Abstract:
A thrust roller bearing comprising: a pair of races each of which has an annular plate; a cage interposed between the pair of races, formed by a single annular plate including a bent portion bent in an intermediate region in a radial direction of the cage such that a plurality of projecting portions to project in a thickness direction of the cage are formed on a first side and a second side of the cage, and a plurality of pockets stamped out in the thickness direction at predetermined intervals in a circumferential direction of the cage; and a plurality of rollers received respectively in the pockets; wherein the at least two projecting portions are formed on each of the first side and the second side of the cage; and a plurality of race-contacting projection portions are arranged on a surface of at least one of said at least two projecting portions on at least one of the first side and the second side of the cage at predetermined intervals in the circumferential direction of the cage.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a thrust roller bearing, and more particularly to a thrust roller bearing in which a shearing resistance due to the viscosity of oil between a cage and races is reduced, thereby reducing a rotation torque. 
         [0003]    2. Related Art 
         [0004]    At present, the environment is much affected by automobiles, and therefore it is an important problem to achieve a low fuel consumption design. In an automatic transmission, ten or more thrust roller bearings are used on the average, and with respect to such thrust roller bearings, also, attempts have been made to achieve a low torque design in order that the reduction of a rotation torque can contribute to the low fuel consumption design. 
         [0005]    As means for achieving the low torque design, there have been proposed a system in which an end surface of each roller and an end surface of a pocket of a cage are disposed in point-contact with each other, a double-row arrangement of rollers, etc. 
         [0006]    A conventional thrust roller bearing comprises a pair of first and second races of an annular shape having their respective raceway surfaces opposed to each other, a plurality of rollers disposed between the two races and arranged at intervals in a circumferential direction, and an annular cage for holding and guiding the rollers. 
         [0007]    The first race has an axially-extending cylindrical flange formed at its outer periphery by bending, and an inner peripheral portion of the first race is flush with the flat raceway surface. The second race is merely formed into a flat plate-shape, and is received in the first race. 
         [0008]    The cage for holding and guiding the rollers (which are disposed between the raceway surfaces of the first and second races, and roll on the raceway surfaces while receiving a load) has a generally M-shaped cross-section, and has a plurality of pockets formed by stamping and arranged at predetermined intervals in the circumferential direction. The rollers are received in the pockets, respectively. 
         [0009]    Thus, the cage has the generally M-shaped cross-section, and with this construction the rigidity of the cage is increased, and a large load capacity is secured while achieving a compact design of the thrust bearing (see, for example, JP-A-7-332357 Publication). 
         [0010]    The cage of the above conventional thrust roller bearing has the generally M-shaped cross-section, and an inner peripheral portion and an outer peripheral portion of the cage fully contact the raceway surface of the first race, and a central portion of the cage disposed between the inner and outer peripheral portions thereof does not contact the second race, and the rollers received in the respective pockets contact the second race. Therefore, the inner and outer peripheral portions of the cage thus fully contact the raceway surface of the first race, and therefore there has been encountered a problem that a rotation torque is increased by a shearing resistance due to the viscosity of oil. 
         [0011]    And besides, in the case where the cage of the thrust roller bearing has a generally inverted M-shaped cross-section, a central portion of the cage fully contacts the raceway surface of the first race, and an inner peripheral portion and an outer peripheral portion of the cage do not contact the second race, and the rollers received in the respective pockets contact the second race. In this case, thus, the central portion of the cage fully contacts the raceway surface of the first race, which has also invited a problem that a rotation torque is increased by a resistance offered by the viscosity of oil. 
       SUMMARY OF THE INVENTION 
       [0012]    This invention has been made in order to solve these problems, and an object of the invention is to provide a thrust roller bearing in which full contact between a cage and races is eliminated so as to reduce a shearing resistance due to the viscosity of oil between the cage and the races, thereby reducing a rotation torque. 
         [0013]    According to one aspect of the present invention, there is provided a thrust roller bearing including: a pair of races each of which has an annular plate; 
         [0014]    a cage interposed between the pair of races, formed by a single annular plate including a bent portion bent in an intermediate region in a radial direction of the cage such that a plurality of projecting portions to project in a thickness direction of the cage are formed on a first side and a second side of the cage, and a plurality of pockets stamped (blanked) out in the thickness direction at predetermined intervals in a circumferential direction of the cage; and 
         [0015]    a plurality of rollers received respectively in the pockets; 
         [0016]    wherein the at least two projecting portions are formed on each of the first side and the second side of the cage; and 
         [0017]    a plurality of race-contacting projection portions are arranged on a surface of at least one of said at least two projecting portions on at least one of the first side and the second side of the cage at predetermined intervals in the circumferential direction of the cage. 
         [0018]    Furthermore, the race-contacting projection portions are convex portions formed by punching (pressing) part of the projecting portion or protruding portions formed by notching part of an end portion of the projecting portion. 
         [0019]    Furthermore, one of the at least two projecting portions of the bent portion on one of the first side and the second side of the cage is projected in an amount larger than the amount of projecting of the other projecting portion in stepped relation thereto to provide a race-contacting projection portion. 
