Abstract:
A gerotor pump comprises a rotor chamber having a longitudinally extending central axis, an outer rotor rotatable in the rotor chamber with a radial clearance between an outer periphery of the outer rotor and the rotor chamber, an inner rotor rotatable in the outer rotor and cooperably engageable therewith to define a plurality of pumping chambers having respective volumes that vary when, in use, the inner rotor rotates relative to the outer rotor. The inner rotor is rotatable about an axis of rotation spaced from the central axis and contained in a first plane that contains the central axis and the rotor chamber is configured such that the radial clearance in at least one direction in the first plane is greater than at least one radial clearance in a second plane that extends perpendicular to the first plane and contains the central axis.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to gerotor pumps and particularly, but not exclusively, for gerotor pumps for use in diesel fuel injection systems. 
       BACKGROUND TO THE INVENTION 
       [0002]    The gerotor pump is a known type of positive displacement pump that can, for example, be used as a transfer pump in a diesel common rail fuel delivery system. In such systems, a gerotor pump can be used to suck fuel from the fuel tank and serves as a primary stage pump providing a fuel supply at a high enough pressure to fill a main high pressure fuel pump, which may be a plunger type pump. 
         [0003]      FIG. 1  is a schematic representation of a known gerotor pump configuration illustrating the pump geometry. The gerotor pump comprises an inner gear rotor  10  having n teeth  12  and an outer gear rotor  14  having n+1 teeth  16 . In the illustrated example, the inner gear rotor  10  has six teeth  12  and the outer gear rotor  14  has seven teeth  16 . The outer gear rotor  14  is housed in a circular section rotor chamber defined by a bore  18  in a housing part  20 . There is a constant radial clearance  22  between the circular outer periphery of the outer gear rotor  14  and the circular wall defining the bore  18 . It will be appreciated that for ease of representation the clearance  22  is shown much exaggerated in the drawing. 
         [0004]    A drive shaft  24  is secured to the inner gear rotor  10 . The drive shaft  24  is supported for rotation by the housing  20  such that it can rotate the inner gear rotor  10  and, by engagement of the teeth  12 ,  16 , the outer gear rotor  14  so that both the rotors are rotated in the bore  18  by the drive shaft. The axis of rotation of the inner gear rotor  10  is offset in the vertical direction (as viewed in  FIG. 1 ) with respect to the axis of rotation of the outer gear rotor  14  and the axis of the bore  18 , which coincides with the axis of rotation of the outer gear rotor  14 . The offset is indicated by reference numeral  26 . 
         [0005]    In use, as the rotors  10 ,  14  are rotated relative to one another by the drive shaft  24 , pumping chambers  28  are formed between the respective sets of teeth  12 ,  16 . The relative rotation of the inner and outer gear rotors  10 ,  14  causes the pumping chambers  28  to cyclically increase and then decrease in size. An inlet port (not shown) is provided in the housing  20  in the region of a rotational position of the rotors  10 ,  14  at which the pumping chambers  28  are relatively large and an outlet port (not shown) is provided in the housing  20  in the region of a rotational position at which the pumping chambers are relatively small. Typically, the ports are located approximately 180° apart and are kidney shaped. 
         [0006]    As the size of a pumping chamber  28  increases, a vacuum is created so that as the pumping chamber sweeps past the inlet port, the fluid to be pumped is sucked into the pumping chamber. As the size of the pumping chamber decreases, the fluid is pumped (compressed if the fluid is a gas) and then swept out of the pumping chamber as the pumping chamber passes over the outlet port. The arrangement of the rotors  10 ,  14  and the inlet and outlet ports is such that a gerotor pump can provide a relatively pulseless output. 
