Patent Publication Number: US-10760584-B2

Title: Speed increaser and centrifugal compressor

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a speed increaser and a centrifugal compressor equipped with the speed increaser. 
     A speed increaser that transmits the rotation of a low-speed shaft to a high-speed shaft has been known in the art pertaining to the present invention. Japanese Patent Application Publication 2003-314446 discloses a speed increaser including a peripheral wall that is rotatable with the rotation of a low-speed shaft, a high-speed shaft that is disposed within the peripheral wall, and a roller that is disposed between the peripheral wall and the high-speed shaft and has an outer peripheral surface that is in contact with the inner peripheral surface of the peripheral wall and the outer peripheral surface of the high-speed shaft. The roller and the high-speed shaft are held by tightening of the peripheral wall. 
     According to the speed increaser of the above-cited publication, an oil film (elastohydrodynamic lubrication film) is formed at a contact area between the peripheral wall and the roller and at a contact area between the roller and the high-speed shaft with the rotation of the peripheral wall, and the rotation is transmitted through the oil film. 
     In order for the oil film to be formed stably during the rotation, pressure to some extent need be applied to the contact area between the inner peripheral surface of the peripheral wall and the outer peripheral surface of the roller and also to the contact area between the outer peripheral surface of the roller and the outer peripheral surface of the high-speed shaft. 
     Since the high-speed shaft and the roller are in contact with each other at their outer peripheral surfaces, the area surface of the contact area tends to become small. On the other hand, the peripheral wall and the roller are in contact with each other at the inner peripheral surface of the peripheral wall and the outer peripheral surface of the roller, so that the contact area between the peripheral wall and the roller is greater than that of the contact area between the high-speed shaft and the roller. Consequently, the pressure at the contact area between the peripheral wall and the roller tends to become small. 
     In this case, if a tightening of the peripheral wall is increased so as to increase the pressure at the contact area between the peripheral wall and the roller, there is a fear that power loss between the high-speed shaft and the roller may be increased and the high-speed shaft may be subjected to an increased load. 
     The present invention, which has been made in light of the above problems, is directed to providing a speed increaser that successfully transmits power from the peripheral wall to the high-speed shaft and a centrifugal compressor equipped with such speed increaser. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, there is provided a speed increaser including a high-speed shaft, an annular peripheral wall surrounding the high-speed shaft and rotatable with a rotation of a low-speed shaft, the peripheral wall having an inner peripheral surface and a projection extending inwardly from the inner peripheral surface in a radial direction of the high-speed shaft, and a roller disposed between the high-speed shaft and the peripheral wall and having an outer peripheral surface that is in contact with both the projection and an outer peripheral surface of the high-speed shaft. 
     In accordance with another aspect of the present invention, there is provided a centrifugal compressor including a low-speed shaft, an electric motor driving to rotate the low-speed shaft, a speed increaser including a high-speed shaft, an annular peripheral wall surrounding the high-speed shaft and rotatable with a rotation of the low-speed shaft, the peripheral wall having an inner peripheral surface and a projection extending inwardly from the inner peripheral surface in a radial direction of the high-speed shaft. The centrifugal compressor further includes a roller disposed between the high-speed shaft and the peripheral wall and having an outer peripheral surface that is in contact with both the projection and an outer peripheral surface of the high-speed shaft, and an impeller mounted to the high-speed shaft. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic longitudinal sectional view of a speed increaser and a centrifugal compressor according to an embodiment of the present invention; 
         FIG. 2A  is a cross-sectional view taken along line II-II of  FIG. 1 ; 
         FIG. 2B  is a partially enlarged view of the  FIG. 2A ; 
         FIG. 3  is a fragmentary enlarged schematic cross-sectional view showing a first ring contact area according to a first embodiment; 
         FIG. 4  is a fragmentary enlarged schematic cross-sectional view showing a first ring contact area according to a second embodiment; 
         FIG. 5  is a fragmentary enlarged schematic cross-sectional view showing a projection of a speed increaser according to another example of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following will describe a speed increaser and a centrifugal compressor equipped with such speed increaser according to a first embodiment of the present invention. The centrifugal compressor of the present embodiment is mounted on a fuel cell vehicle (FCV) having a fuel cell battery and sends air to the fuel cell battery. 
     Referring to  FIG. 1 , there is shown a centrifugal compressor  10  that includes a low-speed shaft  11 , a high-speed shaft  12 , an electric motor  13  driving to rotate the low-speed shaft  11 , a speed increaser  14  increasing the rotation speed of the low-speed shaft  11  and transmitting it to the high-speed shaft  12  and a compression part  15  compressing a fluid (air in the present embodiment) with the rotation of the high-speed shaft  12 . For the sake of the illustration, the low-speed and high-speed shafts  11 ,  12  are shown in side view in  FIG. 1 . The low-speed and high-speed shafts  11 ,  12  are made of a metal such as a steel or a steel alloy. 
     The centrifugal compressor  10  includes a housing  20  that forms the outer shell of the centrifugal compressor  10 . The housing  20  accommodates therein the low-speed and high-speed shafts  11 ,  12 , the electric motor  13 , the speed increaser  14  and the compression part  15 . The housing  20  has a generally cylindrical shape and has end surfaces  20   a ,  20   b  at the axial ends of the housing  20 , respectively. 
     The housing  20  includes a motor housing  21  accommodating therein the electric motor  13 , a speed increaser housing  22  accommodating therein the speed increaser  14  and a compressor housing  23  having a intake port  23   a  through which a fluid is drawn in. The intake port  23   a  is formed in the end of the compressor housing  23  at the end surface  20   a . The compressor housing  23 , the speed increaser housing  22  and the motor housing  21  are arranged in this order in axial direction of the housing  20  as seen from the intake port  23   a . The housing  20  further includes a plate  24  that is interposed between the speed increaser housing  22  and the compressor housing  23 . 
     The centrifugal compressor  10  of the present embodiment is mounted in a horizontal position on the vehicle in a manner so that the axial direction of the housing  20  corresponds to the horizontal direction of the vehicle. Additionally, the vertical direction shown in  FIG. 1  corresponds to the vertical direction of the vehicle. 
     The motor housing  21  has a generally bottomed cylindrical shape and includes bottom portion  21   a . The outer surface of the bottom portion  21   a  of the motor housing  21  forms end surface  20   b  of the housing  20  that is located opposite end from the end surface  20   a  having the intake port  23   a . The speed increaser housing  22  also has a generally cylindrical shape and includes a bottom portion  22   a.    
     The motor housing  21  and the speed increaser housing  22  are connected together with the opened end of the motor housing  21  disposed in abutment with the bottom portion  22   a  of the speed increaser housing  22 . The inner surface of the motor housing  21  and the surface of the bottom portion  22   a  of the speed increaser housing  22  facing the motor housing  21  cooperate to form a motor chamber S 1  in which the electric motor  13  is disposed. The low-speed shaft  11  is accommodated in the motor chamber S 1  in a manner so that the rotation axis of the low-speed shaft  11  extends in the axial direction of the housing  20 . 