         [0020]    According to another aspect of the invention, there is provided a thrust roller bearing including; 
         [0021]    a pair of races each of which has an annular plate; 
         [0022]    a cage interposed between the pair of races, formed by a single annular plate including a bent portion bent in an intermediate region in a radial direction of the cage such that a plurality of projecting portions to project in a thickness direction of the cage are formed on a first side and a second side of the cage, and a plurality of pockets stamped out in the thickness direction at predetermined intervals in a circumferential direction of the cage; and 
         [0023]    a plurality of rollers received respectively in the pockets; 
         [0024]    wherein the at least two projecting portions are formed on each of the first side and the second side of the cage; and 
         [0025]    wherein a plurality of cage-contacting projection portions are arranged on a surface of at least one of said pair of races at predetermined intervals along a circumferential direction of the races. 
         [0026]    The cage-contacting projection portions are convex portions. 
         [0027]    As described above, in the thrust roller bearing of the invention, when the thrust roller bearing is assembled in such a manner that the cage having the rollers received in the pockets is interposed between the pair of races, the race-contacting projection portions formed on at least one of the two projecting portions on at least one of the first side and second side of the cage abut against an inner surface (raceway surface) of the corresponding race, and peripheral surfaces of the rollers received in the respective pockets of the cage contact the inner surfaces of the pair of races. Thus, the inner surface of the race does not fully contact the projecting portion of the cage having the race-contacting projection portions, and instead only the plurality of race-contacting projection portions contact the race. Therefore, a shearing resistance due to the viscosity of oil between the cage and the race is reduced, so that a rotation torque can be reduced. 
         [0028]    Furthermore, the race-contacting projection portions are the convex portions formed by punching part of the projecting portion or the protruding portions formed by notching part of the end portion of the projecting portion, and therefore the race-contacting projection portions can be easily formed on the race-contacting surface of the projecting portion of the cage. 
         [0029]    Furthermore, one of two or more projecting portions of the bent portion on one of the first side and the second side of the cage is projected in an amount larger than the amount of projecting of the other projecting portion in stepped relation thereto to provide the race-contacting projection portion. Therefore, the race-contacting projection portion can be easily formed by providing the two projecting portions of different projecting amounts. 
         [0030]    Furthermore, in the thrust roller bearing of the invention, the plurality of cage-contacting projection portions are formed on a surface of at least one of the pair of races, and are arranged at predetermined intervals in the circumferential direction. Therefore, when the thrust roller bearing is assembled in such a manner that the cage having the rollers received in the respective pockets is interposed between the pair of races, the plurality of cage-contacting projection portions (formed on at least one of the pair of races) abut against a race-contacting surface of the corresponding projecting portion of the cage, and peripheral surfaces of the rollers received in the respective pockets of the cage contact inner surfaces of the pair of races. Thus, the inner surface of the race having the cage-contacting projection portions does not fully contact the corresponding projecting portion of the cage, and instead only the plurality of cage-contacting projection portions contact the race-contacting surface of the projecting portion of the cage. Therefore, a shearing resistance due to the viscosity of oil between the cage and the race is reduced, so that a rotation torque can be reduced. 
         [0031]    Furthermore, the cage-contacting projection portions are convex portions, and therefore can be easily formed on the cage-contacting surface of the race. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a vertical cross-sectional view showing a first embodiment of a thrust roller bearing of the present invention. 
           [0033]      FIG. 2  is a plan view of a cage of the thrust roller bearing of  FIG. 1 . 
           [0034]      FIG. 3  is a vertical cross-sectional view showing a second embodiment of a thrust roller bearing of the invention. 
           [0035]      FIG. 4  is a plan view of a cage of the thrust roller bearing of  FIG. 3 . 
           [0036]      FIG. 5  is a vertical cross-sectional view showing an important portion of a third embodiment of a thrust roller bearing of the invention. 
           [0037]      FIG. 6  is a vertical cross-sectional view showing an important portion of a fourth embodiment of a thrust roller bearing of the invention. 
           [0038]      FIG. 7  is a vertical cross-sectional view showing an important portion of a fifth embodiment of a thrust roller bearing of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0039]      FIG. 1  is a vertical cross-sectional view showing a first embodiment of a thrust roller bearing of the present invention, and  FIG. 2  is a plan view of a cage of this thrust roller bearing. 
         [0040]    As shown in  FIG. 1 , the thrust roller bearing  1  of this first embodiment comprises a pair of first and second races  2  and  3  each having an annular shape, a plurality of needle rollers  4  disposed between the two races  2  and  3 , and the cage  5  holding the needle rollers  4 . 
         [0041]    The first race  2  includes a body portion  2   a  formed by an annular thin plate, and an outer cylindrical portion  2   b  formed at an outer periphery of the body portion  2   a  and bent to extend toward the second race  3 . The first race  2  is formed into an integral construction (that is, all of the portions of the first race  2  are formed integrally with each other), for example, by pressing a steel plate into a predetermined shape. The first race  2  is fitted in a counterbore portion  7   a  formed in a housing  7  which is a stationary member. 
