Patent Publication Number: US-2007116393-A1

Title: Needle roller bearing, crank shaft supporting structure, and split method of outer ring of needle roller bearing

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a crank shaft supporting structure used in an engine of a car and the like.  
      2. Description of the Background Art  
      As shown in  FIG. 25 , a crank shaft  101  comprises a shaft  102 , a crank arm  103 , a crank pin  104  for arranging a con-rod between the adjacent crank arms  103 . As shown in  FIGS. 26 and 27 , the shaft  102  is rotatably supported by a sliding bearing  105 . Since the sliding bearing  105  has high load capacity, it is suitable for use under a high load environment. Furthermore, a cylinder block  107  (referred to as an “engine block” also hereinafter) and a bearing cap  108  are mounted on the outside the sliding bearing  105 .  
      Referring to  FIG. 27 , since the sliding bearing  105  cannot support an axial load, a thrust washer  106  is arranged between the crank arm  103 , and the engine block  107  and the bearing cap  108  in order to prevent the crank shaft  101  from moving in the axial direction. In addition, at least one thrust washer  106  may be arranged when the plurality of shafts  102  are provided.  
      In addition, in accordance with the increasing demand for a car that is low in fuel cost and has low noise level and less oscillation in view of the environment in recent years, instead of the sliding bearing  105  to support the shaft  102 , it is proposed to use a needle roller bearing  111  comprising an outer ring  112 , needle rollers  113  arranged along the inner diameter surface of the outer ring  112  and a retainer  114  retaining the interval of the adjacent needle rollers  113  as shown in  FIGS. 28 and 29 .  
      According to the needle roller bearing  111 , since the needle roller  113  and the track surface are linearly in contact with each other, there is an advantage that high load capacity and high rigidity can be provided for a small bearing projected area, so that it is widely used in various kinds of fields such as a car or a two-wheel vehicle engine. In addition, although the needle roller bearing  111  is low in load capacity as compared with the sliding bearing  105 , since friction resistance at the time of rotation is small, rotation torque and a fueling amount to the support part can be reduced.  
      However, as shown in  FIG. 26 , since the crank arms  103  are provided at both ends of the shaft  102 , the needle roller bearing  111  cannot be pressed in the axial direction. Thus, a bearing that can be used in such case is disclosed in U.S. Pat. No. 1,921,488, for example.  
      According to the needle roller bearing disclosed in the U.S. Pat. No. 1,921,488, it comprises an outer ring  112  split into outer ring members  112   a  and  112   b  by split lines  112   c  extending in the axial direction of the bearing as shown in  FIG. 30  and a retainer  114  comprising two semicircle-shaped split retainers as shown in  FIGS. 32A and 32B . Alternatively, the needle roller bearing may comprise an outer ring  112  split into outer ring members  112   a  and  112   b  by split lines inclined at a predetermined angle in the axial direction.  
      According to the needle roller bearing disclosed in the U.S. Pat. No. 1,921,488, when it is incorporated in the shaft  102  sandwiched by the crank arms  103  of the crank shaft  101 , the retainer  114  housing the needle rollers and the outer ring members  112   a  and  112   b  can be incorporated in the diameter direction, respectively.  
      At this time, since both outer rings  112  and retainers  114  are split into two parts, it is necessary to provide means for preventing the retainer  114  from falling off when the outer ring  112  is incorporated. This complicates the incorporating operation procedures and needs a special member for preventing the retainer  114  from falling off in some cases, which increases the number of operation steps and an operation cost.  
      In addition, as shown in  FIG. 33A , the outer ring  112  ideally has a perfect cylindrical shape. However, in practice, the outer ring members  112   a  and  112   b  are shifted in the diameter direction and the split parts are shifted to generate a step part as shown in  FIG. 33B . Furthermore, this step part becomes large as incorporating precision gets worse.  
      In this case, when the needle roller  113  passes through the step part, an abnormal sound is generated. The abnormal sound becomes loud as the step part becomes large and as the bearing rotation is speeded up, which becomes a big problem for the bearing that supports the shaft rotating at high speed such as the crank shaft  101 .  
      In addition, according to the retainer  114  shown in  FIGS. 32A and 32B , the split parts of the retainer  114  could be shifted in the axial direction at the time of the bearing rotation. Thus, an eccentric load is applied to the outer ring  112  and the crank shaft  101 , and a trouble such as peeling or flaking could be generated at an early stage.  
      In addition, the needle roller bearing  111  having the above constitution is elastically deformed by the load applied from the crank shaft  101  at the time of rotation. At this time, in the case of the retainer  114 , for example the split part could be largely deformed and corresponding end surfaces come in contact with each other to generate a metallic sound.  
      Furthermore, when the metals come in contact to each other, the contact part could be abraded and a lubricant agent could deteriorate because abrasion powder is mixed in. Since this becomes conspicuous as the rotation of the bearing is speeded up, the above is a serious problem for the needle roller bearing  111  supporting the crank shaft  101 .  
      Thus, when the needle roller bearing  111  is used to support the crank shaft  101 , as shown in  FIG. 34 , recessed parts are provided in both of the outer ring  112  and the engine block  107  and fixed by a fixing pin  115 . Since the needle roller bearing  111  can support the axial load at a flange  112   d , it is not necessary to provide a thrust washer and the like.  
      However, according to the needle roller bearing  111 , the outer ring  112  having the flange  112   d  is highly rigid and great force is required to split the outer ring  112  into the two outer ring members  112   a  and  112   b . Furthermore, when great force is applied to the outer ring  112  to split it, the outer ring  112  could be deformed.  
      Meanwhile, as shown in  FIG. 35 , a needle roller bearing  116  having the same constitution as that of the needle roller bearing  111  basically but having no flange at both ends of an outer ring  117  may be used. However, in this case, since there is no means for preventing the retainer  119  from moving in the axial direction, the needle roller  118  could fall off the track surface of the outer ring  117 .  
      A method of splitting the outer ring  112  is disclosed in Japanese Unexamined Patent Publication No. 7-317778, for example. According to the Japanese Unexamined Patent Publication No. 7-317778, as shown in  FIG. 36A , V-shaped grooves  112   e  each having a V-shaped sectional configuration are formed on both end surfaces of the outer ring  112  and as shown in  FIG. 36B , the outer ring  112  is split into two outer ring members  112   a  and  112   b  when pressure is applied to the parts in which the V-shaped grooves  112   e  are formed from both sides in the diameter direction.  
