Patent Publication Number: US-2020300299-A1

Title: Bearing for clutch

Description:
TECHNICAL FIELD 
     The present invention relates to a bearing for a clutch. 
     RELATED ART 
     A clutch used with a friction plate to be mounted to a vehicle and the like is configured to axially press a diaphragm spring of a clutch cover by a release fork which is an input member, thereby releasing an urging force of the diaphragm spring from the friction plate to thus cut off power transmission. 
     The release fork is arranged at a fixed side such as a vehicle body, and the diaphragm spring is mounted to the clutch cover configured to rotate integrally with a flywheel or the like of an engine with being mounted thereto. Therefore, when the diaphragm spring configured to rotate together with the clutch cover is directly pressed by the release fork, the wear occurs at a contact part between the release fork and the diaphragm spring. Thus, for preventing the wear, a clutch release bearing is arranged between the diaphragm spring and the release fork, a rotating ring is integrally rotated with being in contact with the diaphragm spring, and a guide sleeve to which an input from the release fork is to be applied is contacted to a fixed ring. 
     When using the clutch release bearing for a clutch in a state where the friction plate is immersed in lubricant, the clutch release bearing is used under a lubrication environment in which wear pieces from the friction plate are included (contaminated). Therefore, the wear pieces are deposited in the clutch release hearing, so that the clutch release bearing may be damaged. Also, when the lubricant stays in the bearing, rotating torque increases due to a stirring resistance. In Patent Document 1, one axial side part of an outer ring is formed to have a substantial U-shape, and an overlapping part at which an outer peripheral surface of a folded back part of the outer ring and an inner peripheral surface of an inner ring axially overlap with each other is provided with a predetermined radial gap to limit inflow of the lubricant, so that the deposition of the wear pieces in the clutch release bearing is suppressed and the flow of the lubricant in the entire clutch device is controlled. 
     CITATION LIST 
     Patent Documents 
     Patent Document 1: DE-A-102014209418 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, according to Patent Document 1, the radial gap between the outer peripheral surface of the folded back part of the outer ring and the inner peripheral surface of the inner ring is constant, and outflow (supply to a raceway) of the lubricant caused to flow into the radial gap is not considered. Also. Patent Document 1 does not describe reduction in the stirring resistance against the lubricant caused to flow into the raceway by a cage. 
     The present invention has been made in view of the above situations, and an object thereof is to provide a bearing for a clutch capable of reducing rotating torque by controlling an oil amount and a flow of lubricant to be supplied to the bearing for a clutch and thus reducing a stirring resistance of the lubricant. 
     Means for Solving the Problems 
     The object of the present invention is achieved by following configurations. 
     (1) A bearing for a clutch comprising: 
     an outer ring including an outer ring raceway formed on an inner peripheral surface thereof, an outer ring small-diameter part extending toward one axial side with respect to the outer ring raceway, an outer ring large-diameter part extending toward the other axial side with respect to the outer ring raceway, a radial wall part extending from an axial end portion of the outer ring small-diameter part toward an inner diameter-side, and a folded-back part extending from a radially inner end portion of the radial wall part toward the other axial side, the outer ring being a press-molded product; 
     an inner ring including an inner ring raceway formed on an outer peripheral surface thereof, an inner ring small-diameter part extending toward one axial side with respect to the inner ring raceway, and an inner ring large-diameter part extending toward the other axial side with respect to the inner ring raceway; 
     a plurality of balls rollably arranged between the outer ring raceway and the inner ring raceway, the plurality of balls being in contact with both the raceways at predetermined contact angles, and 
     a cage configured to rollably hold the plurality of balls, 
     wherein the folded-back part of the outer ring radially overlaps the inner ring small-diameter part with a radial gap between the folded-back part and an inner peripheral surface of the inner ring small-diameter part, and 
     wherein the radial gap of an inner diameter-side entry formed by an axial end of the folded-back part and the inner peripheral surface of the inner ring small-diameter part is smaller than the radial gap of an inner diameter-side exit formed by an axial end of the inner ring small-diameter part and an outer peripheral surface of the folded-back part. 
     (2) The bearing for a clutch of the above (1), wherein the inner ring small-diameter part is formed with an inner ring taper part, and a diameter of an inner peripheral surface of the inner ring taper part increases toward the axial end of the inner ring small-diameter part. 
     (3) The bearing for a clutch of the above (1), wherein the folded-back part of the outer ring is formed with an outer ring taper part, and a diameter of an outer peripheral surface of the outer ring taper part increases toward the axial end of the folded-back part. 
     (4) The bearing for a clutch of one of the above (1) to (3), wherein a ratio of the radial gap of the inner diameter-side exit to the radial gap of the inner diameter-side entry is 1:1.2 to 1:5.0. 
