Tapered roller bearing

A tapered roller bearing is provided has a grinding undercut with an undercut width A of 0.5 mm or less from a reference point to a large flange surface. The reference point is the intersection point of the imaginary line extending from the generatrix of the raceway surface of the inner ring toward the grinding undercut, and the imaginary line extending from the generatrix of the large flange surface toward the grinding undercut.

TECHNICAL FIELD

The present invention relates to a tapered roller bearing.

BACKGROUND ART

A rolling bearing that supports a rotary part needs to be selected taking into consideration the direction and size of the load which the rolling bearing receives, and the space where the bearing is installed. If a rolling bearing is used to support a rotary part disposed in a transmission (MT, AT, DCT, CVT, a hybrid transmission, etc.) or a differential for an automobile, the rolling bearing is required to be a small-sized bearing even under use conditions where the bearing receives radial, axial, and moment loads. Therefore, as such a rolling bearing, a tapered roller bearing is used, which is capable of receiving both radial and axial loads, and has an excellent capacity for such loads.

In such a tapered roller bearing, a thrust force that pushes the tapered rollers toward the larger-diameter side is generated during operation. Therefore, the inner ring is formed with a large flange for guiding, while supporting, the large end surfaces of the tapered rollers in their revolution direction (circumferential direction), in which the tapered rollers revolve around the center axis of the bearing. The large flange has a large flange surface with which the large end surfaces of the tapered rollers are brought into sliding contact. The inner ring is formed with a grinding undercut extending around the entire circumference, and connecting the large flange surface and the raceway surface of the inner ring to each other.

In general, the shapes of the large end surface of each tapered roller and the large flange surface of the inner ring are designed such that the large end surface and the large flange surface come into contact with each other geometrically at only one point. During operation, the large end surface of each tapered roller comes into sliding contact with the large flange surface of the inner ring in the revolution direction. The sliding contact portions are each present within a generally elongated oval area having a radial short axis with its center located at the above contact point. If the sliding contact portions are not sufficiently lubricated, heat could build up, thereby causing a sharp rise in temperature.

If, as in an automotive transmission, the tapered roller bearing is operated at a high speed and the temperature of lubricating oil is high, a good lubrication mode may not be maintained between the sliding contact portions of the large flange surface of the inner ring and the large end surfaces of the tapered rollers, thereby generating boundary lubrication, so that the sliding contact portions may not be lubricated sufficiently. In order to improve the seizure resistance during operation at a high speed, measures are taken in the shapes and surface properties of the large end surfaces of the tapered rollers and the large flange surface of the inner ring (see discussion of several documents below).

In Japanese Unexamined Patent Application Publication No. 2000-170774, when considering the radius of curvature R of the large end surface of each tapered roller, and the distance RBASEfrom the vertex of the cone angle of the tapered roller to its contact portion with the large flange surface, by setting R/RBASEwithin the range of 0.75 to 0.87, it is possible to generate a good wedge effect when lubricating oil is dragged between the large flange surface of the inner ring and the large end surfaces of the tapered rollers, thereby improving the oil film thickness (and thus reducing heat buildup) at the sliding contact portions of these surfaces.

In Japanese Unexamined Patent Application Publication No. 2000-170775, by forming an undercut surface shaped such that the distance between the undercut surface and the large end surface of each tapered roller increases from the radially outer edge of the large flange surface toward the radially inner edge of a chamfer of the large flange, it is possible to increase the effect of the lubricating oil being pulled onto the contact portions of the large end surfaces of the tapered rollers and the large flange surface of the inner ring, and thus to improve the oil film forming capability.

In Japanese Unexamined Patent Application Publication No. 2018-136027, by setting the above ratio R/RBASEwithin the range of 0.75 to 0.87, and also, by setting, when considering the actual radius of curvature RACTUALof the large end surface of each tapered roller, RACTUAL/R at 0.5 or more, it is possible to reduce heat buildup at the large end surfaces of the tapered rollers and the large flange surface of the inner ring even under severe lubrication conditions, and thus to improve seizure resistance. Especially by introducing a flange portion lubrication coefficient as an indicator showing the level of severity of the lubrication condition, it is possible to increase the workable range of the ratio of RACTUAL/R, and thus to select bearing specifications according to the use conditions.

However, in automotive transmissions or differentials, in order to improve fuel efficiency, there is a growing tendency to reduce the viscosity of lubricating oil or the amount of lubricating oil in a unit, and this tendency is expected to continue. Therefore, rolling bearings are expected to be used under increasingly severer lubrication conditions. Especially in tapered roller bearings, it is increasingly important to ensure oil film thickness at the contact portions of the large end surfaces of the tapered rollers and the large flange surface of the inner ring, and to reduce a rise in temperature due to lubricating oil.

In view of the above-described background, it is an object of the present invention to provide a tapered roller bearing designed such that, even if the tapered roller bearing is used under sever lubrication conditions, a sharp rise in temperature is prevented and the bearing rotates smoothly.

SUMMARY OF THE INVENTION

In order to achieve the above object, the present invention provides a tapered roller bearing comprising: an inner ring; an outer ring; a plurality of tapered rollers disposed between the inner ring and the outer ring; and a cage in which the tapered rollers are received, wherein each of the tapered rollers has: a conical rolling surface; a chamfer continuous with a large-diameter side of the rolling surface; and a large end surface continuous with the chamfer, and wherein the inner ring has: a conical raceway surface; a large flange surface configured to receive the large end surfaces of the tapered rollers; and a groove-shaped grinding undercut connecting the large flange surface and the raceway surface to each other, characterized in that the grinding undercut has an undercut width A of 0.5 mm or less from a reference point to the large flange surface, the reference point being an intersection point of an imaginary line extending from a generatrix of the raceway surface toward the grinding undercut, and an imaginary line extending from a generatrix of the large flange surface toward the grinding undercut.

In the above bearing, since the undercut width A of the grinding undercut of the inner ring is set at a particularly small dimension, i.e., 0.5 mm or less, the width of the large flange surface is wide enough to receive the large end surfaces of the tapered rollers. Therefore, it is possible to optimize the contact relationship between the large flange surface and the large end surfaces of the tapered rollers; and generate a good wedge effect between the large flange surface and the large end surfaces of the tapered rollers so as to improve the oil film forming capability.

