of a second embodiment of the present invention. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIGS. 1 and 2 , one embodiment of the cylindrical planetary traction drive A comprises an outer ring member  1 , a sun roller member  2 , a first planetary roller  5 , two second planetary rollers  3  &amp;  4 , and a carrier member  27 . The outer ring member  1  further comprises a first cylindrical raceway  9  surrounding the axis of rotation, and a first fixed flange  10  and a second fixed flange  11 . 
   The sun member  2  includes a second cylindrical raceway  12 , a third fixed flange  13 , a fourth fixed flange  14 , and a shaft  15 . The first planetary roller  5  has a third cylindrical raceway  16 , and the second planetary rollers  3  and  4  have a fourth cylindrical raceway  18  and  17  respectively. The first planetary roller  5  and the two second planetary rollers  3  and  4  are placed between and in contact with first cylindrical raceway  9  and second cylindrical raceway  12 . 
   At least one of either the planetary roller  5  or the planetary rollers  3  and  4  is flexible compared to other rollers. In the embodiment shown herein, the first planetary roller  5  is the flexible roller and serves as a loading roller. Thus, it will hereafter be called the loading planetary roller  5 . The loading planetary roller  5  deforms noticeably when diametrically squeezed. 
   At least one of the second planetary rollers  3  and  4  is a supporting roller. The supporting roller is relatively rigid and hardly deforms under diametrical load. In the embodiment shown herein, both planetary rollers  3  and  4  are supporting rollers and will hereafter be called the supporting planetary rollers  3  and  4 . The supporting planetary rollers  3  and  4  are firmly fixed by shafts  22  and  25  respectively to the carrier member  27 . Each of the supporting planetary rollers  3  and  4  is mounted onto its respective shaft  22  or  25  with a bearing  26  and two snap rings  24 . 
   The carrier member  27  contains a base plate  6  and a cover plate  7 . There are extrusions  19 ,  20 , and  21  protruding perpendicular from the back face of the base plate  6 . The cover plate  7  is fastened to the top of the extrusions using fasteners  8 , forming cavities for receiving the loading planetary roller  5  and the supporting planetary rollers  3  and  4 . The first cylindrical raceway  9  on the outer ring member  1  is eccentric to the second cylindrical raceway  12  on the sun member  2 . The space between the first cylindrical raceway  9  and the second cylindrical raceway  12  forms a wedge gap  23  ( FIG. 5 ). 
   The width of the wedged-gap h is expressed as: 
   
     
       
         
           
             
               
                 h 
                 = 
                 
                   2 
                   ⁢ 
                   r 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   cos 
                   ⁢ 
                   
                     δ 
                     2 
                   
                 
               
             
             
               
                 ( 
                 
                   1 
                   ⁢ 
                   a 
                 
                 ) 
               
             
           
         
       
     
   
   It can be approximated by the following cosine function:
 
 h≈R   2   −R   1   +e  cos α 1   (1b)
 
where R 1  is the radius of the sun roller raceway  12 ; R 2  is the radius of the first cylindrical raceway  9 ; and e represents the eccentricity between the raceways  12  &amp;  9  of the sun roller member and the outer ring.
 
   The wedge angle δ for planetary roller at azimuth position α 1  is given by: 
                 δ   =     Arccos   ⁡     [           (       R   1     +   r     )     2     +       (       R   2     -   r     )     2     -     ⅇ   2         2   ⁢     (       R   1     +   r     )     ⁢     (       R   2     -   r     )         ]               (   2   )               
where r is the effective radius of the planetary roller at this azimuth position, and can be expressed in terms of α 1  as:
 
   
     
       
         
           
             
               
                 r 
                 = 
                 
                   
                     
                       R 
                       2 
                       2 
                     
                     - 
                     
                       R 
                       1 
                       2 
                     
                     - 
                     
                       ⅇ 
                       2 
                     
                     + 
                     
                       2 
                       ⁢ 
                       e 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         R 
                         1 
                       