     (5) The bearing for a clutch of the above (1), wherein the inner ring small-diameter part is formed with an inner ring taper part, and a diameter of an inner peripheral surface of the inner ring taper part increases toward the axial end of the inner ring small-diameter part, and 
     wherein the folded-back part of the outer ring is formed with an outer ring taper part, and a diameter of an outer peripheral surface of the outer ring taper part increases toward the axial end of the folded-back part. 
     (6) The bearing for a clutch of the above (5), wherein a ratio of the radial gap of the inner diameter-side exit to the radial gap of the inner diameter-side entry is 1:1.4 to 1:10.0. 
     (7) The bearing for a clutch of one of the above (1) to (6), wherein the inner ring has a flange part extending from an axial end portion of the inner ring large-diameter part toward an outer diameter-side and facing an axial end of the outer ring large-diameter part with an axial gap between the flange part and the axial end of the outer ring large-diameter part, and 
     wherein the axial gap is larger than the radial gap of the inner diameter-side exit. 
     (8) The bearing for a clutch of one of the above (1) to (7), wherein the cage has a small circular ring part, a large circular ring part and a plurality of column parts configured to axially connect the small circular ring part and the large circular ring part, and 
     wherein a sum of cross-sectional areas of gaps between the small circular ring part and the outer ring and inner ring is smaller than a sum of cross-sectional areas of gaps between the large circular ring part and the outer ring and inner ring. 
     (9) The bearing for a clutch of one of the above (1) to (8), wherein the outer ring and the inner ring are formed by pressing a metal plate of an alloy material or a steel material, in which carbon of 0.7 to 0.9 weight %, manganese of 0.3 to 0.9 weight %, chromium of 0.3 to 1.0 weight % and silicone of 0.01 to 0.15 weight % are contained, and an ironing rate of the folded-back part is equal to or higher than 60%. 
     (10) The bearing for a clutch of one of the above (1) to (9), wherein the inner ring has a flange part extending from an axial end portion of the inner ring large-diameter part toward an outer diameter-side and facing an axial end of the outer ring large-diameter part with an axial gap between the flange part and the axial end of the outer ring large-diameter part, and 
     wherein at least one of an outer surface of the radial wall part of the outer ring and an outer surface of the flange part of the inner ring is formed with a locking part capable of engaging with a counter member. 
     Effects of the Invention 
     According to the bearing for a clutch of the present invention, the outer ring has the outer ring small-diameter part extending toward one axial side with respect to the outer ring raceway, the radial wall part extending from the axial end portion of the outer ring small-diameter part toward the inner diameter-side, and the folded-back part extending from the inner end portion of the radial wall part toward the other axial side and radially overlapping the inner ring small-diameter part with the radial gap between the folded-back part and the inner peripheral surface of the inner ring small-diameter part. Also, the radial gap of the inner diameter-side entry formed by the axial end of the folded-back part and the inner peripheral surface of the inner ring small-diameter part is smaller than the radial gap of the inner diameter-side exit formed by the axial end of the inner ring small-diameter part and the outer peripheral surface of the folded-back part. Thereby, an amount of lubricant to be supplied from the radial gap of the inner diameter-side entry to the bearing for a clutch is limited, and the lubricant is smoothly discharged from the radial gap of the inner diameter-side exit to an inside (a part to be lubricated) of the bearing for a clutch, so that an oil amount and a flow of the lubricant are controlled to reduce a stirring resistance of the lubricant, thereby suppressing rotating torque. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an enlarged sectional view of main parts of a bearing for a clutch in accordance with a first embodiment of the present invention, and  FIG. 1B  is an enlarged view of the I part of  FIG. 1A . 
         FIG. 2  is an enlarged sectional view of main parts of a bearing for a clutch in accordance with a second embodiment of the present invention. 
         FIG. 3  is an enlarged sectional view of main parts of a bearing for a clutch in accordance with a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a bearing for a clutch in accordance with each embodiment of the present invention will be described in detail with reference to the drawings. 
     First Embodiment 
     As shown in  FIG. 1A , a bearing  10  for a clutch of a first embodiment is an angular ball bearing including a substantially circular ring-shaped outer ring  11 , a substantially circular ring-shaped inner ring  12  formed concentrically with the outer ring  11 , a plurality of balls  15  which is rollably arranged between an outer ring raceway  11   a  formed on an inner peripheral surface of the outer ring  11  and an inner ring raceway  12   a  formed on an outer peripheral surface of the inner ring  12  and which is in contact with both the raceways  11   a ,  12   a  at predetermined contact angles, and a cage  16  configured to hold the balls  15  with predetermined intervals. The respective raceways  11   a ,  12   a  and rolling surfaces of the balls  15  of the bearing  10  for a clutch are lubricated by lubricant to be supplied. 