         [0042]    The second race  3  is formed generally in symmetrical relation to the first race  2 , with the needle rollers  4  interposed therebetween. The second race  3  includes a body portion  3   a  formed by an annular thin plate, and an inner cylindrical portion  3   b  formed at an inner periphery of the body portion  3   a  and bent to extend toward the first race  2 . The second race  3  is formed into an integral construction (that is, all of the portions of the second race  3  are formed integrally with each other), for example, by pressing a steel plate into a predetermined shape. A shaft convex portion  9   a  formed at an end of a shaft member  9  (which is to be borne by this thrust bearing) is fitted in a bore of the inner cylindrical portion  3   b  of the second race  3 . 
         [0043]    The cage  5  is formed from a single annular plate, and a bent portion  11  of a generally M-shape is formed at a radially-intermediate region thereof by drawing, the generally M-shaped bent portion  11  being bent in an axial direction. Thus, the cage  5  has the generally M-shaped bent portion  11 , and with this construction the rigidity of the cage  5  is increased, and a large load capacity is secured while achieving a compact design of the thrust bearing. 
         [0044]    The generally M-shaped bent portion  11  includes a U-shaped radially-outer projecting portion  11   a  and a U-shaped radially-inner projecting portion  11   b  which are formed respectively at radially-outward and radially-inward portions thereof and project a generally equal distance toward the second race  3  in a direction of the thickness of the cage  5 , and further includes a U-shaped intermediate projecting portion  11   c  disposed between the two projecting portions  11   a  and  11   b  and projecting toward the first race  2  in the direction of the thickness of the cage  5 , a first slanting portion  11   d  interconnecting the radially-outer projecting portion  11   a  and the intermediate projecting portion  11   c,  a second slanting portion  11   d  interconnecting the radially-inner projecting portion  11   b  and the intermediate projecting portion  11   c,  an outer peripheral horizontally-projecting portion  11   e  extending horizontally from an outer edge of the radially-outer projecting portion  11   a,  and an inner peripheral horizontally-projecting portion  11   f  extending horizontally from an inner edge of the radially-inner projecting portion  11   b.    
         [0045]    Further, three axially-projecting convex portions (projections)  13  are formed on a race-contacting surface of the radially-outer projecting portion  11   a  of the generally M-shaped bent portion  11  of the cage  5 , and are arranged at predetermined intervals in the circumferential direction. Also, three axially-projecting convex portions  13  are formed on a race-contacting surface of the outer peripheral horizontally-projecting portion  11   e  of the generally M-shaped bent portion  11 , and are arranged at predetermined intervals in the circumferential direction. These convex portions  13  can be easily formed by punching in the pressing process. 
         [0046]    In the cage  5 , a plurality of pockets  12  are formed in that portion of the generally M-shaped bent portion  11  lying between a region near to the outer edge of the radially-outer projecting portion  11   a  and a region near to the inner edge of the radially-inner projecting portion  11   b,  and are arranged at predetermined intervals in the circumferential direction. The pockets  12  are formed by stamping (blanked) out the relevant portions of the generally M-shaped bent portion  11  in the direction of the thickness of the cage  5 , and each pocket  12  has a generally rectangular shape when viewed from the top. The needle rollers  4  are rotatably received respectively in the pockets  12  in such a manner as not to be disengaged therefrom. 
         [0047]    When the needle rollers  4  are received respectively in the pockets  12  of the cage  5  of the above construction, and then the thrust roller bearing  1  is assembled in such a manner that the cage  5  is interposed between the first race  2  and the second race  3 , an end portion of the outer cylindrical portion  2   b  of the first race  2  is disposed out of contact with the body portion  3   a  of the second race  3 , and also an end portion of the inner cylindrical portion  3   b  of the second race  3  is disposed out of contact with the body portion  2   a  of the first race  2 . 
         [0048]    The needle rollers  4  received respectively in the pockets  12  of the cage  5  are arranged in such a manner that axes of these needle rollers  4  are disposed radially with respect to the axis of the shaft member  9 , and peripheral surfaces of the needle rollers  4  are in contact with inner surfaces (serving as raceway surfaces) of the body portions  2   a  and  3   a  of the first and second races  2  and  3 . 
         [0049]    Therefore, the needle rollers  4  can support an axial load of the shaft portion  9  via the body portions  2   a  and  3   a  of the first and second races  2  and  3 . In this condition, when the first and second races  2  and  3  rotate relative to each other, the needle rollers  4  rotate, so that a frictional resistance to the shaft member  9  can be reduced. 