         [0007]    In gerotor pumps, it is necessary to control the position of the drive shaft  24  with respect to the bore  18  to ensure an adequate radial clearance  22  is maintained. Failure to maintain the radial clearance  22  results in loading of the rotors against the bore wall, which in turn causes rotor wear and may result in pump seizure. Providing the necessary positional control can be costly as all of the parts making up the tolerance stack must be accurately machined. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention provides a gerotor pump comprising means defining a rotor chamber having a longitudinally extending central axis, outer rotor means rotatable in said rotor chamber with a radial clearance between an outer periphery of said outer rotor means and said rotor chamber, inner rotor means rotatable in said outer rotor means and cooperably engageable therewith to define a plurality of pumping chambers having respective volumes that vary when, in use, the inner rotor means rotates relative to the outer rotor means, said inner rotor means being rotatable about an axis of rotation spaced from said central axis and contained in a first plane that contains said central axis and said rotor chamber being configured such that the said radial clearance in at least one direction in said first plane is greater than at least one radial clearance in a second plane that extends substantially perpendicular to said first plane and contains said central axis. 
         [0009]    The radial clearance has a minimum value and the, or each, said radial clearance in said second plane has said minimum value. 
         [0010]    The invention also includes a gerotor pump comprising a housing defining a rotor chamber, an outer gear rotor received with radial clearance in said rotor chamber and rotatable in said rotor chamber, an inner gear rotor rotatable in said outer gear rotor and having toothing cooperably engaging toothing of said outer gear rotor to form a plurality of variable volume pumping chambers, said outer gear rotor being rotatable about an axis of rotation extending in a first plane, said inner gear rotor being rotatable about an axis of rotation extending in a second plane spaced from said first plane and said rotor chamber being configured such that at least one said radial clearance in said first plane is less than at least one radial clearance in a direction substantially perpendicular to said first plane. 
         [0011]    The radial clearance has a minimum value and the, or each, said radial clearance in said first plane has said minimum value. 
         [0012]    The invention also includes a gerotor pump having an outer gear rotor and an inner gear rotor rotatable in said outer gear rotor and a non-circular rotor chamber, the outer rotor being mounted for rotation in said rotor chamber with a radial clearance between an outer periphery of the outer rotor and opposed portions of a wall defining said rotor chamber and said rotor chamber being configured such that in the average direction in which forces generated by a fluid in the pumping chambers act when said pumping chambers are in flow communication with an outlet port the said radial clearance is less than the said radial clearance in at least one direction in a plane containing the axis of rotation of the inner gear rotor and a longitudinally extending central axis of said rotor chamber. 
         [0013]    The invention also includes a gerotor pump having an outer gear rotor and an inner gear rotor rotatable in said outer gear rotor and a non-circular rotor chamber, the outer rotor being mounted for rotation in said rotor chamber with a radial clearance between an outer periphery of the outer rotor and opposed portions of a wall defining said rotor chamber and said rotor chamber being configured such that in the average direction in which forces generated by a fluid in the pumping chambers act when said pumping chambers are disposed on a high pressure side of the pump the said radial clearance is less than the said radial clearance in at least one direction in a plane containing the axis of rotation of the inner gear rotor and a longitudinally extending central axis of said rotor chamber. 
         [0014]    The invention also includes a gerotor pump comprising means defining a rotor chamber having a longitudinally extending central axis, outer rotor means rotatable in said rotor chamber with a radial clearance between an outer periphery of said outer rotor means and said rotor chamber, inner rotor means rotatable in said outer rotor means and cooperably engageable therewith to define a plurality of pumping chambers having respective volumes that vary when, in use, the inner rotor means rotates relative to the outer rotor means, said inner rotor means being rotatable about a first axis of rotation, and said outer rotor means being rotatable about a second axis of rotation offset from said first axis of rotation in an offset direction, wherein the radial clearance between the outer periphery of the outer rotor means and the rotor chamber has a minimum value in a direction substantially perpendicular to the offset direction. 