     The outer ring raceway  11   a  is located closely to one axial side (a left side, in  FIG. 1A ) with respect to a center O of the plurality of balls  15  in  FIG. 1A , and has a curved surface of about ¼ of a maximum outer shape of the ball  15 . Also, the outer ring  11  has a circular ring-shaped outer ring small-diameter part  11   b  extending from a part configuring the outer ring raceway  11   a  toward one axial side, a circular ring-shaped outer ring large-diameter part  11   e  extending from a part configuring the outer ring raceway  11   a  toward the other axial side (a right side, in  FIG. 1A ), a radial wall part  11   c  extending from an axial end portion of the outer ring small-diameter part  11   b  toward an inner diameter-side, and a folded-back part  11   d  extending from a radially inner end portion of the radial wall part  11   c  toward the other axial side. 
     Therefore, a part of one axial side of the outer ring  11  has a substantially C-shaped section. 
     Also, a part of the folded-back part  11   d  is arranged at an inner diameter-side of an inner ring small-diameter part  12   b  (which will be described later) with a radial gap being interposed therebetween. Thereby, the folded-back part  11   d  and the inner ring small-diameter part  12   b  form an overlapping part  20  (refer to  FIG. 1B ) at which they radially overlap over an axial length W. An outer peripheral surface of the folded-back part  11   d  is formed as a cylindrical surface having a constant outer diameter within a range of the overlapping part  20 . 
     The inner ring raceway  12   a  is located closely to the other axial side with respect to the center O of the plurality of balls  15  in  FIG. 1A , and has a curved surface of about ¼ of the maximum outer shape of the ball  15 . The inner ring  12  has a circular ring-shaped inner ring small-diameter part  12   b  extending from a part configuring the inner ring raceway  12   a  toward one axial side, a circular ring-shaped inner ring large-diameter part  12   d  extending from a part configuring the raceway  12   a  toward the other axial side, and a flange part  12   e  extending radially outward from the other axial end side of the inner ring large-diameter part  12   d . An inner peripheral surface of the inner ring small-diameter part  12   b  is formed with an inner ring taper part  12   c  of which a diameter increases toward an axial end of the inner ring small-diameter part  12   b  and a cylindrical surface part  12   g  having an inner peripheral surface of which an inner diameter is constant. In the meantime, an axial boundary position between the cylindrical surface part  12   g  and the inner ring taper part  12   c  may be located more closely to one axial side than an axial end of the folded-back part  11   d , as shown in  FIG. 1B , or may be located more closely to the other axial side than the axial end of the folded-back part  11   d.    
     In particular, as shown in  FIG. 1B , when the axial boundary position between the cylindrical surface part  12   g  and the inner ring taper part  12   c  is located more closely to one axial side than the axial end of the folded-back part  11   d , the overlapping part  20  is configured by a parallel gap portion  21  and a tapered gap portion  22 . The parallel gap portion  21  has an axial length W 1 , is configured by the other axial side of the overlapping part  20 , and is formed between an inner peripheral surface of the cylindrical surface part  12   g  and an outer peripheral surface of the folded-back part  11   d . The parallel gap portion  21  defines a radial entry gap (hereinafter, referred to as “inner diameter-side entry gap”) C 1  of which a gap dimension is axially constant. 
     On the other hand, the tapered gap portion  22  has an axial length W 2  (=W−W 1 ), is configured by one axial side of the overlapping part  20 , and is formed between an inner peripheral surface of the inner ring taper part  12   c  and an outer peripheral surface of the folded-back part  11   d . A radial gap of the tapered gap portion  22  gradually increases toward one axial side, and defines a radial exit gap (hereinafter, referred to as “inner diameter-side exit gap”) C 2  that is largest at one axial side end portion of the radial gap. 
     In this case, the inner diameter-side entry gap C 1  is set smaller than the inner diameter-side exit gap C 2 . Specifically, like the first embodiment, when the outer peripheral surface of the folded-back part  11   d  is formed to have a constant diameter and the radial gap is changed only by the inner ring taper part  12   c , a ratio of the inner diameter-side exit gap C 2  to the inner diameter-side entry gap C 1  is preferably set to be 1:1.2 to 1:5.0. 
     Also, in this case, a ratio of the length W 1  of the parallel gap portion  21  and the length W of the overlapping part  20  is preferably W 1 /W=0.1 to 0.5, and more preferably 0.2 to 0.4. 
