Abstract:
A gear reducer includes a rotatable planet carrier supporting at least one planetary gear for axial rotation thereon, a stationary ring gear extending around and engageable with the at least one planetary gear, and a moveable ring gear extending around and engageable with the at least one planetary gear. A pitch diameter of the stationary ring gear is substantially identical to a pitch diameter of the moveable ring gear.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 60/991,699, filed at the United States Patent and Trademark Office on Nov. 30, 2007, the entire content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to an improved gear reducer, and more particularly to a high ratio planetary gear reducer. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventional gear reducers include an input, a stationary ring gear, a moveable ring gear, and at least one planetary gear, wherein the planetary gear teeth mesh with stationary and moveable ring gear teeth. Such conventional gear reducers may be used to obtain high gear reduction ratios, but since the stationary ring gear and the moveable ring gear have different numbers of teeth to allow for relative movement between the two gears, typically two sets of planetary gears are required, one set of planetary gears to mesh with each of the stationary ring gear and the moveable ring gear. 
         [0004]    However, devices have been developed whereby a single set of planetary gears mesh with both the stationary ring gear and the moveable ring gear. In one such arrangement, in which the stationary ring gear and the moveable ring gear have different pitch diameters, the standard addendum dimensions of the ring gears are modified to yield equal internal diameters. 
       SUMMARY OF THE INVENTION 
       [0005]    A gear reducer is provided including a rotatable planet carrier supporting at least one planetary gear for axial rotation thereon. A stationary ring gear extends around and is engageable with the at least one planetary gear and a moveable ring gear extends around and is also engageable with the at least one planetary gear. A pitch diameter of the stationary ring gear is substantially identical to a pitch diameter of the moveable ring gear. In one embodiment, the effective point of force transfer between the at least one planetary gear and the stationary ring gear is substantially the same as the effective point of force transfer between the at least one planetary gear and the moveable ring gear. 
         [0006]    Further, the gear reducer may also include a housing for the at least one planetary gear, the stationary ring gear, and the moveable ring gear, and an output driven by the moveable ring gear. A spring compliance member may be between the housing and the output such that the spring compliance member reduces backlash of the gear reducer. The spring compliance member may be generally flat annular resilient member, such as an O-ring. 
         [0007]    In one embodiment, an input is provided for driving the gear reducer. The input may be, for example, an input shaft fixedly attached to the rotatable planet carrier by a set screw or a sun gear engageable with the at least one planetary gear. 
         [0008]    The at least one planetary gear may have a plurality of planetary gear teeth, wherein the stationary ring gear has a plurality of stationary ring gear teeth engageable with the plurality of planetary gear teeth, and wherein the moveable ring gear has a plurality of moveable ring gear teeth engageable with the plurality of planetary gear teeth. The difference between the number of stationary ring gear teeth and the number of moveable ring gear teeth may be a multiple of the number of planetary gears and, in one embodiment, the number of stationary ring gear teeth is not equal to the number of moveable ring gear teeth. 
         [0009]    In one embodiment, the effective point of force transfer between the at least one planetary gear and the stationary ring gear is substantially the same as the effective point of force transfer between the at least one planetary gear and the moveable ring gear. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]      FIG. 1  is a partially exploded orthogonal view of an exemplary embodiment of a gear reducer of the present invention; 
           [0011]      FIG. 2  is a cross-sectional view of another embodiment of a gear reducer located within a housing; 
           [0012]      FIG. 3  is a detail view showing the outline of an exemplary movable ring gear and an exemplary stationary ring gear of the present invention superimposed over one another; 
           [0013]      FIGS. 4 ,  5 , and  6  are cross-sectional views of exemplary arrangements of planetary gears of the present invention. 