     The low-speed shaft  11  is rotatably supported by the housing  20 . A cylindrical first boss  31  is formed extending from the bottom portion  21   a  of the motor housing  21  towards the speed increaser housing  22 . The first boss  31  has a diameter that is greater than the diameter of one end  11   a  of the low-speed shaft  11  that is disposed adjacent to the end surface  20   b  of the housing  20 . The one end  11   a  of the low-speed shaft  11  is inserted in the first boss  31 , and a first bearing  32  is mounted in the first boss  31  and rotatably supports the low-speed shaft  11  at the one end  11   a  thereof. 
     A hole  22   b  having a diameter greater than the other end  11   b  of the low-speed shaft  11  is formed through the bottom portion  22   a  of the speed increaser housing  22 . The bottom portion  22   a  has a cylindrical second boss  33  extending from the outer periphery of the hole  22   b  toward the motor housing  21 , or the first boss  31 . The other end  11   b  of the low-speed shaft  11  is inserted through the second boss  33 , and a second bearing  34 , which support the low-speed shaft  11  rotatably, is disposed between the inner surface of the second boss  33  and the other end  11   b  of the low-speed shaft  11 . The second boss  33  of the bottom portion  22   a  has a projected portion  35  extending radially inwardly at a boundary between the inner peripheral surface of the hole  22   b  and the inner peripheral surface of the second boss  33 . The second bearing  34  is disposed in the space formed by the second boss  33  and the projected portion  35 . 
     As shown in  FIG. 1 , the other end  11   b  of the low-speed shaft  11  is inserted through the hole  22   b  of the speed increaser housing  22 , so that part of the low-speed shaft  11  is positioned in the speed increaser housing  22 . A seal member  36  is interposed between the inner peripheral surface of the hole  22   b  of the speed increaser housing  22  and the other end  11   b  of the low-speed shaft  11  so as to prevent oil from flowing out from the speed increaser housing  22  into the motor chamber S 1 . The seal member  36  is disposed on side of the projected potion  35  that is opposite from the second bearing  34  and in the area between the inner peripheral surface of the hole  22   b  of the speed increaser housing  22  and the other end  11   b  of the low-speed shaft  11 . 
     As shown in  FIG. 1 , a positioning member  37  is fixed on the low-speed shaft  11  so as to position the low-speed shaft  11  relative to the housing  20 . The positioning member  37  is disposed on the side of the second bearing  34  that is adjacent to the electric motor  13 . The positioning member  37  has a disk shape extending radially outwardly from the outer peripheral surface of the low-speed shaft  11 . The positioning member  37  is set in contact with the second bearing  34 . Such contact of the second bearing  34  with the positioning member  37  prevents dislocation of the low-speed shaft  11  in the axial direction thereof which may occur when the low-speed shaft  11  is subjected to an axial force directing from the one end  11   a  toward the other end  11   b.    
     The electric motor  13  includes a rotor  41  that is fixed on the low-speed shaft  11  and a stator  42  that is disposed radially outward of the rotor  41  and fixed to the inner peripheral surface of the motor housing  21 . The rotor  41  and the stator  42  are disposed coaxially with the low-speed shaft  11 . The rotor  41  and the stator  42  face each other in the radial direction of the low-speed shaft  11 . 
     The stator  42  includes a cylindrical stator core  43  and a coil  44  wounded around the stator core  43 . With the coil  44  energized by electric current, the rotor  41  and the low-speed shaft  11  are rotated integrally. 
     The plate  24  of the housing  20  has a disk shape with a diameter that is substantially the same as that of the speed increaser housing  22 . The plate  24  is connected at the first surface  24   a  thereof to the opened end of the bottomed cylindrical speed increaser housing  22 . A speed increaser chamber S 2  is formed by the first surface  24   a  of the plate  24  and the inner surface of the speed increaser housing  22 . 
     The plate  24  has a hole  24   b  through which the high-speed shaft  12  forming a part of the speed increaser  14  is inserted. Part of the high-speed shaft  12  extends out from the hole  24   b  and is positioned in the compressor housing  23 . It is noted that the high-speed shaft  12  forms a part of the speed increaser. 
     A seal member  50  is interposed between the inner peripheral surface of the hole  24   b  and the high-speed shaft  12  so as to seal therebetween, thereby preventing oil from flowing out from the speed increaser housing  22  into the compressor housing  23 . 
     The compressor housing  23  has a generally cylindrical shape and has a hole  51  is formed extending axially through the compressor housing  23 . The end surface  23   b  of the compressor housing  23  forms the end surface  20   a  of the housing  20 . The end of the hole  51  disposed adjacently to the end surface  23   b  serves as the intake port  23   a.    
     The compressor housing  23  and the plate  24  are assembled together with the end surface  23   c  of the compressor housing  23  that is opposite to the end surface  23   b  and the second surface  24   c  of the plate  24  that is opposite to the first surface  24   a  set in contact with each other. With the compressor housing  23  and the plate  24  thus assembled, an impeller chamber S 3  is formed by the inner peripheral surface of the hole  51  and the second surface  24   c . The impeller chamber S 3  accommodates therein an impeller  52 . In other words, the hole  51  serves as the intake port  23   a  and also forms part of the impeller chamber S 3 . The intake port  23   a  and the impeller chamber S 3  are in communication with each other. 
     The diameter of the hole  51  is constant from the end thereof adjacent to the end surface  23   b  of the compressor housing  23  to an axially intermediate position of the hole  51  and increases gradually from the axially intermediate position towards the end of the hole  51  adjacently to the second surface  24   c  of the plate  24 . In other words, the hole  51  has a shape that is similar to a truncated circular cone shape from the axially intermediate position to the end that is adjacent to the second surface  24   c  of the plate  24 . Thus, the impeller chamber S 3  has a generally truncated circular cone shape. 
     The impeller  52  is also of a shape that is similar to a truncated cone and has a base end surface  52   a  and a front end surface  52   b . The diameter of the impeller  52  is reduced from the base end surface  52   a  towards the front end surface  52   b . The impeller  52  has therethrough an insertion hole  52   c  formed extending in the axial direction thereof through which the high-speed shaft  12  is inserted. The impeller  52  is mounted on the high-speed shaft  12  for rotation therewith with part of the high-speed shaft  12  projecting out into the hole  51  inserted through the insertion hole  52   c . With the rotation of the high-speed shaft  12 , the impeller  52  rotates thereby to compress fluid drawn in through the intake port  23   a.    
     The centrifugal compressor  10  has a diffuser passage  53  through which the fluid compressed by the impeller  52  is flowed and a discharge chamber  54  into which the fluid passed through the diffuser passage  53  is discharged. The diffuser passage  53  is formed by the second surface  24   c  of the plate  24  and the surface of the compressor housing  23  that is formed continuously with the inner end of the hole  51  and faces the second surface  24   c  of the plate  24 . The diffuser passage  53  has an annular shape and is disposed radially outward of the impeller chamber S 3  so as to surround the impeller  52  and the impeller chamber S 3 . The impeller chamber S 3  and the discharge chamber  54  are in communication with each other through the diffuser passage  53 . The fluid compressed by impeller  52  is further compressed while passing through the diffuser passage  53  toward the discharge chamber  54  and discharged from the discharge chamber  54 . 
