Tapered roller bearing

An inner ring has a rib face as a concave surface that is formed on the larger diameter side of an inner ring and that contacts large end faces of tapered rollers. A tapered roller bearing satisfies Ri>R≧Rr, where Rr represents a curvature radius of the large end face, Ri represents a curvature radius of the rib face 8 in a longitudinal section of the inner ring 2, R represents a distance from a cone center to a reference point. The distance R is the distance from the cone center to the reference point, where the reference point is the intersection of the rib face and an imaginary line extending from the cone center and along an inner ring raceway surface in the longitudinal section of the inner ring 2. Surface roughness of the rib face is greater than that of the large end face.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2014-264869 filed on Dec. 26, 2014 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to tapered roller bearings.

2. Description of the Related Art

As shown inFIG. 16, a tapered roller bearing90includes an inner ring91, an outer ring93, a plurality of tapered rollers95, and an annular cage96. The inner ring91has a tapered inner ring raceway surface92. The outer ring93has a tapered outer ring raceway surface94. The tapered rollers95roll on the inner ring raceway surface92and the outer ring raceway surface94. The cage96retains the tapered rollers95at intervals in the circumferential direction. The inner ring91has a cone back face rib (hereinafter referred to as a large rib)97formed on the larger diameter side of the inner ring91so as to protrude outward in the radial direction. The large rib97has an annular rib face99that contacts large end faces98of the tapered rollers95.

When the tapered roller bearing90is rotated, the tapered rollers95roll on the raceway surfaces92,94, and the large end faces98of the tapered rollers95slidingly contact the rib face99of the inner ring91. A tapered roller bearing is developed in which the large end faces98are convex surfaces having a predetermined curvature radius and the rib face99is a concave surface having a predetermined curvature radius in order to reduce friction resistance (sliding friction resistance) between the large end faces98and the rib face99(see, e.g., Japanese Utility Model Application Publication No. H05-75520).

A common method to reduce the friction resistance between the large end faces98of the tapered rollers95and the rib face99of the inner ring91is to reduce particularly the surface roughness of the rib face99. Conventionally, the rib face99is therefore super-finished, lapped, etc. after being ground.

However, since the rib face99is a concave surface as described above and is an annular face located inside a recess of the inner ring91, it is difficult to perform super finishing (or lapping) etc. on such a rib face99due to its shape. This is one of the factors that increase manufacturing cost.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a tapered roller bearing capable of reducing friction resistance between large end faces of tapered rollers and a rib face of an inner ring even if the rib face of the inner ring which is to be slidingly contacted by the large end faces of the tapered rollers is not super-finished (or lapped) etc.

According to one aspect of the present invention, a tapered roller bearing includes: an inner ring having a tapered inner ring raceway surface; an outer ring having a tapered outer ring raceway surface; a plurality of tapered rollers configure to roll on the inner ring raceway surface and the outer race raceway surface and each having a large end face formed of a convex surface; and an annular cage that retains the plurality of tapered rollers at intervals in a circumferential direction, wherein the inner ring has a rib face that is provided on a larger diameter side of the inner ring and that contacts the large end face, and surface roughness of the rib face is greater than that of the large end face.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below with reference to the accompanying drawings.FIG. 1is a longitudinal section showing an embodiment of a tapered roller bearing1of the present invention. The tapered roller bearing1includes an inner ring2, an outer ring3, a plurality of tapered rollers4, and an annular cage10. The outer ring3is disposed on the outer peripheral side of the inner ring2so as to be concentric with the inner ring2. The tapered rollers4are arranged between the inner ring2and the outer ring3. The cage10retains the tapered rollers4at intervals in the circumferential direction.

The inner ring2is an annular member made of bearing steel, steel for machine structural use, etc. The inner ring2has a tapered inner ring raceway surface12(a part of a conical surface) along its outer periphery. Like the inner ring2, the outer ring3is also an annular member made of bearing steel, steel for machine structural use, etc. The outer ring3has a tapered outer ring raceway surface13(a part of a conical surface) along its inner periphery. The inner ring raceway surface12faces the outer ring raceway surface13. When the tapered roller bearing1is rotated, the tapered rollers4roll on the inner ring raceway surface12and the outer ring raceway surface13. The tapered rollers4are members made of bearing steel etc. and having the shape of a circular truncated cone. Each tapered roller4has a large end face14on one side in the axial direction and has a small end face15on the other side in the axial direction. The large end face14has a larger diameter, and the small end face15has a smaller diameter. The large end face14is a convex surface.