      When the outer ring  112  is split by the above method, the vicinity of the split part is largely deformed inward in the diameter direction as shown in  FIG. 37A . In addition, when the outer ring  112  is incorporated in the cylinder block  107  and the bearing cap  108 , the diameter in the vicinity of the split part becomes smaller than a designed value and the diameter in the center becomes larger than the designed value as shown in  FIG. 37B .  
      In this case, since the space formed between the shaft  102  and the inner diameter surface of the outer ring  112  in which the needle rollers  113  roll (referred to as the “rolling space” hereinafter) is varied in the circumferential direction, the rolling of the needle roller  113  becomes unstable. As a result, a noise or oscillation could be generated at the time of the rotation of the bearing, or a trouble such as flaking or seizing due to the lack of an oil film could be generated. In addition, when the thickness of the outer ring  112  is decreased, the deformation due to the splitting becomes large and this problem becomes serious.  
      As another bearing to support the shaft  102  of the crank shaft  101 , a roller bearing  125  disclosed in Japanese Unexamined Patent Publication No. 2004-232724, for example is employed in some cases.  
      As shown in  FIGS. 38 and 39 , the roller bearing  125  disclosed in the Japanese Unexamined Patent Publication No. 2004-232724 comprises a two-split outer ring (not shown), a plurality of rollers  126  arranged along the inner diameter surface of the two-split outer ring, and a two-split retainer  127 . According to the roller bearing  125  having the above constitution, since the outer ring and the retainer  127  can be incorporated from the diameter direction of the shaft  102 , it is said that the roller bearing is suitable for use to support the shaft  102 .  
      A problem such as the damage of the two-split retainer  127  due to the contact of the end surfaces of the two-split retainer  127  in the circumferential direction at the time of the rotation of the bearing has been pointed out. Thus, according to the Japanese Unexamined Patent Publication No. 2004-232724, the section modulus of a pillar part close to the end surface of the two-split retainer  127  in the circumferential direction is increased to prevent the damage of the retainer  127 .  
      In the Japanese Unexamined Patent Publication No. 2004-232724, assuming that the load applied to a pillar part  127   a  that is closest to the end surface of the retainer  127  in the circumferential direction is the highest, when the width of the pillar part  127   a  that is closest to the end surface in the circumferential direction is “Wa”, the width of a pillar part  127   b  adjacent to the pillar part  127   a  is “Wb”, and the width of another pillar part  127   c  in the circumferential direction is “Wc”, they are set so as to satisfy the relation Wa&gt;Wb&gt;Wc to make the section modulus of the pillar part  127   a  greater than those of the other pillar parts  127   b  and  127   c.    
      However, according to a rotation test performed for a bearing having a retainer in which all pillar parts have the same width, it is reported that the second pillar part from the end surface of the two-split retainer in the circumferential direction was damaged, so that it has been confirmed that the load applied to the pillar part  127   b  is highest in the bearing for supporting the shaft  102  of the crank shaft  101  actually.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a crank shaft supporting structure in which an incorporating operation is easy through the use of a needle roller bearing that prevents a retainer from falling off at the time of incorporating.  
      It is another object of the present invention to provide a crank shaft supporting structure having a needle roller bearing that can prevent a retainer from moving in an axial direction even when an outer ring has no flange.  
      It is still another object of the present invention to provide a crank shaft supporting structure in which an abnormal sound generated when a needle roller passes through a step part of an outer ring is prevented.  
      It is still another object of the present invention to provide a needle roller bearing in which the split parts of a retainer are prevented from being in contact with each other. In addition, it is an object to provide a crank shaft supporting structure in which such needle roller bearing is used to reduce noise.  
      It is still another object of the present invention to provide a crank shaft supporting structure having high reliability through the use of a needle roller bearing having a retainer in which split parts are prevented from being shifted in the axial direction.  
      It is still another object of the present invention to provide a crank shaft supporting structure having high durability and reliability through the use of a needle roller bearing in which each part is designed so as to have strength according to a load applied to a retainer.  
      It is still another object of the present invention to provide a method of splitting an outer ring of a needle roller bearing in which the vicinity of a split part is not likely to be deformed.  
      A needle roller bearing according to the present invention comprises an outer ring having a plurality of outer ring members split by split lines extending in the axial direction of the bearing and a plurality of needle rollers arranged on the track surface of the outer ring so that they can roll. Thus, a load is applied to the end surface of the outer ring in the direction crossing the end surface to split the outer ring.  
      Since a load is not applied in the diameter direction when the outer ring member is formed, a deformed amount can be small in the vicinity of the split parts. As a result, the needle roller bearing enables the needle rollers to smoothly roll.  
      Preferably, the outer ring has a V-shaped groove having a V-shaped sectional configuration at its end surface and the angle θ of the V-shaped groove is within a range of 5°≦θ≦150°, and the width “w” of the outer ring in the axial direction and the depth “d” of the V-shaped groove has a relation d/w≦0.2.  
      When the angle θ is too large, since the degree of stress concentration generated at the root part of the V-shaped groove becomes small, the load required to split the outer ring becomes high. Meanwhile, when the angle θ is too small, it becomes difficult to form the V-shaped groove. Thus, in view of the split processability of the outer ring and processability of the V-shaped groove, the angle θ of the V-shaped groove is preferably within the range of 5°≦θ≦150°. In addition, when the depth “d” of the V-shaped groove is too large, the needle roller and the V-shaped groove interfere with each other and the rolling defect of the needle roller could be generated. Thus, the problem can be solved by setting the depth such that d/w≦0.2.  
      Preferably, the thickness “t” of the outer ring is t≦5 mm. As the thickness “t” of the outer ring becomes small, the deformed amount becomes large in the vicinity of the split parts. Thus, when the present invention is applied to the outer ring having the thickness of t≦5 mm, a higher effect can be expected.  
      Preferably, the needle roller bearing further comprises a retainer having a cut part extending in the axial direction on the circumference and a buffer member at the end surface of the cut part. According to the above constitution, since the end surfaces of the cut part of the retainer are not directly in contact with each other, a metallic sound is prevented from being generated and abrasion at the contact part can be prevented.  
      According to the present invention, since the buffer member is arranged at the cut part of the retainer, the needle roller bearing prevents the metallic sound due to the contact of the end surfaces. In addition, when such bearing is used to support the shaft of the crank shaft, the crank shaft supporting structure can be low in noise level.  
