Thrust roller bearing cage and method for manufacturing the same

A thrust roller bearing cage (11) a thrust roller bearing (20) and a plurality of pockets (21) accommodating rollers (13). The thrust roller bearing cage (11) includes: a radially outer area bent portion (41) formed by bending an area located radially outside the pockets (21) inward in a radial direction; and projecting portions (44) that are formed in a tip end of the radially outer area bent portion (41) and project inward in the radial direction so as to contact end faces (16) of the rollers (13) accommodated in the pockets. Radially outer edges (21a) of the pockets (21) are located radially outside base end parts (44a) of the projecting portions (44).

TECHNICAL FIELD

The present invention relates to thrust roller bearing cages (hereinafter sometimes simply referred to as the “cages”) and methods for manufacturing the same, and more particularly relates to a thrust roller bearing cage that is manufactured by using a press and a method for manufacturing the same.

BACKGROUND ART

For example, thrust roller bearings that support thrust loads are sometimes placed at such locations in automatic transmissions for automobiles, car air conditioner compressors, etc. that are subjected to thrust loads. For improved fuel efficiency and power saving, it is desired to reduce running torque of such thrust roller bearings. A thrust roller bearing includes bearing rings arranged in the direction of the rotation axis of the thrust roller bearing, a plurality of needle rollers that roll on raceway surfaces of the bearing rings, and a cage that retains the plurality of needle rollers. Some cages are manufactured by bending a steel sheet and then punching out pockets that accommodate the rollers.

A technique relating to the cages included in such thrust roller bearings is disclosed in, e.g., Japanese Unexamined Patent Publication No. H10-220482 (Patent Literature 1). The thrust roller bearing cage of Patent Literature 1 includes an annular body formed by cutting, punching, etc. and projecting portions formed in a radially outer part of the annular body. The projecting portions are formed such that those parts of the radially outer end face of the folded part of the annular body which are located at the positions of the pockets project inward in the radial direction, and the tip ends of the projecting portions face approximately the centers of the end faces of the rollers. That is, the projecting portions of Patent Literature 1 face the inside of the pockets beyond the radially outer end faces of the pockets. Patent Literature 1 discloses that this thrust roller bearing cage reduces running torque of the rollers as the tip ends of the projecting portions contact the rollers at positions near the rotation centers of the rollers.

CITATION LIST

Patent Literatures

Patent Literature 1: Japanese Unexamined Patent Application Publication No. H10-220482

SUMMARY OF INVENTION

Technical Problem

However, lubricant may not flow satisfactorily in the thrust roller bearing of Patent Literature 1.

It is therefore an object of the present invention to provide a thrust roller bearing cage that allows a satisfactory flow of lubricant and a method for manufacturing the same.

Solution to Problem

The inventors found that the problem of an unsatisfactory flow of lubricant in the thrust roller bearing of Patent Literature 1 is significant in areas where the projecting portions contact the rollers. The inventors completed the present invention through their intensive research regarding how to achieve a satisfactory flow of lubricant without in the areas where the projecting portions contact the rollers.

A thrust roller bearing cage according to the present invention is a thrust roller bearing cage included in a thrust roller bearing and including a plurality of pockets accommodating rollers. The thrust roller bearing cage includes: a radially outer area bent portion formed by bending an area located radially outside the pockets inward in a radial direction; and projecting portions that are formed in a tip end of the radially outer area bent portion and project inward in the radial direction so as to contact end faces of the rollers accommodated in the pockets. Radially outer edges of the pockets are located radially outside base end parts of the projecting portions.

A method for manufacturing a thrust roller bearing cage according to the present invention is a method for manufacturing a thrust roller bearing cage included in a thrust roller bearing and including a plurality of pockets accommodating rollers. The method includes the steps of: preparing a cage material that will later become the cage; forming an outer shape of the cage material so that the outer shape has portions that will later become projecting portions projecting inward in a radial direction so as to contact end faces of the rollers accommodated in the pockets; forming the pockets in the cage material so that radially outer edges of the pockets are located radially outside base end parts of the projecting portions; and forming a radially outer area bent portion by bending an area of the cage material which is located radially outside the pockets inward in the radial direction.

According to the thrust roller bearing cage of the present invention and the method for manufacturing the same, the radially outer edges of the pockets are located radially outside the base end parts of the projecting portions. Clearance between the radially outer edge of each pocket and the base end part of each projecting portion can therefore serve as a lubricant flow path. This ensures that a sufficient lubricant flow path is present in the pockets with the projecting portions being in contact with the end faces of the rollers, whereby a satisfactory lubricant flow can be achieved. The thrust roller bearing cage of the present invention and the method for manufacturing the same can thus achieve a satisfactory flow of lubricant.