                       ⁢ 
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         α 
                         1 
                       
                     
                   
                   
                     2 
                     ⁢ 
                     
                       ( 
                       
                         
                           R 
                           2 
                         
                         + 
                         
                           R 
                           1 
                         
                         - 
                         
                           ⅇ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             α 
                             1 
                           
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 
                   3 
                   ⁢ 
                   a 
                 
                 ) 
               
             
           
         
       
     
   
   The wedge angle δ represents a contact geometry condition. 
   The geometry coefficient μ G  is defined as: 
   
     
       
         
           
             
               
                 
                   μ 
                   G 
                 
                 = 
                 
                   tan 
                   ⁡ 
                   
                     ( 
                     
                       δ 
                       2 
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   As can be seen from equations (1) to (4), for α 1  between 0 and 90 (or 0 and −90), when α 1  moves away from α 1 =0 position, the width of the gap h reduces, the wedge angle increases and thus geometry coefficient μ G  increases. 
   In the embodiment shown in  FIG. 1 , the flexible loading planetary roller  5  is assembled in the wedge gap  23  at an azimuth angle between α 1 =−90 to 90 degrees, preferably in vicinity of α 1 =0. The center of the flexible loading planetary roller  5  is left floating along the wedge gap  23 . The flexible loading planetary roller  5  is sufficiently flexible in the radial direction. When squeezed its effective diameter reduces in the corresponding direction. It is recommended that at home position, α 1 =0, the flexible loading planetary roller  5  is slightly squeezed. 
   The supporting planetary rollers  3  and  4  are arranged in wedge gap  23  generally between azimuth positions of α 1 =90 to 270 degrees. The centers of each of the supporting planetary rollers  3  and  4  are fixed to the carrier member  27 . 
   The planetary traction drive A is preferred to operate with the carrier member  27  stationary. During operation, the traction force tangent to the third cylindrical raceway  16  of the loading planetary roller  5  always drags the loading planetary roller  5  into the convergent wedge gap  23 . The loading planetary roller  5  is thus squeezed, generating substantial contacting force normal to the contact surfaces. If the eccentricity e in relationship to geometry of the planetary train is favorable, a balance is achieved where the maximum available traction force is equal to or greater than the operating traction force. This condition is called frictional self-loading. The relationship for ensuring such fictional self-loading is set forth by:
 
μ G ≦μ T   (5a)
 
where μ T  is the maximum available friction coefficient at the contacts.
 
   For optimal efficiency and service life of the planetary traction drive A, it is always desirable to have geometry coefficient μ G  close to but slightly smaller than the maximum available traction coefficient μ T  under various load conditions. That is:
 
μ G ≦≈μ T   (5b)
 
   During operation, the loading planetary roller  5  is entrained into the convergent wedge gap  23  and squeezed. Consequently, the effective diameter (or radius) of the loading planetary roller  5  reduces. The loading planetary roller  5  thus moves to a new azimuth position, establishing a new balance. At the new position, the width of wedge gap  23  is narrower and the contact geometry coefficient μ G  is increased. 
   The diametrical reduction of the loading planetary roller  5  can be estimated by: 
                   2   ⁢   d   ⁢           ⁢   r     =       (       π   4     -     2   π       )     ⁢       W   ⁢           ⁢     r   3         E   ⁢           ⁢   I                 (   6   )               
where W is the contact load; E is the Young&#39;s elastic modulus; and I is the area moment of inertia of ring cross section for the flexible first planetary roller  5 .
 