      A crank shaft supporting structure according to the present invention comprises a crank shaft having a shaft and crank arms positioned at both ends of the shaft and the needle roller bearing for supporting the crank shaft rotatably as set forth in claim  1 . Focusing on the needle roller bearing, the needle roller bearing further comprises a retainer whose both ends project from the end surface of the outer ring to be in contact with the crank arms.  
      According to the above constitution, since the movement of the retainer in the axial direction is prevented by the wall surface of the crank arm, even when the outer ring has no flange, the needle roller does not fall off the track surface of the outer ring. As a result, the crank shaft supporting structure can keep the smooth rolling of the needle roller.  
      According to the present invention, since both ends of the retainer abut on the crank arms to prevent the retainer from moving in the axial direction, the crank shaft supporting structure can keep the smooth rolling.  
      A crank shaft supporting structure according to another aspect of the present invention comprises a crank shaft and the needle roller bearing for supporting the crank shaft rotatably as set forth in claim  1 . Focusing on the needle roller bearing, the needle roller bearing further comprises an integral retainer having a cut part extending in the axial direction on the circumference.  
      According to the needle roller bearing having the above constitution, the retainer is elastically deformed to be incorporated in the crank shaft and then the outer ring member is incorporated in the diameter direction. At this time, since the retainer does not fall off because of disassembly, the crank shaft supporting structure enables a simple incorporating operation.  
      According to the present invention, since the retainer is the integral type, the retainer is prevented from falling off when the outer ring is incorporated, so that the crank shaft supporting structure enables a simple incorporating operation.  
      A crank shaft supporting structure according to still another aspect comprises a crank shaft and the needle roller bearing for supporting the crank shaft rotatably as set forth in claim  1 . The split lines of the outer ring are provided apart from a maximum radial load point of the needle roller bearing to both sides in the circumferential direction by 50° or more.  
      As described above, when the split line of the outer ring is arranged at a position apart from the maximum radial load point, the abnormal sound generated when the needle roller passes through the step part can be prevented. As a result, the noise level of the crank shaft supporting structure can be low. In addition, the “maximum radial load point” used in this specification means a point to which the highest radial load is applied on the circumference of the outer ring of the needle roller bearing incorporated in the crank shaft.  
      Preferably, the split lines are provided apart from a symmetric position to the maximum radial load point across the bearing center to both sides in the circumferential direction by 50° or more. In general, a high radial load is applied to a point symmetric to the maximum radial load point across the bearing center. Thus, when the split line of the outer ring is arranged at a position apart from this point, the noise level of the crank shaft supporting structure can be low.  
      According to the present invention, since the step part of the outer ring is arranged at a position apart from the maximum radial load point, the crank shaft supporting structure in which the abnormal sound to be generated when the needle roller passes through the step part is prevented can be provided.  
      A crank shaft supporting structure according to still another aspect of the present invention comprises a crank shaft and the needle roller bearing for supporting the crank shaft rotatably as set forth in claim  1 . Focusing on the needle roller bearing, the needle roller bearing further comprises a retainer having cut parts extending in the axial direction on the circumference, a projected part at one cut part and a recessed part for receiving the projected part, at the other cut part, and the gap δ between the projected part and the recessed part in the axial direction is such that 0≦δ≦0.2 mm.  
      As described above, when the gap δ between the projected part and the recessed part in the axial direction is set such that 0≦δ≦0.2 mm, the cut parts of the retainer can be prevented from being shifted. As a result, since the trouble such as peeling or flaking can be prevented, the crank shaft supporting structure can have long life and high reliability. In addition, it is preferable that the retainer is formed of a resin material in view of processability.  
      According to the present invention, since the gap between the projected part and the recessed part in the axial direction is set within the predetermined range, the retainer can be prevented from being shifted in the axial direction, so that the crank shaft supporting structure can have a long life and high reliability.  
      A crank shaft supporting structure according to still another aspect of the present invention comprises a crank shaft and the needle roller bearing for supporting the crank shaft rotatably as set forth in claim  1 . Focusing on the needle roller bearing, the needle roller bearing further comprises a retainer formed by circumferentially connecting a plurality of retainer segments each having a plurality of pockets for housing the needle rollers and comprising an arc-shaped ring part and a plurality of pillar parts projecting from the end surface of the ring part in the axial direction. The pillar part comprises two first pillar parts positioned closest to both end surfaces of the ring part in the circumferential direction, two second pillar parts adjacent to the two first pillar parts, respectively and third pillar parts arranged between the two second pillar parts, and the width of the second pillar part in the circumferential direction is larger than those of the other pillar parts.  
      As described above, since the strength of the second pillar part to which the highest load is applied at the time of the bearing rotation is increased, the crank shaft supporting structure can be highly durable and reliable.  
      Preferably, the retainer segment comprises a first pocket formed between the first pillar part and the second pillar part, a second pocket formed between the second pillar part and the third pillar part adjacent to the second pillar part, and a third pockets formed between the adjacent third pillar parts. When it is assumed that the central angle formed between the end surface of the ring part in the circumferential direction and the first pocket is “α”, the central angle formed between the first pocket and the second pocket is “β” and the central angle formed between the second pocket and the third pocket adjacent to the second pocket is “γ”, the relations such that α≠β, β≠γ, and γ≠α are satisfied.  
      The diameters of all of the needle rollers have to be the same in view of keeping the smooth rotation of the needle roller bearing. When all of the needle rollers have the same diameter, the opening widths of all of the pockets have to be the same. Since the retainer of the needle roller bearing used in the crank shaft supporting structure according to the present invention have different dimensions in the pillar parts in the circumferential direction, in order to make the opening widths of the pockets uniform, the pitches (α, β, γ) of the adjacent pockets have to be irregular.  
      Preferably, the needle roller bearing further comprises an outer ring in which an annular member is formed by cutting and a plurality of split lines extending in the axial direction on the circumference of the annular member are formed by natural splitting. When the outer ring is split by the above method, since the manufacturing steps can be simplified, the crank shaft supporting structure is low in cost.  
      Preferably, the retainer segment comprises SNCM or SCM as a starting material and formed through a carburizing or carbonitriding treatment. When the retainer segment is manufactured by the above method, the strength of the whole retainer segment can be enhanced. As a result, the crank shaft supporting structure can have higher reliability.  
      Preferably, the retainer is formed of a resin material. Since the resin material has high elastic deformability, it is very suitable for the retainer material to be incorporated according to the above procedure.  
      Preferably, the crank shaft is used in a multiple cylindered engine. The crank shaft used in the multiple cylindered engine has a shaft whose both ends are sandwiched by the crank arms and whose number is increased in proportion to the number of cylinders. When the above needle roller bearing is used in such crank shaft, a higher effect can be expected.  