In the thrust roller bearing cage according to the present invention, it is preferable that areas of the projecting portions which are to contact the end faces of the rollers be subjected to a press-flattening process.

It is preferable that the method for manufacturing the thrust roller bearing cage according to the present invention further include the step of press-flattening areas of the projecting portions which are to contact the end faces of the rollers.

Since the areas of the projecting portions which are to contact the end faces of the rollers are subjected to the press-flattening process, this can reduce the risk that discontinuity of a lubricant film will be caused during rotation of the bearing by the sliding motion of the end faces of the rollers on the areas of the projecting portions which contact the end faces of the rollers. This improves lubricating properties in the contact areas and reduces what is called aggression of the rollers against the projecting portions of the cage. Such a thrust roller bearing cage can thus further reduce the running torque of the bearing.

As used herein, the “press-flattening process” means a process in which, in the step of forming the radially outer area bent portion, the projecting portions are pressed outward in the radial direction by using a radially outer surface of a die that serves as a stopper to control the amount of collapse, in order to smooth the rough surfaces of the projecting portions before and after the process. Specifically, the press-flattening process can smooth a press-sheared surface or a fracture surface, which is formed in the step of forming the outer shape, to arithmetic mean roughness Ra (JIS B 0601) of about 2 μm or less.

In the thrust roller bearing cage according to the present invention, it is preferable that the projecting portions be formed by bending a radially outer area of the cage obliquely inward in the radial direction.

In the method for manufacturing the thrust roller bearing cage according to the present invention, it is preferable that, in the step of forming the radially outer area bent portion, the area of the cage material which is located radially outside the pockets be bent obliquely inward in the radial direction.

The radially outer area bent portion is formed by bending the area located radially outside the pockets obliquely inward in the radial direction. Accordingly, the projecting portions formed in the tip end of the radially outer area bent portion have a smaller contact area with the rollers than in the case where the area located radially outside the pockets is not tilted (the tilt angle is 0°) as in Patent Literature 1. When the rollers are biased from the central axis of the cage toward the outside in the radial direction by a rotational centrifugal force of the rollers, friction is generated between each roller and the part of each projecting portion which contacts the roller. However, since the projecting portions have a smaller contact area with the rollers, rotational resistance can be reduced. Running torque can therefore be reduced.

Advantageous Effects of Invention

The thrust roller bearing cage of the present invention and the method for manufacturing the same allow a satisfactory flow of lubricant.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below with reference to the drawings. In the figures described below, the same or corresponding portions are denoted with the same reference characters, and description thereof will not be repeated.

FIG. 1shows a part of a thrust roller bearing cage11according to an embodiment of the present invention.FIG. 1shows the cage11as viewed in the direction of the rotation axis of the cage11.FIG. 2is a sectional view of the thrust roller bearing cage11shown inFIG. 1.FIG. 2shows the cage11taken along line II-II inFIG. 1. Specifically, a portion having a pocket, which will be described later, is shown in section on the right side ofFIG. 2, and a portion having a pillar, which will be described later, is shown in section on the left side ofFIG. 2.FIG. 3is an enlarged sectional view showing a part of the thrust roller bearing cage11shown inFIG. 2. The enlarged sectional view inFIG. 3shows the area indicated by III inFIG. 2. InFIGS. 2 and 3, the rotation axis12of the cage11is shown by an alternate long and short dash line. For ease of understanding,FIG. 3shows a needle roller13accommodated in a pocket21that will be described later, and a part of a pair of bearing rings14,15disposed on both sides of the cage11in the direction of the rotation axis of the cage11.FIG. 4is an enlarged plan view showing a part of the thrust roller bearing cage11shown inFIGS. 1 and 2. The enlarged plan view inFIG. 4shows the area indicated by IV inFIG. 1. For ease of understanding,FIG. 4shows a needle roller13accommodated in a pocket21that will be described later. The direction perpendicular to the plane of paper ofFIGS. 1 and 4and the vertical direction inFIGS. 2 and 3are the direction of the rotation axis of the cage11. The direction shown by arrow A1or its opposite direction inFIG. 1is the circumferential direction. For ease of understanding, the upper side inFIGS. 2 and 3is defined as the upper side in the axial direction. That is, the direction shown by arrow A2inFIGS. 2 and 3is the upward direction. The lateral direction inFIGS. 2 and 3is the radial direction. The direction shown by arrow A3inFIG. 3is the radially outward direction.