         [0015]    The invention also includes a gerotor pump comprising a housing defining a rotor chamber, an outer gear rotor received with radial clearance in said rotor chamber and rotatable in said rotor chamber, an inner gear rotor rotatable in said outer gear rotor and having toothing cooperably engaging toothing of said outer gear rotor to form a plurality of variable volume pumping chambers, said outer gear rotor being rotatable about an axis of rotation extending in a first plane, said inner gear rotor being rotatable about an axis of rotation extending in a second plane spaced from said first plane in an offset direction, and said rotor chamber being configured such that the radial clearance has a minimum value in a direction substantially perpendicular to the offset direction. 
         [0016]    It will be appreciated that preferred and/or optional features described herein in relation to a particular embodiment, variant or aspect of the invention are equally applicable to the other embodiments, variants or aspects of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In order that the invention may be well understood, some embodiments thereof, which are given by way of example only, will now be described with reference to the drawings in which: 
           [0018]      FIG. 1  is a schematic representation of a prior art gerotor pump illustrating aspects of the pump geometry; 
           [0019]      FIG. 2  is a schematic illustration of a diesel fuel injection system comprising a gerotor pump according to the invention; 
           [0020]      FIG. 3  is a schematic representation of a part of a gerotor pump according to the invention illustrating aspects of the pump geometry; 
           [0021]      FIG. 4  is a schematic illustration of aspects of the geometry of the gerotor pump of  FIG. 3 ; 
           [0022]      FIG. 5  is a schematic illustration of a prior art gerotor pump having the configuration shown in  FIG. 1  and showing the effects of mispositioning of parts of the pump; and 
           [0023]      FIG. 6  is a schematic illustration of the gerotor pump of  FIGS. 3 and 4  showing the effects of the same mispositioning of parts as in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0024]      FIG. 2  shows a portion of a common rail diesel fuel injection system  50 . The fuel injection system  50  comprises a fuel tank  52  containing a fuel strainer  54 . The fuel tank  52  has an outlet connected to low pressure piping leading to a gerotor pump  56 . The gerotor pump  56  feeds a high pressure plunger-type pump  58 , which supplies high pressure diesel to a fuel rail  60  via a fuel filter  62 . The fuel rail  60  is connected to a plurality of fuel injectors  64  arranged to deliver diesel into an engine (not shown). In addition to the components shown, the fuel injection system  50  comprises an electronic controller, transducers (such as pressure transducers) connected to the electronic controller and return lines for returning unused diesel fuel to the tank  52 . Those features and other possible features will be familiar to those skilled in the art and since they do not form a part of the invention will not be described in any detail herein. 
         [0025]      FIG. 3  is a schematic representation of a part of the gerotor pump  56  illustrating aspects of the pump geometry. The gerotor pump  56  comprises an inner gear rotor  66  having n teeth  68  and an outer gear rotor  70  having n+1 teeth  72 . In the illustrated embodiment, the inner gear rotor  66  has six teeth  68  and the outer gear rotor  70  has seven teeth  72 . It is to be understood that these numbers are not to be taken as limiting and the number of teeth can be varied as desired. 
         [0026]    The outer gear rotor  70  is housed in a generally elliptical section rotor chamber defined by a bore  74  in a member  76  of a pump housing. In the illustrated embodiment, the housing member  76  is a circular plate and the bore  74  extends between the major surfaces of the plate. There is a radial clearance  78  between the circular outer periphery  80  of the outer gear rotor  70  and the opposing wall defining the bore  74 . It will be appreciated that for ease of representation the clearance  78  is shown much exaggerated in the drawing. 
         [0027]    A drive shaft  84  is secured to the inner gear rotor  66  by any suitable means, for example by a key or splines (not shown). The drive shaft  84  is supported for rotation by the housing member  76  such that it can rotate the outer gear rotor  70  by engagement of the teeth  68 ,  72  of the rotors  66 ,  70  so that both rotors rotate in the bore  74 . 
