Abstract:
A gerotor assembly includes a star, a ring, and an annular plug member, as well as an o-ring. The star member defines a center opening of a first diameter which is connectable to a low-pressure fluid reservoir. The ring member circumscribes the star member. The ring member defines, in conjunction with a stationary end cap of a fluid control device, a fluid channel connectable to a high-pressure fluid supply. The plug member is circumscribed by the star member, and defines a center bore of a second diameter less than the first diameter. The o-ring is positioned between the star and the plug members. The plug member forms a fluid seal against the end cap. A fluid control device includes the above gerotor assembly and a valve housing section. A method of assembling the gerotor assembly and fluid control device are also disclosed.

Description:
TECHNICAL FIELD 
     The present invention relates generally to a gerotor assembly for use within a fluid control device, and in particular to a semi-plugged star gerotor and a method of assembling the same. 
     BACKGROUND 
     Star gerotors are positive-displacement fluid pumping devices having meshed inner and outer rotors. The inner and outer rotors are typically referred to as a star member and a ring member, respectively. Each rotor has a fixed center point that is eccentric with respect to the center point of the other rotor. The star member has n teeth, and is circumscribed by the ring member having (n+1) lobes. Rotation of one rotor drives the other, with a low relative speed maintained between the two rotors. The volume defined between the mating teeth/lobes of the engaged rotors creates a vacuum during gerotor rotation, and thus a resultant suction or intake stage for each revolution of the gerotor. 
     A steering control unit (SCU) of a hydrostatic power steering system is one type of fluid control device that commonly uses a star gerotor in its construction. An SCU may experience slip between its rotating gerotor members and a stationary member, e.g., an end cap which is secured adjacent to the gerotor. For example, when a steering cylinder controlled via a valve housing section of the SCU reaches the limit of its range of travel, a steering wheel controlled via the SCU may still rotate beyond this limit. Such additional rotation is often a result of internal fluid leakage between the star member and an adjacent surface of the stationary end cap. 
     SUMMARY 
     A gerotor assembly is provided herein for use with a fluid control device such as the SCU noted above. The gerotor assembly disclosed herein is semi-plugged, i.e., a hybrid between a solid plug-style star seal design and a conventional sealing ring, as set forth in detail below. The gerotor assembly includes a star member, a ring member, an annular plug member, and an o-ring. The star member has (n) teeth, and defines a center opening of a first diameter. The center opening is in fluid communication with a low-pressure fluid reservoir when the gerotor assembly is installed in the fluid control device. The ring member circumscribes the star member, and has (n+1) lobes that mesh with the (n) teeth, as is well understood in the art of gerotors. 
     The ring member is configured to define, in conjunction with a stationary end cap of the fluid control device, a high-pressure fluid channel, i.e., a fluid channel that is connectable to a high-pressure fluid supply. The annular plug member is circumscribed by the star member, and defines a center bore of a second diameter that is smaller than the first diameter. The o-ring is positioned between the star member and the annular plug member. The annular plug member is thus configured to form a semi-plugged fluid seal against the stationary end cap of the fluid control device, with various performance benefits as explained below. 
     A fluid control device is also disclosed. The fluid control device includes a gerotor star member, a gerotor ring member, an annular plug member, an o-ring, and a valve housing section. The star member defines a center opening of a first diameter, with the center opening in fluid communication with a low-pressure fluid reservoir. The ring member circumscribes the star member, and has (n+1) lobes that engage with the (n) teeth of the star member. The plug member is circumscribed by the star member, and defines a center bore of a second diameter less than the first diameter. 
     The center bore is in fluid communication with the low-pressure fluid reservoir via the center opening. The o-ring is positioned between the star member and the annular plug member. The o-ring is in fluid communication with the high-pressure fluid reservoir via a high-pressure fluid channel, and with the low-pressure fluid reservoir via the center opening. The valve housing section has a stationary end cap and a wear plate, with the end cap positioned immediately adjacent to the annular plug member to define the high-pressure fluid channel in conjunction with the star member. The high-pressure fluid channel is in fluid communication with a high-pressure fluid reservoir. 