Specifically, when considering a cone angle β of each of the rolling surfaces, and an acute angle ρ of an imaginary line connecting together a vertex of the cone angle β and a contact point of the large flange surface and the large end surface of each of the tapered rollers, relative to the generatrix of the raceway surface, a relationship between β and ρ is preferably β/6≥ρ. Since the angle ρ, which denotes the radial height of the contact point between the large flange surface and the large end surface of each tapered roller, relative to the reference point, is smaller than β/6, it possible to prevent a rise in the sliding velocity at the sliding contact portions of the large flange surface and the large end surfaces, thus reducing heat buildup at the large flange surface, and thereby to prevent a sharp rise in temperature.

It is preferable that, when considering an approach angleaof the grinding undercut relative to the large flange surface of the inner ring, and an approach angle b of the grinding undercut relative to the raceway surface, a relationship betweenaand b isa>b, and, when considering the undercut width A from the reference point to the large flange surface, and an undercut width B from the reference point to the raceway surface, a relationship between A and B is A<B. When manufacturing the bearing, in order to set the undercut width A at 0.5 mm or less, it should be taken into consideration that, if the ground amount of the large flange surface overshoots or undershoots relative to the target value during machining, the width of the large flange surface changes depending on the approach angleaof the grinding undercut. Since the larger the approach anglearelative to the large flange surface, the smaller the amount of change in the width of the large flange surface due to any overshoot or undershoot of the ground amount of the large flange surface, the approach angleais preferably set at a large value. Also, in order to easily discharge chips produced while forming the grinding undercut by turning, it is preferable to satisfy the relationships of a>b and A<B.

When considering a depth c of the grinding undercut relative to the raceway surface of the inner ring, and a depth d of the grinding undercut relative to the large flange surface, a relationship between c and d is preferably c>d. By satisfying this relationship, it is possible to reduce the stress of the large flange of the inner ring caused by loads applied from the large end surfaces of the tapered rollers to the large flange surface of the inner ring, and to improve the strength of the large flange of the inner ring.

The depth d of the grinding undercut relative to the large flange surface of the inner ring is preferably 0.3 mm or less. If this depth is 0.3 mm or less, it is possible to reliably improve the strength of the large flange of the inner ring.

The approach angleaof the grinding undercut relative to the large flange surface of the inner ring is preferably within a range of 20°≤a≤50°. Within this range, it is possible to easily control the undercut width A during grinding of the large end surface.

A width W of the large flange surface preferably satisfies the following Formula 1:
W≥{Dw×(½)×Tan θ/(L/Dw)},  <Formula 1>
where θ is an acute angle of the generatrix of the raceway surface relative to a center axis of the inner ring; Dw is a large-end diameter of the rolling surface of each of the tapered rollers; and L is a roller length of each of the tapered rollers. If the width W satisfies Formula 1, it is possible to make the large flange surface sufficiently opposed to the large end surfaces of the tapered rollers. Therefore, even if the sliding contact portions of the large end surfaces of the tapered rollers and the large flange surface of the inner ring are displaced radially outwardly of the of the large flange, it is possible to keep a good contact state therebetween.

A grain size number of old austenite crystal grains in the large flange surface of the inner ring is preferably No. 6 or more. Such a large flange surface is suitable for delaying its surface damage due to metal contact with the large end surfaces of the tapered rollers.

The large flange surface of the inner ring is preferably formed by a nitrided layer having a nitrogen content of 0.05 wt % or more. Such a large flange surface is suitable for delaying its surface damage due to metal contact with the large end surfaces of the tapered rollers.

The large flange surface of the inner ring has a surface roughness of 0.1 μm Ra or less, and the large end surface of each of the tapered rollers has a surface roughness of 0.12 μm Ra or less. Within these ranges, it is possible to improve oil film formation between the large flange surface and the large end surfaces of the tapered rollers.

It is preferable that, when considering set radii of curvature R of the large end surfaces of the respective tapered rollers, and base radii of curvature RBASEfrom the vertexes of the cone angles of the respective rolling surfaces to the large flange surface of the inner ring, the R/RBASEvalues are 0.70 or more and 0.95 or less, and, when considering actual radii of curvature RACTUALof the large end surfaces of the respective tapered rollers, at least one of the RACTUAL/R values is 0.3 or more and less than 0.5. In the present invention, since it is possible to improve the oil film forming capability on the side of the large flange surface, it is possible to set each of R/RBASEand RACTUAL/R within a wide range compared to the tapered roller bearing of Unexamined Patent Application Publication No. 2018-136027 discussed above. As a result, it is possible to improve the yield rate of the tapered rollers, and thus provide the tapered roller bearing at a relatively low cost.

Since the tapered roller bearing of the present invention has improved seizure resistance under severe lubrication conditions, the tapered roller bearing can be suitably used to support a rotary shaft of a transmission or a differential for an automobile

Effects of the Invention

By using the above structure in the present invention as described above, it is possible to optimize the contact relationship between the large flange surface of the inner ring and the large end surfaces of the tapered rollers, and improve oil film forming capability. Therefore, even if the tapered roller bearing is used under severe lubrication conditions, it is possible to prevent a sharp rise in temperature, and rotate the bearing smoothly.

DETAILED DESCRIPTION OF THE INVENTION

The tapered roller bearing embodying the present invention is now described with reference to the attached drawings.

As illustrated inFIG.2, this tapered roller bearing includes an inner ring10; an outer ring20; a plurality of tapered rollers30disposed between the inner ring10and the outer ring20; and a retainer40in which these tapered rollers30are received. This tapered roller bearing is intended for use in a transmission or a differential for automobiles, mainly for passenger vehicles, and has an outer diameter of 150 mm or less.

As illustrated inFIGS.2and3, the inner ring10is a bearing ring having, on its outer periphery, a conical raceway surface11; a large flange12having a diameter larger than the diameter of the large-diameter-side edge of the raceway surface11on its large-diameter side; a grinding undercut13formed from the base of the large flange12to the raceway surface11; a small flange14having a diameter larger than the diameter of the small-diameter-side edge of the raceway surface11on its small-diameter side; and a small-diameter-side grinding undercut15formed from the base of the small flange14to the raceway surface11.

As illustrated inFIG.2, the outer ring20is a bearing ring having a conical raceway surface21on its inner periphery. Lubricating oil is supplied to the bearing interior space between the inner ring10and the outer ring20from the outside of the bearing.

Each tapered roller30is a rolling element having a conical rolling surface31; a chamfer32continuous with the large-diameter side of the rolling surface31; a large end surface33continuous with the chamfer32; and a small end surface34formed on the side opposite from the large end surface33. The large end surface33and the small end surface34include both ends of the tapered roller30that define the roller length L of the tapered roller30.