           [0014]      FIG. 7  is a cross-sectional view of another exemplary gear reducer of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    As shown in  FIGS. 1-7 , embodiments of the present invention are directed to a gear reducer typically having a gear reduction ratio of greater than 100 that may be accomplished by a single stage gear reduction or by a multiple stage gear reduction. The gear reducer includes at least one planetary gear orbiting an input axis, the planetary gear simultaneously engaging a stationary ring gear and a movable ring gear within a housing. As teeth of the planetary gear(s) engage teeth of the stationary ring gear and teeth of the moveable ring gear, the moveable ring gear rotates with respect to the stationary ring gear. The moveable ring gear has a different number of teeth than the stationary ring gear, yet it has the same operating pitch diameter as the stationary ring gear, allowing the planetary gear teeth to adequately mesh with the stationary ring gear teeth and the moveable ring gear teeth. 
         [0016]    With reference now to  FIGS. 1 and 2 , the gear reducer  10  includes a housing  12  containing the gears of the gear reducer, the housing remaining stationary relative to a rotating input  14  and a rotating output  16 . In one exemplary embodiment, the housing  12  may be made of a metallic material, such as aluminum or steel, or any other material suitably rigid to provide stability for the gear reducer  10  against the forces created by rotation of the input  14 . Examples of such materials include synthetic polymeric materials, such as resinous materials. The housing  12  includes a first end cap  13  with a peripheral O-ring  15  sealing a first open end of the housing and a second end cap  17  threaded to the housing and sealing a second open end of the housing. As will be understood by one of ordinary skill in the art, the first and second end caps  13 ,  17  may have appropriately sized openings to accommodate the input  14  and output  16 . The input  14  may be driven by an exterior driving component, such as a motor or drive shaft located outside the housing  12 . Additionally, the output  16  may be integral with a moveable ring gear  44  ( FIG. 1 ), or may be connected to the moveable ring gear by a pin  19  ( FIG. 2 ), such that rotation of the moveable ring gear results in rotation of the output  16 , as described in more detail below. 
         [0017]    In one exemplary embodiment, as shown in  FIG. 1 , the input  14  may be a sun gear  14   a  having circumferential sun gear teeth  22  that mesh with circumferential planetary gear teeth  24  on three planetary gears  18  orbiting the sun gear. Accordingly, the number of revolutions of the sun gear  14   a  is greater than the number of orbits of each planetary gear  18  around the sun gear, resulting in a gear reduction between the input  14  (sun gear  14   a ) and the planetary gears  18 . According to this embodiment, the teeth of the planetary gear  18  have a constant profile along the axes of the gears. 
         [0018]    In another exemplary embodiment, as shown in  FIG. 2 , the input  14  may be a shaft  14   b  or other driving device attached directly to a planetary gear carrier  20  such that the number of revolutions of the shaft  14   b  equals the number of orbits of the planetary gears  18  around the shaft. The shaft  14   b  may be attached to the planetary gear carrier  20  by, for example, a set screw  24 , or the shaft may be integral with the planetary gear carrier. 
         [0019]    The planetary gear carrier  20  is rotatably mounted within the housing  12  about a central longitudinal axis  28 . The planetary gear carrier  20  includes a base  30  having a front surface  34  facing toward an entry point of the input  14  into the housing  12  and a rear surface  36  facing toward an exit point of the output  16  from the housing. At least one planetary gear support  32  is mounted on and extends perpendicularly from the front surface  34  ( FIG. 1 ) or the rear surface  36  ( FIG. 2 ) of the base  30 . Each planetary gear support  32  is adapted to carry a single planetary gear  18 , and, in one exemplary embodiment, comprises an elongate cylinder having a portion embedded within the base  30  and a portion protruding from the base on which the planetary gear is rotatably mounted. Where a multiple-planetary gear configuration is employed ( FIG. 1 ), the multiple planetary gear supports  32  are equally radially spaced from each other on the base  30 . 
         [0020]    A stationary ring gear  40  and the moveable ring gear  44  are mounted within the housing  12  in a configuration to constantly engage the planetary gear(s)  18  as described in more detail below. The stationary ring gear  40  is fixedly secured to the housing  12  and contains a plurality of internal, involute stationary ring gear teeth  38 . The moveable ring gear  44  is rotatably mounted within the housing  12 , supported by a plurality of bearings  46  abutting an interior surface  48  of the housing, and contains a plurality of internal, involute moveable ring gear teeth  42 . 
