Patent Publication Number: US-9845804-B2

Title: Positive displacement pump assembly with movable end plate for rotor face clearance control

Description:
This application is is a National Stage Application of PCT/US2013/038589, filed 29 Apr. 2013, which claims benefit of U.S. Patent Application Ser. No. 61/640,330 filed on 30 Apr. 2012 and which applications are incorporated herein by reference. To the extent appropriate, a claim of priority is made to the above disclosed applications. 
    
    
     TECHNICAL FIELD 
     The present teachings generally include a positive displacement pump assembly, such as a supercharger assembly for an engine. 
     BACKGROUND 
     Positive displacement pumps can be used to add fluid pressure under certain operating conditions. A supercharger is one type of positive displacement pump that is used to boost air pressure at an engine air intake. Positive displacement air pumps typically have meshing, multi-lobed rotors within a rotor housing. Air is moved from an inlet to an outlet; clearance between the rotors and the rotor housing is designed to prevent air from following unintended paths. Air leakage around the rotor end faces is one unintended path and a cause of positive displacement air pump inefficiency. 
     The rotors are mounted on rotor shafts. The rotors and rotor shafts may tend to expand and contract due to thermal fluctuations. The rotor housing may also tend to expand and contract, and may do so at different rates than the rotors or rotor shafts, especially if formed from a different material. One solution has been to leave a gap between the rotor face and the rotor housing at the inlet end of the housing that is sufficiently large to allow the rotor shafts and the housing to expand relative to one another. The rotor shafts are typically fixed axially to the rotor housing at one end by bearings, referred to herein as axial bearings. Needle bearings between the rotor shafts and the rotor housing on the other end allow the rotor shafts to expand and contract axially relative to the rotor housing. 
     SUMMARY 
     A positive displacement pump assembly is provided that allows axial expansion and contraction of rotors and rotor shafts relative to the housing along the length of the rotor cavity while reducing a change in axial clearance at faces of the rotors. The positive displacement pump assembly includes a rotor housing defining a rotor cavity, and an end plate configured to at least partially close one end of the rotor cavity. Rotors are fixed to and supported on rotor shafts and extend through the rotor cavity. A first pair of bearings fixes the rotor shafts axially to the end plate. A second pair of bearings fixes the rotor shafts axially to the rotor housing, preventing relative axial movement between the rotor shafts and the rotor housing. The end plate is axially movable with the rotor shafts when the rotor shafts vary in axial length due to thermal fluctuations so that changes in the axial clearance at the end faces of the rotors due to thermal fluctuations is substantially reduced. The effect of material selection and associated thermal expansion rates of the rotors, rotor shafts, and the rotor housing on the clearance is thus significantly reduced, and leakage through the clearance will thus be minimized with a corresponding increase in efficiency of the positive displacement pump assembly. 
     The above features and advantages and other features and advantages of the present teachings are readily apparent from the following detailed description of the best modes for carrying out the present teachings when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional illustration of a positive displacement pump assembly taken at lines  1 - 1  in  FIG. 5  in accordance with one aspect of the present teachings. 
         FIG. 2  is a schematic perspective illustration of an axial end plate of the positive displacement pump assembly of  FIG. 1 . 
         FIG. 3  is another schematic perspective illustration of the axial end plate of  FIG. 2 . 
         FIG. 4  is a schematic perspective illustration of the positive displacement pump assembly of  FIG. 1 , with an end portion of the rotor housing removed. 
         FIG. 5  is a schematic perspective illustration of the positive displacement pump assembly of  FIG. 5 , with the end portion attached to a midportion of the rotor housing. 
         FIG. 6  is a schematic cross-sectional illustration of a positive displacement pump assembly in accordance with another aspect of the present teachings. 
         FIG. 7  is a schematic perspective illustration of an end plate of the positive displacement pump assembly of  FIG. 6 . 