The inner ring2has a cone back face rib (hereinafter referred to as a large rib)7. The large rib7is formed on the larger diameter side in the outer periphery of the inner ring2. The large rib7adjoins one end of the inner ring raceway surface12in the axial direction and protrudes outward in the radial direction. A rib face8serving as an end face on the inner ring raceway surface12side of the large rib7is a concave surface. The rib face8is an annular surface as the large rib7has an annular shape. A grinding undercut portion (hereinafter referred to as a recessed portion)9having a concave shape in section is formed along the entire circumference in the corner between the inner ring raceway surface12and the rib face8of the inner ring2. The inner ring2further has a cone front face rib (hereinafter referred to as a small rib)5. The small rib5is formed on the smaller diameter side in the outer periphery of the inner ring2. The small rib5adjoins the other end of the inner ring raceway surface12in the axial direction and protrudes outward in the radial direction.

The large end faces14of the tapered rollers4can contact the rib face8. When the tapered roller bearing1is rotated, the tapered rollers4roll on the raceway surfaces12,13and the rib face8slidingly contacts the large end faces14. It is herein assumed that the inner ring2is rotated. The large end faces14are convex surfaces, and the rib face8is a concave surface. Accordingly, when the rib face8contacts (slidingly contacts) the large end faces14, the contact surface (sliding-contact surface) between the rib face8and each of the large end faces14has an elliptical shape. That is, a contact ellipse M (see, e.g.,FIG. 3) is formed between each of the large end faces14and the rib face8. Lubricating oil (including grease) is supplied to the tapered roller bearing1. When the tapered roller bearing1(the inner ring2) is rotated, the lubricating oil flows in the circumferential direction along the annular rib face8.

The cage10has a pair of annular portions21,22and a plurality of bars23. The bars23connect the annular portions21,22. Those regions surrounded by the annular portions21,22and each pair of bars23adjacent to each other in the circumferential direction serve as pockets that accommodate the tapered rollers4. This cage10can retain the plurality of tapered rollers4at intervals (regular intervals) in the circumferential direction. The above configuration is common to tapered roller bearings1of other embodiments described below.

FIG. 2is a diagram illustrating the shape of the rib face8of the inner ring2and the shape of the large end face14of the tapered roller4. As shown inFIG. 2, the shape of the rib face8of the inner ring2and the shape of the large end face14of the tapered roller4are set based on a distance R, where R represents the distance from a cone center C of the tapered roller4to a reference point K.FIG. 2is a model diagram showing the tapered roller4(in section) superimposed on the longitudinal section of the inner ring2. The longitudinal section of the inner ring2herein refers to a section including the center line of the inner ring2.

As used herein, the reference point K refers to the intersection of an imaginary line L and the rib face8in the longitudinal section of the inner ring2(seeFIG. 2). The imaginary line L is a line extending from the cone center C of the tapered roller4and along the inner ring raceway surface12. This “rib face8” includes an extended line Yi extended from the rib face8in the longitudinal section. In the present embodiment, the recessed portion9is formed as described above. The reference point K is therefore the intersection of the imaginary line L and the extended line Yi. The “cone center C” of the tapered roller4means the vertex of the conical shape of the tapered roller4. The distance R is the distance between two points, namely the cone center C and the reference point K. For example, in the case where the cone center C has coordinates (0, 0) and the reference point K has coordinates (x, y) on the xy coordinate system, the distance R satisfies the relational expression of R2=x2+y2.

The shape of the rib face8of the inner ring2and the shape of the large end face14of the tapered roller4will be described below. In the longitudinal section of the inner ring2, a curvature radius Ri of the rib face8is set to a value larger than the distance R (Ri>R). Preferably, the curvature radius Ri of the rib face8is set so as to satisfy the relationship of 100%<Ri≦300% with respect to the distance R (100% of R<Ri≦300% of R). A curvature radius Rr of the large end face14of the tapered roller4is set to a value that is the same as or smaller than the distance R (Ri≦R). Preferably, the curvature radius Rr of the large end face14is set to a value in the range of 80 to 100% of the distance R (80% of R≦Rr≦100% of R). The curvature radius Rr of the large end face14is also set to a value smaller than the curvature radius Ri of the rib face8(Rr<Ri). InFIG. 2, the center Cr of curvature of the large end face14coincides with the cone center C.

As described above, the curvature radius Ri of the rib face8of the inner ring2is set to a value larger than the distance R, and the curvature radius Rr of the large end face14of the tapered roller4is set to a value that is the same as or smaller than the distance R. The relationship of “Ri>R≧Rr” is thus satisfied.

In the present embodiment, the center G of the contact ellipse M (crosshatched region shown inFIG. 3) formed by contact between the large end face14and the rib face8is located on an imaginary straight line X passing through the center Cr of curvature of the large end face14and the center Ci of curvature of the rib face8. As shown inFIG. 2, the center G of the contact ellipse M is thus located near the center of the rib face8in the radial direction.