         [0021]    Each planetary gear  18  contains a plurality of external, involute planetary gear teeth  24  adapted to simultaneously engage the stationary ring gear teeth  38  and the moveable ring gear teeth  42 . In order for the moveable ring gear  44  to rotate relative to the stationary ring gear  40 , the moveable ring gear has a different number of teeth than the stationary ring gear. Despite the different numbers of teeth on the stationary and moveable ring gears  40 ,  44 , the planetary gear teeth  24  adequately meshes with the teeth  38 ,  42  on both the stationary and moveable ring gears to prevent jamming of the gear mechanism. In accordance with embodiments of the present invention, and also with reference to  FIG. 3 , the teeth  38 ,  42  of the stationary ring gear  40  and the moveable ring gear  44  are configured to have identical or substantially identical pitch diameters to allow the planetary gear teeth  24  to mesh with the stationary and moveable ring gear teeth  38 ,  42 . 
         [0022]    Pitch diameter, sometimes referred to as “theoretical pitch diameter,” is defined as the number of teeth on a gear divided by diametral pitch. As is commonly understood, gear tooth sizes are designated by diametral pitch, which is the number of teeth per inch of diameter of the pitch circle. The pitch circle is the circle whose periphery is the pitch surface, or the surface of an imaginary circle that rolls without slippage with a pitch circle of a mating gear. Another form of pitch diameter relates to force transfer between gears, which can be referred to as an “operating pitch diameter.” Operating pitch diameter may be defined as the effective point of force transfer between gears. When the words “pitch diameter” are used alone in this specification, they mean the operating pitch diameter. Thus, when the stationary ring gear and the moveable ring are defined as having “substantially identical pitch diameters,” this means that the effective point of force transfer between the planetary gear  18  and the stationary ring gear  40  is the substantially the same as the effective point of force transfer between the planetary gear and the moveable ring gear  44 . 
         [0023]    An exemplary embodiment of the stationary ring gear  40  and the moveable ring gear  44  having a substantially identical operating pitch diameter is shown in  FIG. 3 . As shown in the figure, a 96 diametral pitch tool may be used to cut 96 teeth  38  into the stationary ring gear  40 , resulting in a pitch diameter  45  of 1 inch, indicated by the arcuate line “PD” dividing the teeth. Similarly, a 93 diametral pitch tool has been used to cut 93 teeth  42  into a moveable ring gear  44 , also resulting in a pitch diameter  45  of 1 inch, indicated by the line “PD.” Accordingly, such a configuration allows the teeth  24  of a single planetary gear  18 , or a single set of planetary gears, to adequately mesh with the teeth  38 ,  42  of the stationary and moveable ring gears  40 ,  44 , and, upon rotation of the planetary gear(s), causes rotation of the moveable ring gear with respect to the stationary ring gear. Additionally, since the profiles of the teeth  38 ,  42  are similar and since the gears  40 ,  44  have the same pitch diameter, the pressure angles between the planetary gear teeth  24  and the stationary and moveable ring gear teeth are substantially similar, resulting in a smooth operation of the gear reducer  10 . 
         [0024]    With reference now to  FIGS. 4-6 , exemplary configurations of planetary gears  18  are shown in accordance with the present invention. To properly assemble the gears, the respective tooth sums of each ring gear and the sun gear (if applicable) divided by the number of planetary gears results in a whole number. Thus, if two planetary gears  18  are used, the difference in the number of teeth of the stationary ring gear  40  and the moveable ring gear  44  equals two or a multiple of two. 
         [0025]    In the gear configurations shown in  FIGS. 4-6 , the stationary ring gear and the moveable ring gear are oriented adjacent to each other such that the planetary gear(s) contacts both the stationary ring gear and the moveable ring gear. However, as will be appreciated, the gears may be oriented in a variety of configurations each achieving the same results as the configurations depicted in  FIGS. 4-6 . 
         [0026]    With reference now to  FIG. 4 , a single planetary gear  18  is shown contacting the sun gear  14   a,  the stationary ring gear  40 , and the moveable ring gear  44  such that the planetary gear teeth  24  mesh with sun gear teeth  22  and stationary and ring gear teeth  38 ,  42 . In such a configuration, the difference between the number of stationary and moveable ring gear teeth  38 ,  42  is one. 