         [0050]    The three convex portions  13  formed on the race-contacting surface of the radially-outer projecting portion  11   a  of the cage  5  and arranged at the predetermined intervals in the circumferential direction abut against the inner surface (raceway surface) of the body portion  3   a  of the second race  3 . Also, the three convex portions  13  formed on the race-contacting surface of the outer peripheral horizontally-projecting portion  11   e  of the cage  5  and arranged at the predetermined intervals in the circumferential direction abut against the inner surface (raceway surface) of the body portion  2   a  of the first race  2 . The intermediate projecting portion  11   c  of the cage  5  is disposed near to the inner surface (raceway surface) of the body portion  2   a  of the first race  2 , and the radially-inner projecting portion  11   b  of the cage  5  is disposed near to the inner surface (raceway surface) of the body portion  3   a  of the second race  3 . 
         [0051]    As described above, in the thrust roller bearing  1  of this first embodiment, the three axially-projecting convex portions  13  are formed on the race-contacting surface of each of the radially-outer projecting portion  11   a  and the outer peripheral horizontally-projecting portion  11   e  of the generally M-shaped bent portion  11  of the cage  5 , and are arranged at the predetermined intervals in the circumferential direction. Therefore, when the thrust roller bearing is assembled in such a manner that the cage  5  having the needle rollers  4  received in the respective pockets  12  is interposed between the first and second races  2  and  3 , the convex portions  13  formed on the radially-outer projecting portion  11   a  of the cage  5  abut against the inner surface (raceway surface) of the body portion  3   a  of the second race  3 , while the convex portions  13  formed on the outer peripheral horizontally-projecting portion  11   e  of the cage  5  abut against the inner surface (raceway surface) of the body portion  2   a  of the first race  2 , and the peripheral surfaces of the needle rollers  4  received in the respective pockets  12  of the cage  5  contact the inner surface of the body portion  2   a  of the first race  2  and the inner surface of the body portion  3   a  of the second race  3 . Thus, the inner surfaces of the body portions  2   a  and  3   a  of the first and second races  2  and  3  do not fully contact the outer peripheral horizontally-projecting portion  11   e  and the radially-outer projecting portion  11   a  of the cage  5 , respectively, and instead the three convex portions  13  on the radially-outer projecting portion  11   a  are merely disposed in point-contact with the second race  3 , while the three convex portions  13  on the outer peripheral horizontally-projecting portion  11   e  are merely disposed in point-contact with the first race  2 . Therefore, a shearing resistance due to the viscosity of oil between the cage  5  and the first and second races  2  and  3  is reduced, so that the rotation torque can be reduced. 
         [0052]    In the above first embodiment, although the three convex portions  13  are formed on the race-contacting surface of each of the radially-outer projecting portion  11   a  and the outer peripheral horizontally-projecting portion  11   e  of the cage  5  from the standpoint of stability, a desired number of convex portions  13  can be formed in so far as this number is not smaller than three. Furthermore, although the three convex portions  13  are formed on the radially-outer projecting portion  11   a  of the cage  5 , these three convex portions  13  may be formed on the radially-inner projecting portion  11   b.    
       Second Embodiment 
       [0053]      FIG. 3  is a vertical cross-sectional view showing a second embodiment of a thrust roller bearing of the invention, and  FIG. 4  is a plan view of a cage of this thrust roller bearing. 
         [0054]    In this second embodiment, those constituent portions similar to those of the first embodiment will be designated by identical reference numerals, respectively, and explanation thereof will be omitted, and only different constituent portions will be described. 
         [0055]    In the first embodiment, the convex portions  13  are formed on the radially-outer projecting portion  11   a  and the outer peripheral horizontally-projecting portion  11   e  of the generally M-shaped bent portion  11  of the cage  5 . In the second embodiment, however, convex portions  23  are formed on a radially-inner projecting portion  21   b  and an inner peripheral horizontally-projecting portion  21   f  of a generally inverted M-shaped bent portion  21  of the cage  5 . 
         [0056]    The cage  5  of this second embodiment has the generally inverted M-shaped bent portion  21  bent in an axial direction. The generally inverted M-shaped bent portion  21  includes a U-shaped radially-outer projecting portion  21   a  and the U-shaped radially-inner projecting portion  21   b  which are formed respectively at radially-outward and radially-inward portions thereof and project a generally equal distance toward a first race  2  in a direction of the thickness of the cage  5 , and further includes a U-shaped intermediate projecting portion  21   c  disposed between the two projecting portions  21   a  and  21   b  and projecting toward a second race  3  in the direction of the thickness of the cage  5 , a first slanting portion  21   d  interconnecting the radially-outer projecting portion  21   a  and the intermediate projecting portion  21   c,  a second slanting portion  21   d  interconnecting the radially-inner projecting portion  21   b  and the intermediate projecting portion  21   c,  an outer peripheral horizontally-projecting portion  21   e  extending horizontally from an outer edge of the radially-outer projecting portion  21   a,  and the inner peripheral horizontally-projecting portion  21   f  extending horizontally from an inner edge of the radially-inner projecting portion  21   b.    