         FIG. 8  is a schematic perspective illustration of the positive displacement pump assembly of  FIG. 6  with the end portion of the rotor housing removed. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numbers refer to like components throughout the several views,  FIG. 1  shows a positive displacement pump assembly  10 . In this embodiment, the positive displacement pump assembly  10  is a supercharger assembly for an engine, although the positive displacement pump assembly  10  may be used to pump other fluids and in other applications. The positive displacement pump assembly  10  has a first rotor  12  that meshes with a second rotor  14 . Each of the rotors  12 ,  14  has multiple lobes. The first rotor  12  is mounted on and rotates with a first rotor shaft  16 . The second rotor  14  is mounted on and rotates with a second rotor shaft  18  that is generally parallel with the first rotor shaft  16 . 
     The rotors  12 ,  14  and rotor shafts  16 ,  18  are contained within a multi-component positive displacement pump housing  20 . The housing  20  includes a front cover  22 , a midportion  24  that can be referred to as a rotor housing portion, and an end portion  26 . The front cover  22  and the end portion  26  are fastened with bolts or otherwise secured to the midportion  24 . 
     An input shaft  28  that can be driven by an engine belt or other drive input is operatively connected to the first rotor shaft  16  through a coupling  30 . A torsion spring  32  is connected at one end to the front cover  22  of the positive displacement pump housing  20  and at another end to the input shaft  28 . The torsion spring  32  damps vibrations of the input shaft  28 . A first timing gear  34  is mounted on and rotates with the first rotor shaft  16  and meshes with a second timing gear  36  mounted on and rotating with the second rotor shaft  18  to cause rotation of the second rotor shaft  18 . 
     The midportion  24  defines a rotor cavity  38  through which the rotor shafts  16 ,  18  extend and in which the rotors  12 ,  14  rotate. A fluid such as air is driven through the rotor cavity  38  from an inlet  39  (shown in  FIG. 5  and referred to herein as an air inlet) in the end portion  26  to an outlet  42  in the midportion  24  (shown in hidden lines in  FIG. 5  and referred to herein as an air outlet). Air that can be passed from the air inlet  40  to the air outlet  42  by passing between the mesh of the rotors  12 ,  14 , or air that exits out of the rotor cavity  38  by passing back to the inlet  39  along first axial end faces  40 A,  40 B of the rotors  12 ,  14  or along second axial end faces  43 A,  43 B of the rotors  12 ,  14  is referred to as “leakage” and decreases the efficiency of the positive displacement pump assembly  10 . In order to minimize such leakage, changes in an axial clearance  45  between the midportion  24  and the second end faces  43 A,  43 B due to thermal fluctuations, as well as changes in an axial clearance  48  at the first end faces  40 A,  40 B due to thermal fluctuations are minimized while the positive displacement pump assembly  10  still accommodates axial expansion and contraction of the rotors  12 ,  14  and rotor shafts  16 ,  18  relative to the rotor housing  20  due to the thermal fluctuations. 
     Specifically, an end plate  44  is axially fixed for movement with the rotor shafts  16 ,  18  by a first pair of bearings  46 A,  46 B positioned between the rotor shafts  16 ,  18  and the end plate  44 . The bearings  46 A,  46 B are press fit into stepped openings  50 A,  50 B in the end plate  44 . The stepped openings  50 A,  50 B are best shown in  FIGS. 1 and 3 . The bearings  46 A,  46 B are configured to permit the rotor shafts  16 ,  18  to rotate relative to the end plate  44 , but fix the axial position of the rotor shafts  16 ,  18  relative to the end plate  44 . Changes in a first predetermined clearance  48  between the first end faces  40 A,  40 B and a face  51  of the end plate  44  (best shown in  FIG. 2 ) due to thermal fluctuations are thus minimized. The clearance  48  is very small relative to the surrounding components, and is indicated in  FIG. 1  as a line at the end faces  40 A,  40 B. A second axial clearance  52  between an internal surface  54  of the end portion  26  and a face  56  of the end plate  44  can vary in size as the end plate  44  moves toward or away from the surface  54  because the end plate  44  is not axially fixed to the rotor housing  20 . The surface  51  of the end plate  44  defines an end  58  of the rotor cavity  38 . 
