Thrust roller bearing

A thrust roller bearing includes a plurality of rollers and a cage including a plurality of cage pockets for housing the rollers. The rollers each include a cylindrical portion, an outer crowning portion, and an inner crowning portion. The cage pockets each include an outer surface, an inner surface facing a second end surface of the roller, and a pair of side surfaces facing an outer peripheral surface of the roller. The side surfaces each include a recessed surface that is not contactable with the roller, and flat surfaces that are provided on the outer side in the radial direction and on the inner side in the radial direction, respectively, and are contactable with the roller. A first dimension is larger than a second dimension.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2019-034293 filed on Feb. 27, 2019 and Japanese Patent Application No. 2019-033896 filed on Feb. 27, 2019, each including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a thrust roller bearing.

2. Description of Related Art

Thrust roller bearings are able to receive an axial load generated between a first member and a second member that rotate relative to each other. The thrust roller bearings are able to be downsized especially when rollers are needle rollers. Further, the thrust roller bearings can cope with rotation at a high speed and have large load capacity. The thrust roller bearings are applied to transmission devices of an automobile and an industrial construction machine, and also widely used in other rotary devices (refer to Japanese Unexamined Patent Application Publication No. 2018-66501 (JP 2018-66501 A)).

SUMMARY

When a thrust roller bearing (hereinafter also simply referred to as a “bearing”) rotates, a roller rolls between a raceway surface of the first member and a raceway surface of the second member. The roller that has a columnar shape tends to move straight along the raceways. However, the roller is housed in a cage pocket included in a cage, and the cage regulates the movement of the roller so that the roller moves in a circumferential direction. During this movement, an outer peripheral surface of the roller makes a partial contact with a side surface of the cage pocket that faces the outer peripheral surface of the roller, and the roller slides on a contact portion of the cage pocket. When a sliding frictional resistance at the contact portion between the cage pocket and the roller increases, low torque characteristics of the bearing deteriorate.

In recent years, there are demands for achieving the low torque characteristics of the thrust roller bearing in various rotary devices so as to improve rotation efficiency, that is, to reduce a rotation loss. For this reason, it is desired to reduce the sliding frictional resistance at the contact portion between the cage pocket and the roller as much as possible. Therefore, the present disclosure reduces the sliding frictional resistance at the contact portion between the roller and the cage to achieve the low torque characteristics.

An aspect of the present disclosure is a thrust roller bearing. The thrust roller bearing includes a plurality of rollers disposed between a first raceway surface and a second raceway surface that face each other, and a cage including a plurality of cage pockets that houses the respective rollers such that a central axis of each of the rollers extends along a radial direction. The rollers each include a cylindrical portion provided in the middle of the roller in an axial direction of the roller, an outer crowning portion provided on an outer side of the cylindrical portion in the radial direction, and an inner crowning portion on an inner side of the cylindrical portion in the radial direction. The cage pockets each include an outer surface provided with a protrusion that is contactable with a first end surface of the roller on the outer side in the radial direction, an inner surface that faces a second end surface of the roller on the inner side in the radial direction, and a pair of side surfaces facing an outer peripheral surface of the roller. The side surfaces each include a recessed surface that is provided in the middle of the cage pocket in the radial direction and is not contactable with the roller and flat surfaces that are provided on the outer side in the radial direction and on the inner side in the radial direction, respectively, and are contactable with the roller. A first dimension is larger than a second dimension. The first dimension is a dimension along the radial direction from the first end surface that is contactable with the protrusion to a boundary between the outer crowning portion and the cylindrical portion. The second dimension is a dimension along the radial direction from a contact point where the first end surface contacts the protrusion to a boundary between the flat surface on the outer side in the radial direction and the recessed surface.

With the above configuration, when the thrust roller bearing rotates, the rollers roll between the first raceway surface and the second raceway surface while the first end surface of each of the rollers is brought into contact with the protrusion provided in the cage pocket by a centrifugal force. Since the first dimension is larger than the second dimension, the outer crowning portion of the roller is brought into contact with the flat surface in the cage pocket on the outer side in the radial direction. The outer crowning portion includes an arc generatrix. Therefore, the roller and the cage can be in contact (point contact) with each other in a region of the cage pocket on the outer side in the radial direction in a form of contact that forms a contact ellipse. This makes it possible to reduce a sliding frictional resistance between the rollers and the cage, which achieves low torque characteristics.