         [0057]    Further, the three axially-projecting convex portions (projections)  23  are formed on a race-contacting surface of the radially-inner projecting portion  21   b  of the generally inverted M-shaped bent portion  21  of the cage  5 , and are arranged at predetermined intervals in the circumferential direction. Also, the three axially-projecting convex portions  23  are formed on a race-contacting surface of the inner peripheral horizontally-projecting portion  21   f  of the generally inverted M-shaped bent portion  21 , and are arranged at predetermined intervals in the circumferential direction. These convex portions  13  can be easily formed by punching in a pressing process. 
         [0058]    When needle rollers  4  are received respectively in pockets  12  of the cage  5  of the above construction, and then the thrust roller bearing  1  is assembled in such a manner that the cage  5  is interposed between the first race  2  and the second race  3 , an end portion of an outer cylindrical portion  2   b  of the first race  2  is disposed out of contact with a body portion  3   a  of the second race  3 , and also an end portion of an inner cylindrical portion  3   b  of the second race  3  is disposed out of contact with a body portion  2   a  of the first race  2 . 
         [0059]    The needle rollers  4  received respectively in the pockets  12  of the cage  5  are arranged in such a manner that axes of these needle rollers  4  are disposed radially with respect to an axis of a shaft member  9 , and peripheral surfaces of needle rollers  4  are in contact with inner surfaces (serving as raceway surfaces) of the body portions  2   a  and  3   a  of the first and second races  2  and  3 . 
         [0060]    Therefore, the needle rollers  4  can support an axial load of the shaft portion via the body portions  2   a  and  3   a  of the first and second races  2  and  3 . In this condition, when the first and second races  2  and  3  rotate relative to each other, the needle rollers  4  rotate, so that a frictional resistance to the shaft member  9  can be reduced. 
         [0061]    The three convex portions  23  formed on the race-contacting surface of the radially-inner projecting portion  21   b  of the cage  5  and arranged at the predetermined intervals in the circumferential direction abut against the inner surface (raceway surface) of the body portion  2   a  of the first race  2 . Also, the three convex portions  3  formed on the race-contacting surface of the inner peripheral horizontally-projecting portion  21   f  of the cage  5  and arranged at the predetermined intervals in the circumferential direction abut against the inner surface (raceway surface) of the body portion  3   a  of the second race  3 . The intermediate projecting portion  21   c  of the cage  5  is disposed near to the inner surface (raceway surface) of the body portion  3   a  of the second race  3 , and the radially-outer projecting portion  21   a  of the cage  5  is disposed near to the inner surface (raceway surface) of the body portion  2   a  of the first race  2 . 
         [0062]    As described above, in the thrust roller bearing  1  of this second embodiment, the three axially-projecting convex portions  23  are formed on the race-contacting surface of each of the radially-inner projecting portion  21   b  and the inner peripheral horizontally-projecting portion  21   f  of the generally inverted M-shaped bent portion  21  of the cage  5 , and are arranged at the predetermined intervals in the circumferential direction. Therefore, when the thrust roller bearing is assembled in such a manner that the cage  5  having the needle rollers  4  received in the respective pockets  12  is interposed between the first and second races  2  and  3 , the convex portions  23  formed on the race-contacting surface of the radially-inner projecting portion  21   b  of the cage  5  abut against the inner surface (raceway surface) of the body portion  2   a  of the first race  2 , while the convex portions  23  formed on the race-contacting surface of the inner peripheral horizontally-projecting portion  21   f  of the cage  5  abut against the inner surface (raceway surface) of the body portion  3   a  of the second race  3 , and the peripheral surfaces of the needle rollers  4  received in the respective pockets  12  of the cage  5  contact the inner surface of the body portion  2   a  of the first race  2  and the inner surface of the body portion  3   a  of the second race  3 . Thus, the radially-inner projecting portion  21   b  of the cage  5  does not fully contact the body portion  2   a  of the first race  2 , and also the inner peripheral horizontally-projecting portion  21   f  does not fully contact the body portion  3   a  of the second race  3 , and instead the three convex portions  23  on the radially-inner projecting portion  21   b  are merely disposed in point-contact with the first race  2 , while the three convex portions  23  on the inner peripheral horizontally-projecting portion  21   f  are merely disposed in point-contact with the second race  3 . Therefore, a shearing resistance due to the viscosity of oil between the cage  5  and the first and second races  2  and  3  is reduced, so that a rotation torque can be reduced. 
         [0063]    In the above second embodiment, although the three convex portions  23  are formed on the race-contacting surface of each of the radially-inner projecting portion  21   b  and the inner peripheral horizontally-projecting portion  21   f  of the cage  5  from the standpoint of stability, a desired number of convex portions  23  can be formed in so far as this number is not smaller than three. 
         [0064]    Furthermore, although the three convex portions  23  are formed on the radially-inner projecting portion  21   b  of the cage  5 , these three convex portions  23  may be formed on the radially-outer projecting portion  21   a.  Furthermore, although the three convex portions  23  are formed on the inner peripheral horizontally-projecting portion  21   f  of the cage  5 , these three convex portions  23  may be formed on the intermediate projecting portion  21   c.    
       Third Embodiment 
       [0065]      FIG. 5  is a vertical cross-sectional view showing an important portion of a third embodiment of a thrust roller bearing of the invention. 