     A second pair of bearings  57 A,  57 B is positioned between the midportion  24  and the rotor shafts  16 ,  18  and axially fixes the rotor shafts  16 ,  18  to the midportion  24 . The bearings  57 A,  57 B are referred to herein as axial bearings. The bearings  57 A,  57 B are press fit into stepped openings  59 A,  59 B of the midportion  24  near second axial ends  65 A,  65 B of the rotor shafts  16 ,  18 . The bearings  57 A,  57 B are configured to permit the rotor shafts  16 ,  18  to rotate relative to the midportion  24 , but fix the axial position of the rotor shafts  16 ,  18  relative to the midportion  24 . Seals  63 A,  63 B are positioned around the rotor shafts  16 ,  18  in the stepped openings  59 A,  59 B between the axial bearings  57 A,  57 B and the rotor cavity  38 . Oil can fill the stepped openings  59 A,  59 B around the seals  63 A,  63 B, and the seals  63 A,  63 B prevent oil leakage into the rotor cavity  38 . 
     As the temperature of the positive displacement pump assembly  10  increases, the rotors  12 ,  14  and rotor shafts  16 ,  18  and the rotor housing  20  may expand axially an amount dependent on the linear thermal expansion coefficient of the materials from which they are formed. Expansion of the rotors  12 ,  14 , rotor shafts  16 ,  18  and rotor housing  20  is also dependent on a temperature gradient that may exist along the length of the rotors  12 ,  14  and the housing  20  due to the fact that compressed air (or other fluid) at the outlet  42  of the housing  20  is much hotter than the air (or other fluid) at the inlet  39  of the housing  20 . This may cause the ends  60 A,  60 B of the shafts  16 ,  18  to move axially toward or away from the surface  54  of the end portion  26 , varying the clearance  52 . The width of the clearance  52  does not affect leakage of the positive displacement pump assembly  10 . By reducing variations of the clearance  48 , and instead allowing the width of the clearance  52  to vary freely with the thermal fluctuations, the end plate  44  can provide a high efficiency for the positive displacement pump assembly  10 . 
     In one nonlimiting example, the rotor shafts  16 ,  18  can be a first material, such as steel, and the rotor housing  20  can be a second material, such as an aluminum alloy. These materials have different rates of linear thermal expansion and contraction, quantified as coefficient of linear thermal expansion. For example, the coefficient of linear thermal expansion of steel may be 13×10 −6  meters per meter per degree Kelvin, while that of Aluminum may be 22.2×10 −6  meters per meter per degree Kelvin, and that of an Aluminum alloy somewhere therebetween. However, the end plate  44  is fixed to move axially with the rotor shafts  16 ,  18  and so will significantly reduce variations in the clearance  48  despite these different rates of expansion and contraction. The end plate  44  can be the same material as the rotors  12 ,  14  to best maintain the clearance  48 . 
       FIGS. 2 and 3  show the unique shape of the outer perimeter  70  of the end plate  44 . A first portion  72  of the outer perimeter  70  is shaped to follow the contours of an inner surface  74  of the rotor housing  20 . Specifically, the shape of the first portion  72  matches the adjoined cylindrical cavities that form the rotor cavity  38  to house the rotors  12 ,  14 , and also matches a recess  75  in the end portion  26  in which the end plate  44  is housed. The end plate  44  partially closes the open end of the midportion  24  to define the end  58  of the rotor cavity  38 . The end plate  44  is sized so that the first portion  72  of the perimeter  70  can slide axially relative to the inner surface of the end portion  26  at the recess  75  but minimizes air leakage around the perimeter  70 . The inner surface  74  of an alternative embodiment of the end portion  126  is shown in  FIG. 7 . The end portion  126  has the same inner surface  74  as the end portion  26 . 