First, the configuration of the thrust roller bearing cage11according to the embodiment of the present invention will be described with reference toFIGS. 1 to 4. The thrust roller bearing cage11according to the embodiment of the invention is in the shape of a disc and has a through bore22extending straight through its central area in the thickness direction of the cage11. A rotary shaft, not shown, is inserted through the through bore22.

The cage11includes a pair of annular portions23,24with different diameters, and a plurality of pillars25formed at intervals in the circumferential direction so as to form the pockets21that accommodate the needle rollers13therein and connecting the pair of annular portions23,24.

The pockets21are substantially rectangular as viewed in the axial direction. The pockets21are arranged radially about the rotation axis12of the cage11. The pockets21have upper roller stoppers26and lower roller stoppers27,28on their side wall surfaces. The upper roller stoppers26prevent the needle rollers13accommodated in the pockets21from falling out upward in the axial direction, while the lower roller stoppers27,28prevent the needle rollers13accommodated in the pockets21from falling out downward in the axial direction. The upper roller stoppers26are formed in the middle parts in the radial direction of the pockets21. The lower roller stoppers27are formed in the radially inner parts of the pockets21, and the lower roller stoppers28are formed in the radially outer parts of the pockets21. The upper roller stoppers26and the lower roller stoppers27,28are formed on the side wall surfaces located on both sides in the circumferential direction of each pocket21so as to project into the pockets21.

The needle rollers13are pressed into the pockets21to fit in the pockets21. End faces of each needle roller13, specifically, an outer end face16in the bearing and an inner end face17in the bearing, are flat.

The cage11has concave and convex portions that are formed by bending a plate in the thickness direction thereof a few times. Specifically, the cage11includes four disc portions31,32,33,34extending in the radial direction and four cylindrical portions36,37,38,39extending in the axial direction. The four disc portions31to34are arranged in this order from the inside in the radial direction so that the first disc portion31has the smallest inside diameter, followed by the second disc portion32, the third disc portion33, and the fourth disc portion34in ascending order. The four cylindrical portions36to39are arranged in order of the first cylindrical portion36, the second cylindrical portion37, the third cylindrical portion38, and the fourth cylindrical portion39from the inside in the radial direction. The first cylindrical portion36and the second cylindrical portion37extend straight in the axial direction. The third cylindrical portion38is slightly tilted so that its radially inner part is located below its radially outer part in the axial direction. The fourth cylindrical portion39, which is the outermost cylindrical portion in the radial direction, is slightly tilted so that its radially inner part is located above its radially outer part in the axial direction.

More specifically, the first cylindrical portion36extends in the axial direction. The first disc portion31is formed so as to be continuous with the upper edge of the first cylindrical portion36and to extend in the radially outward direction. The second cylindrical portion37is formed so as to be continuous with the radially outer edge of the first disc portion31and to extend downward in the axial direction. The second disc portion32is formed so as to be continuous with the lower edge of the second cylindrical portion37and to extend in the radially outward direction. The third cylindrical portion38is formed so as to be continuous with the radially outer edge of the second disc portion32and to extend upward in the axial direction. The third disc portion33is formed so as to be continuous with the upper edge of the third cylindrical portion38and to extend in the radially outward direction. The fourth cylindrical portion39is formed so as to be continuous with the radially outer edge of the third cylindrical portion33and to extend downward in the axial direction. The fourth disc portion34is formed so as to be continuous with the lower edge of the fourth cylindrical portion39and to extend in the radially outward direction. The first disc portion31is substantially flush with the third disc portion33. The second disc portion32is substantially flush with the fourth disc portion35.

The upper roller stoppers26are formed in the third disc portion33. The lower roller stoppers27are formed in the second disc portion32, and the lower roller stoppers28are formed in the fourth disc portion34.

The inner annular portion23in the radial direction includes the first disc portion31, a part of the second disc portion32, the first cylindrical portion36, and the second cylindrical portion37. The outer annular portion24in the radial direction includes a part of the fourth disc portion34, a radially outer area bent portion41, and projecting portions44. The radially outer area bent portion41and the projecting portions44will be described later. Each pillar25includes a part of the second disc portion32, the third disc portion33, a part of the fourth disc portion34, the third cylindrical portion38, and the fourth cylindrical portion39.

The cage11includes the radially outer area bent portion41that is formed by bending a radially outer area of the cage11inward in the radial direction. In other words, the cage11has the radially outer area bent portion41that is formed by bending an area located radially outside the pockets21inward in the radial direction. The radially outer area bent portion41is a standing wall standing in the axial direction and is formed to extend continuously in an annular shape.