     A method is also disclosed herein, including providing a gerotor star member defining an annular shelf and a center opening of a first diameter, and circumscribing the star member with a gerotor ring member such that (n+1) lobes of the ring member engage with (n) teeth of the star member. The method includes positioning an o-ring on a surface of the star member, and providing an annular plug member that defines a center bore of a diameter less than the first diameter. The annular plug member is placed on the o-ring such that the annular plug member is circumscribed by the star member to thereby form the gerotor assembly. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a fluid control device using a semi-plugged gerotor assembly of the type disclosed herein; 
         FIG. 2  is a schematic plan view illustration of the present gerotor assembly; 
         FIG. 3  is a partial schematic cross-sectional side view illustration of a portion of the fluid control device shown in  FIG. 2 , including the gerotor assembly and a stationary end cap of the fluid control device of  FIG. 1 ; and 
         FIG. 4  is a schematic cross-sectional illustration of a portion of the fluid control device shown in  FIG. 1 , including the portion shown in  FIG. 3 . 
     
    
    
     DESCRIPTION 
     Referring to the Figures, wherein like reference numbers correspond to similar components,  FIG. 1  is a schematic illustration of a fluid control device  11 . The fluid control device  11  includes a semi-plugged gerotor assembly  13 . The gerotor assembly  13  has an annular plug member  18  forming a star seal. As described in detail below, the annular plug member  18  is configured to reduce internal fluid leakage within the fluid control device  11  in which the plug member  18  is installed. 
     In one possible embodiment, the fluid control device  11  may be configured as a steering control unit (SCU) for use in a hydrostatic power steering system. The gerotor assembly  13  may be included as part of an SCU to reduce undesirable steering wheel rotation while reducing friction losses relative to conventional designs, thereby increasing energy efficiency. Other fluidic systems in which fluid leakage from a high-pressure side to a low-pressure side is a critical design concern, such as fluid motors pumping systems, may likewise benefit from use of the semi-plugged gerotor assembly  13  and its annular plug member  18  as described herein. 
     Within the fluid control device  11  shown in  FIG. 1 , the gerotor assembly  13  may be bolted or otherwise securely fastened to a valve housing section  70 , e.g., via bolts  17 . A stationary wear plate  80  is disposed between the gerotor assembly  13  and the valve housing section  70 . The gerotor assembly  13  is positioned between the wear plate  80  and a stationary end cap  24 . The valve housing section  70  may define various fluid ports, including a fluid inlet port  72 , a fluid return port  74 , and various cylinder control ports, e.g., control ports  76  and  78 , either on one side of the valve housing section  70  or distributed as shown. A fluid device subassembly  10  is formed by the gerotor assembly  13  and the end cap  24 , and is described below with reference to  FIGS. 3 and 4 . 
     Although not show in  FIG. 1  for simplicity, the interior of the valve housing section  70  defines a bore containing any required valves and associated control devices for actuating the device being controlled, e.g., a rotatable spool and a cooperating, relatively rotatable follow up valve member, as is well understood in the art. The follow up valve member may be driven using a main drive shaft (not shown), with the main shaft splined to and rotatable in conjunction with the semi-plugged gerotor assembly  13 . 
     Referring to  FIG. 2 , the gerotor assembly  13  includes an internally-toothed outer rotor, which is referred to hereinafter as a ring member  12 . The gerotor assembly  13  further includes an externally-toothed inner rotor, i.e., a star member  14 . The star member  14  is eccentrically disposed within the ring member  12  for orbital and rotational movement therein. Both the star member  14  and the ring member  12  may be constructed of steel, powder metal, or another suitable metallic material. 
     The star member  14  defines an annular, axial wall  62 . The axial wall  62  defines a center opening (arrow  20 ) as shown in  FIG. 4 . The star member  14  may includes splines  22  (see  FIGS. 3 and 4 ) to allow the star member  14  to engage with mating splines of a main drive shaft (not shown) positioned within the valve housing section  70  of  FIG. 1 . As understood in the art of gerotors, a plurality (n) of teeth  15  of the star member  14  mesh with or engage with a larger plurality (n+1) of teeth or lobes  21  of the ring member  12  to define multiple fluid volume chambers (arrows  23 ). The fluid volume chambers (arrows  23 ) are in fluid communication with the valve housing section  70  of  FIG. 1  through passages (not shown) defined by the wear plate  80  shown in the same Figure. 