The tapered rollers30are arranged in a single row between the inner and outer raceway surfaces11and21. The retainer40is an annular bearing component that uniformly keeps the circumferential distances between the tapered rollers30. The tapered rollers30are received, respectively, in pockets of the retainer40circumferentially equidistantly spaced apart from each other.

While the retainer40in the shown example is a cage (cage-shaped member) formed by punching, the material and manufacturing method of the retainer40are not particularly limited.

As used herein, the terms “axial” and “axially” are related to the direction along the center axis (rotation axis) CL of the inner ring10; the terms “radial” and “radially” are related to a direction orthogonal to the center axis CL; and the terms “circumferential” and “circumferentially” are related to the direction around the center axis CL. The tapered roller bearing is designed such that the center axis CL of the inner ring10corresponds to the rotation axis of the tapered roller bearing.

The inner and outer raceway surfaces11and21are surfaces with which the rolling surfaces31of the tapered rollers30can come into rolling contact, and to which radial loads are applied from the rolling surfaces31.

As illustrated inFIG.4, in the positional relationship where the center axes of the inner ring10, the outer ring20and each of the tapered rollers30lie in the same imaginary axial plane, and the center axes (not shown) of the tapered rollers30are opposed to, and aligned in a straight line with, a point O1on the center axis CL of the inner ring10, the vertices of the conical shapes of the inner and outer raceways11and21and the rolling surfaces31of the tapered rollers30coincide with the point O1. Each tapered roller30is designed such that, inFIG.4, the large end surface33of the tapered roller30is defined based on the spherical surface of a set radius of curvature R having its center on the straight line connecting together the point O1and the center axis of the tapered roller30.

The conical shapes of the inner and outer raceway surfaces11and21and the rolling surfaces31of the tapered rollers30are not limited to shapes generated by a straight generatrix, and it is to be understood that such conical shapes include shapes having crowning. The “generatrix” refers to a line segment that generates a certain curved surface as a trajectory of its motion about a center axis. For example, the generatrix of the raceway surface11is a line segment lying on an imaginary axial plane including the center axis CL of the inner ring10and forming the raceway surface11, and the generatrix of the rolling surface31of each tapered roller30is a line segment lying on an imaginary plane including the center axis of the tapered roller30and forming the rolling surface31. As the shape of the crowning mentioned above, a full-crowning shape or a cut-crowning shape as disclosed in Japanese Unexamined Patent Application Publication No. 2018-136027 (hereinafter JP '027) by the applicant of the present application may be used. As the cut-crowning shape of the rolling surface31, logarithmic crowning, such as the shape obtained by a numerical formula in Japanese Patent No. 5037094 cited in JP '027, may be used.

As illustrated inFIGS.2and3, the large flange12of the inner ring10has a large flange surface12athat receives the large end surfaces33of the tapered rollers30; a radially outer surface12bthat defines the outer diameter of the large flange12; and a flange-side chamfer12cthat connects together, around the entire circumference, the radially outer edge of the large flange surface12a, and the radially outer surface12b. The end face of the large flange12opposite from the large flange surface12aforms a portion of the side surface of the inner ring10.

The large flange surface12ais a surface with which the large end surfaces33of the tapered rollers30are brought into sliding contact in the circumferential direction. The generatrix of the large flange surface12ais a straight line inclined relative to the radial direction. Therefore, the large flange surface12ais a conical surface having the same center axis as the raceway surface11. The large flange surface12amay have any geometrical shape provided it is capable of coming into contact, at only one point, with the large end surface33of each tapered roller30. For that purpose, its generatrix may be changed into, e.g., a concave generatrix (in this case, the large flange surface comes into surface contact with the roller large end surface, but, for convenience, such contact is also interpreted as a point contact at the contact position between the concave bottom and the roller large end surface), or the generatrix may be a convex generatrix.

The grinding undercut13of the inner ring10is groove-shaped and connects together the large flange surface12aand the raceway surface11. The groove-shaped grinding undercut13extends around the entire circumference, and is formed for grinding and super-finishing the raceway surface11and the large flange surface12a. The grinding undercut13has depths relative to the raceway surface11and the large flange surface12a, respectively.

As illustrated inFIG.2, the small flange14of the inner ring10prevents the tapered rollers30from falling off from the raceway surface11to the small-diameter side, thereby forming an assembly of the tapered rollers30, the cage40, and the inner ring10. The small flange14is not an essential element of the inner ring, and thus the small-diameter-side grinding undercut15, which is adopted if the small flange is formed, is also not an essential element.

The inner ring10, the outer ring20, and the tapered rollers30are formed by first forging, then turning, and finally grinding, their predetermined portions.

The raceway surface11and the large flange surface12aof the inner ring10are formed by turning and grinding a forged object, and are polished by super-finishing.

As illustrated inFIGS.1and3, the grinding undercut13of the inner ring10is formed by turning based on a predetermined generatrix shape. After turning, the generatrix of the grinding undercut13is defined by a large-diameter-side straight line portion inclined from the large flange surface12a; a small-diameter-side straight line portion inclined from the raceway surface11; and a circular arc-shaped line portion coupling together the large-diameter-side straight line portion and the small-diameter-side straight line portion. Grinding and super-finishing are not actively performed on the grinding undercut13, but, when grinding the raceway surface11and the large flange surface12a, the grinder slightly rounds the large-diameter-side end of the ground portion of the raceway surface, and the inner-diameter-side end of the ground portion of the large flange surface. Therefore, although substantially the entire surface of the grinding undercut13is a turned surface, the connection portions of the grinding undercut13connected to the raceway surface11and the large flange surface12ahave slightly rounded ground surfaces or super-finished surfaces.

The intersection point (inFIG.1) of the imaginary line extending from the generatrix of the raceway surface11of the inner ring10toward the grinding undercut13, and the imaginary line extending from the generatrix of the large flange surface12atoward the grinding undercut13is referred to as the reference point O2. The approach angle of the grinding undercut13relative to the large flange surface12ais referred to as the approach anglea. The approach angle of the grinding undercut13relative to the raceway surface11is referred to as the approach angle b. The depth of the grinding undercut13relative to the raceway surface11is referred to as the depth c. The depth of the grinding undercut13relative to the large flange surface12ais referred to as the depth d. The undercut width of the grinding undercut13from the reference point O2to the large flange surface12ais referred to as the undercut width A. The undercut width of the grinding undercut13from the reference point O2to the raceway surface11is referred to as the undercut width B.