The radially outer area bent portion41of the present embodiment is formed by bending the area located radially outside the pockets21obliquely inward in the radial direction. Specifically, the radially outer area bent portion41is formed by bending the radially outer edge of the fourth disc portion34, which is the outermost disc portion in the radial direction, upward in the axial direction to a predetermined angle.

The angle of the radially outer area bent portion41, namely the angle between a radially inner surface42of the radially outer area bent portion41and an upper surface43of the fourth disc portion34, is shown by an angle B1inFIGS. 2 and 3. This angle may be 0°, but is preferably an acute angle.

As shown inFIG. 3, a bottom part41aof the radially outer area bent portion41is located radially outside a radially outer edge21aof each pocket21. The bottom part41aof the radially outer area bent portion41is the position along which the radially outer area of the cage11is bent. In this case, since the radially outer area bent portion41and the fourth disc portion34form a lubricant reservoir, lubricant can be stably supplied to the contact portions between the projecting portions44described later and the end faces16of the rollers13, whereby wear resistance of the cage can be improved. Moreover, strength of the radially outer area bent portion41can be improved.

The bottom part41aof the radially outer area bent portion41may be located at the same position in the radial direction as the radially outer edges21aof the pockets21. In this case, since a large lubricant flow path can be formed, a more satisfactory flow of lubricant can be achieved.

The radially outer area bent portion41has the projecting portions44formed in its tip end. The projecting portions44project inward in the radial direction into radially outer areas of the pockets21so as to contact the end faces16of the needle rollers13accommodated in the pockets21. That is, the projecting portions44abut on the end faces of the rollers accommodated in the pockets21to restrict movement of the rollers toward the outside in the radial direction. Specifically, the projecting portions44are shaped to extend continuously from the inner peripheral edge of the radially outer area bent portion41toward the inside in the radial direction. That is, the radially outer area bent portion41and the projecting portions44are formed as a single-piece member.

The projecting portions44are formed at circumferential positions so that the tips of the projecting portions44are located in the middle parts in the circumferential direction of the pockets21. Specifically, the projecting portions44are formed so that their corners45on the surface42side, namely the innermost parts in the radial direction of the projecting portions44(the innermost corners45in the radial direction of the projecting portions44), contact the centers of the end faces16of the needle rollers13accommodated in the pockets21. In this example, the corners45are the corners of the projecting portions44which are located closer to the fourth disc portion34.

The corners45have been press-flattened. The corners45subjected to the press-flattening process have no sharply pointed parts and smoothly connect to the surfaces forming the corners45. This reduces aggression of the corners45against members that are contacted by the corners45.

As shown inFIGS. 3 and 4, a radially outer base end part44aof each projecting portion44is located radially inside the radially outer edge21aof each pocket21. The base end part44aof each projecting portion44is the boundary with the radially outer area bent portion41. In other words, the radially outer edges21aof the pockets21are located radially outside the radially outer edges of the projecting portions44. That is, the radially outer areas of the pockets21overlap the entire projecting portions44and a radially inner area of the radially outer region bent portion41. As shown inFIG. 3, clearance between the radially outer end face16of each roller13and the radially outer edge21aof each pocket21serves as a lubricant flow path, and the radially outer edge21aof each pocket21extends to a position located radially outside the base end part44aof each projecting portion44. A lubricant flow path is thus widened by an amount corresponding to clearance H shown inFIG. 3.

As shown inFIGS. 1 and 2, the cage11has three pilot holes51,52. The three pilot holes51,52serve as engagement portions for alignment. One of the pilot holes is not shown inFIG. 1. The three pilot holes51,52are formed at intervals in the circumferential direction and extend straight through the cage11in the thickness direction of the cage11. The three pilot holes51,52open in a circular shape. The three pilot holes51,52are formed substantially equally spaced apart from each other. In this example, the three pilot holes51,52are formed at intervals of 120 degrees about the rotation axis12of the cage11. Specifically, the pilot holes51,52are formed in the middle part in the radial direction of the innermost first disc portion31in the radial direction. For example, the diameter of the pilot holes51,52is φ2.5 mm or φ3 mm.

For example, a thrust roller bearing20having such a cage11includes the plurality of needle rollers13, the upper bearing ring14, and the lower bearing ring15. When the thrust roller bearing20is in operation, the needle rollers13accommodated in the pockets21roll on a raceway surface18of the upper bearing ring14in the axial direction and a raceway surface19of the lower bearing ring15in the axial direction. The cage11rotates around its rotation axis12. Each of the needle rollers13accommodated in the pockets21revolves while rotating around its axis. The needle rollers13are subjected to a radially outward centrifugal force. The centers of the end faces16of the needle rollers13make sliding contact with the projecting portions44of the cage11, specifically the innermost corners45in the radial direction of the projecting portions44of the cage11. That is, the corners45of the projecting portions44are the areas that contact the end faces16of the needle rollers13.