According to the above configuration, the center G of the contact ellipse M formed by contact between the large end face14and the rib face8is located on the imaginary straight line X passing through the center Cr of curvature of the large end face14of the tapered roller4and the center Ci of curvature of the rib face8of the inner ring2. The center G of the contact ellipse M can thus be located near the center in the radial direction of the rib face8of the inner ring2. This can suppress contact of the large end face14with the outer and inner corners in the radial direction of the rib face8, and can therefore effectively prevent an edge load between the large end face14of the tapered roller4and the rib face8of the inner ring2.

In the tapered roller bearing1of the present embodiment, surface roughness σ2of the rib face8of the inner ring2is greater than surface roughness σ1of the large end face14of each tapered roller4(σ2>σ1). Specifically, the surface roughness (arithmetic average roughness) σ2of the rib face8is preferably 0.1 μmRa or more and 0.5 μmRa or less. On the other hand, the surface roughness (arithmetic average roughness) al of the large end face14of the tapered roller4is preferably less than 0.1 μmRa and 0.01 μmRa or more. Since the surface roughness σ2of the rib face8is 0.1 μmRa or more, the rib face8can be a ground surface produced by grinding. The large end face14of the tapered roller4is also a ground surface.

As shown inFIG. 4, in the tapered roller bearing1of the present embodiment, the large end face14of the tapered roller4has a plurality of recesses17. All of the recesses17are formed within the range of the contact ellipse M formed by contact between the large end face14and the rib face8.FIG. 4shows the large end face14of the tapered roller4as viewed in the axial direction. InFIG. 4, the contact ellipse M is shown by a long dashed double-short dashed line. When the tapered roller bearing1(the inner ring2) is rotated, the tapered rollers4roll between the inner ring2and the outer ring3. The contact ellipse M is thus formed in an annular region on the large end face14of each tapered roller4. InFIG. 4, this annular region is a region between a first imaginary circle K1and a second imaginary circle K2, and the recesses17are formed in this annular region.

In the embodiment shown inFIG. 4, the recesses17are very small recessed portions having a shape conforming to a part of a spherical surface. In the large end face14, the plurality of recesses17are formed along a third imaginary circle K3and a fourth imaginary circle K4which are located between the first and second imaginary circles K1, K2. For example, the recesses17are formed by punching, and the large end face14is ground after punching.

The recesses17may be in other forms. For example, as shown inFIG. 5, the recesses17may be elongated grooves (grooves in a radial pattern) that are elongated in the radial direction. In this case as well, the recesses17are formed in the region between the first and second imaginary circles K1, K2.

In the forms shown inFIGS. 4 and 5, the recesses17are formed in the region between the first and second imaginary circles K1, K2. The lubricating oil held in the recesses17can thus stay in the range of the contact ellipse M. The surface roughness al (less than 0.1 μmRa) of the large end face14of the tapered roller4is the surface roughness of the large end face14other than the recesses17.

According to the above configuration, when the tapered roller bearing1(in the present embodiment, the inner ring2) is rotated, each tapered roller4rolls on the inner ring raceway surface12. Accordingly, the rib face8and the large end face14of each tapered roller4theoretically do not slide on each other at the intersection of the inner ring raceway surface12and the rib face8(the reference point K), namely at the inner peripheral end position of the rib face8. In the tapered roller bearing1of the present embodiment, however, the large end face14of each tapered roller4partially contacts the rib face8(in the range of the contact ellipse M shown inFIG. 3) at a position radially outward of the inner peripheral end position (the reference point K). At this position radially outward of the inner peripheral end position, the rib face8(the inner ring2) rotates faster than the large end face14of the tapered roller4based on the rotational speed at the inner peripheral end position. The rib face8and the large end face14of the tapered roller4thus contact each other while sliding on each other (slidingly contact each other) due to this difference in speed.

In the tapered roller bearing1of the present embodiment, the surface roughness σ2of the rib face8that rotates faster is made greater than the surface roughness σ1of the large end face14of each tapered roller4which is a ground surface (σ2>σ1). This improves the capability of the rib face8to draw the lubricating oil that is present therearound. That is, since the rib face8has great surface roughness σ2, the rib face8can catch the lubricating oil that is present therearound. As shown in the conceptual diagram ofFIG. 6, as the inner ring2is rotated, the lubricating oil P caught by the rib face8can be supplied between the rib face8and the large end face14of each tapered roller4. InFIG. 6, the rib face8is rotating faster than the large end face14, and the lubricating oil P to be supplied therefore flows in the direction shown by an arrow X1.

The present embodiment satisfies “Ri>R≧Rr” (seeFIG. 2) as described above. The contact ellipse M formed by contact between the large end face14of the tapered roller4and the rib face8of the inner ring2is therefore an ellipse that is short in the circumferential direction of the inner ring2(the rib face8) and is long in the direction perpendicular to the circumferential direction. That is, ellipticity of the contact ellipse M can be increased. The ellipticity of the contact ellipse M (seeFIG. 3) is the ratio (b/a) of the dimension (vertical width) b of the contact ellipse M in the radial direction of the annular rib face8to the dimension (horizontal width) a in the tangential direction to an imaginary circle passing through the ellipse center G on the annular rib face8.