The approach anglesaand b, the undercut widths A and B, and the depths c and d are physical quantities to define the shape of the grinding undercut13. Of these physical quantities, since the degrees of the above-described roundness at the connection portions of the grinding undercut13connected to the raceway surface11and to the large flange surface12aare unstable, it is difficult to use these connection portions to define the approach anglesaand b. Therefore, the inclination angles of the turned surface of the grinding undercut13relative to the large flange surface12aand the raceway surface11are used as the approach anglesaand b, respectively.

Specifically, the approach angleaof the grinding undercut13is the angle (acute angle) of the large-diameter-side straight line portion of the generatrix of the grinding undercut13, relative to the radially inner edge of the large flange surface12a. The approach angle b of the grinding undercut13is the angle (acute angle) of the small-diameter-side straight line portion of the generatrix of the grinding undercut13, relative to the large-diameter-side edge of the raceway surface11.

The undercut width A of the grinding undercut13is the distance from the radially inner edge of the large flange surface12ato the reference point O2in the direction along the generatrix of the large flange surface12a. The undercut width B of the grinding undercut13is the distance from the large-diameter-side edge of the raceway surface11to the reference point O2in the direction along the generatrix of the raceway surface11.

The approach angleaof the grinding undercut13is larger than its approach angle b. If the ground amount of the large flange surface12aby grinding (amount by which the large flange surface12ais ground in the direction orthogonal to the generatrix of the large flange surface12a) overshoots or undershoots the target value, the width W (seeFIG.3) of the large flange surface12achanges depending on the approach angleaof the grinding undercut13. The width W of the large flange surface12ais the distance between both ends of the generatrix of the large flange surface12a. Since, in the shown example, the generatrix of the large flange surface12ais a straight line, the length of the generatrix corresponds to the width W. The larger the approach anglea, seeFIG.1, the smaller the amount of change in the width W of the large flange surface12a. In other words, the larger the approach anglea, the smaller the influence of the overshoot or undershoot of the ground amount of the large flange surface12arelative to the target value, on the undercut width A.

The approach angleaof the grinding undercut13is preferably 20 degrees or more and 50 degrees or less. Within this range, the influence of any overshoot or undershoot of the ground amount of the large flange surface12a, on the undercut width A, is moderate, thus making it possible to easily control the undercut width A. The approach angleais more preferably 30 degrees or more and 40 degrees or less.

The depth c of the grinding undercut13is the depth of the grinding undercut13relative to the large-diameter-side edge of the raceway surface11, in the direction orthogonal to the imaginary line extending from the generatrix of the raceway surface11. The depth d of the grinding undercut13is the depth of the grinding undercut13relative to the radially inner edge of the large flange surface12a, in the direction orthogonal to the imaginary line extending from the generatrix of the large flange surface12a.

The depth c of the grinding undercut13is larger than its depth d. This is in order to prevent thinning of the wall thickness of the inner ring10between the side surface of the inner ring10and the grinding undercut13. In order to keep this wall thickness sufficiently large, the depth d is preferably 0.3 mm or less.

The undercut width A of the grinding undercut13is smaller than its undercut width B. Setting the undercut width A smaller than the undercut width B is advantageous in making the approach anglealarger than the approach angle b. The grinding undercut13is formed by turning. Chips generated by turning can be more easily discharged from the grinding undercut13toward the raceway surface11, where a relatively wide space is available, than toward the large flange surface12a. Therefore, by discharging the chips toward the raceway surface11, the grinding undercut13can be more efficiently formed by turning. By satisfying the approach anglesa>b and the undercut widths A<B regarding the grinding undercut13, the discharge pressure of the chips during turning is relatively small on the side of the approach angle b and the undercut width B, and thus the chips are easily discharged toward the raceway surface11. It is thus possible to improve the turning machinability, and reduce the machining cost.

The undercut width A of the grinding undercut13is 0.5 mm or less. The reason why such a small undercut width A is used is to decrease the inner diameter of the large flange surface12a, thereby, as illustrated inFIG.3, sufficiently widening the width W of the large flange surface12a, which is opposed to the large end surfaces33of the tapered rollers30. Widening the width W of the large flange surface12ais advantageous in that, even if the positions of the sliding contact portions of the large flange surface12aand the large end surfaces33of the tapered rollers30are displaced, a good contact state is maintained between the large flange surface12aand the large end surfaces33of the tapered rollers30.

The undercut width A of the grinding undercut13is smaller than the width RC of the chamfer32of each tapered roller30in the direction along the generatrix of the large flange surface12a. This is in order to prevent the sliding contact portions of the large flange surface12aand the large end surfaces33of the tapered rollers30from being displaced to the radially inner edge of the large flange surface12a. The width RC of the chamfer32of each tapered roller30can be set at, e.g., 0.7 mm or less.

InFIGS.2and4, the acute angle of the generatrix of the raceway surface11relative to the center axis CL of the inner ring10is denoted by θ. The large-end diameter of the rolling surface31of each tapered roller30at its large end is denoted by Dw. In the geometric relationship among the inclination angle θ of the raceway surface11, the large-end diameter Dw of the rolling surface31, and the roller length L (seeFIG.2), the width W (seeFIG.3) of the large flange surface12asatisfies the following Formula 1:
W≥{Dw×(½)×Tan θ/(L/Dw)}  <Formula 1>

Formula 1 determines the lower limit of the width W of the large flange surface12ain maintaining a good contact state between the large flange surface12aof the inner ring10and the large end surfaces33of the tapered rollers30shown inFIGS.2and3. Specifically, while a radial load (or a dynamic equivalent load which is a combined load of a radial load and an axial load) is being applied to this tapered roller bearing, due to the inclination angle θ of the raceway surface11, the radial load is distributed and applied to the raceway surface11and the large flange surface12a. In Formula 1, the ratio of this distribution is denoted by Tan θ, and Tan θ is multiplied by the large-end diameter Dw of the rolling surface31, which is closely related to the bearing load capacity. Since, normally, the load applied to the tapered roller bearing during its operation is approximately half or less of the bearing load capacity, the large-end diameter Dw of the rolling surface31is multiplied by (½). Further, because the larger the roller length L is, the higher, in the above-described distribution ratio, the ratio of the load received by the raceway surface11, the relationship between the roller length L and the large-end diameter Dw of the rolling surface31is inserted in Formula 1 as (L/Dw)−1. Formula 1 thus sets the lower limit value of the width W of the large flange surface12aaccording to the applied load. By satisfying Formula 1, even if the sliding contact portions of the large end surfaces33and the large flange surface12amove radially outwardly of the large flange, e.g., by the skew of the tapered rollers30or the inclination of the large flange12of the inner ring10due to a large moment load, it is possible to keep a good contact state therebetween.