Next, a method for manufacturing the thrust roller bearing cage11according to the embodiment of the present invention will be described. The thrust roller bearing cage11is manufactured by using a transfer press. The transfer press is a relatively inexpensive press machine with a less complicated configuration.FIG. 5is a flowchart illustrating representative steps of the method for manufacturing the thrust roller bearing cage11according to the embodiment of the present invention.

Referring toFIG. 5, a cage material, which will later become the cage11, is first prepared (cage material preparing step: step S1). For example, the cage material is a thin flat steel sheet. At this stage, the cage material may be either a plate cut into a substantially rectangular shape or a circular plate, as the final outer shape of the cage is formed in an outer shape forming step (step S4) that will be performed later.

Next, concave and convex portions are formed in the cage material in the thickness direction of the cage material (concave and convex portions forming step: step S2). This ensures that the cage11has a large length dimension in the direction of its rotation axis even if the cage11is in the shape of a thin plate, whereby the cage11can appropriately retain the rollers.

Specifically, in this step, the cage material is subjected to a drawing process. In this case, the concave and convex portions can be formed more efficiently.FIG. 6is a sectional view of the cage material after the concave and convex portions forming step. The section shown inFIG. 6corresponds to the section shown inFIG. 2. Specifically, referring toFIG. 6, a flat plate-like cage material56is subjected to a drawing process to form first to fourth disc portions61to64and first to fourth cylindrical portions66to69. A circular through bore57is formed in the central part of the cage material56so as to extend therethrough in the thickness direction. That is, in this case, the cage material56has what is called a mountain-and-valley shape made by bending the cage material56a plurality of times in the axial direction.

Thereafter, pilot holes that will serve as engagement portions are formed (pilot hole forming step: step S3).FIG. 7is an enlarged sectional view showing a part of the cage material56after the pilot hole forming step. The section shown inFIG. 7corresponds to the area VII inFIG. 2. A pilot hole71that will serve as an engagement portion is formed in the middle part in the radial direction of the first disc portion61so as to extend straight through the first disc portion61in the thickness direction. Three of the pilot holes71are formed in total at intervals of 120 degrees in the circumferential direction so as to be substantially equally spaced apart from each other.

Subsequently, the outer shape of the cage material56is formed (outer shape forming step: step S4). In this step, the outer shape of the cage material56is formed so that the outer shape has portions that will later become projecting portions projecting inward in the radial direction so as to contact the end faces of the rollers accommodated in pockets to be formed in a pocket forming step (step S5). The pocket forming step will be described later.

FIG. 8is an enlarged sectional view showing a part of the cage material56after the outer shape forming step. The section shown inFIG. 8corresponds to the area VII shown inFIG. 2, and is a section taken along line VIII-VIII inFIG. 9. Specifically, the cage material56is punched straight in the thickness direction so that the cage11can be formed into the final outer shape by a radially outer area bending step (step S7) etc. that will be performed later. In this case, the outer shape of the cage material56can be formed relatively easily and accurately. A radially outer edge72of the cage11, specifically, a radially outer edge72of the fourth disc portion64, is thus formed.

When forming the outer shape, the cage material56is punched so as to form the portions that will later become projecting portions70. That is, in this case, the outer shape forming step is also a projecting portions forming step, i.e., the step of forming the projecting portions.FIG. 9shows a part of the cage material56after the pocket forming step, which is the step following the outer shape forming step.FIG. 9corresponds toFIG. 1. When punching the cage material56so as to form the projecting portions70, the cage material56is aligned in the circumferential direction by using the plurality of pilot holes71. Specifically, a plurality of guide pins (not shown), which serve as what is called pencil-like alignment jigs having a pointed end and having a tapered shape with its diameter gradually increasing from the pointed end, are prepared and are gradually inserted into the plurality of pilot holes71from one side in the thickness direction. The cage material56is thus aligned by using the plurality of guide pins and is punched into the overall outer shape with a punching machine (not shown) in view of the positions, shape, etc. of the projecting portions70. Accordingly, even if the cage material56is slightly out of alignment with the punching machine with respect to the proper positions where the projecting portions70are supposed to be formed, the cage material56can be aligned with the punching machine with respect to the proper positions where the projecting portions70are supposed to be formed, as the pencil-like guide pins having a pointed end are gradually inserted into the pilot holes71. The punching process can thus be performed. In this example, since the cage material56has the three pilot holes71, rotation etc. of the cage material56is prevented during alignment. The cage material56can thus be aligned more accurately.