In the thrust roller bearing according to the above aspect, the outer crowning portion may include a first portion that has an arc generatrix having a first radius of curvature and is adjacent to the cylindrical portion, and a second portion that has an arc generatrix having a second radius of curvature smaller than the first radius of curvature and that is adjacent to the first portion. A third dimension may be smaller than the second dimension. The third dimension may be a dimension along the radial direction from the first end surface that is contactable with the protrusion to a boundary between the first portion and the second portion. According to the above configuration, the first portion in the outer crowning portion of the roller is brought into contact with the flat surface of the cage pocket on the outer side in the radial direction. The first portion has a larger radius of curvature of the arc generatrix compared to the second portion. With this configuration, the contact ellipse formed between the flat surface and the first portion can be relatively made larger. Accordingly, a surface pressure at the contact portion between the roller and the cage is reduced, which can suppress occurrence of wear.

In the thrust roller bearing according to the above aspect, a raised roundness may be provided at the boundary between the flat surface on the outer side in the radial direction and the recessed surface. According to the above configuration, even when the outer crowning portion of the roller contacts the boundary, the edge load does not occur.

In the thrust roller bearing according to the above aspect, a fourth dimension may be smaller than a fifth dimension. The fourth dimension may be a dimension along the radial direction from the first end surface that is contactable with the protrusion to a boundary between the inner crowning portion and the cylindrical portion. The fifth dimension may be a dimension along the radial direction from the contact point where the first end surface contacts the protrusion to a boundary between the flat surface on the inner side in the radial direction and the recessed surface. According to the above configuration, since the fourth dimension is smaller than the fifth dimension, the inner crowning portion of the roller is brought into contact with the flat surface in the cage pocket on the inner side in the radial direction. The inner crowning portion includes an arc generatrix. Therefore, the roller and the cage can be brought in contact (point contact) with each other in a region of the cage pocket on the inner side in the radial direction in the form of contact that forms the contact ellipse. This makes it possible to reduce a sliding frictional resistance between the rollers and the cage, which achieves low torque characteristics.

In the thrust roller bearing according to the above aspect, the inner crowning portion may include a third portion that has an arc generatrix having a third radius of curvature and is adjacent to the cylindrical portion, and a fourth portion that has by an arc generatrix having a fourth radius of curvature smaller than the third radius of curvature and that is adjacent to the third portion. A sixth dimension may be larger than the fifth dimension. The sixth dimension may be a dimension along the radial direction from the first end surface that is contactable with the protrusion to a boundary between the third portion and the fourth portion. According to the above configuration, the third portion in the inner crowning portion of the roller is brought into contact with the flat surface of the cage pocket on the inner side in the radial direction. The third portion has a larger radius of curvature of the arc generatrix compared to the fourth portion. With this configuration, the contact ellipse formed between the flat surface and the third portion can be relatively made larger. Accordingly, a surface pressure at the contact portion between the roller and the cage is reduced, which can suppress occurrence of wear.

In the thrust roller bearing according to the above aspect, a raised roundness may be provided at the boundary between the flat surface on the inner side in the radial direction and the recessed surface. According to the above configuration, even when the inner crowning portion of the roller contacts the boundary, the edge load does not occur.

With the present disclosure, the sliding frictional resistance at the contact portion between the roller and the cage is reduced, which achieves the low torque characteristics of the thrust roller bearing.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1is a cross-sectional view showing an example of a thrust roller bearing. A thrust roller bearing10(hereinafter also simply referred to as “bearing10”) shown inFIG. 1includes an annular cage12and a plurality of rollers11. The bearing10of a present disclosure further includes an annular first bearing ring (housing bearing washer)5disposed on an one side of the bearing10in an axial direction of the bearing10(the upper side inFIG. 1) and a second bearing ring (shaft bearing washer)6disposed on the other side of the bearing10in the axial direction (the lower side inFIG. 1).