     The length W 1  of the parallel gap portion  21  is secured by 0.1 or greater with respect to the length W of the overlapping part  20 , so that it is easy to control an inflow amount of the lubricant. Also, the length W 1  of the parallel gap portion  21  is set to 0.5 or smaller with respect to the length W of the overlapping part  20 , so that it is possible to design an inclination of the inner ring taper part  12   c  so as to be a gentle gradient. 
     Also, a ratio of the length W 1  of the parallel gap portion  21  and the inner diameter-side entry gap C 1  is preferably (the length W 1  of the parallel gap portion  21 )/(the inner diameter-side entry gap C 1 )=1 to 5, and more preferably 2 to 4. 
     The length W 1  of the parallel gap portion  21  is secured by 1 or greater with respect to the inner diameter-side entry gap C 1 , so that it is easy to control the inflow amount of the lubricant. Also, the length W 1  of the parallel gap portion  21  is set to 5 or smaller with respect to the inner diameter-side entry gap C 1 , so that it is easy to avoid contact between the inner ring small-diameter part  12   b  and the folded-back part  11   d.    
     Also, a ratio of the length W of the overlapping part  20  and an inner ring plate thickness t of the cylindrical surface part  12   g  of the inner ring small-diameter part  12   b  is preferably (the length W of the overlapping part  20 )/(the inner ring plate thickness t)=1 to 2, and more preferably 1.2 to 1.8. 
     Also, the flange part  12   e  of the inner ring  12  faces an axial end of the outer ring large-diameter part  11   e , and an axial gap (hereinafter, referred to as “outer diameter-side exit gap”) C 3  is formed between the flange part  12   e  and the axial end of the outer ring large-diameter part  11   e.    
     The outer diameter-side exit gap C 3  is set greater than the inner diameter-side exit gap C 2 . That is, dimensions of the respective gaps increase in order of the inner diameter-side entry gap C 1 , the inner diameter-side exit gap C 2 , and the outer diameter-side exit gap C 3  (C 1 &lt;C 2 &lt;C 3 ). 
     The outer ring  11  and the inner ring  12  are made by pressing and heat-treating a plate material of an alloy material or a steel material, in which carbon of 0.7 to 0.9 weight %, manganese of 0.3 to 0.9 weight %, chromium of 0.3 to 1.0 weight % and silicone of 0.01 to 0.15 weight % are contained. For example, PCR5 may be used. 
     The reasons to include the respective elements and to define the contents thereof are described. In order to obtain high hardness of HRC60 or higher necessary for the outer ring  11  and the inner ring  12  by a quenching treatment, a carbon amount of 0.7 weight % or more is required. However, when the carbon amount exceeds 0.9 weight %, the deep drawability is lowered. Silicone is added as a deoxidizing agent upon the steel making and is normally contained in an amount of 0.01 weight % or more. However, when silicone is contained in an amount more than 0.15 weight %, ferrite is reinforced and the deep drawability is lowered. Therefore, the content of silicone is set to 0.15 weight % or less. Manganese is added as a deoxidizing agent, like silicone, thereby improving the hardenability. However, when manganese is added too much, a deformation resistance is increased. Therefore, an upper limit thereof is set to 0.9 weight %. Chromium is added in an amount of 0.3 weight % or more so as to improve the hardenability. However, when the content of chromium exceeds 1.0 weight %, the deep drawability is lowered. Therefore, an upper limit thereof is set to 1.0 weight %. 
     Like this, the outer ring  11  is made by the material having high ductility and workability, so that it is possible to set an ironing rate of the folded-back part  11   d  to 60% or higher, and preferably 60 to 65%, and the workability is improved. In the meantime, the ironing rate means a reduction rate of a plate thickness after ironing to a plate thickness before ironing. 
     At a clutch part, the inner ring and outer ring of the bearing are not fitted with a shaft and a housing, and are in contact with the other components only at flat surface parts of both sides of the bearing after mounting. When the outer ring  11  is configured as a fixed ring and the inner ring  12  is configured as a rotating ring, it is required that the outer ring  11  should be connected to the other component (for example, a guide sleeve) without sliding and the inner ring  12  should rotate without sliding relative to the other component (for example, a diaphragm spring). For this reason, in general, a load is axially applied so as to apply frictional forces between the outer ring  11  and the guide sleeve and between the inner ring  12  and the diaphragm spring. However, when the high axial load is applied to the bearing  10  for a clutch, the rotating torque increases, so that a fuel consumption of a vehicle may be deteriorated. 
