Abstract:
A gerotor pump includes an outer rotor having a first toothed surface and lobes that extend inwards. An inner rotor is eccentrically aligned relative to the outer rotor and includes a second toothed surface and lobes that extend outwards. Planetary gears are located between the outer rotor and the inner rotor. Each planetary gear has a third toothed surface that intermeshes with the first toothed surface and the second toothed surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to pumps and, more particularly, to gerotor pumps having eccentrically aligned rotor gears. 
         [0002]    Gerotor pumps comprising eccentrically aligned rotor gears are widely known and used, for example, as fluid pumps. Conventional gerotor pumps typically include an inner rotor having lobes that extend radially outward and an outer rotor that has lobes that extend radially inward. The inner rotor rotates about an eccentric axis relative to the outer rotor to create compression chambers between the lobes of the outer rotor and lobes of the inner rotor. The eccentric rotation decreases the compression chamber size between a low pressure suction side of the pump and a high pressure discharge side of the pump to pump the fluid. 
         [0003]    Conventional gerotor pumps have several significant drawbacks. For one thing, it is difficult to maintain a seal between the inner rotor and the outer rotor during operation, especially at low speed, high pressure conditions. This may allow fluid to prematurely escape from the compression chambers, which reduces the pumping efficiency. Additionally, some gerotor pumps that incorporate planetary gears between the rotors do not form seals between the surfaces of the planetary gears and the rotors. Planetary gear gerotor pumps are also susceptible to seizing up when radial forces between the rotors and the planetary gears become too high. As a result, pump maintenance or replacement may be necessary. 
       SUMMARY OF THE INVENTION 
       [0004]    An example gerotor pump includes an outer rotor having a first toothed surface and lobes that extend inward. An inner rotor is eccentrically aligned relative to the outer rotor and includes a second toothed surface and lobes that extend outwards. Planetary gears are located between the outer rotor and the inner rotor. Each planetary gear has a third toothed surface that engages the first toothed surface and the second toothed surface. 
         [0005]    An example gerotor pump system includes a first gerotor pump and a second gerotor pump arranged in parallel with the first gerotor pump. Each gerotor pump includes planetary gears that revolve between an outer rotor and an inner rotor. The planetary gears of the first gerotor are oriented out of phase relative to the planetary gears of the second gerotor. Additional gerotor pumps may also be used in the parallel arrangement. 
         [0006]    An example method for use with a gerotor pump includes the step of revolving toothed planetary gears along a path that extends between a toothed inner rotor and a toothed outer rotor to pump a fluid. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows. 
           [0008]      FIG. 1  illustrates an axial cross-sectional view of an example gerotor pump. 
           [0009]      FIG. 2  illustrates a radial cross-sectional view of the gear sets of the gerotor pump depicted in  FIG. 1 . 
           [0010]      FIG. 3  illustrates an example gerotor pump system having multiple gerotor pumps in parallel. 
           [0011]      FIG. 4  illustrates example output fluid flow curves of the gerotor pump system of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]      FIGS. 1 and 2  illustrate simplified schematic views of selected portions of an example gerotor pump  20  for efficiently pumping a fluid and avoiding maintenance problems such as seizure. In this example, the gerotor pump  20  includes a housing  22  having a pocket  24  that contains an inner rotor  28  having lobes  29  that extend outward, and an outer rotor  30  (i.e., gear sets). In this example, the outer rotor  30  is a ring gear having lobes  31  that extend inwards. A selected number, N, of planetary gears  32  are received between the inner rotor  28  and the outer rotor  30  for revolution about the inner rotor  28  and simultaneous revolution within the outer rotor  30 . 
         [0013]    In this example, a cover  34  retains the rotors  28 ,  30  and planetary gears  32  within the pocket  24 . The cover  34  is secured to the housing  22  in a known manner to provide a sealed chamber in which the rotors  28 ,  30  and planetary gears  32  operate. 
         [0014]    The housing  22  includes an inlet port  44  and an outlet port  46 . Each of the inlet port  44  and the outlet port  46  includes a first slot  48   a  and a second slot  48   b  that is parallel to and radially inward of the first slot  48   a.  This “split slot” configuration provides the advantage of providing an unrestrictive flow path while preventing the planetary gears  32  from falling into the ports  44  and  46  as they revolve next to the ports  44  and  46 . Alternatively, the inlet port  44 , the outlet port  46 , or both are ported through the cover  34  instead of the housing  22  (as seen in phantom at  44 ′ and  46 ′), depending on the particular needs of a design. 