The first bearing ring5and the second bearing ring6rotate relative to each other about an central axis C0of the bearing10. In the present disclosure, a direction along the central axis C0of the bearing10is referred to as “axial direction”. The term “axial direction” includes a direction parallel to the central axis C0. A direction orthogonal to the central axis C0is denoted as a “radial direction about the central axis C0” (hereinafter simply referred to as “radial direction”). A circumferential direction about the central axis C0, that is, a direction of the relative rotation between the first bearing ring5and the second bearing ring6is denoted as a “circumferential direction of the bearing10” (hereinafter simply referred to as a “circumferential direction”). In the present disclosure, a central axis of the cage12is coincident with the central axis C0of the bearing10.

The first bearing ring5includes an annular first body portion5aand a first rib portion5b. The first rib portion5bhas a short cylinder shape, and extends toward the other side in the axial direction from an outer end of the first body portion5ain the radial direction. A first raceway surface7that is flat and annular is provided on a surface of the first body portion5aon the other side in the axial direction. The second bearing ring6includes an annular second body portion6aand a second rib portion6b. The second rib portion6bhas a short cylinder shape, and extends toward the one side in the axial direction from an inner end of the second body portion6ain the radial direction. A second raceway surface8that is flat and annular is provided on a surface of the second body portion6aon the one side in the axial direction. The cage12and the rollers11are disposed between the first raceway surface7and the second raceway surface8. When the bearing10rotates, the rollers11rolls between the first raceway surface7and the second raceway surface8in a state where the rollers11are held by the cage12.

In the bearing10, the first bearing ring5and the second bearing ring6may be omitted. In this case, although not shown, a first member included in a device in which the bearing10is provided serves as a substitute for the first bearing ring5, and a second member included in the device serves as a substitute for the second bearing ring6. The first member is formed with the annular first raceway surface7, and the second member is formed with the annular second raceway surface8.

FIG. 2is a perspective view showing a part of the cage12holding the rollers11therein. The cage12has a plurality of cage pockets13. The cage pockets13are arranged radially about the central axis C0(seeFIG. 1). The cage12includes an inner annular body38disposed on an inner side in the radial direction, an outer annular body39disposed on an outer side in the radial direction, and a plurality of cage bars40connecting the inner annular body38and the outer annular body39. A space between the inner annular body38and the outer annular body39and between the cage bars40that are adjacent to each other in the circumferential direction serves as each of the cage pockets13.

The rollers11has a columnar shape. InFIGS. 1 and 2, the roller11has a first end surface21on the outer side in the radial direction, a second end surface22on the inner side in the radial direction, and an outer peripheral surface20. The rollers11according to the present disclosure may be cylindrical rollers or long cylindrical rollers, in addition to the needle rollers. Each of the cage pockets13houses one roller11. The rollers11are housed in the respective cage pockets13so that the central axis C1of each of the rollers11is coincident with the radial direction.

As described above, in the bearing10according to the present disclosure, the rollers11are disposed between the first raceway surface7and the second raceway surface8that face each other. The cage12has a plurality of cage pockets13. The cage pockets13each hold the roller11with a central axis C1of the roller11arranged along the radial direction. For this reason, an appropriate clearance is provided between the outer peripheral surface20of the roller11and the cage bar40. The rollers11, the first bearing ring5, and the second bearing ring6are made of steel. The cage12may be made of metal (made of steel). However, in the present disclosure, the cage12is made of resin so as to reduce a sliding frictional resistance.

The shape of the roller11will be further described.FIG. 3is a view showing the cage pocket13and the roller11in a cross-section that is orthogonal to the central axis C0of the bearing10(seeFIG. 1) and includes the central axis C1of the roller11. The roller11includes a cylindrical portion15in the middle, an outer crowning portion16, and an inner crowning portion17. The cylindrical portion15is interposed between the outer crowning portion16and the inner crowning portion17.

The cylindrical portion15is a portion provided in the middle of the roller11in the axial direction of the roller11, and is formed to include a linear generatrix. An outer peripheral surface15aof the cylindrical portion15has a cylindrical surface that is parallel to the central axis C1of the roller11. The outer crowning portion16is a portion provided adjacent to the cylindrical portion15on the outer side in the radial direction, and is formed to include an arc generatrix. An outer peripheral surface16aof the outer crowning portion16has a shape in which its diameter gradually decreases toward the first end surface21side. The inner crowning portion17is a portion provided adjacent to the cylindrical portion15on the inner side in the radial direction, and is formed to include an arc generatrix. An outer peripheral surface17aof the inner crowning portion17has a shape in which its diameter gradually decreases toward the second end surface22side.