When the inner ring2is rotated, the lubricating oil on the rib face8flows in the circumferential direction of the inner ring2. The flow of the lubricating oil that is caught by the rib face8and supplied between the rib face8and the large end face14of each tapered roller4is therefore restricted in a wide range (the range of the dimension b) due to the shape of the contact ellipse M. This increases the thickness of an oil film between the large end face14of each tapered roller4and the rib face8of the inner ring2. The lubricating oil that is supplied between the rib face8and the large end face14of each tapered roller4can thus reduce friction resistance between the rib face8and the large end face14of each tapered roller4even if the surface roughness of the rib face8of the inner ring2is not significantly reduced by, e.g., super finishing etc. as in conventional examples. This eliminates the need for super finishing etc. of the rib face8, whereby manufacturing cost of the tapered roller bearing1can be reduced.

The ellipticity of the contact ellipse M is preferably 0.2 or more, and more preferably 0.3 or more. As in a second embodiment described below, the ellipticity of the contact ellipse M may be larger than 1. By increasing the ellipticity, the flow of the lubricating oil in the circumferential direction (along the rib face8) can be restricted in a wide range, and the thickness of the oil film can be increased.

In the present embodiment, the plurality of recesses17are formed in a predetermined range of the large end face14as described above (seeFIGS. 4, 5). The lubricating oil caught by the rib face8and supplied between the rib face8and the large end face14can thus be held in the recesses17. This can further improve lubrication performance between the rib face8and the large end face14.

Since the general configuration of a tapered roller bearing according to the second embodiment is similar to that of the tapered roller bearing1according to the first embodiment, description thereof will be omitted. The second embodiment is different from the first embodiment in the shape of the rib face8of the inner ring2and the shape of the large end face14of each tapered roller4. This difference will be described below.

FIG. 7is a diagram illustrating the shape of the rib face8of the inner ring2, the shape of the large end face14of each tapered roller4, etc.FIG. 7is a model diagram showing the tapered roller4(in section) superimposed on the longitudinal section of the inner ring2. For illustration, the shape of each part is shown in a simplified manner inFIG. 7. The large end face14of the tapered roller4is actually in contact with the rib face8of the inner ring2. For illustration, however, the large end face14of the tapered roller4is shown separated from the rib face8of the inner ring2inFIG. 7. The longitudinal section of the inner ring2herein refers to a section including the center line LO of the inner ring2.

The large end face14is a part of a spherical surface, and Rr represents the curvature radius of this large end face14. The large end face14therefore has the curvature radius Rr in a longitudinal section of the tapered roller4(FIG. 7) which is in the same section as the longitudinal section of the inner ring2. The large end face14also has the curvature radius Rr in a section perpendicular to the longitudinal section (FIG. 7) and passing through the center line L2of the tapered roller4.

Ri represents the curvature radius of the rib face8in the longitudinal section of the inner ring2(FIG. 7).FIG. 8is a schematic diagram illustrating the shape of the rib face8. As described above, the rib face8has the curvature radius Ri in the longitudinal section of the inner ring2(FIG. 7). As shown inFIG. 8, however, the rib face8has a curvature radius Rx as viewed radially inward in the direction perpendicular to an imaginary line L3(the direction shown by an arrow V1; hereinafter referred to as an imaginary radial direction). The imaginary line L3is a line connecting the cone center C and the center G of the contact ellipse M. The rib face8therefore has two curvature radii Ri, Rx depending on the direction in which the rib face8is viewed.

InFIG. 7, Rx represents the distance from the cone center C to the center G of the contact ellipse M formed by contact between the large end face14and the rib face8. The cone center C is the cone center of the tapered roller4and means the vertex of the conical shape of the tapered roller4. The cone center C is also the intersection of an extended line L1of the inner ring raceway surface12and the center line LO of the inner ring2in the longitudinal section of the inner ring2. The cone center C, the center Cr of the large end face14, and the center Ci of the rib face8are located at different positions. The center Cr is located on an extended line of the center line L2of the tapered roller4.

This tapered roller bearing1satisfies “Rx>Ri>Rr.” As shown inFIG. 11, the contact ellipse M formed by contact between the large end face14of the tapered roller4and the rib face8of the inner ring2is therefore an ellipse that is short in the circumferential direction of the inner ring2and is long in the direction perpendicular to the circumferential direction (the perpendicular direction to the circumferential direction). This will be described later.