         [0015]    The inner rotor  28  is operatively coupled with a drive shaft  47  along an axis A 1 . The outer rotor  30  rotates about a central axis A 2  that is eccentric relative to the inner rotor  28  rotational axis A 1 , and the planetary gears  32  revolve about a central axis A 3 . In the disclosed example, the axes A 1 , A 2 , and A 3  align collinearly along a line L ( FIG. 2 ) and are offset from each other. In this example, the axis A 1  is offset a magnitude, e 1 , from axis A 3 , and the axis A 2  is offset an equal magnitude e 1  from axis A 3 . In one example, the offset value e 1  is used to model the profile shape of the lobes  29  and  31 . In a further example, the profile shapes of the lobes  29  and  31  are modeled from the offset value e 1  using a known modeling technique, such as SAE 99P-464 entitled “Modeling and Simulation of Gerotor Gearing in Lubricating Oil Pumps.” 
         [0016]    In the illustrated example, the gerotor pump  20  includes five planetary gears  32  (i.e., N=5); however, it is to be understood that the benefits described in this description will also be applicable to pumps having different numbers of planetary gears  32 . The number of planetary gears may be selected during a design stage of the gerotor pump  20  and determines the configuration of the rotors  26 . In one example, for N planetary gears  32 , the inner rotor  28  has N−1 lobes  29  and the outer rotor  30  has N+1 lobes  31 . Thus, in the illustrated example, there are four lobes  29  of the inner rotor  28  and six lobes  31  of the outer rotor  30 . 
         [0017]    The planetary gears  32  each include teeth  50   a.  The teeth  50   a  intermesh with corresponding teeth  50   b  and  50   c  on the inner rotor  28  and the outer rotor  30 , respectively. 
         [0018]    Similar to the relationship between the number of planetary gears  32  and the number of lobes  29  and  31 , a number X of teeth  50   a  on the planetary gears  32  determines the number of teeth  50   b  and  50   c  on the inner rotor  28  and outer rotor  30 , respectively. In one example, for X teeth  50   a  and N planetary gears  32 , the inner rotor has X·(N−1) teeth  50   b  and the outer rotor  30  has X·(N+1) teeth  50   c.  The relationship between the number N of planetary gears  32  and its number X of teeth  50   a  and the number of lobes  29  and  31  and number of teeth  50   b  and  50   c  of the inner rotor  28  and the outer rotor  30 , respectively, provides the benefit of forming a tight seal between the planetary gears  32  and the rotors  28 ,  30  to increase the pumping efficiency. 
         [0019]    The relationship between the number N of planetary gears  32  and its number X of teeth  50   a  and the number of lobes  29  and  31  and number of teeth  50   b  and  50   c  of the inner rotor  28  and the outer rotor  30 , respectively, in the disclosed example also provides a desirable rotational speed relationship. For X teeth  50   a  and N planetary gears  32  that rotate about the axis A 3  with a speed Z, the inner rotor  28  rotates at a speed of Z·N/(N−1) and the outer rotor rotates at a speed of Z·N/(N+1). In this example, each of the planetary gears  32  travels over one of the lobes  29  of the inner rotor  28  and one of the lobes  31  of the outer rotor  30  with each revolution about the axis A 3 . 
         [0020]    In operation, the drive shaft  47  rotates the inner rotor  28 . This in turn drives the planetary gears  32  to revolve along a path  60  about central axis A 3  and rotates the outer rotor  30  about its axis A 2 . In the illustrated configuration, the planetary gears  32  accelerate from a “short side” (i.e., the bottom in  FIG. 2 ) to a “long side” (i.e., the top in  FIG. 2 ) and decelerate from the “long side” to the “short side.” As the planetary gears  32  revolve, fluid enters through the inlet port  44  into compression chambers  62  between the planetary gears  32 . The planetary gears  32  reduce the size of the compression chamber  62  along the path  60  between the inlet port  44  and the outlet port  46  to compress the fluid. The compressed fluid is then discharged through the outlet port  46 . 
         [0021]    The correspondence between the number of planetary gears  32  and the number of lobes  29  and  31 , and the correspondence between the number of teeth  50   a  on the planetary gears  32  and the number of teeth  50   b  and  50   c  on the inner rotor  28  and the outer rotor  30  provides the benefit of maintaining a desired operational relationship between the planetary gears  32 , the inner rotor  28 , and the outer rotor  30 . As seen in  FIG. 2 , the planetary gears  32  maintain a tangential relationship with the inner rotor  28  and the outer rotor  30  along the path  60 . Each of the planetary gears  32  maintains a first tangent point P 1  between each of the planetary gears  32  and the inner rotor  28  and a second tangent point P 2  between each of the planetary gears  32  and the outer rotor  30  such that the tangent points P 1  and P 2  are collinear (designated with lines  64 ) with a central axis A 4  of each of the planetary gears  32  entirely along the path  60 . The lines  64  intersect at point C, also known as the pitch circle contact point. Maintaining this tangential relationship provides the benefit of directing radial forces from the inner rotor  28  to the outer rotor  30  through the centers of the planetary gears  32  to prevent sliding and maintain a tight seal between the interlocking teeth  50   a,    50   b,  and  50   c.  This in turn prevents fluid escape from the compression chambers  62  to provide efficient pumping, which is a drawback with some prior gerotor pumps. 