      According to the present invention, since the strength of the second pillar part to which the highest load is applied at the time of the bearing rotation is enhanced, the crank shaft supporting structure can be superior in durability and high in reliability.  
      A method of splitting an outer ring of a needle roller bearing according to the present invention is a method of splitting an outer ring of a needle roller bearing comprising the outer ring having a plurality of outer ring members split by split lines extending in the axial direction of the bearing, and a plurality of needle rollers arranged on the track surface of the outer ring so that they can roll. More specifically, the method comprises a step of splitting a cylindrical material by applying a load to the end surface of the cylindrical material in the direction crossing the end surface to split the outer ring.  
      For example, the method comprises a step of forming a notch extending in the diameter direction, on one end surface of the cylindrical material in the axial direction, a step of setting the outer ring such that the end surface having the notch is arranged on the lower side and a space is provided in the vicinity of the notch, and a step of splitting the cylindrical material by applying the load to the end surface not having the notch.  
      Alternatively, the method comprises a step of forming notches extending in the diameter direction, on both end surfaces of the cylindrical material in the axial direction, a step of setting the outer ring such that the one end surface is arranged on the lower side and a space is provided in the vicinity of the notch, and a step of splitting the cylindrical material by applying the load to the other end surface except for the notch.  
      Since a load is not applied to the outer ring in the diameter direction according to the above methods, a deformed amount can be small in the vicinity of the split part. As a result, the needle roller can stably roll and a trouble can be prevented.  
      According to the present invention, since the load is applied to the end surface of the cylindrical material to split the outer ring by the method of splitting the outer ring of the needle roller bearing, the deformation in the vicinity of the split line can be prevented and the needle roller can stably roll. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a view showing a crank shaft supporting structure in its incorporated state according to one embodiment of the present invention;  
       FIG. 2  is a view showing an outer ring used in a needle roller bearing in  FIG. 1 ;  
       FIG. 3  is a view showing the outer ring in  FIG. 2  split by natural splitting;  
       FIG. 4  is an enlarged view of a split part in  FIG. 3 ;  
       FIG. 5  is a side view showing a retainer of the needle roller bearing in  FIG. 1 ;  
       FIG. 6  is a front view showing the retainer of the needle roller bearing in  FIG. 1 ;  
       FIG. 7  is an enlarged view showing a part Q in  FIG. 6 ;  
       FIG. 8A  is a view showing the incorporated crank shaft supporting structure taken in an axial direction according to one embodiment of the present invention;  
       FIG. 8B  is a view showing the incorporated crank shaft supporting structure taken in a direction perpendicular to the axial direction according to one embodiment of the present invention;  
       FIG. 9  is a view showing a crank shaft supporting structure according to another embodiment of the present invention;  
       FIG. 10A  is a front view showing a retainer of a needle roller bearing in  FIG. 9 ;  
       FIG. 10B  is a side sectional view showing the retainer of the needle roller bearing in  FIG. 9 ;  
       FIG. 11  is a view showing a state in which the crank shaft supporting structure in  FIG. 9  is incorporated;  
       FIG. 12  is a view showing a needle roller bearing to be used in  FIG. 13 ;  
       FIG. 13  is a view showing a crank shaft supporting structure according to another embodiment of the present invention;  
       FIG. 14A  is a front view showing a retainer of the needle roller bearing in  FIG. 12 ;  
       FIG. 14B  is a side sectional view showing the retainer of the needle roller bearing in  FIG. 12 ;  
       FIG. 15  is a view showing a distribution of radial loads applied to the needle roller bearing in  FIG. 12 ;  
       FIG. 16  is a view showing a test result to confirm the effect of the present invention;  
       FIG. 17  is a view showing a crank shaft supporting structure according to another embodiment of the present invention;  
       FIG. 18  is a view showing a state in which needle rollers are housed in a retainer used in the needle roller bearing;  
       FIG. 19  is a view showing widths of pillar parts of a retainer segment used in the needle roller bearing;  
       FIG. 20  is a view showing pitches between pockets of the retainer segment used in the needle roller bearing;  
       FIG. 21A  is a front view showing a method of splitting the outer ring of the needle roller bearing according to one embodiment of the present invention;  
       FIG. 21B  is a plan view showing the method of splitting the outer ring of the needle roller bearing according to one embodiment of the present invention;  
       FIG. 22A  is a view showing roundness before the outer ring split by the method in  FIGS. 21A and 21B  is incorporated;  
       FIG. 22B  is a view showing roundness after the outer ring split by the method in  FIGS. 21A and 21B  has been incorporated;  
       FIG. 23A  is a front view showing a method of splitting the outer ring of the needle roller bearing according to another embodiment of the present invention;  
       FIG. 23B  is a plan view showing the method of splitting the outer ring of the needle roller bearing according to another embodiment of the present invention;  
       FIG. 24  is a view showing various kinds of dimensions of the outer ring used in  FIGS. 21A and 21B ;  
       FIG. 25  is a view showing a conventional crank shaft;  
       FIG. 26  is an enlarged view showing a part P in  FIG. 25 ;  
       FIG. 27  is a view showing a conventional crank shaft supporting structure in which a thrust washer is disposed between a crank arm and an engine block;  
       FIG. 28  is a view showing a conventional needle roller bearing to support the shaft of the crank shaft;  
       FIG. 29  is a view showing a conventional needle roller bearing to support the shaft of the crank shaft;  
       FIG. 30  is a view showing a conventional split outer ring;  
       FIG. 31  is a view showing a conventional split outer ring;  
       FIG. 32A  is a view showing one side of a conventional split retainer;  
       FIG. 32B  is a view showing an abutting part of the conventional retainer;  
       FIG. 33A  is a view showing an example in which the outer ring members are combined with accuracy;  
       FIG. 33B  is a view showing an example in which the outer ring members are combined out of alignment;  
       FIG. 34  is a view showing a conventional crank shaft supporting structure in which a needle roller bearing having flanges at both ends supports a shaft;  
       FIG. 35  is a view showing a conventional crank shaft supporting structure in which a needle roller bearing having no flanges at both ends supports the shaft;  
       FIG. 36A  is a view showing a V-shaped groove of an outer ring before split;  
       FIG. 36B  is a view showing a conventional method of splitting an outer ring;  
       FIG. 37A  is a view showing roundness before the outer ring split by the method in  FIGS. 36A and 36B  is incorporated;  
       FIG. 37B  is a view showing roundness after the outer ring split by the method in  FIGS. 36A and 36B  has been incorporated;  
       FIG. 38  is a view showing a conventional needle roller bearing; and  
       FIG. 39  is a view showing a retainer of the needle roller bearing in  FIG. 38 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A crank shaft supporting structure according to one embodiment of the present invention will be described with reference to  FIG. 1  hereinafter.  