The outer peripheral surface20of the roller11includes the outer peripheral surface15aof the cylindrical portion15, the outer peripheral surface16aof the outer crowning portion16, and the outer peripheral surface17aof the inner crowning portion17.

The outer crowning portion16is configured of two portions. That is, the outer crowning portion16includes a first portion26that is adjacent to the cylindrical portion15and a second portion27that is adjacent to the first portion26. An outer peripheral surface of the first portion26includes an arc generatrix having a first radius of curvature r1in the cross-section including the central axis C1of the roller11. An outer peripheral surface of the second portion27includes an arc generatrix having a second radius of curvature r2that is smaller than the first radius of curvature r1in the cross-section including the central axis C1of the roller11. The outer peripheral surface15aof the cylindrical portion15and the outer peripheral surface of the first portion26are connected in a differentiable manner in the cross-section including the central axis C1of the roller11. The outer peripheral surface of the first portion26and the outer peripheral surface of the second portion27are connected in a differentiable manner in the cross-section including the central axis C1of the roller11.

The inner crowning portion17is configured of two portions. That is, the inner crowning portion17includes a third portion28that is adjacent to the cylindrical portion15and a fourth portion29that is adjacent to the third portion28. An outer peripheral surface of the third portion28includes an arc generatrix having a third radius of curvature r3in the cross-section including the central axis C1of the roller11. An outer peripheral surface of the fourth portion29includes an arc generatrix having a fourth radius of curvature r4that is smaller than the third radius of curvature r3in the cross-section including the central axis C1of the roller11. The outer peripheral surface15aof the cylindrical portion15and the outer peripheral surface of the third portion28are connected in a differentiable manner in the cross-section including the central axis C1of the roller11. The outer peripheral surface of the third portion28and the outer peripheral surface of the fourth portion29are connected in a differentiable manner in the cross-section including the central axis C1of the roller11. In the present disclosure, the first radius of curvature r1has the same value as that of the third radius of curvature r3, and the second radius of curvature r2has the same value as that of the fourth radius of curvature r4.

FIG. 4is a view of the cage pocket13of the cage12as viewed from the one side in the axial direction.FIG. 5is a view of the cage pocket13of the cage12as viewed from the other side in the axial direction. InFIGS. 4 and 5, the roller11is indicated as a hidden outline (by long dashed double-short dashed lines). The cage pocket13is configured as a space surrounded by an outer surface33, an inner surface34, and a pair of side surfaces31,32.

The outer surface33is configured by a surface of the outer annular body39on the inner side in the radial direction. A curved protrusion37is provided on the outer surface33. The protrusion37is contactable with the first end surface21of the roller11. The protrusion37according to the present disclosure has a curved surface that extends along a spherical surface as a contacting surface with the roller11. The innermost position on the curved surface of the protrusion37in the radial direction is located on a virtual plane that is located in the same distance from opposing surfaces of the cage bars40that are adjacent to each other in the circumferential direction. When the bearing10rotates, the roller11tends to move outward in the radial direction by a centrifugal force. The first end surface21of the roller11then makes a point contact with the protrusion37, which positions the roller11in the radial direction. The inner surface34is configured of a surface of the inner annular body38on the outer side in the radial direction. The inner surface34faces the second end surface22of the roller11.

The first side surface31on the one side in the circumferential direction is configured of a surface of the cage bar40that is located on the one side in the circumferential direction with respect to the roller11. The surface of the cage bar40faces toward the other side in the circumferential direction. The first side surface31faces the outer peripheral surface20of the roller11. The second side surface32on the other side in the circumferential direction is configured of a surface of the cage bar40that is located on the other side in the circumferential direction with respect to the roller11. The surface of the cage bar40faces toward the one side in the circumferential direction. The second side surface32faces the outer peripheral surface20of the roller11.

As shown inFIG. 4, each of the cage pockets13is provided with a pair of first raised portions41so as to face each other in the circumferential direction. The first raised portions41are provided in the middle of the respective cage bars40in the radial direction. The first raised portion41is provided so as to protrude from the cage bar40in the circumferential direction on the one side in the axial direction. In the cage pocket13, the distance between the pair of first raised portions41is smaller than the diameter of the roller11. With this configuration, the roller11housed in the cage pocket13is prevented from falling off toward the one side in the axial direction by the first raised portions41.