         [0022]      FIG. 3  illustrates a simplified schematic view of an embodiment having a gerotor pump system  21  comprising multiple gerotor pumps  20   1 ,  20   2 , and  20   3  arranged in parallel. In the illustrated example, the gerotor pumps  20   1 ,  20   2 , and  22   3  are similar to the gerotor pump  20  described in the above example. In this example, the gerotor pumps  20   1 ,  20   2 , and  20   3  have progressively offset planetary gear  32  sets. That is, the planetary gears  32  of the gerotor pump  20   2  are offset by an angle relative to the planetary gear sets  32  of the gerotor pumps  20   1  and  20   3 . Likewise, the planetary gears  32  of the gerotor  20   1  are offset from the planetary gears  32  of the gerotor pump  20   3 . The drive shaft  47 ′ drives all three of the gerotor pumps  20   1 ,  20   2 , and  20   3  in this example. Fluid enters into each inlet port  44  of the gerotor pumps  20   1 ,  20   2 , and  20   3  from a common inlet manifold  66  and is discharged from each outlet port  46  into a common outlet manifold  68 . 
         [0023]    Generally, a single gerotor pump  20  produces fluid flow ripples as the chambers  62  discharge the fluid through the outlet port  46 . In some instances, it is desirable to reduce the magnitude of the ripples (i.e., a difference between a maximum fluid flow and a minimum fluid flow through the outlet port  46 ) to, for example, promote quieter operation. 
         [0024]    In the disclosed example, each gerotor pump  20  within the gerotor pump system  21  has the same number N planetary gears  32 . This provides the benefit of minimizing fluid flow ripple issuing from a gerotor pump system  21 . 
         [0025]    In one example demonstrated by  FIG. 4 , the gerotor system  21  includes an odd number M of gerotor pumps  20   1  through  20   M . The gerotor pumps  20   1  through  20   M  have progressively offset planetary gear  32  sets. In the disclosed example, the offset is an angle with respect to the direction of rotation of the drive shaft  47 ′ and is a function of the number M of gerotor pumps  20  in the gerotor pump system  21  and the number N of planetary gears  32  in each gerotor pump  20 . In a further example, the offset angle equals 2·360°/(M·N). 
         [0026]    In this example, M=3 and N=5 whereby the desired progressive offset angle is 2·360°/(3·5)=48° such that the planetary gears  32  of the gerotor pumps  20   1 ,  20   2 , and  20   3  are oriented 48° out of phase from each other. For example, if the direction of rotation of the drive shaft  47 ′ is clockwise, the planetary gears of the second gerotor pump  20   2  are oriented 48° in a clockwise direction from the first gerotor pump  20   1 , and the planetary gears of the third gerotor pump  20   3  are oriented 48° in a clockwise direction from the second gerotor pump  20   2 . Thus as will be apparent from an inspection of  FIG. 4  below, the out of phase orientation provides the benefit of offsetting the fluid flow ripples produced by each of the gerotor pumps  20   1 ,  20   2 , and  20   3  to reduce the magnitude of the resulting output fluid flow ripple. 
         [0027]    The example illustrated in  FIG. 4  shows a graph of relative volume of fluid flow versus radians (relative to rotation of the inner rotors  28 ) for the three gerotor pumps  20   1 ,  20   2 , and  20   3 . The curves near the bottom of the graph represent the relative volume of fluid flow curves of the compression chambers  62  of a single gerotor pump  20  as the compression chamber  62  receives, compresses, and discharges fluid. The three curves near the top represent the total relative volume flow (which is proportional to fluid flow) of the respective gerotor pumps  20   1 ,  20   2 , and  20   3 . The three curves are offset by 48° in this example because of the 48° offset angle between the planetary gears  32  of the gerotor pumps  20   1 ,  20   2 , and  20   3 . As can be appreciated, the individual curves near the bottom of the graph depict the fact that in each of the gerotor pumps  20  there is finite asymmetry in fluid flow from each of the compression chambers  62 . Thus, it will be appreciated that progressively offsetting each of the M gerotor pumps  20   1  through  20   M  at an angle of 2·360°/(M·N) rather than by an angle of 360°/(M·N) results in dispersing sets of three absolute minimum fluid flow cusps  70  (i.e., in this case at 2·360°/(M·N)=48° rather than sets of three in succession at 360°/(M·N)=24° followed by sets of three reduced magnitude cusps  72 ). In any case, it can be observed that the difference D 1  in magnitude between the peaks and valleys of the three fluid flow curves is significantly smaller than the difference D 2  between the peaks and valleys of any single curve. Thus, using multiple gerotor pumps  20   1 ,  20   2 , and  20   3  provides the benefit of reducing the magnitude of output fluid flow ripple. It is to be understood that although the example illustrates use of three gerotor pumps  20   1 ,  20   2 , and  20   3 , in general, fewer pumps or additional pumps may be used as desired. 
         [0028]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.