As in the first embodiment, in the tapered roller bearing1of the present embodiment, the surface roughness σ2of the rib face8of the inner ring2is greater than the surface roughness σ1of the large end face14of each tapered roller4(σ2>σ1). Specifically, the surface roughness (arithmetic average roughness) σ2of the rib face8is preferably 0.1 μmRa or more and 0.5 μmRa or less. On the other hand, the surface roughness (arithmetic average roughness) al of the large end face14of the tapered roller4is preferably less than 0.1 μmRa and 0.01 μmRa or more. Since the surface roughness σ2of the rib face8is 0.1 μmRa or more, the rib face8can be a ground surface produced by grinding. The large end face14of the tapered roller4is also a ground surface.

As in the first embodiment, in the tapered roller bearing1of the present embodiment, the large end face14of the tapered roller4has the plurality of recesses17, as shown inFIG. 4(orFIG. 5). All of the recesses17are formed within the range of the contact ellipse M formed by contact between the large end face14and the rib face8.

The relationship between “Rx>Ri>Rr” and the shape of the contact ellipse M will be described below. As described above, the large end face14of the tapered roller4is a part of a spherical surface. The large end face14therefore has the curvature radius Rr in the longitudinal section of the inner ring2(FIG. 7). The large end face14also has the curvature radius Rr in the section perpendicular to this longitudinal section and passing through the center line L2of the tapered roller4. The rib face8of the large rib7has the curvature radius Ri in the longitudinal section of the inner ring2(seeFIG. 7). However, the rib face8of the inner ring2has the curvature radius Rx as viewed radially inward in the direction perpendicular to the imaginary line L3connecting the cone center C and the center G of the contact ellipse M (the direction shown by the arrow V1) (seeFIG. 8).

Simple examples of how the large end face14(convex surface) of the tapered roller4contacts the rib face8(concave surface) of the inner ring2will be described by using a generalized convex surface F and a generalized convex surface f shown inFIGS. 9A, 9B.

As shown inFIG. 9A, the convex surface F has a curvature radius Rr, and the concave surface f has a curvature radius Ri. For example, in the case where the curvature radius Rr of the convex surface F is “1,000” and the curvature radius Ri of the concave curve f is “1,003,” the difference β1between the curvature radii Rr, Ri is 3 (=1,003−1,000). InFIG. 9B, the convex surface F has the same curvature radius Rr as inFIG. 9A, but the concave surface f has a curvature radius Rx (larger than Ri). For example, in the case where the curvature radius Rr of the convex surface F is “1,000” and the curvature radius Rx of the concave curve f is “1,005” inFIG. 9B, the difference δ2between the curvature radii Rr, Rx is 5 (=1,005−1,000).

As in the case ofFIG. 9A, if the difference δ1(=3) between the curvature radii Rr, Ri is small (as compared to the difference δ2(=5) between the curvature radii Rr, Rx inFIG. 9B), the contact width (contact length) b between the convex surface F and the concave surface f is large (as compared to the case ofFIG. 9B). As in the case ofFIG. 9B, if the difference δ2(=5) between the curvature radii Rr, Rx is large (as compared to the difference δ1(=3) between the curvature radii Rr, Ri inFIG. 9A), the contact width (contact length) a between the convex surface F and the concave surface f is small (as compared to the case ofFIG. 9A) (a<b). That is, in the case where the convex surface F and the concave curve f which have different curvature radii contact each other, the contact width b between the convex surface F and the concave surface f is relatively large (wide) if the difference between the curvature radius of the convex surface F and the curvature radius of the concave surface f is small (FIG. 9A). However, the contact width a between the convex surface F and the concave surface f is relatively small (narrow) if the difference between the curvature radius of the convex surface F and the curvature radius of the concave surface f is large (FIG. 9B). The smaller the difference between the curvature radius of the convex surface F and the curvature radius of the concave surface f is, the larger the contact width b is. The larger the difference between the curvature radius of the convex surface F and the curvature radius of the concave surface f is, the smaller the contact width a is.

The present embodiment will be described in view of the above description of the generalized case. The present embodiment (seeFIG. 7) satisfies the relationship of “Rx>Ri>Rr” as described above. That is, the distance (curvature radius) Rx from the cone center C to the center G of the contact ellipse M and the curvature radius Ri of the rib face8are larger than the curvature radius Rr of the large end face14of the tapered roller4(Rx>Rr, Ri>Rr). Moreover, Rx is larger than Ri (Rx>Ri). In the case where the forms shown inFIGS. 9A, 9Bare applied to the present embodiment,FIG. 9Acorresponds to the longitudinal section of the inner ring2(FIG. 7), andFIG. 9Bcorresponds to the section as viewed in the imaginary radial direction (the direction shown by the arrow V1inFIGS. 7, 8).