     The following will describe the speed increaser  14  according to a first embodiment. The speed increaser  14  of the present embodiment is of a so-called traction drive type (friction roller type). As shown in  FIG. 1 , the speed increaser  14  includes a ring member  60  that is connected to the other end  11   b  of the low-speed shaft  11 . The ring member  60  is made of a metal. 
     The ring member  60  includes a base  61  that has a disk shape and is connected to the other end  11   b  of the low-speed shaft  11  and a peripheral wall  62  that extends from the outer periphery of the base  61  toward the plate  24 . The base  61  and the peripheral wall  62  are rotatable with the rotation of the low-speed shaft  11 . 
     The peripheral wall  62  has a ring shape. The peripheral wall  62  has an inner peripheral surface  63  and the outer peripheral surface  64 . The peripheral wall  62  has a base end portion  65  that is disposed adjacently to the base  61  and a distal end portion  66  that is disposed opposite from the base  61 . 
     According to the present embodiment, the peripheral wall  62  has an inner diameter that is substantially constant at any position in the axial direction Z. The inner diameter of the peripheral wall  62  is greater than the diameter of the other end  11   b  of the low-speed shaft  11 . For the sake of the description, part of the peripheral wall  62  having the wall thickness that is substantially the same as the base end portion  65  will be referred to as a body portion  67  and part of the peripheral wall  62  connecting between the body portion  67  and the distal end portion  66  will be referred to as a connecting portion  68 . 
     According to the present embodiment, the ring member  60  and the low-speed shaft  11  are connected so that the rotation axis of the base  61  of the ring member  60  and the rotation axis of the low-speed shaft  11  coincide with each other. In this case, the rotation axis of the peripheral wall  62  is disposed coaxially with the rotation axis of the low-speed shaft  11 . The inner diameter and the outer diameter of the peripheral wall  62  are concentric with respect to the rotation axis of the low-speed shaft  11 . 
     Since the ring member  60  is connected to the low-speed shaft  11 , the positioning member  37  that positions the low-speed shaft  11  relative to the housing  20  may serve as a positioning member of the ring member  60 . In other words, the positioning member  37  positions the ring member  60  in its axial position relative to the housing  20 . 
     The speed increaser  14  includes the high-speed shaft  12 , and part of the high-speed shaft  12  is disposed within the peripheral wall  62 . In other words, the peripheral wall  62  surrounds the high-speed shaft  12 . As shown in  FIGS. 2A and 2B , the speed increaser  14  includes a plurality of rollers, namely the first roller  71 , the second roller  72 , the third roller  73 , disposed between the high-speed shaft  12  and the peripheral wall  62 . Specifically, the rollers  71 ,  72 ,  73  are in contact with both a projection  69  of the peripheral wall  62 , which will be described later, and an outer peripheral surface  12   a  of the high-speed shaft  12 . The rollers  71 ,  72 ,  73  have a cylindrical shape. The rotation axes of the rollers  71 ,  72 ,  73  extend parallel to each other in the axial direction Z of the high-speed shaft  12 . For the sake of description, the projection  69  is illustrated in enlarged size in  FIG. 1 through 3 . 
     The rollers  71 ,  72 ,  73  are spaced angularly at a regular interval (120 degree in the present embodiment) in the circumferential direction of the high-speed shaft  12 . The rollers  71 ,  72 ,  73  are made of a metal. Specifically, the rollers  71 ,  72 ,  73  are made of the same metal material as the high-speed shaft  12  and the ring member  60  (peripheral wall  62 ). Oil is supplied to the high-speed shaft  12 , the peripheral wall  62  and the rollers  71 ,  72 ,  73 . 
     As shown in  FIGS. 1 and 2 , the speed increaser  14  includes a support member  80  that supports the rollers  71 ,  72 ,  73  rotatably in cooperation with the plate  24 . The support member  80  is disposed radially inward of the peripheral wall  62  of the ring member  60 . The support member  80  includes a disk-shaped support base  81  having a diameter smaller than the inner diameter of the peripheral wall  62  and three columnar support portions  82 ,  83 ,  84  extending axially from the support base  81  toward the plate  24 . The support base  81  is disposed facing on one side thereof the base  61  of the ring member  60  and other side thereof the plate  24 . The support portions  82 ,  83 ,  84  extend from a surface  81   a  of the support base  81  that faces the first surface  24   a  and are disposed in the spaces each of which defined by the inner peripheral surface  63  of the peripheral wall  62  and the outer peripheral surfaces of any two adjacent rollers  71 ,  72 ,  73 . 
     As shown in  FIG. 2A , the first support portion  82  is disposed in a space formed by the inner peripheral surface  63  of the peripheral wall  62 , the outer peripheral surface  71   a  of the first roller  71  and the outer peripheral surface  72   a  of the second roller  72  with a uniform clearance formed therebetween, respectively. 
     The second support portion  83  is disposed in a space formed by the inner peripheral surface  63  of the peripheral wall  62 , the outer peripheral surface  72   a  of the second roller  72  and the outer peripheral surface  73   a  of the third roller  73  with a uniform clearance formed therebetween, respectively. 
     The third support portion  84  is disposed in a space formed by the inner peripheral surface  63  of the peripheral wall  62 , the outer peripheral surface  71   a  of the first roller  71  and the outer peripheral surface  73   a  of the third roller  73  with a uniform clearances formed therebetween, respectively. 
     As shown in  FIGS. 1 and 2A , a threaded hole  92  is formed for each of the support portions  82 ,  83 ,  84  for engagement with a bolt  91  serving as a fastener. A threaded hole  93  is formed in the plate  24  for each threaded hole  92 . The threaded holes  93  are formed in the plate  24  in alignment with the threaded holes  92  in the support portions  82 ,  83 ,  84 , respectively, and the end surfaces of the support portions  82 ,  83 ,  84  disposed adjacently to the first surface  24   a  of the plate  24 , the bolts  91  engaged both the threaded holes  92  of the support portions  82 ,  83 ,  84  and their corresponding threaded holes  93  of the plate  24 , so that the support portions  82 ,  83 ,  84  are fixed to the plate  24 , respectively. 
     The first, second and third rollers  71 ,  72 ,  73  are disposed between the plate  24  and the support base  81  of the support member  80  and rotatably supported by a first roller bearing  94  fixed to the plate  24  and a second roller bearing  95  fixed to the support base  81 , respectively. 
     Specifically, as shown in  FIG. 1 , the first roller  71  has first and second projections  101 ,  102  having a cylindrical shape and extending in the axial direction Z from the center of the opposite end surfaces of the first roller  71 , respectively. 