Subsequently, pockets are formed (pocket forming step: step S5). In this step, the pockets are formed so that the radially outer edges of the pockets will be located radially outside the base end parts of the projecting portions with the tip (the opposite side from the bottom part) of a radially outer area bent portion (step S7), described later, facing inward in the radial direction. In the present embodiment, the pockets are formed so that the radially outer edges of the pockets are located radially inside the bottom part of the radially outer area bent portion that will be formed in the radially outer area bending step (step S7) described later.

FIG. 10is an enlarged sectional view showing a part of the cage material after the pocket forming step. The section shown inFIG. 10corresponds to the area III shown inFIG. 2and is a section taken along line X-X inFIG. 9. In this example, each pocket73is punched out along a part of the second disc portion62, the third disc portion63, and a part of the fourth disc portion64and also along the third cylindrical portion68and the fourth cylindrical portion69so as to extend straight through the cage material56in the thickness direction. Although not shown inFIG. 10, the upper roller stoppers and the lower roller stoppers, which are shaped so as to project into the pockets73in the circumferential direction, are formed simultaneously with the pockets73. That is, the pockets73are punched out in view of the shape of the upper roller stoppers and the lower roller stoppers so as to conform to the outer shape of needle rollers13that are to be accommodated in the pockets73. The plurality of pockets73may be punched out either all at once or one by one.

When forming the pockets73in the cage material56, the pilot holes71are also used to align the cage material56to be punched with a punching machine (not shown) for punching out the pockets. That is, the pockets73are formed with respect to the positions of the pilot holes71. As in the case of the outer shape forming step, alignment in the circumferential direction is performed by using the plurality of pilot holes71. Specifically, a plurality of guide pins serving as sharp pencil-like alignment jigs are prepared and the tip ends of the guide pins are gradually inserted into the plurality of pilot holes71from one side in the thickness direction as described above. The cage material56is thus aligned by using the plurality of guide pins, and the pockets73are punched out with the punching machine in view of the positions, shape, etc. of the pockets73. The pockets73are thus formed in phase with the projecting portions70in the circumferential direction, so that an appropriate positional relationship can be established between the pockets73and the projecting portions70. Accordingly, the projecting portions70can be accurately and efficiently formed in terms of the positional relationship of the projecting portions70with the pockets73. Since the projecting portions44are accurately formed at the appropriate positions, end faces16of the needle rollers13can appropriately contact the projecting portions44when the bearing is in operation. The plurality of pockets73may be punched out either all at once or one by one.

In the present embodiment, the pilot holes71are formed in an area located radially inside the pockets73. In this case, the pilot holes71can be formed by making effective use of the available area of the cage11.

In the present embodiment, the pilot holes71are formed so as not to overlap the pockets73in the circumferential direction. This can avoid local strength reduction in the circumferential direction of the cage11. The positional relationship of the pockets73with the pilot holes71can be determined as desired. Specifically, in this example, the plurality of pockets73are formed so that each of the pilot holes71is located at a position corresponding to the middle in the circumferential direction between adjoining ones of the pockets73.

Subsequently, as shown inFIGS. 9 and 10, an annular groove79is formed at a position radially outside the pockets73in the cage material (groove forming step: step S6). In this step (step S6), the groove79is formed at such a position that the cage material is to be bent along the groove79when forming a radially outer area bent portion41in the radially outer area bending step (step S7) described below. Although performing the groove forming step (step S6) makes it easier to bend a radially outer area of the cage material56inward in the radially outer area bending step (step S7) described below, the groove forming step (step S6) may be omitted. The steps S4to S6may be performed in any order.

Subsequently, an area of the cage material56which is located radially outside the pockets73is bent inward in the radial direction to form the radially outer area bent portion (radially outer area bending step) (step S7). In this step, it is preferable that the area of the cage material56which is located radially outside the pockets73be bent obliquely inward in the radial direction to an acute tilt angle to form the radially outer area bent portion. In the case where the groove forming step (step S6) is performed, the radially outer area of the cage material56is bent along the groove79to form the radially outer area bent portion.