The upper limit value of the width W of the large flange surface12amay be any value (in millimeters) for the purpose of supporting and guiding the large end surfaces33of the tapered rollers30, but this upper limit value is preferably not more than three times the lower limit value of Formula 1. If the width W of the large flange surface12a(and thus the outer diameter of the large flange surface12a) is too large, lubricating oil will not easily reach the sliding contact portions of the large flange surfaces12aand the large end surfaces33of the tapered rollers30, thus making it impossible to ensure a good lubrication state.

The acute angle of the large flange surface12arelative to the radial direction is referred to as the flange surface angle α (seeFIG.3). The difference in radial height between the reference point O2and the contact point of the large flange surface12aand the large end surface33of each tapered roller30is referred to as the contact point height H (seeFIG.3). The contact point height H is determined in a one-to-one relationship with the combination of the flange surface angle α and a base radius of curvature RBASEof the large end surface33of each tapered roller30. InFIGS.2and4, the cone angle of the rolling surface31of each tapered toller30is denoted by β. The cone angle β of the rolling surface31is the central angle defined by the conical shape of the rolling surface31with the vertex O1of the cone angle β at the center. The letter “ρ” indicates the angle (acute angle) of the imaginary line extending from the contact point between the large flange surface12aof the inner ring10and the large end surface33of the tapered roller30to the vertex O1of the cone angle β, relative to the generatrix of the raceway surface11is denoted by. As illustrated inFIG.3, the angle ρ corresponds to the contact point height H. If the large flange surface has a convex curvature toward the large end surface of each tapered roller, or a concave curvature away from the large end surface, the angle ρ will be the angle of the imaginary line extending from the contact point between the large end surface and the deepest portion or the highest portion of the large flange surface.

The circumferential sliding velocity at the contact point between the large flange surface12aand the large end surface33of each tapered roller30depends on the contact point height H. If the above contact point is at the reference point O2, which is the imaginary intersection point between the raceway surface11and the large flange surface12aof the inner ring10(contact point height H=0), the sliding velocity is zero, and the higher the contact point height H from the reference point O2is, the higher the sliding velocity is at the contact point. By using a small undercut width A as described above, it is possible to widen the large flange surface12atoward the grinding undercut13, thereby reducing the contact point height H. Therefore, the contact point of the large flange surface12aand the large end surface33of each taped roller30is located a low position which satisfies β/6≥ρ. Setting the contact point at such a low position is effective in reducing the sliding velocity at the sliding contact portions of the large flange surface12aand the large end surfaces33of the tapered rollers30, thereby reducing heat buildup at the large flange surface12aand preventing a sharp rise in the temperature of the large flange surface12a.

If the width RC of the chamfer32of each tapered roller30is set to be 0.7 mm or less, the contact point of the large flange surface12aand the large end surface33of the tapered roller30can be set at a further lower position that satisfies β/7≥ρ.

Ratio R/RBASE, i.e., the ratio of the set radius of curvature R at the large end surface33of each of the tapered rollers30(one of which is shown inFIG.4), to the base radius of curvature RBASEfrom the vertex O1of the cone angle β of the rolling surface31to the large flange surface12aof the inner ring10; and ratio RACTUAL/R, i.e., the ratio of the actual radius of curvature RACTUALof the large end surface33to the set radius of curvature R, can be set within the numerical ranges disclosed in JP '027 by the applicant of the present application. Since the details and technical significance of these ratios R/RBASEand RACTUAL/R are disclosed in JP '027, the ratios R/RBASEand RACTUAL/R are only summarily described in this embodiment.

Specifically, the set radius of curvature R (seeFIG.4) of the large end surface33of the tapered roller30is the dimension of the large end surface33if it is composed of an ideal spherical surface. When consideringFIG.5, in which:

(i) points P1, P2, P3and P4are the ends of the large end surface33;

(ii) point P5is the midpoint between points P1and P2;

(iii) point P6is the midpoint between points P3and P4;

(iv) R152is the radius of curvature of the circle passing through the points P1, P5and P2;

(v) R364is the radius of curvature of the circle passing through points P3, P6and P4; and

(vi) R1564is the radius of curvature of the circle passing through points P1, P5, P6and P4,

then the above-described ideal spherical surface satisfies the relation: R=R152=R364=R1564. Points P1and P4are the connection points between the large end surface33and the chamfer32. Points P2and P3are connection points between the large end surface33and an undercut35. Actually, however, as illustrated inFIG.6, since shear drops are formed at both ends of the large end surface33during grinding, the radius of curvature R152, R364of the large end surface33on each side thereof is not equal to, and is smaller than, the radius of curvature R1564of the entire large end surface33. The radius of curvature R152, R364of the large end surface33on each side thereof after machining is referred to as the actual radius of curvature RACTUAL.

The set radius of curvature R and the actual radii of curvature RACTUALare obtained as follows: The radius of curvature R1564inFIG.6is the radius of an approximate circle passing through four points P1, P5, P6and P4inFIG.5. Measurement of R152=R364:=R1564was performed using a surface roughness measuring device called “Surftest” (model SV-3100; produced by Mitutoyo Corporation). Specifically, using this measuring device, the shape along the generatrix of the large end surface33of each tapered roller30was obtained, points P1, P2, P3and P4were plotted, and then midpoints P5and P6were plotted. The radius of curvature R152was calculated as the radius of the circular arc-shaped curved line passing through points P1, P5and P2(the radius of curvature R364was also calculated in a similar manner). The radius of curvature R1564was calculated as the radius of an approximate circular arc-shaped curved line based on values obtained by plotting the four points using the command “multiple inputs”. The shape of the large end surface33along its generatrix was measured once in the diameter direction.

InFIG.3, the large flange surface12aof the inner ring10comes into sliding contact only with the portion of the large end surface33of the tapered roller30located on one side thereof, and having the radius of curvature R152, R364. The radii of curvature of the portions of the large end surface33which actually come into contact with the large flange surface12aare the actual radii of curvature RACTUAL(R152, R364), which are smaller than the set radius of curvature R (R1564). Due to this difference in radius, the actual contact surface pressure between the large flange surface12and the large end surface33and the actual skew angle of the tapered roller30are larger than the ideal values in design. If the contact surface pressure and/or the skew angle is large in an environment where an oil film is not sufficiently formed, this destabilizes the sliding contact between the large end surface33and the large flange surface12a, thus reducing the oil film parameter. If the oil film parameter falls below 1, the lubrication between the large end surface33and the large flange surface12abecomes boundary lubrication, in which metal contact starts, so that the risk of seizure increases. The oil film parameter is denoted by Λ (=h/σ) defined by the ratio of the oil film thickness h obtained by the elastohydrodynamic lubrication theory, to the composite roughness σ which is the root-mean-square roughness value of the large end surface33and the large flange surface12a. The verification of the workable range of the ratio of the actual radii of curvature RACTUALto the set radius of curvature R is affected by the level of severity in the lubrication state between the large end surface33and the large flange surface12aat the peak of lubricating oil use temperature.