As shown inFIG. 5, each of the cage pockets13is provided with a pair of second raised portions42so as to face each other in the circumferential direction. The second raised portions42are provided on the respective cage bars40on the outer side in the radial direction. The second raised portions42are provided so as to protrude from the cage bar40in the circumferential direction on the other side in the axial direction. The distance between the pair of second raised portions42is smaller than the diameter of the roller11. With this configuration, the roller11housed in the cage pocket13is prevented from falling off toward the other side in the axial direction by the second raised portions42. As shown inFIG. 5, each of the cage pockets13is provided with a pair of third raised portions43so as to face each other in the circumferential direction. The third raised portions43are provided on the respective cage bars40on the inner side in the radial direction. The third raised portions43are provided so as to protrude from the cage bar40in the circumferential direction on the other side in the axial direction. The distance between the pair of third raised portions43is smaller than the diameter of the roller11. With this configuration, the roller11housed in the cage pocket13is prevented from falling off toward the other side in the axial direction by the third raised portions43.

FIG. 3shows a state where a central axis C2of the cage pocket13along the radial direction coincides with the central axis C1of the roller11(this state is hereinafter referred to as a “coincident state”). The central axis C2of the cage pocket13is a line on the virtual plane that is located in the same distance from opposing surfaces of the cage bars40that are adjacent to each other in the circumferential direction, and extends in the radial direction including the innermost position of the protrusion37in the radial direction. In this state, the roller11protrudes from a surface of the cage12on the one side in the axial direction toward the one side in the axial direction, and also protrudes from a surface of the cage12on the other side in the axial direction toward the other side in the axial direction. Further, in this state, a clearance is provided between the first side surface31of the cage pocket13and the outer peripheral surface20of the roller11, and between the second side surface32of the cage pocket13and the outer peripheral surface20of the roller11. The first side surface31included in the cage pocket13on the one side in the circumferential direction includes a central recessed surface35a, and flat surfaces36a,36bthat interpose the central recessed surface35atherebetween. Similar to the first side surface31on the one side in the circumferential direction, the second side surface32included in the cage pocket13on the other side in the circumferential direction includes a central recessed surface35b, and flat surfaces36c,36dthat interpose the central recessed surface35btherebetween. The first side surface31and the second side surface32have the same configuration. Therefore, the configuration of the first side surface31on the one side in the circumferential direction will be described as an example.

The flat surface36ais a surface that is provided in the cage pocket13on the outer side in the radial direction and is contactable with the roller11. The flat surface36bis a surface that is provided in the cage pocket13on the inner side in the radial direction and is contactable with the roller11. The recessed surface35ais provided in the cage pocket13in the middle in the radial direction. The recessed surface35ais a surface that is recessed from the flat surfaces36a,36b, and that is not contactable with the roller11. The recessed surface35aincludes an outer inclined surface45, an inner inclined surface46, and a middle surface47. On the outer inclined surface45, a width dimension of the cage pocket13in the circumferential direction gradually increases from the flat surface36adisposed on the outer side in the radial direction. On the inner inclined surface46, the width dimension of the cage pocket13in the circumferential direction gradually increases from the flat surface36bdisposed on the inner side in the radial direction. The middle surface47is disposed between the outer inclined surface45and the inner inclined surface46. In the coincident state, the flat surfaces36a,36band the middle surface47are parallel to the central axis C1of the roller11. Each of the flat surfaces36a,36bis configured of a surface to be flat both in the radial direction and the axial direction, excluding the second raised portions42and the third raised portions43(seeFIGS. 4 and 5).

A raised roundness (round chamfering)48is provided at a boundary B2between the flat surface36aon the outer side in the radial direction and the recessed surface35a(outer inclined surface45). Further, a raised roundness (round chamfering)49is provided at a boundary B5between the flat surface36bon the inner side in the radial direction and the recessed surface35a(inner inclined surface46). The second side surface32on the other side in the circumferential direction has the same configuration as that of the first side surface31on the one side in the circumferential direction with respect to the roundness at the boundary B2and the boundary B5as described above. That is, the raised roundness48is provided at the boundary B2between the flat surface36con the outer side in the radial direction and the recessed surface35b. Further, the raised roundness49is provided at the boundary B5between the flat surface36don the inner side in the radial direction and the recessed surface35b. The dimension of the roundness48and49, that is, the dimension of round chamfering, is 1 millimeter or more and 5 millimeters or less, for example.