      The crank shaft supporting structure shown in  FIG. 1  comprises a crank shaft  15 , a cylinder block  16   a , a bearing cap  16   b , and a needle roller bearing  11  arranged between the crank shaft  15  and the bearing cap  16   b  and supporting the crank shaft  15  rotatably.  
      The needle roller bearing  11  comprises an outer ring  12  having a plurality of outer ring members  12   a  split by split lines extending in the axial direction of the bearing, a plurality of needle rollers arranged on the track surface of the outer ring  12  so that they can roll, and an integral retainer  14  having a cut part  14   a  extending in the axial direction on the circumference.  
      According to the needle roller bearing  11 , since the needle roller  14  and the track surface are linearly in contact with each other, high load capacity and high rigidity can be provided for its small bearing projected area, so that it is suitable for use in a car, a two-wheel vehicle engine and the like.  
      The outer ring shown in  FIG. 1  will be described with reference to FIGS.  2  to  4 . In addition,  FIG. 2  is a view showing a state of the outer ring  12  before split,  FIG. 3  is a view showing a state of the outer ring  12  split by natural splitting, and  FIG. 4  is an enlarged view of the split part of the outer ring  12 .  
      Referring to  FIG. 2 , a cylindrical annular member is formed by a cutting process and the like to provide the outer ring  12 . In addition, since the inner diameter surface of the outer ring  12  functions as the track surface of the needle roller bearing  14 , it is ground for secure the smooth rotation of the needle rollers  14 .  
      Referring to  FIG. 3 , a plurality of split lines extending in the axial direction are formed on the circumference of the annular member by applying shock load to the outer diameter surface or the end surface of the annular member. Thus, the outer ring members  12   a  are provided. According to this embodiment, the outer ring member  12   a  is in the form of a semicircle having a central angle of 180°. Referring to  FIG. 4 , since the end surface of the split part of the outer ring member  12   a  is not ground, the configuration of it is indented because of natural splitting. When the bearing is used, the cylindrical outer ring is provided by abutting the corresponding end surfaces. In addition, the above manufacturing method is called “natural splitting”.  
      The retainer  14  shown in  FIG. 1  will be described with reference to FIGS.  5  to  7 . In addition,  FIG. 5  is a side view of the retainer  14 ,  FIG. 6  is a front view of the retainer  14 , and  FIG. 7  is an enlarged view showing a part “Q” in  FIG. 6 . First, referring to  FIG. 5 , the retainer  14  is an integral retainer having the cut part  14   a  at one portion on the circumference. The retainer  14  is formed of a resin material.  
      Referring to  FIG. 6 , the retainer  14  has a projected part  14   b  on one side of the cut part Q and a recessed part  14   c  on the other side thereof to receive the projected part  14   b , and when it is incorporated, the projected part  14   b  is fitted in the recessed part  14   c  so that they are fixed. Thus, as shown in  FIG. 7 , the gap δ between the projected part  14   b  and the recessed part  14   c  in the axial direction is set such that 0≦δ≦0.2 mm.  
      In this constitution, the shift of the retainer  14  in the cut part Q in the axial direction can be minimized. Thus, the trouble of the outer ring  12  or the crank shaft  15  such as peeling, flaking can be prevented, so that the crank shaft supporting structure has a long life and high reliability.  
      Here, although it is ideal that the gap δ between the projected part  14   b  and the recessed part  14   c  in the axial direction is zero, it is very difficult to implement the above precision in view of manufacturing error and the like. However, when δ≦0.2 mm, since an eccentric load applied to the outer ring  12  or the crank shaft  15  is small, the trouble caused by the shift of the retainer  14  can be sufficiently prevented from being generated.  
      A method of incorporating the needle roller bearing  11  having the above constitution into the crank shaft  15  will be described.  
      First, the retainer  14  that has incorporated needle roller  13  in each pocket previously is prepared. Then, the retainer  14  is incorporated such that the cut parts  14   a  are elastically deformed to the degree it can be incorporated in the crank shaft  15 . At this time, the projected part  14   b  and the recessed part  14   c  of the retainer  14  are engaged and fixed to the crank shaft  15 . Finally, the outer ring members  12   a  are incorporated in the crank shaft  15  in the diameter direction and then the cylinder block  16   a  and the bearing cap  16   b  are incorporated.  
      As a result, as shown in  FIGS. 8A and 8B , the crank shaft  15 , the retainer  14 , the outer ring members  12   a , and the inner diameter surface of the cylinder block  16   a  and the bearing cap  16   b  are arranged concentrically, so that the needle rollers  13  can roll stably, According to the above incorporating steps, the needle roller bearing  11  can be incorporated into a shaft whose both ends are sandwiched by crank arms. Furthermore, there is no possibility that the retainer  14  falls off when the outer ring member  12   a  is incorporated. Therefore, the incorporating operation is simple and it is not necessary to provide a member especially for preventing the retainer  14  from falling off. As a result, the number of operating steps and the operation cost can be reduced.  
      At this time, although the retainer  14  may be a metal retainer manufactured by pressing or cutting a metal material, when it is a resin retainer manufactured by injection molding a resin material having an elastic deformation property, the incorporating operation becomes simple.  
      A crank shaft supporting structure according to another embodiment of the present invention will be described with reference to  FIGS. 9, 10A  and  10 B.  
      The crank shaft supporting structure shown in  FIG. 9  comprises a crank shaft  31  having a shaft  32  and crank arms  33  positioned at both ends of the shaft  32 , and a needle roller bearing  41  supporting the shaft  32  of the crank shaft  31  rotatably.  
      The needle roller bearing  41  comprises an outer ring  42 , a plurality of needle rollers  43  arranged on the track surface of the outer ring  42  so that they can roll, and a retainer  44  whose both ends project from the ends of the outer ring  42  and have contact with the crank arms  33 . In addition the outer ring  42  is fixed to the engine block  34  and the bearing cap  36  with a fixing pin  35 .  
      The outer ring  42  is the split type outer ring formed by the “natural splitting” shown in FIGS.  2  to  4 . Since this outer ring  42  does not have any flange at the end in the axial direction, great force is not needed when it is split into two. This provides the effect that the outer ring  42  is prevented from being deformed at the time of splitting in addition to the effect that the manufacturing can be simplified. Furthermore, when it does not have the flange, since the roller can be as long as possible in a limited space, the needle roller bearing  41  can provide a large load capacity.  