   The geometry coefficient μ G  corresponding to this diametrical reduction is: 
   
     
       
         
           
             
               
                 
                   μ 
                   G 
                 
                 = 
                 
                   tan 
                   ⁢ 
                   
                     { 
                     
                       
                         1 
                         2 
                       
                       ⁢ 
                       
                         Arccos 
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         R 
                                         1 
                                       
                                       + 
                                       
                                         R 
                                         2 
                                       
                                       + 
                                       ⅇ 
                                       - 
                                       
                                         2 
                                         ⁢ 
                                         d 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         r 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                                 + 
                                 
                                   
 
                                 
                                 ⁢ 
                                 
                                   
                                     ( 
                                     
                                       
                                         R 
                                         1 
                                       
                                       + 
                                       
                                         R 
                                         2 
                                       
                                       - 
                                       ⅇ 
                                       + 
                                       
                                         2 
                                         ⁢ 
                                         d 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         r 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                                 - 
                                 
                                   4 
                                   ⁢ 
                                   
                                     ⅇ 
                                     2 
                                   
                                 
                               
                             
                             
                               
                                   
                               
                               ⁢ 
                               
                                 2 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       R 
                                       1 
                                     
                                     + 
                                     
                                       R 
                                       2 
                                     
                                     + 
                                     ⅇ 
                                     - 
                                     
                                       2 
                                       ⁢ 
                                       d 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       r 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       R 
                                       1 
                                     
                                     + 
                                     
                                       R 
                                       2 
                                     
                                     - 
                                     ⅇ 
                                     + 
                                     
                                       2 
                                       ⁢ 
                                       d 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       r 
                                     
                                   
                                   ) 
                                 
                               
                               ⁢ 
                               
                                   
                               
                             
                           
                           ] 
                         
                       
                     
                     ⁢ 
                     
                         
                     
                     } 
                   
                 
               
             
             
               
                 ( 
                 
                   7 
                   ⁢ 
                   a 
                 
                 ) 
               
             
           
         
       
     
   
   With equations (6) and (7a), the increase in geometry coefficient μ G  with the contact load can be quantified.  FIG. 7  shows the variation of μ G , with contact load W for planetary rollers with different cross section moment of inertia I. 
   The maximum geometry coefficient is bounded by 
   
     
       
         
           
             
               
                 
                   μ 
                   
                     G 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     max 
                   
                 
                 = 
                 
                   tan 
                   ⁢ 
                   
                     { 
                     
                       
                         1 
                         2 
                       
                       ⁢ 
                       
                         Arccos 
                         ⁡ 
                         
                           [ 
                           
                             1 
                             - 
                             
                               2 
                               ⁢ 
                               
                                 
                                   ( 
                                   
                                     e 
                                     
                                       
                                         R 
                                         1 
                                       
                                       + 
                                       
                                         R 
                                         2 
                                       
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                             
                           
                           ] 
                         
                       
                     
                     } 
                   
                 
               
             
             
               
                 ( 
                 
                   7 
                   ⁢ 
                   b 
                 
                 ) 
               
             
           
         
       
     
   
   On the other hand, a large body of research results (Tevaarwerk, NASA CR-165226, 1981) showed that the maximum available traction coefficient μ T  increases with contact pressure or contact load. The change in μ T  with contact load for a traction fluid is also plotted in  FIG. 7  with symbols and a dotted line. 
   The flexible, floating planetary roller design of the current invention thus provides an opportunity for an adaptive frictional self-loading mechanism. That is, to adaptively change contact geometry coefficient to match the change in maximum available traction coefficient. As a result, not only constrain (5a) can be met, but also the geometry coefficient can be kept close to the maximum available traction coefficient (constraint (5b)) for the entire load spectrum. Adaptive self-loading can be achieved by choosing a flexible planetary roller with adequate cross section moment of inertia I. With this flexible roller it is possible to the change contact geometry coefficient and have the change match or compensate for the change in the maximum available traction coefficient when contact load changes. For example, the second curve in  FIG. 7  with I=13 mm 4  matches the change in maximum traction coefficient. This curve demonstrates the adaptive design principle. 
   The flexible loading planetary roller  5  can be as simple as a ring, however, other design alternatives are also possible. 
   The torque capacity of a traction drive is determined by the maximum allowable stress at the contact. Since planetary rollers for an eccentric planetary traction drive may have different sizes, the maximum contact stress for each planetary roller is not necessarily the same. To improve fatigue life, it is desirable to select the planetary roller size and place these rollers in proper azimuth positions so that every planetary roller has approximately the same contact stress. 
   Assume under a specified load (usually the maximum weighted load), the effective radius of the flexible loading planetary roller  5  is r 5  (after being squeezed), and its azimuth position is α 5  (see  FIG. 6 ). The radii of the supporting planetary rollers  3  and  4  are determined from the following equations in conjunction with equation (3).
 