FIG. 6shows a state where the central axis C1of the roller11is inclined with respect to the central axis C2of the cage pocket13that extends along the radial direction (this state is hereinafter referred to as a “non-coincident state”). When the roller11skews within the range of the cage pocket13, the non-coincident state as described above is established. The term “skew” in the present disclosure denotes the state where the roller11is inclined with respect to the normal rotation axis of the roller11. The normal rotation axis coincides with the central axis C2of the cage pocket13. When the bearing10rotates, the roller11rolls between the first raceway surface7(seeFIG. 1) and the second raceway surface8in the state where the first end surface21of the roller11is brought into contact with the protrusion37of the cage pocket13by a centrifugal force. As shown inFIG. 6, when the roller11is inclined in the cage pocket13, the outer crowning portion16of the roller11is brought into contact with the flat surface36aand the inner crowning portion17of the roller11is brought into contact with the flat surface36d, while the first end surface21of the roller11is in contact with the protrusion37. In this state, the roller11is positioned in the cage pocket13and rotates.

With referring back toFIG. 3, the dimensions of each portion of the roller11and the cage pocket13will be described. A first dimension X1as defined below with respect to the roller11is larger than a second dimension X2as defined below with respect to the cage pocket13(X1>X2). The first dimension X1is a dimension along the radial direction from the first end surface21of the roller11where the protrusion37of the cage pocket13contacts to the boundary B1between the outer crowning portion16and the cylindrical portion15. The second dimension X2is a dimension along the radial surface from a contact point P1where the first end surface21of the roller11contacts the protrusion37of the cage pocket13to the boundary B2(provided, however, that the boundary B2is a boundary between the flat surface36a(36c) on the outer side in the radial direction and the recessed surface35a(35b)).

Since the first dimension X1is larger than the second dimension X2as described above, the outer crowning portion16of the roller11is brought into contact with the flat surface36aof the cage pocket13on the outer side in the radial direction when the bearing10rotates and the roller11is inclined in the cage pocket13as shown inFIG. 6. The outer crowning portion16includes, as already described above, an arc generatrix. This makes it possible for the outer crowning portion16of the roller11and the flat surface36aof the cage pocket13to contact (make a point contact) with each other in a form of contact that forms a contact ellipse. Consequently, the sliding frictional resistance at the contact portion between the roller11and the cage12is reduced, which achieves low torque characteristics.

Further, when the bearing10rotates in the reverse direction and the roller11is inclined in the direction opposite to the direction shown inFIG. 6, the outer crowning portion16is brought into contact with the flat surface36con the opposite side (not illustrated). Even in this case, the outer crowning portion16of the roller11and the flat surface36cof the cage pocket13are in contact (point contact) with each other in the form of contact that forms the contact ellipse.

Further, in the present disclosure (seeFIG. 3), a third dimension X3as defined below is smaller than the second dimension X2(X3<X2). The third dimension X3is a dimension along the radial direction from the first end surface21that contacts the protrusion37to the boundary B3between the first portion26and the second portion27included in the outer crowning portion16.

According to this configuration, as shown inFIG. 6, the first portion26of the outer crowning portion16of the roller11is brought into contact with the flat surface36aof the cage pocket13because the inclination angle of the roller11is small (for example, the inclination angle is smaller than 3 degrees). As already described above, the first portion26has a larger radius of curvature of the arc generatrix compared to the second portion27(r1>r2). With this configuration, the contact ellipse formed between the flat surface36aand the first portion26can be relatively made larger. Accordingly, a surface pressure at the contact portion between the roller11and the cage12is reduced, which can suppress occurrence of wear. In this configuration, the relationship (first dimension X1>second dimension X2>third dimension X3) is established.

Further, in the present disclosure (seeFIG. 3), a fourth dimension X4as defined below with respect to the roller11is smaller than a fifth dimension X5as defined below with respect to the cage pocket13(X4<X5). The fourth dimension X4is a dimension along the radial direction from the first end surface21of the roller11with which the protrusion37of the cage pocket13is contactable to the boundary B4between the inner crowning portion17and the cylindrical portion15. The fifth dimension X5is a dimension along the radial direction from the contact point P1where the first end surface21of the roller11contacts the protrusion37of the cage pocket13to the boundary B5(provided, however, that the boundary B5is a boundary between the flat surface36don the inner side in the radial direction and the recessed surface35b).