The width dimension b of the contact ellipse M in the longitudinal section of the inner ring2(FIG. 9A) is larger than the width dimension a of the contact ellipse M in the section as viewed in the imaginary radial direction (FIG. 9B). That is, as shown inFIG. 11, the contact ellipse M formed by contact between the large end face14of the tapered roller4and the rib face8of the inner ring2is an ellipse that is short in the circumferential direction of the inner ring2and is long in the direction perpendicular to the circumferential direction. The contact ellipse M that is longer in the direction perpendicular to the circumferential direction of the inner ring2than in the circumferential direction of the inner ring2is thus formed between the rib face8of the inner ring2and the large end face14of the tapered roller4. The ellipticity (b/a) of the contact ellipse M is larger than 1 (ellipticity >1).

A contact ellipse of a conventionally used tapered roller bearing will be described. A tapered roller bearing is proposed in which a rib face of an inner ring is a concave surface in a longitudinal section of the inner ring as disclosed in Japanese Utility Model Application Publication No. H05-75520. However, a rib face of an inner ring is typically a flat surface rather than a concave surface in currently commercially available tapered roller bearings. In this case, the curvature radius Ri (seeFIG. 7) of the rib face of the inner ring is infinite. According toFIGS. 9A, 9B, since the curvature radius Ri is infinite, Rr<Ri, and Rx<Ri, the width dimension b of the contact ellipse M is smaller than the width dimension a of the contact ellipse M (b<a). The contact ellipse M is therefore an ellipse that is long in the circumferential direction of the inner ring and is short in the direction perpendicular to the circumferential direction of the inner ring. The ellipticity of the contact ellipse M is thus smaller than 1 (ellipticity <1). The ellipticity is less than 0.2 in conventional typical tapered roller bearings.

On the other hand, in the present embodiment, the contact ellipse M formed by contact between the large end face14of the tapered roller4and the rib face8of the inner ring2is an ellipse that is short in the circumferential direction of the inner ring2and is long in the direction perpendicular to the circumferential direction (seeFIG. 11), as described above. That is, the ellipticity is larger than 1. As described above, when the tapered roller bearing1is rotated, the lubricating oil on the rib face8flows in the circumferential direction of the inner ring2. According to the shape of the contact ellipse M of the present embodiment (seeFIG. 11), the flow of the lubricating oil is restricted in a wide range (the range of the contact width b) of the rib face8. This increases the thickness of an oil film between the large end face14of the tapered roller4and the rib face8of the inner ring2, and can thus effectively reduce friction resistance between the large end face14of the tapered roller4and the rib face8of the inner ring2.

According to the conventional example shown inFIG. 10, the contact ellipse M is an ellipse that is long in the circumferential direction and is short in the direction perpendicular to the circumferential direction. Almost all of the lubricating oil flowing along the rib face8therefore passes the contact ellipse M, and the tapered roller bearing of the conventional example therefore hardly functions to increase the thickness of the oil film.

According to the above configuration, when the tapered roller bearing1(in the present embodiment, the inner ring2) is rotated, each tapered roller4rolls on the inner ring raceway surface12. Accordingly, the rib face8and the large end face14of each tapered roller4theoretically do not slide on each other at the intersection of the inner ring raceway surface12and the rib face8(the reference point K), namely at the inner peripheral end position of the rib face8. In the tapered roller bearing1of the present embodiment, however, the large end face14of each tapered roller4partially contacts the rib face8(in the range of the contact ellipse M shown inFIG. 11) at a position radially outward of the inner peripheral end position (the reference point K). At this position radially outward of the inner peripheral end position, the rib face8(the inner ring2) rotates faster than the large end face14of the tapered roller4based on the rotational speed at the inner peripheral end position. The rib face8and the large end face14of the tapered roller4thus contact each other while sliding on each other (slidingly contact each other) due to this difference in speed.

In the tapered roller bearing1of the present embodiment, the surface roughness σ2of the rib face8that rotates faster is made greater than the surface roughness σ1of the large end face14of each tapered roller4which is a ground surface (σ2>σ1). This improves the capability of the rib face8to draw the lubricating oil that is present therearound. That is, since the rib face8has greater surface roughness σ2, the rib face8can catch the lubricating oil that is present therearound. As shown in the conceptual diagram ofFIG. 6, as the inner ring2is rotated, the lubricating oil caught by the rib face8can be supplied between the rib face8and the large end face14of each tapered roller4.

The present embodiment satisfies “Rx>Ri>Rr” (seeFIGS. 7, 8) as described above. The contact ellipse M formed by contact between the large end face14of the tapered roller4and the rib face8of the inner ring2is therefore an ellipse that is short in the circumferential direction of the inner ring2(the rib face8) and is long in the direction perpendicular to the circumferential direction. That is, the ellipticity of the contact ellipse M can be increased. The ellipticity of the contact ellipse M is the ratio (b/a) of the dimension (vertical width) b of the contact ellipse M in the radial direction of the annular rib face8to the dimension (horizontal width) a in the tangential direction to the imaginary circle passing through the ellipse center G on the annular rib face8.