      Meanwhile, the retainer  44  is a metal retainer manufactured by pressing or cutting a metal material, and it is formed by combining two split retainers  44   a  split at cut parts  44   b  in the circumferential direction as shown in  FIG. 10A . In addition, as shown in  FIG. 10B , the retainer  44  has a pocket  44   c  housing the needle roller  43 .  
      According to the above needle roller bearing  41 , both ends of the retainer  44  are in contact with the crank arms  33 , even when the flange is not provided in the outer ring  42 , the retainer  44  can be prevented from moving in the axial direction. As a result, since the needle roller  43  is prevented from falling off the track surface of the outer ring  42 , the needle roller  43  can roll smoothly.  
      A method of incorporating the needle roller bearing  41  having the above constitution to the crank shaft  31  will be described with reference to  FIG. 11  hereinafter.  
      First, one outer ring member  42   a  and the split retainer  44   a  incorporating the needle rollers  43  previously are put on the engine block  34 . Then, the crank shaft  31  is put thereon and the other outer ring member  42   a  and the split retainer  44   a  incorporating the needle rollers  43  previously are put thereon. Finally, they are fixed by the bearing cap  36 .  
      Although the above retainer  44  is the metal retainer manufactured by pressing or cutting the metal material in the above example, the present invention is not limited to this. For example, it may be a resin retainer manufactured by injection molding a resin material having high elastic deformability.  
      In addition, although the retainer  44  is the two-split type of retainer  44  having the two cut parts  44   b  on the circumference in the above example, the present invention is not limited to this. For example, it may be an integral retainer having one cut part on the circumference.  
      A crank shaft supporting structure according to another embodiment of the present invention will be described with reference to  FIG. 13 .  
      The crank shaft supporting structure shown in  FIG. 13  comprises a crank shaft  25  having a shaft  26 , crank arms  27  positioned on both ends of the shaft  26  and a crank pin  28  arranged on the other side of the shaft  26  across the crank arm  27 , a needle roller bearing  21  supporting the crank shaft  25  rotatably, a crank case  29  and a crank case cap  30 .  
      The needle roller bearing  21  comprises an outer ring  22  having a plurality of outer ring members  22   a  split by split lines extending in the axial direction of the bearing, a plurality of needle rollers  23  arranged on the track surface of the outer ring  22  so that they can roll, and a retainer  24  having pockets for housing the plurality of needle rollers as shown in  FIG. 12 . In addition, the outer ring  22  is the split type outer ring formed by the “natural splitting” shown in FIGS.  2  to  4 .  
      Meanwhile, the retainer  24  is formed by combining two split retainers  24   a  split by cut parts  24   b  in the circumferential direction as shown in  FIG. 14A . In addition, as shown in  FIG. 14B , it has pockets  24   c  for housing the needle rollers  23 .  
      A method of incorporating the above needle roller bearing  21  into the crank shaft  25  will be described hereinafter.  
      First, the needle roller  23  is incorporated in each pocket of the retainer  24 . Then, one outer ring member  22   a  is incorporated in the crank case  29 , and one split retainer  24   a , the crank shaft  25 , the other split retainer  24   a , and the other outer ring member  22   a  are set thereon. Finally, the crank case cap  30  is incorporated to fix them.  
      At this time, the split lines of the two outer ring members  22   a  are provided at positions apart from the maximum radial load point of the crank shaft supporting structure  25  to both sides in the circumferential direction by 50° or more. In this constitution, even when there is a step part at the abutting part of the outer ring members  22   a , abnormal noise to be generated when the needle roller  23  passes the step part can be prevented. As a result, the crank shaft supporting structure can be low in noise level.  
      Furthermore, as shown in  FIG. 15 , since a high load is applied at a point opposite to the maximum radial load point across the center of the bearing in general, the split lines of the outer ring members  22   a  are provided at positions apart from that point to both sides in the circumferential direction by 50° or more. That is, they may be provided in the range of 50° to 130° on both sides of the maximum radial load point.  
      Then, in order to confirm the effect of the present invention, a test for measuring the noise during the rotation of the bearing was performed, changing the positional relation between the maximum radial load point of the crank shaft and the split line of the outer ring member  22   a.    
      In addition, the crank shaft supporting structures used in the test includes a structure in which the maximum radial load point and the split line correspond to each other (at the point of 0 20  in the drawing), and structures in which both are shifted from each other by 30°, 50°, 70°, and 90°. In addition, the test was performed at the bearing rotation speeds of 1000 rpm, 1800 rpm, and 5000 rpm. The result is shown in Table 1 and  FIG. 16 .  
                   TABLE 1                          Positional relation           between the maximum radial   Bearing rotation speeds                             load point and the split line   1000 rpm   1800 rpm   5000 rpm                                     0°   73.0 dB   80.6 dB   82.8 dB       30°   60.1 dB   67.0 dB   82.1 dB       50°   58.3 dB   61.1 dB   67.5 dB       70°   56.5 dB   59.3 dB   62.5 dB       90°   54.6 dB   56.0 dB   58.0 dB                  
 
      Referring to Table 1 and  FIG. 16 , it has been confirmed that the noise level is low in the structure in which the split line of the outer ring member  22   a  is apart from the maximum radial load point by 50° or more. In addition, it has been confirmed that even when the rotation speed is changed, the noise level is not changed so much. Furthermore, it has been confirmed that the noise level is the lowest when the split line and the maximum radial load point are apart from each other by 90°.  
      In addition, the retainer  24  may be a metal retainer manufactured by pressing or cutting a metal material or a resin retainer manufactured by injection molding a resin material having high elastic deformability.  
      A variation of the crank shaft supporting structure shown in  FIG. 1  will be described with reference to  FIG. 17  hereinafter. In addition, since its basic constitution is the same as that of the crank shaft supporting structure shown in  FIG. 1 , the description of the same parts will be omitted and a difference point will be described.  
      Referring to  FIG. 17 , a needle roller bearing  11  for supporting a crank shaft  15  rotatably comprises an outer ring having a plurality of outer ring members  12   a  split by split lines extending in the axial direction of the bearing, a plurality of needle rollers  13  arranged on the track surface of the outer ring so that they can roll, a split type retainer  14  having a plurality of cut parts  14   a  extending in the axial direction on the circumference, and a buffer member  14   d  at the end surface of the cut part  14   a.    