If the generatrix of the large flange surface12ahas a constant straight-line shape, the lubrication state between the large end surface33and the large flange surface12ais determined by the actual radii of curvature RACTUALand the use temperature of the lubricating oil. Since predetermined lubricating oil is basically used in transmissions and differentials, the viscosity of the lubricating oil is also predetermined. Assuming, as the maximum condition at the peak of the use temperature of the lubricating oil, an extremely severe temperature condition under which the peak lasts for 3 minutes (180 seconds) at 120° C., in the lubrication state where the viscosity characteristic of the lubricating oil is added to this assumed peak temperature condition, the threshold value for setting the ratio RACTUAL/R, i.e., the ratio of the actual radius of curvature RACTUALto the set radius of curvature R, so as not to generate a sharp rise in temperature, is obtained as a flange lubrication coefficient obtained by flange lubrication coefficient, i.e., flange lubrication coefficient=viscosity at 120° C.×(oil film thickness h)2/180 seconds. The oil film thickness h is obtained by Karna's formula. In view of the contact surface pressure between the large end surface33and the large flange surface12a, the oil film thickness h, the skew angle, and the oil film parameter, it is possible to set RACTUAL/R within a workable range by setting this ratio such that the flange lubrication coefficient exceeds 8×10−9(threshold value).

Turbine oil of ISO viscosity grade VG32, which is a lubricating oil often used in transmissions, has a 120° C. viscosity of 7.7 cSt (=7.7 mm2/s), which is low. Thus, the lubrication state where the viscosity of the lubricating oil is added to the assumed peak temperature condition will be extremely sever conditions. Therefore, the above-described ratio RACTUAL/R is preferably 0.8 or more. For SAE 75W-90, which is a gear lubricating oil often used in differentials, RACTUAL/R is preferably 0.5 or more.

The ratio R/RBASE, i.e., the ratio of the set radius of curvature R (seeFIG.4) of the large end surface33of each tapered rollers30(one of which is shown inFIG.4), to the base radius of curvature RBASEfrom the vertex O1of the cone angle β of the rolling surface31to the large flange surface12aof the inner ring10, relates, as illustrated inFIG.7, to the oil film forming capability at the sliding contact portion of the large end surface33and the large flange surface12a. The maximum Hertzian stress p at the sliding contact portion of the large end surface33and the large flange surface12adecreases as R/RBASEincreases. Also, the skew angle increases as R/RBASEdecreases.

The vertical axis ofFIG.7shows the ratio t/t0, which is the ratio of the oil film thickness t of an oil film formed between the sliding contact portion of each of the large end surfaces33(one of which is shown inFIG.4) and the large flange surface12a, to the oil film thickness t0of the oil film when R/RBASEis 0.76. As shown inFIG.7, the oil film thickness t becomes maximum when R/RBASEis 0.76, and the oil film thickness t decreases sharply when R/RBASEexceeds 0.9. In order to set the oil film thickness at the optimum value, R/RBASEis particularly preferably set at 0.75 or more and 0.87 or less.

In the tapered roller bearing of the present invention, since the large flange surface12ais optimized such that a good contact state is maintained between the large flange surface12aand the large end surfaces33of the tapered rollers30, by, as described above, decreasing the undercut width A of the grinding undercut13, thereby widening the width W of the large flange surface12atoward the grinding undercut13, it is possible to expand the allowable range of each of R/RBASEand RACTUAL/R.

Specifically, R/RBASEcan be set at 0.70 or more and 0.95 or less, and is preferably 0.70 or more and 0.90 or less, most preferably 0.75 or more and 0.87 or less.

RACTUAL/R can be set at 0.3 or more, and is preferably 0.5 or more, most preferably 0.8 or more. For a completed tapered roller30in which RACTUAL/R is within the range of 0.3 or more and less than 0.5, even if there is some disturbance causing displacement of the sliding contact portion, e.g., the skew of the tapered roller30or the inclination of the large flange surface12due to a large moment load, since the large flange surface12ais optimized as described above, it is possible to maintain a good contact state between the large flange surface12and the large end surface33of the tapered roller30.

This means that one or more of the plurality of completed tapered rollers30can have an R/RBASEvalue of 0.70 or more and 0.95 or less, and/or can have an RACTUAL/R value of 0.3 or more and less than 0.5. Thus, it is possible to improve the yield rate of the tapered rollers30.

The above-described oil film parameter depends on the composite roughness of the large end surfaces33of the tapered rollers30and the large flange surface12aof the inner ring10. By mirror-finishing the large end surfaces33and the large flange surface12a, it is possible to improve oil film formation, and ensure a suitable oil film thickness. Specifically, the large flange surface12ahas a surface roughness of 0.1 μm Ra or less, preferably 0.08 μm Ra or less. The large end surfaces33have a surface roughness of 0.12 μm Ra or less, preferably 0.1 μm Ra or less. The surface roughness refers to arithmetic mean roughness Ra defined in JIS B0601:2013 “Geometric property specifications (GPS) of product—surface properties: Contour curve method—term, definition and surface properties parameter”.

In order to prevent the large end surfaces33of the tapered rollers30from coming into sliding contact (edge abutment) with the radially outer edge of the large flange surface12aof the inner ring10, an undercut surface may be formed between the large flange surface12aand the flange-side chamfer12cinFIG.1. This modification is shown inFIG.8. As shown inFIG.8, an undercut surface12dis formed between the large flange surface12aand the flange-side chamfer12c. The undercut surface12dbends toward the radially outer surface12bsuch that the amount of its bend increases from the radially outer edge of the large flange surface12atoward the flange-side chamfer12c. The generatrix of the undercut surface12dis a circular arc-shaped line having a radius of curvature Rd.

When considering:

(i) the imaginary intersection point between the imaginary line extending from the generatrix of the large flange surface12a, and the imaginary line extending from the generatrix of the flange-side chamfer12c;

(ii) the width L1of the flange-side chamfer12c, which is the distance, in the direction along the generatrix of the large flange surface12a, from the above imaginary intersection point to the position equal in diameter to the radially outer surface12b; and
(iii) the width L2of the undercut surface12d, which is the distance, in the direction along the generatrix of the large flange surface12a, from the radially outer edge of the large flange surface12ato the above imaginary intersection point,
in order to prevent the width L2of the undercut surface12dfrom becoming too small, the radius of curvature Rd of the undercut surface12dis preferably 2 mm or less. Also, in order to keep the width L2of the undercut surface12das large as possible, the width L1of the flange-side chamfer12cis preferably 1 mm or less.