When the inner ring2is rotated, the lubricating oil on the rib face8flows in the circumferential direction of the inner ring2. The flow of the lubricating oil that is caught by the rib face8and supplied between the rib face8and the large end face14of each tapered roller4is therefore restricted in a wide range (the range of the dimension b) due to the shape of the contact ellipse M. This increases the thickness of the oil film between the large end face14of each tapered roller4and the rib face8of the inner ring2. In particular, since the ellipticity of the contact ellipse M is larger than 1, the flow of the lubricating oil in the circumferential direction (along the rib face8) can be restricted in a wide range, and the thickness of the oil film can be increased.

The lubricating oil that is supplied between the rib face8and the large end face14of each tapered roller4can thus reduce friction resistance between the rib face8and the large end face14of each tapered roller4even if the surface roughness of the rib face8of the inner ring2is not significantly reduced by, e.g., super finishing etc. as in conventional examples. This eliminates the need for super finishing etc. of the rib face8, whereby manufacturing cost of the tapered roller bearing1can be reduced.

In the present embodiment, the plurality of recesses17are formed in a predetermined range of the large end face14as described above (seeFIGS. 4, 5). The lubricating oil caught by the rib face8and supplied between the rib face8and the large end face14can thus be held in the recesses17. This can further improve lubrication performance between the rib face8and the large end face14.

FIG. 12is an illustration showing a modification of the first embodiment (FIG. 2). Since the general configuration of a tapered roller bearing shown inFIG. 12(third embodiment) is similar to that of the tapered roller bearing1of the first embodiment, description thereof will be omitted. The third embodiment is different from the first embodiment in the shape of the rib face8of the inner ring2and the shape of the large end face14of each tapered roller4. This difference will be described below.FIG. 12is a diagram illustrating the shape of the rib face8of the inner ring2, the shape of the large end face14of the tapered roller4, etc.FIG. 12is a model diagram showing the tapered roller4(in section) superimposed on the longitudinal section of the inner ring2. The longitudinal section of the inner ring2herein refers to a section including the center line of the inner ring2.

The shape of the rib face8of the inner ring2and the shape of the large end face14of the tapered roller4will be described below. R represents the distance from the cone center C of the tapered roller4to a predetermined reference point K separated from the inner ring raceway surface12in the direction along the inner ring raceway surface12. In this case, the shape of the rib face8of the inner ring2and the shape of the large end face14of the tapered roller4are set based on the reference point K and the distance R.

The rib face8of the inner ring2is formed at such a position that the rib face8passes through the reference point K. The curvature radius Ri of the rib face8is set in the range of 100 to 120% of the distance R (Ri≧R). Since the recessed portion9is formed in the present embodiment, the rib face8“passing through the reference point K” includes not only the case where the rib face8actually passes through the reference point K, but also the case where the extended line Yi of the rib face8passes through the reference point K. The “reference point K” is therefore the intersection of the inner ring raceway surface12or the extended line L thereof and the rib face8or the extended line Yi thereof. In the present embodiment, the extended line Yi of the rib face8passes through the reference point K.

The large end face14of the tapered roller4is formed at such a position that the large end face14passes through the reference point K. The curvature radius Rr of the large end face14is set in the range of 80 to 100% of the distance R (Rr≦R). The large end face14“passing through the reference point K” includes not only the case where the large end face14actually passes through the reference point K, but also the case where the extended line Yr of the large end face14passes through the reference point K. In the present embodiment, the extended line Yr of the large end face14passes through the reference point K. In the present embodiment, the curvature radius Ri of the rib face8and the curvature radius Rr of the large end face14satisfy the relationship of “R=(Ri+Rr)/2.” The present embodiment satisfies “Ri>R≧Rr.”

According to the above configuration, as shown inFIG. 13, the large end face14of the tapered roller4slidingly contacts the radially inner end of the rib face8of the inner ring2, which is a part (crosshatched region in the figure) of the range of an imaginary circle about the reference point K. This can reduce the range in which the large end face14slidingly contacts the rib face8(the range in which the large end face14and the rib face8contact each other while sliding on each other) as compared to the case where the large end face14slidingly contacts a radially outer part of the rib face8. Sliding friction between the large end face14of the tapered roller4and the rib face8of the inner ring2can therefore be effectively reduced.

In the tapered roller bearing1of the present embodiment, the surface roughness σ2of the rib face8of the inner ring2is greater than the surface roughness σ1of the large end face14of each tapered roller4(σ2>σ1). Specifically, the surface roughness (arithmetic average roughness) σ2of the rib face8is preferably 0.1 μmRa or more and 0.5 μmRa or less. On the other hand, the surface roughness (arithmetic average roughness) σ1of the large end face14of the tapered roller4is preferably less than 0.1 μm Ra and 0.01 μmRa or more. Since the surface roughness σ2of the rib face8is 0.1 μmRa or more, the rib face8can be a ground surface produced by grinding. The large end face14of the tapered roller4is also a ground surface.