FIG. 11is an enlarged sectional view showing a part of the cage material during the radially outer area bending step.FIGS. 12 and 13are enlarged sectional views illustrating how the radially outer area bending step is performed.FIGS. 14 and 15are enlarged sectional views showing a part of the cage material56after the radially outer area bending step. The sections shown inFIGS. 11 and 14correspond to the area VII inFIG. 2. The section shown inFIG. 15corresponds to the area III inFIG. 2. The sections shown inFIGS. 12 and 13show the positional relationship of an area of the cage material56which is located radially outside the position corresponding to the area III inFIG. 2with holding members101,102and a pressing member103. In this example, as shown inFIG. 11, the annular radially outer edge72of the cage material56is first bent along the entire circumference so as to extend straight in the thickness direction. That is, the angle B2between a radially inner surface75of a radially outer area bent portion74and an upper surface76of the fourth disc portion64is approximately a right angle. For example, the radially outer edge72is bent to a right angle by the following method, although the present invention is not particularly limited to this method. As shown inFIG. 12, the entire cage material56except for a radially outer area of the fourth disc portion64is sandwiched between the holding members101,102in the vertical direction and is held therebetween, and the pressing member103is placed under the radially outer area of the fourth disc portion64. As shown inFIG. 13, the pressing member103is then moved upward. The radially outer area bent portion74can thus be formed at a right angle with respect to the fourth disc portion64.

Subsequently, as shown inFIGS. 14 and 15, the radially outer area bent portion74is tilted further inward in the radial direction, whereby the radially outer area bent portion74is formed. The bending angle (tilt angle), that is, the angle between the radially inner surface75of the radially outer area bent portion74and the upper surface76of the fourth disc portion64is shown by an angle B3inFIGS. 14 and 15. The angle B3corresponds to the angle B1described above. In other words, the angle B1is equal to the angle B3. The angles B1, B3are preferably an acute angle.

In this example, in terms of the positional relationship in the circumferential direction, the projecting portions70are formed at the positions corresponding to the middle parts in the circumferential direction of the pockets73. The projecting portions70are thus formed at appropriate positions. Specifically, the projecting portions70abut on the centers of the end faces16of the needle rollers13at their corners77located closer to the fourth disc portion64. Finally, the areas of the projecting portions70which are to contact the end faces16of the needle rollers13are subjected to a press-flattening process. The thrust roller bearing cage11configured as shown inFIGS. 1 to 4is thus manufactured.

The step of tilting the radially outer edge72of the cage material56inward in the radial direction after the annular radially outer edge72is bent so as to extend straight in the thickness direction and the press-flattening process may be performed successively.FIG. 16is an enlarged sectional view illustrating how the radially outer area bending step is performed.FIG. 17is an enlarged sectional view illustrating how the press-flattening step is performed.FIG. 18is an enlarged sectional view showing the tip of the radially outer area bent portion after the press-flattening step. Specifically, as shown inFIG. 16, after the cage material56is bent so that the radially outer area bent portion74extends at a right angle with respect to the fourth disc portion64, an area of the cage material56which is located radially inside the radially outer area bent portion74is sandwiched between dies104,105in the vertical direction and is held therebetween. At this time, the radially outer edge of the upper die104is located radially inside the radially outer edge of the lower die105. A die106that presses the radially outer area bent portion74downward from above is also placed so as to contact a radially outer surface78of the radially outer area bent portion74. The die106includes a radially inner end face106athat faces the upper die104and extends in the vertical direction, a horizontal face106bthat is continuous with the radially inner end face106aand extends outward in the radial direction, and a radially inner face106cthat faces the radially outer surface78of the radially outer area bent portion74and extends in the vertical direction. A part106dof the die106where the horizontal face106band the radially inner face106cmeet has a round (R) shape. When the die106is moved downward so that the radially inner end face106amoves along a radially outer end face104aof the die104, the radially outer area bent portion74can be tilted inward in the radial direction as guided by the round part106d. Subsequently, as shown inFIG. 17, the die104is moved further downward so that the radially inner corner of the radially outer area bent portion74is smoothed by the radially outer end face104aof the die104and the radially outer corner of the radially outer area bent portion74is smoothed by the horizontal face106bof the die106. As shown inFIG. 18, the projecting portions70subjected to the press-flattening process in the areas of the projecting portions70which are to contact the end faces of the rollers can be formed in this manner.

As described above, in the thrust roller bearing cage11of the present embodiment and the method for manufacturing the same, the radially outer edges21aof the rollers21,73are located radially outside the base end parts44aof the projecting portions44,70. Since the pockets21,73are extended radially outward from the base end parts44aof the projecting portions44,70, clearance between the radially outer edge21aof each pocket21,73and the base end part44aof each projecting portion44,70(clearance H inFIG. 3) can serve as a lubricant flow path. In Patent Literature 1, since the radially outer base end parts of the projecting portions are located at the same position in the radial direction as the radially outer edges of the pockets, the clearance H inFIG. 3is not provided. In the present embodiment, it is ensured that a sufficient lubricant flow path is present in the pockets21,73with the projecting portions44,70being in contact with the end faces16of the rollers13, whereby a satisfactory lubricant flow can be achieved. Satisfactory circulation of the lubricant can therefore be achieved with a small amount of lubricant (lean lubrication condition), namely without increasing the amount of lubricant. A sufficient amount of lubricant can thus be supplied to the contact areas between the projecting portions44,70and the rollers13, whereby running torque can be reduced.