      Furthermore, the gap between abutting parts is filled with the buffer member  14   d . The buffer member  14   d  may be a plate spring made of metal, a FRP such as Viton (registered mark), or a rubber member that is superior in heat resistance such as silicon rubber (RSi) and the like. It may be sandwiched when a projected part  14   b  and a recessed part  14   c  are engaged, or may have been bonded to either one or both end surfaces previously.  
      According to the above constitution, even when a load is applied to the needle roller bearing  11  due to the rotation of the crank shaft  15 , since the corresponding cut parts  14   a  are not in contact with each other, a metallic sound is prevented from being generated. Furthermore, since the contact part can be prevented from being worn, the needle roller bearing  11  has a long life.  
      Next, an example of the needle roller bearing for supporting the crank shaft according to the above each embodiment will be described with reference to FIGS.  18  to  20 . Referring to  FIG. 18 , a needle roller bearing  51  comprises an outer ring (not shown) split into outer ring members (not shown) by two split lines extending in the axial direction, a retainer  53  split into retainer segments  53   a  and  53   b  by two split lines extending in the axial direction similar to the outer ring, and a plurality of needle rollers  54  retained by the retainer  53  and arranged along the inner diameter surface of the outer ring. In addition, the outer ring is the split type outer ring formed by the “natural splitting” shown in FIGS.  2  to  4 .  
      The retainer segment  53   b  shown in  FIG. 18  will be described with reference to FIGS.  18  to  20  hereinafter. In addition,  FIG. 18  is a view showing the retainer segment  53   a  housing the needle rollers  54 ,  FIG. 19  is a view showing the relation of the width dimensions of the pillar parts of the retainer segment  53   a  in the circumferential direction, and  FIG. 20  is a view showing pitches of the pockets of the retainer segment  53   a . In addition, since the retainer segment  53   b  has the same constitution as that of the retainer segment  53   a , its description will be omitted.  
      Referring to  FIG. 18 , the retainer segment comprises a pair of ring parts  55   a  and  55   b  (referred to as the “ring part  55 ” collectively), a plurality of pillar parts  56  projecting from end surface of the ring part  55  in the axial direction and connecting the ring parts  55   a  and  55   b , and a plurality of pockets  60  provided at a region surrounded by the ring part  55  and adjacent pillar parts  56  to house the needle rollers  54 . Each of the ring parts  55   a  and  55   b  is in the form of an arc. According to this embodiment, since the retainer  53  is split into the two retainer segments  53   a  and  53   b , each segment is a semicircle having a central angle of 180°. In addition, the retainer segments  53   a  and  53   b  are connected in the circumferential direction to form the annular retainer  53  when incorporated in the crank shaft.  
      In addition, the pillar part  56  comprises a first roller stopper  57  at the center part in the axial direction to prevent the needle roller  54  from escaping inward in the diameter direction, second roller stoppers  58  at both ends in the axial direction to prevent the needle roller  54  from escaping outward in the diameter direction, and a slanting part  59  to connect the first roller stopper  57  and the second roller stoppers  58 .  
      Referring to  FIG. 19 , the pillar part  56  provided in the retainer segment  53   a  comprises two first pillar parts  56   a  positioned closest to the end surfaces of the ring part in the circumferential direction, two second pillar parts  56   b  positioned adjacent to the first pillar parts  56   a , and a plurality of third pillar parts  56   c  positioned between the second pillar parts  56   b , and those pillar parts have the same configuration.  
      Here, when it is assumed that the width of the first pillar part  56   a  in the circumferential direction is “a”, the width of the second pillar part  56   b  in the circumferential direction is “b”, the width of the third pillar part  56   c  in the circumferential direction is “c” among the pillar parts  56 , they are set so as to satisfy the relation c&lt;a≦b. Here, according to the width of the pillar part  56  in the circumferential direction, the width of the second roller stopper  58  is the largest and the widths of the first roller stopper  57  and the slanting part  59  become smaller in this order. In addition, each dimension of the parts  57 ,  58  and  59  is increased from the inner side in the diameter direction to the outer side in the diameter direction. However, when the corresponding parts of each of the pillar parts  56   a ,  56   b  and  56   c  are compared, the above relation is to be surely satisfied.  
      According to the above constitution, when the width of the second pillar part  56   b  in the circumferential direction is set larger than the widths of the other pillar parts  56   a  and  56   c  in the circumferential direction, the strength of the second pillar part  56   b  in which the highest load is applied at the time of the rotation of the bearing can be increased. Here, although it is also considered that the strength of the whole retainer segment  53   a  is increased by setting the widths of all the pillar parts  56  in the circumferential direction to the same as that of the second pillar part  56   b , when the widths of the pillar parts  56  in the circumferential direction is increased, the number of needle rollers that can be housed is decreased or the roller diameter has to be decreased to maintain the number of the needle rollers, which is not appropriate because the load capacity of the needle roller bearing  51  is lowered. Therefore, as described above, it is preferable that the widths of the pillar parts in the circumferential direction are set according to the load applied to each of the pillar parts  56   a ,  56   b  and  56   c.    
      Referring to  FIG. 20 , the pockets  60  provided in the retainer segment  53   a  comprise two first pockets  60   a  provided between the first pillar part  56   a  and the second pillar part  56   b , two second pockets  60   b  provided between the second pillar part  56   b  and the third pillar part  56   c  adjacent to the second pillar part  56   b , and a plurality of third pockets  60   c  provided between the adjacent third pillar parts  56   c . In addition, this embodiment shows an example in which the number of the third pillar parts  56   c  is three and the number of the third pockets  63   c  is three. In addition, the pockets  60   a ,  60   b  and  60   c  have the same configuration and size to house the needle rollers  54  having the same configuration and size.  
      Here, since the widths of the pillar parts  56   a ,  56   b  and  56   c  in the circumferential direction are different from each other and the widths of the pockets  60   a ,  60   b  and  60   c  in the circumferential direction are the same, the pitches of the pockets  60   a ,  60   b  and  60   c  are irregular. That is, when it is assumed that the central angle between the end surface of the retainer segment  53   a  in the circumferential direction and the first pocket  60   a  is “α”, the central angle between the first pocket  60   a  and the second pocket  60   b  is “β” and the central angle between the second pocket  60   b  and the third pocket  60   c  adjacent to the second pocket  60   b  is “γ”, the relations that α≠β, β≠γ, and γ≠α are satisfied. In addition, these relations are applied to the pitches on the right side in the drawing. In addition, the central angle between the adjacent third pockets  60   c  is the same as “γ”.  