     The plate  24  has a recess  103  that is recessed in the first surface  24   a  of the plate  24 . The recess  103  has a diameter greater than that of the first projection  101  and smaller than that of the first roller  71 . The plate  24  has a cylindrical portion  104  projects from the outer periphery of the plate recess  103 . The cylindrical portion  104  has an inner diameter substantially the same as the diameter of the plate recess  103 . The first projection  101  is disposed in a space formed by the cylindrical portion  104  and the plate recess  103 . The first roller bearing  94  is disposed between the first projection  101  and the inner peripheral surface formed by the cylindrical portion  104  and the plate recess  103 . The first roller bearing  94  is fixed to the plate  24  while supporting the first projection  101  rotatably. 
     The support base  81  has a recess  105  on the side adjacent to the first roller  71  that is recessed in the surface  81   a  of the base  81 . The recess  105  has a diameter greater than that of the second projection  102  and smaller than that of the first roller  71 . The second projection  102  is disposed in the recess  105 . The second roller bearing  95  is disposed between the second projection  102  and the peripheral surface of the recess  105  and fixed to the support member  80  while supporting the second projection  102  rotatably. 
     The second and third rollers  72 ,  73  are rotatably supported in the same manner as the first roller  71 . As shown in  FIG. 2A , the diameters of the rollers  71 ,  72 ,  73  are greater than the diameter of the high-speed shaft  12 . The diameters of the respective rollers  71 ,  72 ,  73  are smaller than the inner radius of the peripheral wall  62  so as to be disposed inside the peripheral wall  62 . 
     It is to be noted that the diameter of the first roller  71  is different from that of the second and third rollers  72 ,  73 , as shown in  FIG. 2A . More specifically, the first roller  71  is formed with a diameter that is greater than that of the second and third rollers  72 ,  73 . Therefore, the high-speed shaft  12  which is pressedly supported by the rollers  71 ,  72 ,  73  is positioned eccentrically with respect to the axis of the peripheral wall  62 . In other words, the rotation axis of the high-speed shaft  12  and the rotation axis of the peripheral wall  62  are not aligned. 
     As shown in  FIG. 1 , the high-speed shaft  12  has a pair of flanges  96  extending radially outwardly from the outer peripheral surface  12   a  thereof. The flanges  96  are spaced apart in the axial direction Z of the high-speed shaft  12 . The first, second and third rollers  71 ,  72 ,  73  are disposed between the paired flanges  96  in the axial direction Z. This prevents the dislocation of the first, second and third rollers  71 ,  72 ,  73  and the high-speed shaft  12  in the axial direction Z. Specifically, for example, the first roller  71  whose end surface on the side adjacent to the impeller  52  is set in contact with its associated flange  96  is prevented from being moved by any thrust force created in the axial direction caused by the rotation of the impeller  52 . 
     The spaced distance between the paired flanges  96  in the axial direction Z is slightly greater than the axial dimension of the first, second and third rollers  71 ,  72 ,  73 . Accordingly, clearances into which oil may be entered are formed between the flanges  96  and the end surfaces of the first, second and third rollers  71 ,  72 ,  73 , respectively. 
     The base  81  of the support member  80  has at the center thereof a hole S 1   b  having a diameter that is greater than the flange  96  so that the flange  96  located adjacently to the base  61  is disposed in the hole  81   b . The other flange  96  positioned adjacently to the impeller  52  is disposed in the hole  24   b  of the plate  24 . 
     The following will describe the peripheral wall  62  of the ring member  60  in detail with reference to  FIGS. 1 through 3 . For the sake of illustration,  FIG. 3  only shows contact of the first roller  71  with the peripheral wall  62 . It is noted that the second and third rollers  72 ,  73  are set in contact with the peripheral wall  62  substantially in the same manner. Unless otherwise indicated, the following description will be made assuming that the shaft  11 ,  12  and the rollers  71 ,  72 ,  73  are at a stop. 
     As shown in  FIGS. 1 to 3 , the peripheral wall  62  of the ring member  60  has the projection  69  that is formed extending inwardly in the radial direction R of the high-speed shaft  12  from the inner peripheral surface  63  of the peripheral wall  62 . As shown in  FIG. 2A , the projection  69  has an annular shape extending in circumferential direction of the peripheral wall  62 . As shown in  FIGS. 1 and 3 , the projection  69  has a semicircular shape as seen in longitudinal section of the speed increaser  14 , or the projection  69  is taken in a plane perpendicular to the circumferential direction of the peripheral wall  62 . The projection  69  of the peripheral wall  62  is in contact with the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the respective first, second and third rollers  71 ,  72 ,  73 . 
     With the projection  69  of the peripheral wall  62  in contact with the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the first, second and third rollers  71 ,  72 ,  73 , the inner peripheral surface  63  of the peripheral wall  62  is spaced apart from the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the first, second and third rollers  71 ,  72 ,  73 . 
     As shown in  FIG. 3 , the distal end portion  66  of the peripheral wall  62  has a wall thickness that is greater than that of the body portion  67 . The wall thickness of the distal end portion  66  and the wall thickness of the body portion  67  are substantially constant at any position in the axial direction Z, respectively. 
     The connecting portion  68  of the peripheral wall  62  connecting the distal end portion  66  and the body portion  67  has a wall thickness that is varied along the axial direction Z. Specifically, the wall thickness of the connecting portion  68  is increased toward the distal end portion  66 . The connecting portion  68  also has a wall thickness that is greater than that of the body portion  67 . The distal end portion  66  and the connecting portion  68  correspond to the thick wall portion and the body portion  67  and the base end portion  65  corresponds to the thin wall portion, respectively, of the present invention. The thick wall portion has a wall thickness that is greater than the thin wall portion. 
     The outer peripheral surface  64  of the peripheral wall  62  is formed stepped corresponding to the varied wall thickness of the peripheral wall  62  along the axial direction Z. Meanwhile, the inner peripheral surface  63  of the peripheral wall  62  has no stepped portion. 
     As shown in  FIG. 3 , the inner peripheral surface  63  of the peripheral wall  62  includes a first inner surface  63   a  corresponding to the distal end portion  66 . The first inner surface  63   a  of the inner peripheral surface  63  faces an axially intermediate part of the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the respective first, second and third rollers  71 ,  72 ,  73  with respect to the axial direction of the high-speed shaft  12 . The inner peripheral surface  63  of the peripheral wall  62  further includes a second inner surface  63   b  and a third inner surface  63   c  corresponding to the body portion  67  and the connecting portion  68 , respectively. The first, second and third inner surfaces  63   a ,  63   b ,  63   c  are formed in the same circle. The first and third inner surfaces  63   a ,  63   c  correspond to the above-mentioned part of the inner peripheral surface corresponding to the thick wall portion according to the present invention. 
     The outer peripheral surface  64  of the peripheral wall  62  includes a first outer surface  64   a  corresponding to the distal end portion  66 , a second outer surface  64   b  corresponding to the body portion  67  and a third outer surface  64   c  corresponding to the connecting portion  68 . The second outer surface  64   b  extends along the axial direction Z of the high-speed shaft  12 . In other words, the second outer surface  64   b  and the axial direction Z extend substantially in parallel. The distance from the axis of the high-speed shaft  12  to the second outer surface  64   b  is substantially constant. 