     The bearing  10  for a clutch of the first embodiment has concave parts  11   f  or hole  12   f , which are locking parts configured to engage with protrusions (not shown) of counter members (the guide sleeve and the diaphragm spring) and provided at at least three places of outer surfaces of the radial wall part  11   c  of the outer ring  11  and the flange part  12   e  of the inner ring  12 . The protrusions of the counter members are engaged to the concave parts  11   f  or the hole  12   f  and the rotation is thus prevented, so that it is not necessary to apply the axial load for sliding prevention and improvement on the fuel consumption is thus expected. Thereby, the axial load necessary for the bearing  10  for a clutch is only a preload for removing an axial backlash of the bearing. 
     Both the locking parts may be configured only by the concave parts  11   f  or only by the hole  12   f  or by a combination of the concave part  11   f  and the hole  12 E The concave part  11   f  and the hole  12   f  may be formed only in the outer surface of the radial wall part  11   c , only in the outer surface of the flange part  12   e  of the inner ring  12 , or in both the outer surfaces of the radial wall part  11   c  and the flange part  12   e . Also, the concave part and the hole may be formed in the counter member and the protrusions functioning as the locking part may be formed on the outer surfaces of the radial wall part  11   c  and the flange part  12   e.    
     Also, the outer surfaces of the radial wall part  11   c  of the outer ring  11  and the flange part  12   e  of the inner ring  12  may be roughened to increase the frictional force. 
     The cage  16  has a small circular ring part  16   a  arranged at each small diameter part-side of the bearing  10  for a clutch, a large circular ring part  16   b  arranged at a large diameter part-side, and a plurality of column parts  16   c  configured to connect the small circular ring part  16   a  and the large circular ring part  16   b  in an axially inclined direction. A pocket  17  for holding the ball  15  is formed by axially inner surfaces of the small circular ring part  16   a  and the large circular ring part  16   b  and circumferential side surfaces of the adjacent column parts  16   c.    
     A sum (S 4 +S 5 ) of a cross-sectional area S 4 , as seen from the axial direction, of a circular ring-shaped gap C 4  formed by an outer peripheral surface of the small circular ring part  16   a  and an inner peripheral surface of the outer ring small-diameter part  11   b  and a cross-sectional area S 5 , as seen from the axial direction, of a circular ring-shaped gap C 5  formed by an inner peripheral surface of the small circular ring part  16   a  and an outer peripheral surface of the inner ring small-diameter part  12   b  is set smaller than a sum (S 6 +S 7 ) of a cross-sectional area S 6 , as seen from the axial direction, of an outer peripheral surface of the large circular ring part  16   b  and an inner peripheral surface of the outer ring large-diameter part  11   e  and a cross-sectional area S 7 , as seen from the axial direction, of a circular ring-shaped gap C 7  formed by an inner peripheral surface of the large circular ring part  16   b  and an outer peripheral surface of the inner ring large-diameter part  12   d . This configuration can be made by causing a radial width of the small circular ring part  16   a  to be different from a radial width of the large circular ring part  16   b . Also, the cross-sectional area S 5 , as seen from the axial direction, of the circular ring-shaped gap C 5  is set smaller than the cross-sectional area S 4 , as seen from the axial direction, of the circular ring-shaped gap C 4 . 
     Meanwhile, in the first embodiment, the respective circular ring-shaped gaps C 4  to C 7  are defined at axially outer end positions (one axial side end portion of the small circular ring part  16   a  and the other axial side end portion of the large circular ring part  16   b ) of the small circular ring part  16   a  and the large circular ring part  16   b.    
     Subsequently, oil amount control of the lubricant by the gaps C 1  to C 7  of the respective parts of the bearing  10  for a clutch is described. 
     The lubricant for lubricating rolling surfaces of the bearing  10  for a clutch is introduced from the inner diameter-side entry gap C 1  and is supplied to the inside through the inner diameter-side exit gap C 2  and the circular ring-shaped gaps C 4  and C 5  between the small circular ring part  16   a  of the cage  16  and the outer ring  11  and inner ring  12 , thereby lubricating the parts to be lubricated (rolling surfaces). The lubricant which lubricated the parts to be lubricated (sliding contact surfaces between the balls  15  and the respective raceways  11   a ,  12   a  and between the balls  15  and the cage  16 ) is discharged to an outside from the circular ring-shaped gaps C 6  and C 7  between the large circular ring part  16   b  of the cage  16  and the outer ring  11  and inner ring  12  and the outer diameter-side exit gap C 3  between the flange part  12   e  and the axial end of the outer ring large-diameter part  11   e.    