Even if those parts of the projecting portions44which contact the rollers become worn through the use of the thrust roller bearing cage11of the present embodiment, the clearance between the radially outer edge21aof each pocket21,73and the radially outer end face16of each roller13, which serves as a lubricant flow path, can be maintained as the pockets21are extended to a position radially outside the base end parts44aof the projecting portions44. Accordingly, even if the radially inner tips (edges on the opposite side from the base end parts44a) of the projecting portions44,70become worn after long term use of the thrust roller bearing cage11, a satisfactory lubricant flow is allowed in the lean lubrication condition, whereby reduced running torque can be maintained.

In the thrust roller bearing cage11of the present embodiment and the method for manufacturing the same, it is preferable that the bottom part41aof the radially outer area bent portion41,74be located radially outside the radially outer edges21aof the pockets21. In this case, an area where lubricant is stored can be formed in the annular portion24, namely the outer annular portion in the radial direction, by the radially outer area bent portion41,74and the fourth disc portion34,64. A sufficient amount of lubricant can thus be supplied to the contact portions between the projecting portions44,70and the end faces16of the rollers13, whereby wear resistance of the cage11can be improved.

In the above embodiment, the corners of the projecting portions which are located closer to the fourth disc portion contact the centers of the end faces of the needle rollers accommodated in the pockets. However, the present invention may have the following configuration.FIG. 19is a sectional view showing a part of a cage having this configuration.FIG. 19corresponds to the section of the cage shown inFIG. 3.

Referring toFIG. 19, a thrust roller bearing cage81according to another embodiment of the present invention has a radially outer area bent portion82having projecting portions83formed at positions corresponding to the positions of pockets85. The projecting portions83contact the centers of end faces16of needle rollers13accommodated in the pockets85at corners84located on the opposite side of the projecting portions83from a fourth disc portion86. The corners84have been press-flattened. This configuration can be achieved by machining the corners84with a jig angled so as to conform to the corner84in the radially outer area bending step.

A pilot hole may be formed at a position where a pocket is supposed to be formed. In other words, one of the plurality of pockets may be used as a pilot hole.FIG. 20shows a part of a cage having this configuration. Referring toFIG. 20, a thrust roller bearing cage91according to still another embodiment of the present invention includes a plurality of pockets92and pillars93each located between adjoining two of the pockets92. A pilot hole95is formed at a position where a pocket92is supposed to be formed in a pillar94located between the pockets92. In this configuration, one of the plurality of pockets92that are formed equally spaced apart from each other in the circumferential direction is replaced with this pilot hole95.

In the above embodiment, the pilot holes extend straight through the cage in the thickness direction. However, the present invention is not limited to this. For example, the pilot holes extending through the cage may have a tapered wall surface. The pilot holes are not limited to the circular holes and may be quadrilateral holes, triangular holes, etc. The pilot holes are formed as engagement portions. However, the present invention is not limited to this. The engagement portions may have other configurations. For example, the engagement portions may be formed by cutouts.

In the above embodiment, a drawing process is performed in the concave and convex portions forming step. However, the present invention is not limited to this. A process other than the drawing process, such as a bending process, may be used to form concave and convex portions.

In the above embodiment, the cage has the concave and convex portions that are formed in the thickness direction. However, the present invention is not limited to this. The cage may not have the concave and convex portions that are formed in the thickness direction, and a cage in the form of what is called a laminate of two plates may be used.

In the above embodiment, the thrust roller bearing having such a cage as described above may not have bearing rings. Rollers other than the needle rollers, such as long rollers etc., may be used.

The embodiments disclosed herein are by way of example in all respects and should not be interpreted as restrictive. The scope of the present invention is defined by the claims rather than by the above embodiments, and the invention is intended to cover all changes and modifications within the spirit and scope of the invention as defined by the claims.

INDUSTRIAL APPLICABILITY

The thrust roller bearing cage according to the present invention and the method for manufacturing the same are effectively utilized to meet demands for thrust roller bearing cages with excellent performance and more efficient methods for manufacturing such a thrust roller bearing cage.

REFERENCE SIGNS LIST

41,74,82Radially Outer Area Bent Portion

44aBase End Part

104aRadially Outer End Face

106aRadially Inner End Face

106cRadially Inner Face