     The first outer surface  64   a  is a circumferential surface that is disposed outward of the second outer surface  64   b  in the radial direction R of the high-speed shaft  12 . The first outer surface  64   a  extends along the axial direction Z of the high-speed shaft  12 . The distance from the axis of the high-speed shaft  12  to the first outer surface  64   a  is constant at any position along the axial direction Z. 
     The third outer surface  64   c  is a surface connecting the first outer surface  64   a  and the second outer surface  64   b . The distance from the high-speed shaft  12  to the third outer surface  64   c  is increased towards the distal end portion  66 . In other words, the third outer surface  64   c  is tapered towards the base end portion  65 . 
     In the peripheral wall  62  having such configuration, the projection  69  is formed in the distal end portion  66  having a wall thickness greater than the body portion  67 . In other words, the projection is formed extending from the inner peripheral wall corresponding to the thick wall portion. Specifically, the projection  69  is formed extending from the first inner surface  63   a  that faces the axially intermediate part of the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the respective rollers  71 ,  72 ,  73  in the radial direction R of the high-speed shaft  12 . 
     According to present embodiment, the projection  69  is in contact with the axially intermediate part of the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the respective first, second and third rollers  71 ,  72 ,  73 . It is noted, however, that the projection  69  may be set in contact with any suitable part of the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the first, second and third rollers  71 ,  72 ,  73 . 
     As shown in  FIG. 1 , parts of the rollers  71 ,  72 ,  73  project out from the peripheral wall  62  toward the plate  24 . In other words, part of the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the first, second and third rollers  71 ,  72 ,  73  are exposed without facing the inner peripheral surface  63  of the peripheral wall  62  in radial direction R of the high-speed shaft  12 . It is noted, however, that the rollers  71 ,  72 ,  73  may be disposed so that the entire outer peripheral surfaces  71   a ,  72   a ,  73   a  of the first, second and third rollers  71 ,  72 ,  73  face the inner peripheral surface  63  of the peripheral wall  62 . 
     As shown in  FIG. 2B , the rollers  71 ,  72 ,  73 , the ring member  60  and the high-speed shaft  12  are assembled into a unit in such a way that and the high-speed shaft  12  is rotatably supported by the rollers  71 ,  72 ,  73 . These parts are fastened into a unit by the tightening of the peripheral wall  62 . The projection  69  is set in contact with the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the respective rollers  71 ,  72 ,  73  at first, second and third ring contact areas Pa 1 , Pa  2 , Pa 3 , respectively. Pressure F 1  is applied from the projection  69  to each of the rollers  71 ,  72 ,  73  at the ring contact areas Pa 1 , Pa  2 , Pa 3  by the tightening of the peripheral wall  62 . As shown in  FIG. 2B , the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the first, second and third rollers  71 ,  72 ,  73  are set in contact with the outer peripheral surface  12   a  of the high-speed shaft  12  at first, second and third shaft contact areas Pb 1 , Pb 2 , Pb 3 . Pressure F 2  is applied to the high-speed shaft  12  at the shaft contact areas Pb 1 , Pb 2 , Pb 3 . 
     As shown in  FIG. 2A , the dimension of the ring contact areas Pa 1 , Pa 2 , Pa 3  in the circumferential direction of the high-speed shaft  12  is greater than that of the shaft contact areas Pb 1 , Pb 2 , Pb 3 . As shown in  FIG. 1 , the shaft contact areas Pb 1 , Pb 2 , Pb 3  extend in axial direction Z. On the other hand, the dimension of the ring contact areas Pa 1 , Pa  2 , Pa 3  in the axial direction Z is smaller than that of the shaft contact areas Pb 1 , Pb 2 , Pb 3  because of the formation of the projection  69 . 
     The ring contact areas Pa 1 , Pa 2 , Pa 3  may be formed greater or smaller than the shaft contact areas Pb 1 , Pb 2 , Pb 3  as long as the ring contact areas Pa 1 , Pa 2 , Pa 3  are relatively similar to those of the shaft contact areas Pb 1 , Pb 2 , Pb 3 , as compared with the case in which the inner peripheral wall of the ring member is formed with a constant radius of curvature in the axial direction. The surface areas of the ring contact areas Pa 1 , Pa 2 , Pa 3  may have the surface area that is substantially the same as the shaft contact areas Pb 1 , Pb 2 , Pb 3 . 
     When the peripheral wall  62  is rotated with the rotation of the low-speed shaft  11  with oil sufficiently supplied to the ring contact areas Pa 1 , Pa  2 , Pa 3 , an oil film (elastohydrodynamic lubrication film, or EHL film) is formed in the clearances between the projection  69  and the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the respective rollers  71 ,  72 ,  73  at positions corresponding to the ring contact areas Pa 1 , Pa  2 , Pa 3 . The projection  69  faces the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the rollers  71 ,  72 ,  73  through the oil film. The rotational force of the peripheral wall  62  of the ring member  60  is transmitted to the rollers  71 ,  72 ,  73  through the oil film thereby to rotate the rollers  71 ,  72 ,  73  in the same rotational direction. 
     When the peripheral wall  62  is rotated with the rotation of the low-speed shaft  11  with oil sufficiently supplied to the shaft contact areas Pb 1 , Pb 2 , Pb 3 , an oil film (EHL film) is formed in the clearances between the outer peripheral surface  12   a  of the high-speed shaft  12  and the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the rollers  71 ,  72 ,  73 , respectively, at positions corresponding to the shaft contact areas Pb 1 , Pb 2 , Pb 3 . In other words, the outer peripheral surface  12   a  of the high-speed shaft  12  face the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the rollers  71 ,  72 ,  73  through the oil film. The rotational power of the first, second and third rollers  71 ,  72 ,  73  is transmitted to the high-speed shaft  12  through the oil film thereby to rotates the high-speed shaft  12 . In this case, the base  61  and the peripheral wall  62  are rotated at the same speed as the low-speed shaft  11  while the first, second and third rollers  71 ,  72 ,  73  are rotated at a speed greater than the low-speed shaft  11 . The high-speed shaft  12  having a diameter smaller than the first, second and third rollers  71 ,  72 ,  73  is rotated at a speed in terms of rpm that is greater than that of the first, second and third rollers  71 ,  72 ,  73 . 
     According to the speed increaser  14 , rotation of the low-speed shaft  11  is transmitted to the high-speed shaft  12  and the high-speed shaft  12  rotates at a speed that is higher than that of the low-speed shaft  11 . 
     In the centrifugal compressor  10  of the present embodiment, oil is circulated within the speed increaser housing  22 . An inlet port  111  is formed in the upper part of the speed increaser housing  22  and an outlet port  112  is formed in the lower part of the speed increaser housing  22 . Oil is introduced through the inlet port  111  and flowed to the contact areas Pa 1 , Pa 2 , Pa 3 , Pb 1  Pb 2 , Pb 3  in the speed increaser  14 . Then, the oil is discharged through the outlet port  112 . In other words, oil is to be supplied to the speed increaser  14 . 