     Here, since the inner diameter-side entry gap C 1  is set smaller than the inner diameter-side exit gap C 2 , the inflow of the lubricant beyond necessity is suppressed by the inner diameter-side entry gap C 1 . Also, the introduced lubricant is smoothly discharged (introduced into a space in the bearing) from the larger inner diameter-side exit gap C 2 . Specifically, the lubricant introduced from the inner diameter-side entry gap C 1  is moved toward one axial side along the inner ring taper part  12   c  by the centrifugal force of the inner ring  12  being rotated, and is then thrown and scattered into the space in the bearing from the axial end of the inner ring small-diameter part  12   b . Also, since the outer diameter-side exit gap C 3  is set greater than the inner diameter-side exit gap C 2 , the lubricant having lubricated the parts to be lubricated is smoothly discharged to the outside of the bearing  10  for a clutch without staying in the bearing  10  for a clutch, so that the stirring resistance of the lubricant is reduced. 
     Also, the sum of the cross-sectional areas S 4  and S 5 , as seen from the axial direction, of the circular ring-shaped gaps C 4  and C 5  between the small circular ring part  16   a  of the cage  16  and the outer ring  11  and inner ring  12  is set smaller than the sum of the cross-sectional areas S 6  and S 7 , as seen from the axial direction of the circular ring-shaped gaps C 6  and C 7  between the large circular ring part  16   b  of the cage  16  and the outer ring  11  and inner ring  12 . Accordingly, an oil amount to be introduced to the part to be lubricated is suppressed to a necessary amount, and the introduced lubricant is discharged from the circular ring-shaped gaps C 6  and C 7  without staying in the parts to be lubricated for a long time. Also, since the cross-sectional area S 5 , as seen from the axial direction, of the circular ring-shaped gap C 5  is set smaller than the cross-sectional area S 4 , as seen from the axial direction, of the circular ring-shaped gap C 4 , the more lubricant flows to the outer ring  11  and the stirring of the lubricant by the column parts  16   c  of the cage  16  is reduced. 
     In particular, the lubricant introduced from the circular ring-shaped gap C 4  is moved to the outer diameter-side along the outer ring raceway  11   a  by the centrifugal force of the bearing being rotated and is then discharged from the circular ring-shaped gap C 6 . On the other hand, most of the lubricant introduced from the circular ring-shaped gap C 5  is guided to the inclined column part  16   c  (along the surface of the inner diameter-side of the column part  16   c ) by the centrifugal force of the bearing being rotated, and is then discharged from the circular ring-shaped gap C 7 . Also, a part of the lubricant introduced from the circular ring-shaped gap C 5  passes (lubricates) between the ball  15  and the pocket  17 , and is then discharged from the circular ring-shaped gap C 6 . In this way, since the lubricant of which the inflow is limited by the circular ring-shaped gaps C 4  and C 5  is positively discharged from the parts to be lubricated, the lubricant does not stay in the vicinity of the column parts  16   c  of the cage  16 . 
     Also, when an area, as seen from the axial direction, of the small circular ring part  16   a  of the cage  16  is denoted as S 16   a , a ratio of the area  16   a , as seen from the axial direction, of the small circular ring part  16   a  to a cross-sectional area SI (SI=S 4 +S 5 +S 16   a ), as seen from the axial direction, between the inner and outer rings at the axial end face position of the small circular ring part  16   a  is preferably S 16   a /SI=0.6 to 0.9, and more preferably 0.7 to 0.8. 
     When S 16   a /SI is set to 0.6 or greater, it is possible to secure an amount of the oil to be attached to the end face of the small circular ring part  16   a , to move the attached oil to the outer diameter-side by the centrifugal force resulting from the revolution of the cage, and to increase the ratio of the oil to pass the circular ring-shaped gap C 4 . On the other hand, when S 16   a /SI is set to 0.9 or smaller, it is possible to prevent a situation where a ratio of the cage section excessively increases to deteriorate the entire flow of the oil. 
     Like this, according to the bearing  10  for a clutch that is used in the lubrication environment of the lubricant, the stirring resistance of the lubricant is reduced, so that the rotating torque is also reduced and the fuel consumption of the vehicle is improved. 
     As described above, according to the bearing  10  for a clutch of the first embodiment, the outer ring  11  has the outer ring small-diameter part  11   b  extending toward one axial side with respect to the outer ring raceway  11   a , the radial wall part  11   c  extending from the axial end portion of the outer ring small-diameter part  11   b  toward the inner diameter-side, and the folded-back part  11   d  extending from the radially inner end portion of the radial wall part  11   c  toward the other axial side and radially overlapping the inner ring small-diameter part  12   b  with the radial gap between the folded-back part and the inner peripheral surface of the inner ring small-diameter part. The inner diameter-side entry gap C 1  formed by the axial end of the folded-back part  11   d  and the inner peripheral surface of the inner ring small-diameter part  12   b  is smaller than the inner diameter-side exit gap C 2  formed by the axial end of the inner ring small-diameter part  12   b  and the outer peripheral surface of the folded-back part  11   d . Accordingly, it is possible to limit the amount of lubricant to be supplied from the inner diameter-side entry gap C 1  to the bearing  10  for a clutch, to smoothly introduce the lubricant from the inner diameter-side exit gap C 2  to the inside (the parts to be lubricated) of the bearing  10  for a clutch, and to control an oil amount and a flow of the lubricant, thereby reducing the stirring resistance of the lubricant and implementing the low torque. 