         [0066]    In this third embodiment, those constituent portions similar to those of the first embodiment will be designated by identical reference numerals, respectively, and explanation thereof will be omitted, and only different constituent portions will be described. 
         [0067]    In the first embodiment, the convex portions  13  are formed on the radially-outer projecting portion  11   a  and the outer peripheral horizontally-projecting portion  11   e  of the generally M-shaped bent portion  11  of the cage  5 . In the third embodiment, however, convex portions  33  are formed on an intermediate projecting portion  31   c  of a generally inverted M-shaped bent portion  31  of a cage  5 , and protruding portions  34  are formed on an outer peripheral L-shaped projecting portion  31   e  of the bent portion  31 . 
         [0068]    The cage  5  of this third embodiment has the generally inverted M-shaped bent portion  31  bent in an axial direction. The generally inverted M-shaped bent portion  31  includes a U-shaped radially-outer projecting portion  31   a  and a U-shaped radially-inner projecting portion  31   b  which are formed respectively at radially-outward and radially-inward portions thereof and project a generally equal distance toward a first race  2  in a direction of the thickness of the cage  5 , and further includes the U-shaped intermediate projecting portion  31   c  disposed between the two projecting portions  31   a  and  31   b  and projecting toward a second race  3  in the direction of the thickness of the cage  5 , a first slanting portion  31   d  interconnecting the radially-outer projecting portion  31   a  and the intermediate projecting portion  31   c,  a second slanting portion  31   d  interconnecting the radially-inner projecting portion  31   b  and the intermediate projecting portion  31   c,  the outer peripheral L-shaped projecting portion  31   e  which first extends horizontally from an outer edge of the radially-outer projecting portion  31   a  and then is bent at right angles to extend toward the first race  2 , and an inner peripheral L-shaped projecting portion  31   f  which first extend horizontally from an inner edge of the radially-inner projecting portion  31   b  and then is bent at right angles to extend toward the first race  2 . 
         [0069]    Further, the three axially-projecting convex portions (projections)  33  are formed on a race-contacting surface of the intermediate projecting portion  31   c  of the generally inverted M-shaped bent portion  31  of the cage  5 , and are arranged at predetermined intervals in the circumferential direction. Also, the three axially-protruding portions  34  are formed on a race-contacting end (or edge) of the outer peripheral L-shaped projecting portion  31   e  of the generally inverted M-shaped bent portion  31 , and are arranged at predetermined intervals in the circumferential direction. These protruding portions  34  are formed by notching the race-contacting end portion of the outer peripheral L-shaped projecting portion  31   e  at predetermined intervals in the circumferential direction, and those portions of the race-contacting end portion which are not notched serve as the protruding portions  34 , respectively. 
         [0070]    The three convex portions  33  formed on the race-contacting surface of the intermediate projecting portion  31   c  of the cage  5  and arranged at the predetermined intervals in the circumferential direction abut against an inner surface (raceway surface) of a body portion  3   a  of the second race  3 . Also, the three protruding portions  34  formed on the race-contacting end of the outer peripheral L-shaped projecting portion  31   e  of the cage  5  and arranged at the predetermined intervals in the circumferential direction abut against an inner surface (raceway surface) of a body portion  2   a  of the first race  2 . The radially-outer projecting portion  31   a  and the radially-inner projecting portion  31   b  of the cage  5  are disposed near to the inner surface (raceway surface) of the body portion  2   a  of the first race  2 . 
         [0071]    As described above, in the thrust roller bearing  1  of this third embodiment, the three convex portions  33  are formed on the race-contacting surface of the intermediate projecting portion  31   c  of the generally inverted M-shaped bent portion  31  of the cage  5 , and are arranged at the predetermined intervals in the circumferential direction. Also, the three protruding portions  34  are formed on the race-contacting end of the outer peripheral L-shaped projecting portion  31   e,  and are arranged at the predetermined intervals in the circumferential direction. Therefore, when the thrust roller bearing is assembled in such a manner that the cage  5  having needle rollers  4  received in respective pockets  12  is interposed between the first and second races  2  and  3 , the three convex portions  33  formed on the race-contacting surface of the intermediate projecting portion  31   c  of the cage  5  abut against the inner surface (raceway surface) of the body portion  3   a  of the second race  3 , while the three protruding portions  34  formed on the race-contacting end of the outer peripheral L-shaped projecting portion  31   e  of the cage  5  abut against the inner surface (raceway surface) of the body portion  2   a  of the first race  2 , and the peripheral surfaces of the needle rollers  4  received in the respective pockets  12  of the cage  5  contact the inner surface of the body portion  2   a  of the first race  2  and the inner surface of the body portion  3   a  of the second race  3 . Thus, the intermediate projecting portion  31   c  and the outer peripheral L-shaped projecting portion  31   e  of the cage  5  do not fully contact the body portions  3   a  and  2   a  of the second and first races  3  and  2 , respectively, and instead the three convex portions  33  are merely disposed in point-contact with the second race  3 , while the three protruding portions  34  are merely disposed in point-contact with the first race  2 . Therefore, a shearing resistance due to the viscosity of oil between the cage  5  and the first and second races  2  and  3  is reduced, so that a rotation torque can be reduced. 