Also, it is preferable to further improve the function by combining together the optimization of the large flange surface12aof the inner ring10as shown inFIGS.1,3and8and heat treatment characteristics of the inner ring10. Specifically, since, if lubrication conditions are severe during the sliding contact between the large end surfaces33of the tapered rollers30and the large flange surface12a, surface damage may occur due to metal contact, it is preferable to make the large flange surface12ahave characteristics that delay surface damage.

Specifically, the grain size number of the old austenite crystal grains at the large flange surface12aof the inner ring10is preferably No. 6 or more. The grain size number of old austenite crystal grains is defined in JIS G0551:2013 as “Steels-Micrographic determination of the apparent grain size”. The “old austenite crystal grains” refer to austenite crystal grains after quenching. The boundaries (grain boundaries) of the old austenite crystal grains are referred to as old austenite crystal grain boundaries, and the old austenite crystal grains are surrounded by the old austenite crystal grain boundaries. The smaller the grain sizes of the old austenite crystal grains (the larger the grain size number), the slower the speed of damage becomes by the crystal grain boundaries. Therefore, the grain size number suitable for an element whose base is a metal and which comes into sliding contact, such as the large flange surface12a, is No. 6 or more, preferably No. 10 or more, more preferably No. 11 or more.

It is desirable that the large flange surface12aof the inner ring10is formed by a nitrided layer having a nitrogen content of 0.05 wt % or more, or that the large flange surface12ahas a nitrogen infiltration depth of 0.1 mm or more. Because the nitrided layer having a nitrogen content of 0.05 wt % or more has tempering softening resistance due to its nitrogen enrichment effect, the resistance to local heat buildup at the sliding contact portion of the large flange surface12aincreases. The nitrided layer is a layer formed on the surface layer of the large flange surface12aand having an increased nitrogen content, and is realized by, e.g., carbonitriding, nitriding or nitrogen infiltrating treatment. The nitrided layer preferably has a nitrogen content of 0.1 wt % or more and 0.7 wt % or less. If the nitrogen content is 0.1 wt % or more, it can be expected that the rolling life improves especially in an environment where foreign matter is present, whereas, if the nitrogen content is more than 0.7 wt %, there is a concern for shortened life due to the formation of holes called voids, or due to reduced hardness resulting from an increased amount of the remaining austenite. The nitrogen content is the value at the surface layer 10 μm of the surface of the large flange surface12aafter grinding, and can be measured by, e.g., EPMA (wavelength dispersion type X-ray micro analyzer).

The inner ring10, the outer ring20and the tapered rollers30shown inFIG.2are formed of high carbon chromium bearing steel (such as SUJ2 material). The inner ring10, the outer ring20and the tapered rollers30are subjected to a heat treatment for forming nitrided layers. This heat treatment may be performed by a method disclosed in JP '027 or by another method. The material of the inner ring10, the outer ring20and the tapered rollers30is not limited to high carbon chromium bearing steel. For example, the inner ring10and the outer ring20may be formed of a carburized steel such as chromium steel or chromium molybdenum steel, and may be subjected to, as the heat treatment, conventional carburizing quenching and tempering.

Tests were conducted to verify the effectiveness of the tapered roller bearing according to the present invention. The verification conditions and the basic specifications of the test bearings in the first test are as follows (hereinafter, seeFIGS.1to3as necessary):

Verification Conditions

Various Parameters of Test Bearings

RACTUAL/R=0.41Width W of large flange surface12a=1.55Surface roughness of large flange surface12a=0.072 μm RaSurface roughness of large end surface33=0.063 μm RaR/RBASE=0.81

Based on, in addition to the above-described basic specifications, the below-shown “applied specifications”, in which the undercut widths A of the grinding undercuts13of the respective test bearings are different from each other, the test bearings were evaluated. The evaluation results are shown in Table 1.

TABLE 1AppliedNo.specificationsEvaluation result1Undercut width⊚ (Calculated life was satisfiedA = 0.28more than enough)Because the temperature of the outerring did not rise, and test time wellexceeded calculated life, the test wasterminated with the bearing unbroken.2Undercut width⊚ (Calculated life was satisfiedA = 0.43more than enough)Because the temperature of the outerring did not rise, and test time wellexceeded calculated life, the test wasterminated with the bearing unbroken.3Undercut width⊚ (Calculated life was satisfiedA = 0.50more than enough)Because the temperature of the outerring did not rise, and test time wellexceeded calculated life, the test wasterminated with the bearing unbroken.4Undercutn width◯ (Calculated life was satisfied enough)A = 0.57Because the temperature of the outerring showed around 110° C., but testtime well exceeded calculated life, thetest was terminated with the bearing unbroken.5Undercut widthΔ (Test time slighly exceeded calculatedA = 0.62life)Because the temperature of the outerring gradually rose to 130° C., thetest was terminated.6Undercut widthX (Calculated life was not satisfied)A = 0.74Because the temperature of the outerring rose to 150° C. sharply, thetest was terminated.

As shown in Table 1, in each of the test bearings 1 to 4, in which the undercut widths A are 0.57 mm or less, it was possible to reduce a rise in temperature even under severe lubrication conditions, and thus to ensure a sufficient bearing life, whereas, in each of the test bearings 5 and 6, in which the undercut widths A are 0.62 mm or more, it was impossible to reduce a rise in temperature, and thus to expect a sufficient bearing life. In other words, it is considered that setting the undercut width A at 0.5 or less is effective in reducing a rise in temperature even under severe lubrication conditions.

The verification conditions and the basic specifications of the test bearings in the second test are as follows:

Verification Conditions

Various Parameters of Test Bearing

RACTUAL/R=0.51Width W of large flange surface12a=1.67Surface roughness of large flange surface12a=0.035 μm RaSurface roughness of large end surface33=0.037 μm RaR/RBASE=0.83

In the second test, based on, in addition to the above-described basic specifications, the below-shown “applied specifications”, in which, with the ratios of the cone angles β to the angles ρ of the respective test bearings set at the same value, the undercut widths A are different from each other, the test bearings were evaluated. The evaluation results are shown in Table 2.