     Also, since the inner ring small-diameter part  12   h  is formed with the inner ring taper part  12   c  at which the diameter of the inner peripheral surface of the inner ring small-diameter part increases toward the axial end of the inner ring small-diameter part  12   b , it is possible to arbitrarily set the ratio of the inner diameter-side entry gap C 1  and the inner diameter-side exit gap C 2 . 
     Also, since the ratio of the inner diameter-side exit gap C 2  to the inner diameter-side entry gap C 1  is 1:1.2 to 1:5.0, it is possible to easily design the radial gap only by the inner ring taper part  12   c.    
     Also, since the outer diameter-side exit gap C 3  between the axial end of the outer ring large-diameter part  11   e  and the flange part  12   e  of the inner ring  12  is larger than the inner diameter-side exit gap C 2 , the lubricant in the bearing  10  for a clutch is smoothly discharged to the outside of the bearing  10  for a clutch without staying in the bearing, so that the stirring resistance of the lubricant is reduced. 
     Also, since the sum of the cross-sectional areas S 4 , S 5  of the gaps C 4 , C 5  between the small circular ring part  16   a  of the cage  16  and the outer ring  11  and inner ring  12  is smaller than the sum of the cross-sectional areas S 6 , S 7  of the gaps C 6 , C 7  between the large circular ring part  16   b  and the outer ring  11  and inner ring  12 , the lubricant in the bearing  10  for a clutch is smoothly discharged, so that the stirring resistance of the lubricant is reduced. 
     Also, since the outer ring  11  and the inner ring  12  are made by pressing the metal plate of the alloy material or steel material, in which carbon of 0.7 to 0.9 weight %, manganese of 0.3 to 0.9 weight %, chromium of 0.3 to 1.0 weight % and silicone of 0.01 to 0.15 weight % are contained, and the ironing rate of the folded-back part  11   d  is equal to or higher than 60%, the folded-back part  11   d  can be easily formed. 
     Also, at least one of the outer surface of the radial wall part  11   c  of the outer ring  11  and the outer surface of the flange part  12   e  of the inner ring  12  is formed with the locking part such as the concave part  11   f  or the hole  12   f  capable of engaging with the protrusion provided at the counter member. Therefore, the rotation is stopped by engaging the protrusion of the counter member and the concave part  11   f  or the hole  12   f . Thereby, it is not necessary to apply the axial load, which has been applied for preventing the sliding, so that the rotating torque is reduced and the fuel consumption is improved. 
     Second Embodiment 
     Subsequently, a bearing for a clutch of a second embodiment is described with reference to  FIG. 2 . In the meantime, in the bearing  10  for a clutch of the second embodiment, the shapes of the folded-back part  11   d  of the outer ring  11  and the inner ring small-diameter part  12   b  are different from those of the bearing  10  for a clutch of the first embodiment, and the other parts are substantially the same as the first embodiment of the present invention. Therefore, the parts, which are the same as or equivalent to the first embodiment, are denoted with the same reference numerals, and the descriptions thereof are simplified or omitted. 
     In the inner ring  12  of the second embodiment, the inner ring small-diameter part  12   b  axially extends in parallel with a central axis of the bearing  10  for a clutch. In the meantime, the folded-back part  11   d  of the outer ring  11  is provided with an outer ring taper part  11   g  formed so that a diameter of the outer peripheral surface of the folded-back part  11   d  increases toward the axial end (rightward, in  FIG. 2 ) of the folded-back part  11   d.    
     The overlapping part W of the folded-back part  11   d  of the outer ring  11  and the inner ring small-diameter part  12   b  is formed with the inner diameter-side entry gap C 1  between the axial end of the folded-hack part  11   d  and the inner peripheral surface of the inner ring small-diameter part  12   b  and the inner diameter-side exit gap C 2  between the axial end of the inner ring small-diameter part  12   b  and the outer peripheral surface of the folded-hack part  11   d  by the outer ring taper part  11   g.    
     Thereby, the inner diameter-side entry gap C 1  is set smaller than the inner diameter-side exit gap C 2 . Specifically, in this case, the inner diameter of the inner peripheral surface of the inner ring small-diameter part  12   b  is constant, and the ratio of the inner diameter-side exit gap C 2  to the inner diameter-side entry gap C 1  is set to 1:1.2 to 1.5.0 only by the outer ring taper part  11   g.    