         [0027]    With reference to  FIG. 5 , two planetary gears  118  are located on generally opposite sides of the sun gear  114   a  such that planetary gear teeth  124  mesh with sun gear teeth  122  and stationary and moveable ring gear teeth  138 ,  142 . In such a configuration, the difference between the number of stationary and moveable ring gear teeth  138 ,  142  is two. In a specific example with reference to  FIG. 5 , when the number of planetary gears  118  (P n ) is 2, the number of teeth  138  in the stationary ring gear  140  (R o ) is 92, the number of teeth  142  in the moveable ring gear  144  (R f ) is 90, and the number of teeth  122  in the sun gear  114   a  (S th ) is 30, the planetary ratio (R p ) is (R f /S th ) +1. As such the planetary ratio R p  in this example is 4. The overall gear ratio (O ar ) is R p *(R o /P n ) or in this case, O ar =4*(92/2)=184. 
         [0028]    Similarly, with reference to  FIG. 6 , three planetary gears  18  are equally spaced around the sun gear  14   a  and the difference between the stationary and moveable ring gear teeth  38 ,  42  is three. Accordingly, the number of planetary gears P n  is 3, and if the number of teeth  238  in the stationary ring gear  240  R o  is 93, the number of teeth  242  in the moveable ring gear  244  R f  is 90, and the number of teeth  222  in the sun gear  214   a  S n  is 30, the planetary ratio R p  is 4 and the overall gear ratio O ar =R p *(R o P n )=4*(93/3)=124. 
         [0029]    As will be apparent to one of ordinary skill in the art, and as discussed herein, the sun gear  14   a  may be replaced by an input shaft  14   b  and a planetary gear carrier  20  to drive the planetary gears  18 . Additionally, more than three planetary gears may also be used. 
         [0030]    Operation of the gear reducer  10  will now be described. As noted above, an external device, such as a motor or input shaft, drives the input  14 , whether the input is a sun gear  14   a  ( FIG. 1 ) or a shaft  14   b  ( FIG. 2 ). Rotation of the input  14  results in rotation of the planetary gear(s)  18  either by contact between the sun gear teeth  22  and the planetary gear teeth  24 , or by rotation of the planetary gear carrier  20  by the input  14 . Since the planetary gear(s)  18  are located such that the planetary gear teeth  24  mesh with both the stationary ring gear teeth  38  and the moveable ring gear teeth  42 , and since the stationary ring gear  40  has a different number of teeth than the moveable ring gear  44 , rotation of the planetary gear(s) results in rotation of the moveable ring gear with respect to the stationary ring gear. Rotation of the moveable ring gear  44  results in simultaneous rotation of the output  16 , either because the output is integral with the moveable ring gear ( FIG. 1 ) or because the moveable ring gear is directly attached to the output ( FIG. 2 ) by, for example, the pin  19 . 
         [0031]    Another exemplary embodiment of the present invention is shown with reference to  FIG. 7 . To the extent that the components shown in  FIG. 7  are the same or substantially similar to those already described, the same reference numerals are used to designate such components. With reference to  FIG. 7 , a single planetary gear  18  is employed, thereby creating the potential for an eccentricity in the gear mechanism. Spring compliance members  50  may be added, for example, between the bearing  46  and the interior surface  48  of the housing  12  to permit dimensioning of the gear reducer  10  for mechanical interference. However, it will be appreciated that spring compliance members may also be located in other places within the gear reducer to achieve the same purpose. In one exemplary embodiment, the spring compliance members  50  may be a plurality of O-rings. However, the spring compliance members  50  may also be flat, annular rubber is members, or any other suitable resilient member. The addition of the spring compliance member  50  permits the gears to be constructed in a state of zero tolerance or with a slight mechanical interference such that any binding between the gears may be substantially avoided by deflection of the moveable ring gear  44 , thereby creating a condition of minimal or zero backlash. 
         [0032]    Although exemplary embodiments in accordance with the present invention have been described, one of ordinary skill in the art will appreciate that various modifications may be made to the embodiments without departing from the spirit and scope of the invention as described and claimed as follows.