     A second portion  78  of the perimeter  70  shown in  FIG. 2  partially defines inlets  80 A,  80 B into the rotor cavity  38 , referred to herein as air inlets. The air inlets  80 A,  80 B are aligned with the air inlet  40  in the end portion  26 . The midportion  24  defines the remainder of the air inlets  80 A,  80 B, as shown in  FIG. 4 . The inner surface of the midportion  24  forms a support rib  82  that runs axially along the rotor cavity  38  and partially separates the adjoined cylindrical cavities of the rotor cavity  38 . The inner surface of the end portion  26  at the air inlet  40  has a support rib  83  that aligns with the support rib  82  when the end portion  26  is fastened to the midportion  24 . The end plate  44  has an extension  84  with a flared end  86  that is configured to conform to the shape of the support rib  83 . The support rib  83  helps to support the end plate  44  within the end portion  26 . 
       FIG. 6  shows a second embodiment of a positive displacement pump assembly  110  that is the same as is described with respect to the positive displacement pump assembly  10  except that rotor shafts  116 ,  118  and an end portion  126  of the positive displacement pump housing  120  have a different configuration. Specifically, first and second rotor shafts  116 ,  118  are used that extend into an end portion  126  that has openings  90 A,  90 B. Ends  160 A,  160 B of the rotor shafts  116 ,  118  extend beyond the end plate  44 . The positive displacement pump housing  120  includes the front cover  22 , the midportion  24  and the end portion  126 . Needle bearings  92 A,  92 B are supported in the openings  90 A,  90 B and surround the rotor shafts  116 ,  118 . The rotor shafts  112 ,  118  are axially fixed relative to the front cover  22  and an end portion  126  by both sets of the bearings  57 A,  57 B and  46 A,  46 B. The needle bearings  92 A,  92 B allow the rotor shafts  116 ,  118  to move axially relative to the end portion  126  and function as an additional positional reference for the rotor shafts  116 ,  118  with respect to the housing  120 . The end portion  126  and the end plate  44  define the clearance  52 , and the rotor end faces  40 A,  40 B and the face  51  of the end plate  44  define the clearance  48  just as in the embodiment of the positive displacement pump assembly  10 . 
     The reference numbers used in the drawings and the specification along with the corresponding components are as follows: 
       10  positive displacement pump assembly 
       12  first rotor 
       14  second rotor 
       16  first rotor shaft 
       18  second rotor shaft 
       20  positive displacement pump housing 
       22  front cover 
       24  midportion 
       26  end portion 
       28  input shaft 
       30  coupling 
       32  torsion spring 
       34  first timing gear 
       36  second timing gear 
       38  rotor cavity 
       39  inlet 
       40 A, B first axial end faces 
       42  outlet 
       43 A, B second axial end faces 
       44  end plate 
       45  clearance 
       46 A, B first axial bearings 
       48  first axial clearance 
       50 A, B stepped openings of end plate 
       51  face of end plate 
       52  second clearance 
       54  internal surface of end portion 
       56  face of end plate 
       57 A, B second pair of axial bearings 
       58  end of rotor cavity 
       59 A, B stepped openings in end portion 
       60 A, B ends of rotor shafts 
       63 A, B seals 
       65 A, B second axial ends 
       70  outer perimeter of end plate 
       72  first portion of outer perimeter 
       74  inner surface of end plate 
       75  recess of end plate 
       78  second portion of outer perimeter 
       80 A, B inlets 
       82  support rib of midportion 
       83  support rib of end portion 
       84  extension 
       86  flared end 
       90 A, B openings in end portion  126   
       92 A, B needle bearings 
       110  positive displacement pump assembly 
       116  first rotor shaft 
       118  second rotor shaft 
       120  positive displacement pump housing 
       126  end portion 
       160 A, B ends of rotor shafts 
     While the best modes for carrying out the many aspects of the present teachings have been described in detail, those familiar with the art to which these teachings relate will recognize various alternative aspects for practicing the present teachings that are within the scope of the appended claims.