         [0028]    Referring to  FIGS. 3 and 4 , the axis of rotation  86  of the outer gear rotor  70  extends in a first plane  88 , which is a horizontal plane as viewed in  FIGS. 3 and 4 . The axis of rotation  86  of the outer gear rotor  70  coincides with the axis of the bore  74 . The axis of rotation  90  of the inner gear rotor  66  is offset in the vertical direction (as viewed in  FIGS. 3 and 4 ) with respect to the axis of rotation  86  of the outer gear rotor  70  and the axis of the bore  74 . The axis  90  extends parallel to the axis  86  in a second plane  92  that extends parallel to the first plane  88 . The perpendicular offset between the respective axes of rotation  86 ,  90  of the rotors (and thus between the first and second planes  88 ,  92 ) is the gerotor offset and is indicated in  FIGS. 3 and 4  by reference numeral  94 . 
         [0029]    In use, as the rotors  66 ,  70  are rotated relative to one another and to the housing  76  by the drive shaft  84 , pumping chambers  96  are formed between the respective sets of teeth  68 ,  72 . The relative rotation of the inner and outer gear rotors  66 ,  70  causes the pumping chambers  96  to cyclically increase and then decrease in size. An inlet port (not shown) is provided in the pump housing  76  in a region of a rotational position of the rotors  66 ,  70  at which the pumping chambers  96  are increasing in volume (relatively large) and an outlet port (not shown) is provided in the housing  76  in a region of a rotational position at which the pumping chambers  96  are decreasing in volume (relatively small). Although not limited to this arrangement, the inlet and outlet ports can be located approximately 180° apart and be generally kidney shaped. 
         [0030]    The configuration of the rotor chamber (in the illustrated embodiment the elliptical cross section of the bore  74 ) is such that the radial clearance  78  between the outer circular periphery  80  of the outer rotor  70  and the wall defining the bore  74  is not constant. As best seen in  FIG. 3  and marked in  FIG. 4 , the radial clearance  78  in the first plane  88  (i.e. the plane containing the axis of rotation  86  of the outer gear rotor  70  and the axis of the bore  74 ) is less than the radial clearance  78 (I) in directions perpendicular to the first plane  88 . In the illustrated embodiment, the radial clearance has a minimum value in the first plane  88  and a maximum value in the radial directions perpendicular to the first plane (i.e. the direction of the plane  97  of the gerotor offset  94 ). Due to the generally elliptical cross section of the bore  74 , the radial clearance  78  varies substantially continuously between its value in the first plane  88  and its value in the radial directions perpendicular to the first plane (i.e. in a plane  97 , which is perpendicular to the first and second planes  88 ,  92 , and contains the gerotor offset  94  as shown in  FIG. 4 ). 
         [0031]    It will be understood that in all radial directions except those of the first plane  88  in which the axis of rotation  86  of the outer gear rotor  70  extends, the non-circular rotor chamber of the gerotor pump  56  has increased radial dimensions as compared with the rotor chamber of the prior art gerotor pump shown in  FIG. 1 . This provides a greater radial clearance  78  in all directions except in the first plane  88 , which allows the movement of the outer gear rotor  70  to be closely contained in the opposed radial directions perpendicular to the gerotor offset  94  while less restraint is provided in the other radial directions. This allows better control of the point of contact between the circular outer periphery  80  of the outer gear rotor  70  and the bore wall, which is important as the reaction force generated by that contact increases as the contact position moves away from the first plane  88 . This allows the pump assembly to tolerate a significantly larger positional error between the drive shaft  84  and the rotor chamber than is possible with the prior art gerotor shown in  FIG. 1 . These effects of the changed radial clearance characteristic of the gerotor pump  56  will be further appreciated from a consideration of  FIGS. 5 and 6 . 