Since the fourth dimension X4is smaller than the fifth dimension X5as described above, the inner crowning portion17of the roller11is brought into contact with the flat surface36dof the cage pocket13on the inner side in the radial direction when the bearing10rotates and the roller11is inclined in the cage pocket13as shown inFIG. 6. As described above, the inner crowning portion17includes an arch generatrix. This makes it possible for the inner crowning portion17of the roller11and the flat surface36dof the cage pocket13to contact (make a point contact) with each other in the form of contact that forms the contact ellipse. Consequently, the sliding frictional resistance at the contact portion between the roller11and the cage12is reduced, which achieves low torque characteristics.

For example, when the bearing10rotates in the reverse direction and the roller11is inclined in the direction opposite to the direction shown inFIG. 6, the inner crowning portion17is brought into contact with the flat surface36bon the opposite side (not illustrated). Also in this case, the inner crowning portion17of the roller11and the flat surface36bof the cage pocket13are in contact (point contact) with each other in the form of contact that forms the contact ellipse.

Further, in the present disclosure (seeFIG. 3), a sixth dimension X6as defined below is larger than the fifth dimension X5(X6>X5). The sixth dimension X6is a dimension along the radial direction from the first end surface21that is contactable with the protrusion37to the boundary B6between the third portion28and the fourth portion29included in the inner crowning portion17.

According to this configuration, as shown inFIG. 6, the third portion28included in the inner crowning portion17of the roller11is brought into contact with the flat surface36dof the cage pocket13because the inclination angle of the roller11is small (for example, the inclination angle is smaller than 3 degrees). As described above, the third portion28has a larger radius of curvature of an arc generatrix compared to the fourth portion29(r3>r4). With this configuration, the contact ellipse formed between the flat surface36dand the third portion28can be relatively made larger. Accordingly, a surface pressure at the contact portion between the roller11and the cage12is reduced, which can suppress occurrence of wear. In this configuration, the relationship (fourth dimension X4<fifth dimension X5<sixth dimension X6) is established.

InFIG. 3, the raised roundness48is provided at the boundary B2between the flat surface36aof the cage pocket13on the outer side in the radial direction and the recessed surface35a. With this configuration, even when the outer crowning portion16of the roller11contacts the boundary B2, an edge load does not occur. Further, as described above, the raised roundness49is provided at the boundary B5between the flat surface36dof the cage pocket13on the inner side in the radial direction and the recessed surface35b. With this configuration, even when the inner crowning portion17of the roller11contacts the boundary B5, an edge load does not occur.

InFIG. 3, according to the configuration in which the first dimension X1is larger than the second dimension X2and the fourth dimension X4is smaller than the fifth dimension X5, the cylindrical portion15of the roller11is located within a range of the cage pocket13in the radial direction where the recessed surface35a(35b) is formed with the first end surface21of the roller11in contact with the protrusion37. Therefore, when the bearing10rotates and the roller11is inclined in the cage pocket13as shown inFIG. 6, the outer crowning portion16and the inner crowning portion17each including an arc generatrix are brought into contact with the flat surface36aof the cage pocket13on the outer side in the radial direction and the flat surface36dof the cage pocket13on the inner side in the radial direction, respectively. As described above, each of the outer crowning portion16and the inner crowning portion17includes an arc generatrix. This makes it possible for the roller11and the cage12to contact (make a point contact) with each other at portions on the outer side in the radial direction and on the inner side in the radial direction in the form of contact that forms the contact ellipse. Consequently, the sliding frictional resistance at the contact portion between the roller11and the cage12is reduced, which achieves low torque characteristics.

As described above, according to the thrust roller bearing10according to the present disclosure, the sliding frictional resistance at the contact portion between the roller11and the cage12is reduced. Consequently, the low torque characteristics of the thrust roller bearing10can be achieved. Further, it is possible to suppress a temperature rise caused by the sliding contact between the roller11and the cage12.

The embodiment disclosed herein is illustrative but is not limitative in all respects. The scope of the present disclosure is not limited to the embodiment described above, and includes any and all modifications within the scope equivalent to the configuration described in the claims.