     The bolts  17  of  FIG. 1  pass through the stationary end cap  24  (see  FIG. 1 ) and through a plurality of bolt holes  25  defined by the ring member  12  in order to fasten the semi-plugged gerotor assembly  13  to the valve housing section  70  shown in  FIG. 1 . Within the star member  14 , the axial wall  62  intersects a radial floor  60  (see  FIGS. 3 and 4 ) to thereby form a radial shelf, with the position of the radial shelf generally indicated by reference number  44  in  FIG. 3 . As used herein, the term “axial wall” refers to a wall extending in the same direction as the axis of rotation of the star member  14 , and the term “radial floor” refers to a floor extending in a direction perpendicular to the same axis. 
     The annular plug member  18  has a bore wall  19  forming a center bore as indicated by arrow  27 . The annular plug member  18  is positioned on the radial floor  60  shown in  FIGS. 3 and 4 . When the semi-plugged gerotor assembly  13  is installed in the fluid control device  11  of  FIG. 1  or another suitable device, a dynamic fluid seal is formed between the annular plug member  18  and the stationary end cap  24  shown in that Figure. Both the structure and the function of the annular plug member  18  are described in detail with reference to  FIGS. 3 and 4 . 
     Assembly to various levels may be accomplished by circumscribing the star member  14  with the ring member  12  such that the lobes  21  of the ring member  12  engage with the teeth  15  of the star member  14 . The o-ring  16  is positioned on the radial shelf  44  (see  FIGS. 3 and 4 ) of the star member  14 . The annular plug member  18  is then placed on the o-ring  16  and the radial shelf  44 . Subsequently connecting the assembled gerotor assembly  13  to the stationary end cap  24  defines a high-pressure fluid channel (arrow  82  of  FIG. 4 ) between the star member  14  and the end cap  24 . The center opening (arrow  20 ) is then connected to a low-pressure fluid reservoir  40  as shown in  FIG. 4 , and the fluid channel (arrow  82 ) of  FIG. 4  is connected to a high-pressure fluid reservoir  30 . 
     Referring to  FIG. 3 , a partial cross-sectional side view is shown of the fluid device subassembly  10  of  FIG. 1 .  FIG. 3  is not intended to be drawn to scale with respect to  FIG. 1 ,  2 , or  4 , but rather to provide a close-up view of certain internal structural portions of the fluid device subassembly  10 . When the semi-plugged gerotor assembly  13  is connected to the stationary end cap  24 , a high-pressure fluid channel is defined between an upper surface  52  of the star member  14  and the underside  50  of the end cap  24 . High-pressure fluid (arrow  31 ) enters the fluid channel, which is indicated by arrow  82  in  FIG. 4 , causing sealing to occur as explained below with reference to  FIG. 4 . 
     The axial wall  62  and the radial floor  60  of the star member  14  form the radial shelf  44 , on which an o-ring  16  is disposed. The o-ring  16  forms a fluid seal between the star member  14  and the annular plug member  18 . The o-ring  16  may be constructed of a suitable wear-resistant elastomeric material having a hardness level sufficient for resisting extrusion in pressurized operation. In one embodiment, the o-ring  16  is provided with a hardness level of at least approximately 90 durometer on the ASTM D2240 type D scale, i.e., 90D hardness. Suitable materials at this hardness level may include, without being limited to, Nitrile Butadiene Rubber (NBR), Hydrogenated NBR (HNBR), polyurethane, etc. 
     The annular plug member  18  is used to form a seal against an underside  50  of the end cap  24 , and may be constructed of steel, powder metal, high hardness resin-based materials, or other suitable materials. The annular plug member  18 , which has a generally L-shaped cross section as shown, includes a first surface  66  and a second surface  68 , which are perpendicular with respect to each other to form a circumferential notch  85  facing the annular radial shelf  44 . The first surface  66  and a second surface  68  are both in direct contact with the o-ring  16 , which is disposed at least partially within the circumferential notch  85 . A third surface  69  of the annular plug member  18  is in direct frictional contact with the underside  50  of the end cap  24 . As used herein, the term “underside” refers to the particular major surface or side of the end cap  24  that is positioned immediately adjacent to the star member  14  within the fluid control device  11  (see  FIG. 1 ) in which the star member  14  is used. 