      According to this embodiment, since the dimensions of the pillar parts  56   a ,  56   b  and  56   c  in the circumferential direction are such that c&lt;a≦b, the central angles are such that γ&lt;α≦β. In addition, the “central angle” in this specification means the angle formed between lines connecting the rotation center “O” of the bearing and the end of the retainer segment  53   a  in the circumferential direction or the rotation centers of the needle rollers  54  housed in the pockets  60   a ,  60   b  and  60   c.    
      The retainer segment  53   a  having the above constitution is formed by pressing or cutting nickel-chrome-molybdenum steel (SMCM) or chrome-molybdenum steel (SCM) used as a starting material. Furthermore, in order to obtain predetermined strength and other mechanical properties, a carburizing treatment or a carbonitriding treatment is performed.  
      In addition, although the widths of the first to third pillar parts  56   a ,  56   b  and  56   c  in the circumferential direction are set so as to satisfy c&lt;a≦b in the above embodiment, a some degree of effect can be expected when the width “b” of the second pillar part  56   b  in the circumferential direction is set so as to be larger than the widths “a” and “c” of the other pillar parts  56   a  and  56   c  in the circumferential direction, and the width “a” of the first pillar part  56   a  and the width “c” of the third pillar part  56   c  are set to the same value.  
      In addition, although the number of the third pillar parts  56   c  is three and the number of the third pockets  60   c  is three in the above embodiment, the present invention is not limited to this. The above number may be any number. The number of the third pockets  60   c  is determined by the widths of the pillar parts  56   a ,  56   b  and  56   c  in the circumferential direction and the roller diameter of the needle roller  54 , for example.  
      Furthermore, although the needle roller bearing  51  comprises the outer ring and the retainer  53  and the needle rollers  54  in the above embodiment, the present invention may be applied to a cage and roller comprising a retainer and needle rollers without an outer ring.  
      A method of splitting the outer ring  12  of the needle roller bearing by the natural splitting will be described with reference to  FIGS. 21A and 21B .  
      First, as shown in  FIG. 21A , two V-shaped grooves  12   b  serving as notches extending in the diameter direction are formed at one end surface of a cylindrical material to become the outer ring  12  in the axial direction at a first step. Then, the cylindrical material is set on a table  67  with the V-shaped groove  12   b  side down at a second step. This table  67  has a groove  67   a  in the center, so that a space is provided in the vicinity of the V-shaped groove  12   b . In addition,  FIG. 21B  is a plan view of  FIG. 21A .  
      At a third step, a load is applied to the end surface in which the V-shaped groove  12   b  is not formed in the direction crossing the end surface by a tool  68 . Thus, stress is concentrated at a root part of the V-shaped groove  12   b  and the outer ring  12  is split from this part as a starting point.  
      According to the above splitting method, since a load is not applied to the outer ring  12  in the diameter direction, the vicinity of the split part of the outer ring  12  is not largely deformed inward in the diameter direction as shown in  FIG. 22A . When the outer ring  12  is incorporated into the crank shaft  15  and the like, high roundness can be maintained as shown in  FIG. 22B . As a result, since the rolling space can be kept constant in the circumferential direction of the bearing, the needle roller  13  can stably rolls, so that a noise or oscillation can be prevented from being generated or a trouble such as flaking or seizing due to the lack of an oil film and the like can be prevented from being generated.  
      In addition, although the above outer ring  12  is split into the two outer ring members  12   b  by forming the two V-shaped grooves  12   b  at one end surface in the axial direction in the above example, the V-shaped grooves may be provided three positions or more to split the outer ring  12  into three outer ring members  12   a  or more.  
      In addition, according to another embodiment of the natural splitting, as shown in  FIGS. 23A and 23   b , V-shaped grooves  12   b  are formed at both end surfaces of a cylindrical material to become the outer ring  12  in the axial direction as notches extending in the diameter direction at a first step. At a second step, the outer ring  12  is put on a table  77  so that a space is provided in the vicinity of one V-shaped groove  12   b . At a third step, a load is applied to the direction crossing the end surface by a tool  78  positioned so as not to be in contact with the other V-shaped groove  12   b  provided in the other end surface to split the outer ring  12 .  
      The configuration of the V-shaped groove in the above embodiments will be described with reference to  FIG. 24  hereinafter.  
      First, the angle θ of the V-shaped groove  12   b  is set within a range 5°≦θ≦150°. In order to generate stress concentration at the root part of the V-shaped groove  12   b , the angle θ may be as small as possible. However, when the angle θ is too small, it is difficult to form the V-shaped groove  12   b . Hence, it is desirable that the angle may be set within the above range in view of split processability of the outer ring  12  and processability of the V-shaped groove  12   b.    
      In addition, when it is assumed that the width of the outer ring  12  in the axial direction is “w”, the depth of the V-shaped groove  12   b  is set within a range of d/w≦0.2. Because, when the depth of the V-shaped groove is large beyond necessity, the needle roller  13  rolls unstably when passes over the V-shape groove  12 .  
      Furthermore, when the present invention is applied to an outer ring  12  having a thickness “t” of 5 mm or less, a higher effect can be expected. When the outer ring  12  is split by the conventional method, as the thickness “t” becomes small, the vicinity of the split part is largely deformed.  
      Although the outer ring  12  comprises the two outer ring members  12   a  in the above embodiment, the present invention is not limited to this. For example, three or more outer ring members may be combined. In addition, although both outer ring members  12   a  are in the form of the semicircle having the central angle of 180° and have the same configuration in the above embodiment, their central angles may be different from each other. Furthermore, the above may be applied to the case where the retainer is split into the retainer segments.  
      In addition, the crank shaft supporting structure according to the present invention can be applied to a crank shaft of an engine of a car or a two-wheel vehicle. In addition, although the number of cylinders may be one or more, when the present invention is applied to the crank shaft used in a multiple cylindered engine having a shaft sandwiched between the crank arms as shown in the part “P” in  FIG. 25 , a higher effect can be expected.  
      Furthermore, according to the present invention, when the above characteristic parts in the above embodiments are arbitrarily combined, a synergetic effect can be expected.  
      Although the embodiments of the present invention have been described with reference to the drawings in the above, the present invention is not limited to the above-illustrated embodiments. Various kinds of modifications and variations may be added to the illustrated embodiments within the same or equal scope of the present invention.  
      The present invention can be advantageously applied to the needle roller bearing for supporting the crank shaft of the engine.