         [0032]      FIG. 5  shows a prior art gerotor pump configuration corresponding to the gerotor pump shown in  FIG. 1 . For ease of reference, the features of the gerotor pump shown in  FIG. 5  are indicated by reference numerals corresponding to those used in  FIG. 1 . In the gerotor pump shown in  FIG. 5 , the axis of rotation of the drive shaft  24 , and so the inner gear rotor  10 , is offset from its correct position due to tolerance stack up. This offset is indicated by the arrow  98 . As a result, contact between the outer periphery of the outer gear rotor  14  and the wall of the bore  18  caused by the pressure in the chamber pockets  28  at their rotational position on the high pressure side of the pump (the forces generated by this pressure are indicated by arrows  100 ) is shifted out of the plane in which the axis of bore  18  extends towards the plane of the gerotor offset. The further the point of contact is from the plane in which the axis of rotation of the bore  18  extends, the greater is the reaction force indicated by arrow  102 . As the reaction force  102  increases, the driving force indicated by arrow  104  also increases. Increased driving forces  104  lead to increased wear of the tips of the teeth  12  of the inner rotor  10 . 
         [0033]      FIG. 6  illustrates what happens in a gerotor pump  56  according to the present invention when the drive shaft  84  is offset in the same way as the drive shaft  24  in  FIG. 5 . In the gerotor pump  56 , the increased radial clearance  78  away from the first plane  88  ( FIG. 3 ) which contains the axis  86  of the bore  74  results in the region of contact between the outer periphery of the outer rotor  80  and the bore  74  being kept closer to the first plane  88 . As compared with the prior art pump geometry illustrated in  FIGS. 1 and 5 , this results in a lower reaction force  102  and driving force  104  for the same positional error. 
         [0034]    It will be appreciated that although the gerotor pump  56  is shown having a rotor chamber with an elliptical cross-section, this is not essential. Other configurations are possible. What is required, is that the radial clearance  78  in a first plane  88  containing the rotor chamber axis  86  (i.e. the plane  88  that extends perpendicular to the plane  97  containing both the rotor chamber axis  86  and the axis of rotation  90  of the inner gear rotor  66 ) is kept relatively small, at least in one direction, and the radial clearance  78 (I) in at least one direction in the plane  97  containing the rotor chamber axis  86  and axis of rotation  90  of the inner gear rotor  66  is relatively larger. 
         [0035]    The embodiment provides a rotor chamber having non-circular cross section that is configured to provide an increased radial clearance between the outer periphery of the outer gear rotor and the facing wall of the rotor chamber in the direction of a plane  97  containing the central axis  86  of the rotor chamber and the axis of rotation  90  of the inner gear rotor. It is not essential that the rotor chamber is non-circular in order to obtain the increased radial clearance. In an alternative embodiment, the outer rotor has a circular outer periphery and the rotor chamber has a circular cross section. However, as compared with the configurations shown in  FIGS. 1 and 5 , the rotor chamber has a larger diameter providing a relatively larger radial clearance. In order to provide the necessary close clearance on the high pressure side of the pump, the drive shaft is shifted to the left (as viewed in  FIGS. 1 and 5 ). In this embodiment, a close radial clearance is maintained on the high pressure side of the pump in a plane containing the axis of rotation of the inner gear rotor while a larger radial clearance is provided in the radial directions of a plane extending perpendicular to that plane and containing the axis of rotation of the inner gear rotor and the longitudinally extending central axis of the rotor chamber. However, this embodiment is non-symmetrical in that a relatively large radial clearance is provided in the radial direction of the plane containing the close clearance that this is on the low pressure side of the pump. As compared with the gerotor pump  56 , this alternative configuration is not so desirable as it increases the pump capacity and the lack of symmetry limits the pump to rotation in just one direction. With the symmetrical configuration shown in  FIG. 3 , the drive shaft can run in either direction. 
         [0036]    It will be appreciated that while particularly suitable for such use, the embodiments of the gerotor pump are not limited to use in diesel fuel injection systems and have general applicability to the known uses of gerotor pumps such as, for example, in fuel injection systems generally and in motor vehicle engine lubrication systems.