         [0072]    In the above third embodiment, from the standpoint of stability, the three convex portions  33  are formed on the race-contacting surface of the intermediate projecting portion  31   c  of the cage  5 , while the three protruding portions  34  are formed on the race-contacting end of the outer peripheral L-shaped projecting portion  31   e.  However, a desired number of convex portions  33  as well as a desired number of protruding portions  34  as can be formed in so far as this number is not smaller than three. 
       Fourth Embodiment 
       [0073]      FIG. 6  is a vertical cross-sectional view showing an important portion of a fourth embodiment of a thrust roller bearing of the invention. 
         [0074]    In this fourth embodiment, those constituent portions similar to those of the third embodiment will be designated by identical reference numerals, respectively, and explanation thereof will be omitted, and only different constituent portions will be described. 
         [0075]    In the third embodiment, the convex portions  33  are formed on the intermediate projecting portion  31   c  of the generally inverted M-shaped bent portion  31  of the cage  5 , and the protruding portions  34  are formed on the outer peripheral L-shaped projecting portion  31   e.  In the fourth embodiment, however, protruding portions  34  are formed on an outer peripheral L-shaped projecting portion  31   e  and an inner peripheral L-shaped projecting portion  31   g  of a generally inverted M-shaped bent portion  31  of a cage  5 . 
         [0076]    The cage  5  of this fourth embodiment has the generally inverted M-shaped bent portion  31  bent in an axial direction. The outer peripheral L-shaped projecting portion  31   e  of the generally M-shaped bent portion  31  first extends horizontally from an outer edge of a radially-outer projecting portion  31   a  and then is bent at right angles to extend toward a first race  2 , while the inner peripheral L-shaped projecting portion  31   g  first extends horizontally from an inner edge of a radially-inner projecting portion  31   b  and then is bent at right angles to extend toward a second race  3 . 
         [0077]    The three axially-protruding portions  34  are formed on a race-contacting end (or edge) of the outer peripheral L-shaped projecting portion  31   e,  and are arranged at predetermined intervals in the circumferential direction. Similarly, the three axially-protruding portions  34  are formed on a race-contacting end (or edge) of the inner peripheral L-shaped projecting portion  31   g,  and are arranged at predetermined intervals in the circumferential direction. The protruding portions  34  are formed by notching the race-contacting end portion of each of the outer and inner peripheral L-shaped projecting portions  31   e  and  31   g  at predetermined intervals in the circumferential direction, and those portions of the race-contacting end portion which are not notched serve as the protruding portions  34 , respectively. 
         [0078]    In this fourth embodiment, the three protruding portions  34  formed on the race-contacting end of the outer peripheral L-shaped projecting portion  31   e  of the cage  5  and arranged at the predetermined intervals in the circumferential direction abut against an inner surface (raceway surface) of a body portion  2   a  of the second race  2 . Also, the three protruding portions  34  formed on the race-contacting end of the inner peripheral L-shaped projecting portion  31   g  of the cage  5  and arranged at the predetermined intervals in the circumferential direction abut against an inner surface (raceway surface) of a body portion  3   a  of the second race  3 . An intermediate projecting portion  31   c  of the cage  5  is disposed near to the inner surface (raceway surface) of the body portion  3   a  of the second race  3 , and the radially-outer projecting portion  31   a  and the radially-inner projecting portion  31   b  of the cage  5  are disposed near to the inner surface (raceway surface) of the body portion  2   a  of the first race  2 . 
         [0079]    As described above, in the thrust roller bearing  1  of this fourth embodiment, the three protruding portions  34  are formed on the race-contacting end of each of the outer peripheral L-shaped projecting portion  31   e  and the inner peripheral L-shaped projecting portion  31   g  of the generally inverted M-shaped bent portion  31  of the cage  5 , and are arranged at the predetermined intervals in the circumferential direction. Therefore, when the thrust roller bearing is assembled in such a manner that the cage  5  having needle rollers  4  received in respective pockets  12  is interposed between the first and second races  2  and  3 , the three protruding portions  34  formed on the outer peripheral L-shaped projecting portion  31   e  of the cage  5  abut against the inner surface (raceway surface) of the body portion  2   a  of the first race  2 , while the three protruding portions  34  formed on the inner peripheral L-shaped projecting portion  31   g  of the cage  5  abut against the inner surface (raceway surface) of the body portion  3   a  of the second race  3 , and the peripheral surfaces of the needle rollers  4  received in the respective pockets  12  of the cage  5  contact the inner surface of the body portion  2   a  of the first race  2  and the inner surface of the body portion  3   a  of the second race  3 . Thus, the outer peripheral L-shaped projecting portion  31   e  and the inner peripheral L-shaped projecting portion  31   g  of the cage  5  do not fully contact the inner surfaces of the body portions  2   a  and  3   a  of the first and second races  2  and  3 , respectively, and instead the three protruding portions  34  on the outer peripheral L-shaped projecting portion  31   e  are merely disposed in point-contact with the first race  2 , while the three protruding portions  34  on the inner peripheral L-shaped projecting portion  31   g  are merely disposed in point-contact with the second race  3 . Therefore, a shearing resistance due to the viscosity of oil between the cage  5  and the first and second races  2  and  3  is reduced, so that a rotation torque can be reduced. 