     The star member  14 , the annular plug member  18 , and the o-ring  16  rotate together with respect to the stationary end cap  24 . The center section or internal diameter (ID) of the star member  14  defined by an inner wall  42  is connected to a low-pressure fluid reservoir  40 , and the all other sides of the star member  14  are connected to a high-pressure fluid reservoir  30 . Both of the reservoirs  30  and  40  are shown schematically in  FIG. 4 . The terms “low” and “high” are relative fluid pressures. In one embodiment, “low pressure” may be approximately 0 to approximately 40 bar, while “high pressure” is any pressure in excess of 40 bar. In another embodiment, 70 to 150 bar may be used as a high pressure range, although high pressure could vastly exceed 150 bar depending on the application. The placement and use of the annular plug member  18  and the o-ring  16  as described herein helps to reduce leakage of high-pressure fluid (arrow  31 ) to the low-pressure fluid reservoir  40  of  FIG. 4 . 
     Referring to  FIG. 4  in conjunction with  FIG. 3 , the stationary end cap  24  extends to include the ring member  12  of  FIG. 2 , and is therefore shown in broken line form in  FIGS. 3 and 4 . The inner wall  42  of the star member  14  defines the center opening (arrow  20 ) of  FIG. 4 . The center opening (arrow  20 ) is in fluid communication with the low-pressure fluid reservoir  40  of  FIG. 3 , such that low-pressure fluid (arrow  41 ) is in communication with the o-ring  16 , the annular plug member  18 , and the end cap  24  via the center opening (arrow  20 ). The o-ring  16  may be preloaded to form a sufficient seal against the star member  14  and the annular plug member  18 . 
     The area of contact between the annular plug member  18  and the end cap  24  should be sufficiently large so as to reduce leakage past the end cap  24 , the star member  14 , and the o-ring  16  from the high-pressure side to the low-pressure side, and yet small enough to minimize friction losses. Thus, the annular plug member  18  forms only a partial plug, i.e., the term “semi-plugged” as used herein. In one embodiment, the diameter of the center bore as defined by the bore wall  19  of the annular plug member  18  is between approximately 60% to approximately 75% of the outer diameter (OD) of the annular plug member  18 . 
     As noted above, the fluid device subassembly  10  shown in  FIG. 4  is in fluid communication with high-pressure fluid (arrows  31 ) delivered from the high-pressure fluid reservoir  30 . A high-pressure fluid channel (arrow  82 ), as shown in  FIG. 4 , is defined between the underside  50  of the end cap  24  and an upper surface  52  of the star member  14  as noted above, with the surfaces  50  and  52  being adjacent to each other. 
     The o-ring  16  is in fluid communication with the high-pressure fluid reservoir  30  of  FIG. 4  via the high-pressure fluid channel (arrow  82 ), and with the low-pressure fluid reservoir  40  via the center opening (arrow  20 ) of the star member  14 . The size of a gap (arrows  84  of  FIG. 3 ) between an underside  64  of the annular plug member  18  and the radial floor  60  of the star member  14  should be minimized to prevent extrusion of the o-ring  16  to the low-pressure side during operation. 
     In operation, high-pressure fluid (arrows  31 ) enters the high-pressure fluid channel (arrow  82 ) and pushes against the o-ring  16 . This forces the annular plug member  18  into frictional contact with the stationary end cap  24 . Fluid leakage from the high-pressure side to the low-pressure side may occur between the o-ring  16  and the star member  14 , between the o-ring  16  and the annular plug member  18 , and/or between the annular plug member and the end cap  24 . 
     However, since the annular plug member  18  is only semi-plugged, as that term is used herein, a relatively large contact area remains present between the annular plug member  18  and the stationary end cap  24 . Fluid leakage is reduced from high-pressure side to the low-pressure side relative to conventional gerotor star seal designs. Additionally, since the contact area between the annular plug member  18  and the end cap  24  is relatively small in the present semi-plugged design relative to a solid-plug design, frictional losses are concurrently reduced in this area. Overall efficiency is thereby increased. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.