     In this way, according to the bearing  10  for a clutch of the second embodiment, since the folded-back part  11   d  of the outer ring  11  is formed with the outer ring taper part  11   g  at which the diameter of the outer peripheral surface of the folded-back part increases toward the axial end of the folded-back part  11   d , it is possible to easily set the inner diameter-side entry gap C 1  smaller than the inner diameter-side exit gap C 2 . 
     The other configurations and operations are the same as the bearing  10  for a clutch of the first embodiment. 
     Third Embodiment 
     Subsequently, a bearing for a clutch of a third embodiment is described with reference to  FIG. 3 . In the bearing  10  for a clutch of the third embodiment shown in  FIG. 3 , the inner peripheral surface of the inner ring small-diameter part  12   b  is formed with the inner ring taper part  12   c  of which the diameter increases toward the axial end of the inner ring small-diameter part  12   b , like the bearing  10  for a clutch of the first embodiment. Also, the folded-back part  11   d  of the outer ring  11  is formed with the outer ring taper part  11   g  so that the diameter of the outer peripheral surface of the folded-back part  11   d  increases toward the axial end (rightward, in  FIG. 3 ) of the folded-back part  11   d , like the bearing  10  for a clutch of the second embodiment. 
     The overlapping part W of the folded-back part  11   d  of the outer ring  11  and the inner ring small-diameter part  12   b  is formed with the inner diameter-side entry gap C 1  between the axial end of the folded-back part  11   d  and the inner peripheral surface of the inner ring small-diameter part  12   b  and the inner diameter-side exit gap C 2  between the axial end of the inner ring small-diameter part  12   b  and the outer peripheral surface of the folded-back part  11   d  by the inner ring taper part  12   c  and the outer ring taper part  11   g.    
     Thereby, the inner diameter-side entry gap C 1  is set smaller than the inner diameter-side exit gap C 2 . Specifically, in this case, the ratio of the inner diameter-side exit gap C 2  to the inner diameter-side entry gap C 1  is set to 1:1.4 to 1:10.0 by the inner ring taper part  12   c  and the outer ring taper part  11   g.    
     According to the bearing  10  for a clutch of the third embodiment, the inner ring taperpart  12   c  and the outer ring taper part  11   g  are provided, so that the inner diameter-side entry gap C 1  is set smaller than the inner diameter-side exit gap C 2 . Also, it is possible to design the ratio of the inner diameter-side exit gap C 2  to the inner diameter-side entry gap C 1  within the range of 1:1.4 to 1:10.0. 
     The other configurations and operations are the same as the bearing  10  for a clutch of the first and second embodiments. 
     In the meantime, the present invention is not limited to the respective embodiments, and can be appropriately modified and improved. For example, the bearing for a clutch of the present invention may be a clutch release bearing in which the force is to be applied to the bearing when a clutch is opened, or a clutch engaging bearing in which the force is to be applied to the bearing when the clutch is fastened. 
     The subject application is based on Japanese Patent Application Nos. 2017-145744 filed on Jul. 27, 2017 and 2018-23982 filed on Feb. 14, 2018, the contents of which are incorporated herein by reference. 
     DESCRIPTION OF REFERENCE NUMERALS 
       10 : bearing for a clutch,  11 : outer ring,  11   a : outer ring raceway,  11   b : outer ring small-diameter part,  11   c : radial wall part,  11   d : folded-back part,  11   e : outer ring large-diameter part,  11   f : concave part (locking part),  11   g : outer ring taper part,  12 : inner ring,  12   a : inner ring raceway,  12   b : inner ring small-diameter part,  12   c : inner ring taper part,  12   d : inner ring large-diameter part,  12   e : flange part,  12   f : hole (locking part),  12   g : cylindrical surface part,  15 : ball,  16 : cage,  16   a : small circular ring part,  16   b : large circular ring part,  16   c : column part,  17 : pocket, C 1 : inner diameter-side entry gap (radial gap of inner diameter-side entry), C 2 : inner diameter-side exit gap (radial gap of inner diameter-side exit), C 3 : outer diameter-side exit gap (axial gap), C 4 : circular ring-shaped gap (gap between small circular ring part and outer ring), C 5 : circular ring-shaped gap (gap between small circular ring part and inner ring), C 6 : circular ring-shaped gap (gap between large circular ring part and outer ring), C 7 : circular ring-shaped gap (gap between large circular ring part and inner ring), S 4 : cross-sectional area of gap C 4 , S 5 : cross-sectional area of gap C 5 , S 6 : cross-sectional area of gap C 6 , S 7 : cross-sectional area of gap C 7