     The present invention offers the following effects. For the sake of the explanation, the effects will be described with reference to the relation between the first roller  71  and the peripheral wall  62  only, though the effects of the present invention can be seen in the second and third rollers  72 ,  73 . 
     (1) The speed increaser  14  includes the annular peripheral wall  62  that is rotatable with the rotation of the low-speed shaft  11 , the high-speed shaft  12  that is disposed radially inward of the peripheral wall  62 , and the first roller  71  that is disposed between the peripheral wall  62  and the high-speed shaft  12 . 
     The speed increaser  14  has the projection  69  extending radially inwardly from the inner peripheral surface  63  of the peripheral wall  62 . The outer peripheral surface  71   a  of the first roller  71  is in contact with the projection  69  and the outer peripheral surface  12   a  of the high-speed shaft  12  when the first roller  71  is at a stop. 
     According to the speed increaser  14  having this configuration in which the projection  69  and the outer peripheral surface  71   a  of the first roller  71  are set in contact with each other, the first ring contact area Pa 1  may be reduced, as compared with the case in which the peripheral wall has an inner diameter that is constant in the axial direction Z of the high-speed shaft  12 . Accordingly, the pressure applied to the first ring contact area Pa 1  may be increased. An oil film may be formed easily at the first ring contact area Pa 1  when the first roller  71  is rotated with relatively small tightening force by the peripheral wall  62 . 
     In addition, according to the present embodiment in which the high-speed shaft  12  and the first roller  71  are in contact with each other at their respective outer peripheral surfaces  12   a ,  71   a , the first contact area Pb 1  tends to become small and, accordingly a relatively high pressure is created at the contact areas Pa 1 , Pb 1  The outer peripheral surface  71   a  of the first roller  71  and the outer peripheral surface  12   a  of the high-speed shaft  12  are curved in directions toward each other, or in the opposite directions, around the first shaft contact Pb 1 . 
     On the other hand, in the structure in which the first roller  71  and the peripheral wall  62  of the ring member  60  are in contact with each other at the outer peripheral surface  71   a  and the inner peripheral surface  63 , the outer peripheral surface  71   a  of the first roller  71  and the inner peripheral surface  63  of the peripheral wall  62  are curved in the same direction at the first ring contact area Pa 1 . The first ring contact area Pa 1  is greater than the first shaft contact area Pb 1 . Thus, the pressure at the first ring contact area Pa 1  becomes smaller that at the first shaft contact area Pb 1 . Small pressure at the first ring contact area Pa 1  may cause failure in formation of a solidified oil film with the rotation of the first roller  71 , with the result that the power from the peripheral wall  62  may fail to be transmitted from to the first roller  71  effectively. 
     Although it may be contemplated to increase the tightening force of the peripheral wall  62  to solve the above-describe problem, such configuration increases the pressure acting at the first shaft contact area Pb 1 , which results in an increase of power loss at the first shaft contact area Pb 1  and an application of excessive load to the high-speed shaft  12 . 
     According to the present invention in which the provision of the projection  69  permits reducing the first ring contact area Pa 1  without having an effect on the first shaft contact area Pb 1 , the pressure at the first ring contact area Pa 1  may be increased without applying excessive load to the high-speed shaft  12 . Thus, the rotation of the first roller  71  forms oil film at locations corresponding to the contact areas Pa 1 , Pb 1 , so that power of the peripheral wall  62  may be transmitted from the peripheral wall  62  to the high-speed shaft  12  effectively. 
     (2) The provision of the projection  69  having an annular shape extending in the circumferential direction of the peripheral wall  62  allows the pressure F 1  to be applied to the first roller  71  constantly irrespective to the rotational position of the peripheral wall  62 , so that stable application of pressure at the first ring contact area Pa 1  may be maintained.
 
(3) The provision of the projection  69  having a semicircular shape in longitudinal section, or in cross section taken in a plane perpendicular to the circumferential direction of the peripheral wall  62  helps to reduce the first ring contact area Pa 1 , so that pressure at the first ring contact area Pa 1  may be increased.
 
(4) The structure in which the peripheral wall  62  includes the body portion  67  and the distal end portion  66  having the wall thickness that is greater than the body portion  67  and the projection  69  is formed extending from the first inner surface  63   a  corresponding to the distal end portion  66  in the inner peripheral surface  63  of the peripheral wall  62 . This permits reduction of the weight of the peripheral wall  62  while securing the pressures F 1 , F 2 .
 
(5) The provision of the projection  69  that is in contact with the axially intermediate part of the outer peripheral surface  71   a  of the first roller  71  in the axial direction Z of the high-speed shaft  12 . This helps to prevent the unbalanced application of pressure to the first shaft contact area Pb 1  such as the pressure being greater at one end than the other end of the first shaft contact area Pb 1  in the axial direction Z
 
     In addition, load is distributed evenly to the first and second roller bearings  94 ,  95  that rotatably support the first roller  71 . This prevents one of the roller bearings  94 ,  95  to be deteriorated prematurely. 
     (6) The centrifugal compressor  10  includes the electric motor  13  that rotates the low-speed shaft  11 , the impeller  52  mounted on the high-speed shaft  12  and the speed increaser  14 . This configuration of the centrifugal compressor  10  enables the impeller  52  to be rotated at a speed that is greater than the rotation speed of the electric motor  13 . The centrifugal compressor  10  may be operated optimally through effective transmission of the rotation of the peripheral wall  62  to the high-speed shaft  12 . 
     The following will describe the speed increaser  14  according to a second embodiment of the present invention with reference to  FIG. 4 . The speed increaser  14  of the second embodiment differs from the first embodiment in the shape of the projection. For the sake of the description,  FIG. 4  only shows contact of the first roller  71  with the peripheral wall  62  of the ring member  60 . It is noted that the second and third rollers  72 ,  73  are in contact with the peripheral wall  62  substantially in the same manner. It is to be noted that a projection of the second embodiment, which is designated by numeral  120 , is illustrated in enlarged size in  FIG. 4 . 
     As shown in  FIG. 4 , the projection  120  has an annular shape extending in the circumferential direction of the peripheral wall  62  and has an end surface  121  and a pair of first and second side surfaces  122 ,  123 . It is noted that the circumferential direction of the peripheral wall  62  corresponds to the circumferential direction of the high-speed shaft  12 . 
     The end surface  121  of the projection  120  is disposed radially inward of the inner peripheral surface  63  of the peripheral wall  62  (the first inner surface  63   a  according to the present embodiment). The end surface  121  extends in circumferential direction of the peripheral wall  62  and also in the axial direction Z of the high-speed shaft  12 . The end surface  121  is in contact with the outer peripheral surface  71   a  of the first roller  71  at the first ring contact area Pa 11 . 