         [0080]    In the above fourth embodiment, although the three protruding portions  34  are formed on the race-contacting end of each of the outer and inner peripheral L-shaped projecting portions  31   e  and  31   g  from the standpoint of stability, a desired number of protruding portions  34  can be formed in so far as this number is not smaller than three. 
       Fifth Embodiment 
       [0081]      FIG. 7  is a vertical cross-sectional view showing an important portion of a fifth embodiment of a thrust roller bearing of the invention. 
         [0082]    In this fifth embodiment, those constituent portions similar to those of the third embodiment will be designated by identical reference numerals, respectively, and explanation thereof will be omitted, and only different constituent portions will be described. 
         [0083]    In the third embodiment, the convex portions  33  are formed on the intermediate projecting portion  31   c  of the generally inverted M-shaped bent portion  31  of the cage  5 , and the protruding portions  34  are formed on the outer peripheral L-shaped projecting portion  31   e.  In the fifth embodiment, however, an outer peripheral horizontally-projecting portion  31   h  and an inner peripheral inverted U-shaped projecting portion  31   i  of a generally inverted M-shaped bent portion  31  are projected in an amount larger than the amount of projecting of an intermediate projecting portion  31   c,  and therefore are disposed in stepped relation thereto, and three convex portions  33  are formed on a race-contacting surface of a radially-outer projecting portion  31   a,  and are arranged at predetermined intervals in a circumferential direction. 
         [0084]    In this fifth embodiment, the outer peripheral horizontally-projecting portion  31   h  and the inner peripheral inverted U-shaped portion  31   i  of the cage  5  abut against an inner surface (raceway surface) of a body portion  3   a  of a second race  3 , and the three convex portions  33  formed on the race-contacting surface of the radially-outer projecting portion  31   a  abut against an inner surface (raceway surface) of a body portion  2   a  of a first race  2 . The intermediate projecting portion  31   c  of the cage  5  is disposed near to the inner surface (raceway surface) of the body portion  3   a  of the second race  3 , and a radially-inner projecting portion  31   b  of the cage  5  is disposed near to the inner surface (raceway surface) of the body portion  2   a  of the first race  2 . 
         [0085]    As described above, in the thrust roller bearing  1  of this fifth embodiment, the outer peripheral horizontally-projecting portion  31   h  and the inner peripheral inverted U-shaped projecting portion  31   i  of the generally inverted M-shaped bent portion  31  of the cage  5  are projected in an amount larger than the amount of projecting of the intermediate projecting portion  31   c,  and therefore are disposed in stepped relation thereto, and the three convex portions  33  are formed on the race-contacting surface of the radially-outer projecting portion  31   a,  and are arranged at the predetermined intervals in the circumferential direction. Therefore, when the thrust roller bearing is assembled in such a manner that the cage  5  having needle rollers  4  received in respective pockets  12  is interposed between the first and second races  2  and  3 , the outer peripheral horizontally-projecting portion  31   h  and the inner peripheral inverted U-shaped portion  31   i  of the cage  5  abut against the inner surface (raceway surface) of the body portion  3   a  of the second race  3 , and the three convex portions  33  formed on the race-contacting surface of the radially-outer projecting portion  31   a  abut against the inner surface (raceway surface) of the body portion  2   a  of the first race  2 , and the peripheral surfaces of the needle rollers  4  received in the respective pockets  12  of the cage  5  contact the inner surface of the body portion  2   a  of the first race  2  and the inner surface of the body portion  3   a  of the second race  3 . Thus, only the race-contacting surfaces of the outer peripheral horizontally-projecting portion  31   h  and inner peripheral inverted U-shaped projecting portion  31   i  of the cage  5  contact the second race  3 , and the radially-outer projecting portion  31   a  of the cage  5  does not fully contact the body portion  2   a  of the first race  2 . Therefore, a shearing resistance due to the viscosity of oil between the cage  5  and the first and second races  2  and  3  is reduced, so that a rotation torque can be reduced. 
         [0086]    In the above first to fourth embodiments, although the convex portions  13 ,  23 ,  33  or the protruding portions  34  are formed on both of the oppositely-projecting portions of the cage  5 , the convex portions  13 ,  23 ,  33  or the protruding portions  34  may be formed on one of the oppositely-projecting portions. Furthermore, although the convex portions  13 ,  23 ,  33  or the protruding portions  34  are formed only on the cage  5 , such convex portions or protruding portions may be formed on one or both of the first and second races  2  and  3 , in which case the convex portions or the protruding portions are formed on those regions other than roller-rolling surfaces.