As in the first embodiment, in the tapered roller bearing1of the present embodiment, the large end face14of the tapered roller4has the plurality of recesses17, as shown inFIG. 4(orFIG. 5). All of the recesses17are formed within the range of the contact ellipse M formed by contact between the large end face14and the rib face8.

According to the above configuration, when the tapered roller bearing1(in the present embodiment, the inner ring2) is rotated, each tapered roller4rolls on the inner ring raceway surface12. Accordingly, the rib face8and the large end face14of each tapered roller4theoretically do not slide on each other at the intersection of the inner ring raceway surface12and the rib face8(the reference point K), namely at the inner peripheral end position of the rib face8. As shown inFIG. 13, in the tapered roller bearing1of the present embodiment, the large end face14of the tapered roller4slidingly contacts the radially inner end of the rib face8, which is a part (crosshatched region in the figure) of the range of the imaginary circle about the reference point K, as described above. Sliding friction between the large end face14and the rib face8is thus reduced.

The surface roughness σ2of the rib face8is made greater than the surface roughness σ1of the large end face14of each tapered roller4which is a ground surface (σ2>σ1). This improves the capability of the rib face8to draw the lubricating oil that is present therearound. That is, since the rib face8has greater surface roughness σ2, the rib face8can catch the lubricating oil that is present therearound. As shown in the conceptual diagram ofFIG. 6, as the inner ring2is rotated, the lubricating oil caught by the rib face8can be supplied between the rib face8and the large end face14of each tapered roller4.

The lubricating oil that is supplied between the rib face8and the large end face14of each tapered roller4can thus reduce friction resistance between the rib face8and the large end face14of each tapered roller4even if the surface roughness of the rib face8of the inner ring2is not significantly reduced by, e.g., super finishing etc. as in conventional examples. This eliminates the need for super finishing etc. of the rib face8, whereby manufacturing cost of the tapered roller bearing1can be reduced.

In the present embodiment as well, the plurality of recesses17(seeFIGS. 4, 5) are formed in a predetermined range of the large end face14. This can further improve lubrication performance between the rib face8and the large end face14.

FIG. 14is a graph (experimental result) illustrating the relationship between the surface roughness [μmRa] of the rib face8and the rotational resistance in the tapered roller bearing.FIG. 15is a graph (experimental result) illustrating the relationship between the surface roughness [μmRa] of the large end face14of the tapered roller4and the rotational resistance in the tapered roller bearing. The rotational resistance is regarded as torque (friction torque) in the experiments. The torque of a conventional product is regarded as “1,” and the ratio of the torque of the tapered roller bearing1of the present embodiment (first embodiment) to the torque of the conventional product is shown on the ordinate. That is, the torque ratio of less than 1 means that the torque (friction torque) of the present embodiment is smaller than that of the conventional product.

As shown inFIG. 14, the torque can be reduced as compared to the conventional product even if the surface roughness of the rib face8is 0.1 μmRa or more. This seems to be because of the following function of the rib face8. In the present embodiment, since the rib face8has great surface roughness, the rib face8can catch the lubricating oil that is present therearound, and the lubricating oil caught by the rib face8is supplied between the rib face8and the large end face14of each tapered roller4. Even if the surface roughness of the rib face8is as large as 1.0 μmRa or more, the torque (friction torque) can be significantly reduced by causing the large end face14of each tapered roller4to have surface roughness of less than 0.1 μmRa.

In the forms shown inFIGS. 4, 5, the area of the recesses17formed in the large end face14of the tapered roller4is preferably 1 to 20% of the area of the contact ellipse M. If the percentage of the area of the recesses17is low (less than 1%), the recesses17do not sufficiently function to hold the lubricating oil. If the percentage of the area of the recesses17is high (more than 20%), the contact surface pressure between the large end face14and the rib face8is increased, which may affect sliding properties.

The tapered roller bearing of the present invention is not limited to the illustrated forms, but may be implemented in other forms without departing from the scope of the present invention. For example, the cage10may have a shape other than that shown in the figures. The above embodiments are described with respect to the case where the rib face8of the large rib7of the inner ring2has the shape of a concave curve in longitudinal section. However, the rib face8may have a linear shape in longitudinal section. In this case as well, the capability of the rib face8to draw the lubricating oil that is present therearound can be improved by making the surface roughness σ2of the rib face8greater than the surface roughness σ1of the large end face14of each tapered roller4(σ2>σ1).

According to the present invention, the lubricating oil that is supplied between the rib face and the large end face of each tapered roller can reduce friction resistance between the rib face and the large end face of each tapered roller even if the surface roughness of the rib face of the inner ring that is to be slidingly contacted by the tapered rollers is not significantly reduced by, e.g., super finishing etc. as in conventional examples. This eliminates the need for super finishing etc. of the rib face, whereby manufacturing cost of the tapered roller bearing can be reduced.