     The ring contact area Pa 11  extends in the circumferential direction of the peripheral wall  62  and also in the axial direction Z of the high-speed shaft  12 . Because the ring contact area Pa 11  has a dimension extending in the axial direction Z of the high-speed shaft  12 , the ring contact area Pa 11  of the second embodiment is larger than the ring contact area Pa 1  of the first embodiment. 
     It is noted that the axial direction of the high-speed shaft  12  corresponds to the width direction of the projection  120 . The end surface  121  is a circumferential surface that extends circumferentially with respect to the axis of the peripheral wall  62 , or the low-speed shaft  11 , and has a width Zx extending in the axial direction Z of the high-speed shaft  12 . 
     The end surface  121  of the projection  120  has flatness. For example, the end surface is so formed that the surface roughness is smaller than the thickness of the oil film formed at the ring contact area Pa 11 . 
     The width Zx of the end surface  121  is determined based on the pressure (load) F 1  applied from the peripheral wall  62  to the first roller  71 , a first Young&#39;s modulus E 1  and a first Poisson&#39;s ratio v 1  of the peripheral wall  62  and a second Young&#39;s modulus E 2  and a second Poisson&#39;s ratio v 2  of the first roller  71 . Specifically, the width Zx of the end surface  121  is determined so as to correspond to the shape of the contact area that is obtained by the equation based on the Hertz&#39;s contact theory, assuming that the projection  120  has a semicircular shape having the radius r 1  and the first roller  71  having the radius r 2 . 
     The first Young&#39;s modulus E 1  and the first Poisson&#39;s ratio v 1  are characteristic values determined by the material of the peripheral wall  62  and the second Young&#39;s modulus E 2  and the second Poisson&#39;s ratio v 2  are characteristic values determined by the material of the first roller  71 . The pressure F 1  is a parameter determined by the tightening of the peripheral wall  62 . Therefore, it may be said that the width Zx of the end surface  121  of the second embodiment is determined by the tightening of the peripheral wall  62 , the materials of the peripheral wall  62  and the first roller  71 , and the radius r 1  of the end surface  121  and the radius r 2  of the first roller  71 . 
     According to the present embodiment, the peripheral wall  62  and the first roller  71  are made of the same material, so that the first and second Young&#39;s moduli E 1 , E 2  are the same values, and the first and second Poisson&#39;s ratios v 1 , v 2  are the same values. The peripheral wall  62  and the first roller  71  need not necessarily be made of the same material. 
     As shown in  FIG. 4 , the pair of side surfaces  122 ,  123  are formed continuously with the opposite ends  121   a ,  121   b  of the end surface  121  in the width direction thereof, or in the axial direction Z of the high-speed shaft  12 , and the inner peripheral surface  63  of the peripheral wall  62 , respectively. Specifically, the first side surface  122  is formed continuous with the first end  121   a  of the end surface  121  and the inner peripheral surface  63  of the peripheral wall  62  and the second side surface  123  is formed continuous with the second end  121   b  of the end surface  121  and the inner peripheral surface  63  of the peripheral wall  62 . 
     The side surfaces  122 ,  123  are curved so that the width of the projection  120 , or the dimension of the projection  120  in the axial direction Z of the high-speed shaft  12 , is reduced gradually towards the end surface  121 . In other words, the projection  120  has a generally crown shape in longitudinal section of the speed increaser  14  having the end surface  121  disposed radially inward of the inner peripheral surface  63  of the peripheral wall  62  and the pair of the side surfaces  122 ,  123 . 
     The paired side surfaces  122 ,  123  are convex with respect to the high-speed shaft  12 . The paired side surfaces  122 ,  123  have a logarithmic curve shape in longitudinal section of the speed increaser  14 , or when the projection  120  is taken in a plane perpendicular to the circumferential direction of the peripheral wall  62 , so that lines which are tangential to the parts of the side surfaces  122 ,  123  adjacent to the end surface  121  extend generally in the same direction as the axial direction of the high-speed shaft  12 . Thus, the opposite ends  121   a ,  121   b  of the end surfaces  121  and the side surfaces  122 ,  123  at the respective boundaries therebetween are smoothly joined without being pointed. In other words, the opposite ends  121   a ,  121   b  and the lines tangential to the part of the side surfaces  122 ,  123  disposed adjacent to the end surfaces  121  intersect at an angle greater than the 90 degree. 
     The above-described second embodiment of the present invention offers the effect below, as well as the effects (1), (2), (4) through (6) described with reference to the first embodiment. For the sake of the explanation, the effects will be described with reference to the relation between the first roller  71  and the peripheral wall  62  only, though the effects of the present invention can be seen in the second and third rollers  72 ,  73 . 
     (7) The projection  120  which extends radially inwardly from the inner peripheral surface  63  of the peripheral wall  62  has the end surface  121  extending in the circumferential direction of the peripheral wall  62  and also in the axial direction Z of the high-speed shaft  12 , and the end surface  121  of the projection  120  is set in contact with the outer peripheral surface  71   a  of the first roller  71 . Because the end surface  121  extends in the axial direction Z of the high-speed shaft  12 , the first ring contact area Pa 11  is larger than the first ring contact area Pa 1  of the first embodiment. This prevents the pressure at the first ring contact area Pa 11  to become excessively large. 
     Pressure at the first ring contact area Pa 11  increases with a reduction of the first ring contact area Pa 11 . Excessively large pressure at the first ring contact area Pa 11  may cause a problem such as deformation of the peripheral wall  62  and the first roller  71  and seizure of the peripheral wall  62  and the first roller  71  by frictional heat. 
     According to the second embodiment in which the end surface  121  extending in the axial direction Z of the high-speed shaft  12  is set in contact with the outer peripheral surface  71   a  of the first roller  71 , the first ring contact area Pa 11  is larger than the first contact are Pa 1  of the first embodiment. In other words, the projection  69  is in line contact with the outer peripheral surface  71   a  of the first roller  71 , meanwhile the projection  120  of the second embodiment is in plane contact with the outer peripheral surface  71   a  of the first roller  71 . This prevents pressure at the first contact area Pa 11  to become excessively large. 
     In addition, because the first ring contact area Pa 11  extends in the axial direction Z of the high-speed shaft  12 , the peripheral wall  62  is unlikely to be inclined relative to the axial direction Z of the high-speed shaft  12 , with the result that the peripheral wall  62  may be positioned stably, thereby preventing the vibration caused by the inclined peripheral wall  62 . 
     (8) The provision of the generally flat end surface  121  permits machining the projection  120  without difficulty. 
     In the speed increaser  14  in which power is transmitted through the oil film (EHL) formed between the projection  120  of the peripheral wall  62  and the outer peripheral surface  71   a  of the first roller  71 , surface roughness of the projection  120  and the outer peripheral surface  71   a  of the first roller  71  need to be small. The end surface  121  having a flat surface extending in the axial direction Z of the high-speed shaft  12  permit achieving suitable surface roughness more easily, as compared with the projection  69  of the first embodiment having an arc shape in longitudinal section. Thus, machining of the projection may be achieved easily. 