 {overscore (R)}   3  sin(α 5 −α 3 )+ {overscore (R)}   4  sin(α 5 −α 4 )=0  (8)
 
 {overscore (R)}   3  cos(α 5 −α 3 )+ {overscore (R)}   4  cos(α 5 −α 4 )+ {overscore (R)}   5 =0  (9)
 
where
 
               R   _     i     =         R   1     ·     r   i           R   1     +     r   i               
(i=3, 4, or 5) represents the composite contact radii for contact between the sun roller  2  and the planetary rollers  3 ,  4 , and  5 , respectively.
 
   The geometry relationship between the radius and azimuth position of any supporting planetary roller is given by equation (3). That is: 
                   r   i     =         R   2   2     -     R   1   2     -     ⅇ   2     +     2   ⁢   e   ⁢           ⁢     R   1     ⁢   cos   ⁢           ⁢     α   i           2   ⁢     (       R   2     +     R   1     -     e   ⁢           ⁢   cos   ⁢           ⁢     α   i         )                 (     3   ⁢   b     )               
Subscript i=3 and 4 refers to the supporting planetary rollers  3  and  4  respectively. To avoid edge stress, first cylindrical raceway  9  and second cylindrical raceway  12  are may be crowned. Or, alternatively, the third and fourth cylindrical raceways  16 ,  17 , and  18  of the planetary rollers  3 ,  4 , and  5  may be crowned.
 
   The outer surface of the outer ring member  1  can have gear teeth to communicate motion of rotation to a driving or driven component. Alternatively, the outer surface of the outer ring member can be made into a pulley to communicate motion of rotation to a belt. 
   A second embodiment of the current invention is shown in  FIG. 8 . The second embodiment comprises an outer ring member  101 , a sun roller member  102 , two supporting planetary rollers  103  and  104 , a flexible loading planetary roller  105 , a carrier member  127 , and a housing  130 . The outer ring member  101  includes a first cylindrical raceway  109 , and a shaft  133  supported on the housing  130  through a double row ball bearing  131 . The sun roller member  102  has a second cylindrical raceway  112  and a shaft  115 . The shaft  115  is supported on the carrier  127  by a ball bearing  132 . 
   The second cylindrical raceway  112  on the sun roller member  102  is surrounded by and set eccentric to the first cylindrical raceway  109  in the outer ring member  101 . The two supporting planetary rollers  103  and  104  are arranged in the wedge gap between the second cylindrical raceway  112  and the first cylindrical raceway  109 . Each supporting planetary roller  103  and  104  has a shaft  122  that is fixed to the carrier member  127 . The supporting rollers  103  and  104  are supported on the shaft  122  by bearings  126 . The flexible loading roller  105  is arranged in the wedge gap between the two supporting rollers. The center of the flexible roller is free to rotate along the wedge gap. 
   The carrier member  127  includes a base plate  106  and a cover plate  107 . The base plate  106  has three protrusions  119 ,  120 , and  121 . The cover plate  107  is bolted to the protrusions on the base plate  106 . The carrier member  127  is mounted to the housing  130  by a fastening means such as bolts. Bearing  131  is axially located by a snap ring  134  and a shoulder  136  in the housing  130 . Bearing  132  is axially fixed by a snap ring  135  and a shoulder  137  in the carrier member  127 .