     (9) The projection  120  has the pair of side surfaces  122 ,  123  that are formed continuous with the opposite ends  121   a ,  121   b  of the end surface  121  and the inner peripheral surface  63  of the peripheral wall  62 . The paired side surfaces  122 ,  123  are curved so that the width of the projection  120  is reduced from the inner peripheral surface  63  of the peripheral wall  62  to the end surface  121 , and the side surfaces  122 ,  123  are convex with respect to the high-speed shaft  12 . 
     The provision of the projection  120  having such shape permits smoothly connecting the opposite ends  121   a ,  121   b  of the end surface  121  with the first and second side surfaces  122 ,  123 , respectively. In other words, the intersection of the line that is tangential to the side surface  122  and the end surface  121  at the boundary therebetween forms an obtuse angle, and the intersection of the line that is tangential to the side surface  123  and the end  121   b  of the end surface  121  at the boundary therebetween forms an obtuse angle. The provision of the projection  120  having such shape prevents concentration of pressure at the opposite ends  121   a ,  121   b  of the end surface  121 , thereby allowing oil to be flowed to part of the ring contact area Pa 11  corresponding to the center of the end surface  121  in the width direction thereof, which may be prevented by the concentration of pressure at the opposite ends  121   a ,  121   b  of the end surface  121 . 
     (10) The provision of the projection  120  in which the pair of side surfaces  122 ,  123  are curved in a logarithmic curve shape in longitudinal section permits smoothly connecting the first and second ends  121   a ,  121   b  of the end surface  121  with the side surfaces  122 ,  123 , respectively, thereby forming an obtuse angle at the intersection of the line tangential to the side surface  122  and end  121   a  of the end surface  121  at the boundary between the side surface  122  and the end  121   a  of the end surface  121  and also at the intersection of the line tangential to the side surface  123  and the end  121   b  of the end surface  121  at the boundary between the side surface  123  and the end  121   b  of the end surface  121 . The provision of the projection  120  having a logarithmic curve shape prevents concentration of pressure at the opposite ends  121   a ,  121   b  of the end surface  121  more effectively.
 
(11) The width Zx of the end surface  121  is determined based on the pressure F 1  applied from the peripheral wall to the first roller  71 , the first Young&#39;s modulus E 1  and the first Poisson&#39;s ratio v 1  of the peripheral wall  62 , the second Young&#39;s modulus E 2  and the second Poisson&#39;s ratio v 2  of the first roller  71 , the radius r 1  of the end surface  121 , and the radius r 2  of the first roller  71 , with the result that an appropriate pressure is secured at the first contact area Pa 11 .
 
     The embodiment of the present invention may be modified in various manners, as exemplified below. 
     The projection need not necessarily have a semicircular shape or a generally crown shape in longitudinal section as in the first and second embodiments, respectively. As shown in  FIG. 5 , for example, the projection, which is designated by numeral  130 , may have a rectangular shape in cross section. In this case, the end surface  131  of the projection  130  shall be set in contact with the outer peripheral surfaces  71   a ,  72   a ,  73   a  of the first, second and hid rollers  71 ,  72 ,  73 . The end surface  131  extends in the circumferential direction of the peripheral wall  62  and also in the axial direction Z of the high-speed shaft  12 . 
     In view of reducing the contact area, however, the projection should preferably have a semicircular shape in cross section rather than a rectangular shape. In view of preventing concentration of the stress on the opposite ends of the end surface in the width direction thereof, the projection should preferably have a generally crown shape in cross section, as with the projection  120  of the second embodiment. 
     The projection  69  of the first embodiment need not necessarily have an annular shape extending in the circumferential direction of the peripheral wall  62 , but may have a hemispherical shape. In this case, a plurality of discrete projections may be formed in the circumferential direction of the peripheral wall  62 . This permits further reducing the contact area Pa 1 , Pa 2 , Pa 3 . Similarly, the projection  120  of the second embodiment need not necessarily have an annular shape extending in the circumferential direction of the peripheral wall  62 , but may be formed by a plurality of discrete projection disposed along the circumferential direction of the peripheral wall. 
     The wall thickness of the peripheral wall  62  may be constant. In other words, the thick wall portion and the thin wall portion need not necessarily be formed. 
     The third outer surface  64   c  need not necessarily be formed by a sloped surface but may be formed by a surface extending perpendicular to the axial direction Z. 
     The projection such as  69  and  120  may be disposed at any suitable location. For example, the projection may be formed projecting from the third inner surface  63   c  or the second inner surface  63   b . Additionally, the projection may be formed projecting from a position of the inner peripheral surface  63  of the peripheral wall  62  that is adjacent to the base end rather than to the front end thereof. 
     It may be so configured that the base  61  of the ring member  60  and the low-speed shaft  11  are formed integrally and the peripheral wall  62  is mounted to the base  61 . Furthermore, the base  61 , the peripheral wall  62  and the low-speed shaft  11  may be separately formed and assembled together. 
     Any suitable oil supply means may be used for the centrifugal compressor  10 . For example, the centrifugal compressor  10  may be provided with an oil pump that supplies oil to the speed increaser housing  22 . 
     The number of the rollers is not limited to three, but any suitable number of rollers may be used. 
     The speed increaser  14  may have a configuration in which at least one of the roller  71 ,  72 ,  73  is driven in accordance with the torque from the low-speed shaft  11 . 
     The rollers  71 ,  72 ,  73  may have substantially the same diameter. In this case, the high-speed shaft  12  may be disposed coaxially with the ring member  60  or the low-speed shaft  11 . 
     The width Zx of the end surface  121  need not necessarily be determined using the radius r 2  of the first roller  71  as a parameter. In other words, the radius of the second roller  72  or the radius of the third roller  73  may be used as a parameter to determine the width Zx of the end surface  121 . If the speed increaser  14  includes the first, second and third rollers  71 ,  72 ,  73  having different diameters, the smallest radius or the largest radius of the first, second and third rollers  71 ,  72 ,  73  may be used as a parameter to determine the width Zx. 
     The pair of side surfaces  122 ,  123  may be curved in an arc shape, instead of a logarithmic curve shape. The paired side surfaces  122 ,  123  may be concave with respect to the high-speed shaft  12  or tapered toward the end surface  121  of the projection  120 . 
     The end surface  121  may have any suitable width Zx. 
     The compression part  15  need not necessarily be formed of an impeller type. For example, the vane type, or scroll type compression part may be used in the centrifugal compressor  10 . 
     The use of the speed increaser  14  is not limited to the centrifugal compressor  10 . For example, the speed increaser  14  may be mounted to a fluid machine such as a pump in which compression of fluid is not performed. 
     The speed increaser  14  and the centrifugal compressor  10  may be mounted to any equipment other than a vehicle. 
     The centrifugal compressor  10  may be used to compress any fluid. For example, the centrifugal compressor  10  may be used for an air conditioner so as to compress a refrigerant gas. 
     The centrifugal compressor may be made including any of the above features.