TABLE 2AppliedNo.specificationsEvaluation result7Undercut widthB/p = 6.5⊚ (Calculated life was satisfiedA = 0.29more than enough)Because the temperature of the outerring did not rise, and test time wellexceeded calculated life, the testwas terminated with the bearingunbroken.8Undercut widthB/p = 6.5⊚ (Calculated life was satisfiedA = 0.44more than enough)Because the temperature of the outerring did not rise, and test time wellexceeded calculated life, the testwas terminated with the bearingunbroken.9Undercut widthB/p = 6.5⊚ (Calculated life was satisfiedA = 0.51more than enough)Because the temperature of the outerring did not rise, and test time wellexceeded calculated life, the testwas terminated with the bearingunbroken.10Undercut widthB/p = 6.5◯ (Calculated life was satisfiedA = 0.59enough)Because the temperature of the outerring showed around 110° C., buttest time well exceeded calculatedlife, the test was terminated withthe bearing unbroken.11Undercut widthB/p = 6.5Δ (Test time slighly exceededA = 0.65calculated life)Because the temperature of the outerring gradually rose to 130° C.,the test was terminated.12Undercut widthB/p = 6.5X (Calculated life was not satisfied)A = 0.78Because the temperature of the outerring rose to 150° C. sharply,the test was terminated.

As shown in Table 2, in each of the test bearings 7 to 10, in which, with the β/ρ value set at 6.5, the undercut width A is 0.59 mm or less, it was possible to reduce a rise in temperature even under severe lubrication conditions, and thus to ensure a sufficient bearing life, whereas, in each of the test bearings 11 and 12, in which, with the β/ρ value set at 6.5, the undercut width A is 0.65 mm or more, it was impossible to reduce a rise in temperature, and thus to expect a sufficient bearing life.

The verification conditions and the basic specifications of the test bearings in the third test are as follows:

Verification Conditions

Various Parameters of the Test Bearings

RACTUAL/R=0.55Width W of large flange surface12a=1.52Surface roughness of large flange surface12a=0.046 μm RaSurface roughness of large end surface33=0.047 μm RaR/RBASE=0.86

In the third test, based on, in addition to the above-described basic specifications, the below-shown “applied specifications”, in which, with the undercut widths A set at the same value, the ratios of the cone angles β to the respective angles ρ of the respective test bearings are different from each other, the test bearings were evaluated. The evaluation results are shown in Table 3.

TABLE 3AppliedNo.specificationsEvaluation result13Undercut widthB/p = 9.0⊚ (Calculated life was satisfiedA = 0.50more than enough)Because the temperature of the outerring did not rise, and test time wellexceeded calculated life, the testwas terminated with the hearingunbroken.14Undercut widthB/p = 7.5⊚ (Calculated life was satisfiedA = 0.50more than enough)Because the temperature of the outerring did not rise, and test time wellexceeded calculated life, the testwas terminated with the bearingunbroken.15Undercut widthB/p = 6.1⊚ (Calculated life was satisfiedA = 0.50more than enough)Because the temperature of the outerring did not rise, and test timewell exceeded calculated life, thetest was terminated with thebearing unbroken.16Undercut widthB/p = 5.7◯ (Calculated life was satisfiedA = 0.50enough)Because the temperature of the outerring showed around 110° C., buttest time well exceeded calculatedlife, the test was terminated withthe bearing unbroken.17Undercut widthB/p = 5.5Δ (Test time slighly exceededA = 0.50calculated life)Because the temperature of the outerring gradually rose to 130° C.,the test was terminated.18Undercut widthB/p = 5.0X (Calculated life was not satisfied)A = 0.50Because the temperature of the outerring rose to 150° C. sharply,the test was terminated.

As shown in Table 3, in each of the test bearings 13 to 16, in which, with the undercut width A set at 0.5 mm, the β/ρ values are 5.7 or more, it was possible to reduce a rise in temperature even under severe lubrication conditions, and thus to ensure a sufficient bearing life, whereas, in each of the test bearings 17 and 18, in which, with the undercut widths A set at 0.5 mm, the β/ρ values are 5.5 or less, it was impossible to reduce a rise in temperature, and thus to expect a sufficient bearing life. In view of the results of the second and third tests, it is considered that setting the undercut width A at 0.5 or less and adopting β/6≥ρ is effective in reducing a rise in temperature even under severe lubrication conditions.

In the tapered roller bearing of the present invention, as described above, since the undercut width A of the grinding undercut13of the inner ring10is set at a particularly small dimension, i.e., 0.5 mm or less, the width W of the large flange surface12is wide enough to receive the large end surfaces33of the tapered rollers30. Therefore, it is possible to optimize the contact relationship between the large flange surface12aand the large end surfaces33; and generate a good wedge effect between the large flange surface12aand the large end surfaces33so as to improve the oil film forming capability at the sliding contact portions of the large flange surface12aand the large end surfaces33. Therefore, even if this tapered roller bearing is used under sever lubrication conditions, it is possible to prevent a sharp rise in temperature and rotate the bearing smoothly.

For example, if the lubrication conditions are particularly severe and the lubrication of the sliding contact portions of the large flange surface12aand the large end surfaces33is or close to the boundary film lubrication, the large flange surface12amay become worn. If the wear of the large flange surface12areaches the grinding undercut13, and the large end surfaces33and the radially inner edge of the large flange surface12acome into edge abutment with each other, a large stress concentration will occur, thereby destabilizing the sliding behavior of the tapered rollers30. This may lead to a sharp rise in temperature. In contrast, in the tapered roller bearing of the present invention, even if the large flange surface12abecomes worn, since the width W of the large flange surface12ais large, the large flange surface12ais sufficiently opposed to the large end surfaces33, and also since the grinding undercut13(undercut width A) is small, the wear of the large flange surface12adoes not reach the boundary between the large flange surface12athe grinding undercuts13(radially inner edge of the large flange surface12a), and the end area of the large flange surface12aon its radially inner side remains. Therefore, even under such particularly sever lubrication conditions, it is possible to properly keep the large flange surface12aand the large end surfaces33in contact with each other.

Also, in this tapered roller bearing, since the relationship between the cone angle β of each tapered roller30and the angle ρ is set at β/6≥ρ, the radial contact point height H relative to the reference point O2, of each large end surface33and the large flange surface12aof the inner ring10is low. Therefore, it possible to prevent a rise in the sliding velocity at the sliding contact portions of the large flange surface12aand the large end surfaces33, thereby reducing heat buildup at the large flange surface12aand thus to prevent a sharp rise in temperature.

As described above, in this tapered roller bearing, it is possible to optimize the contact relationship between the large flange surface12aof the inner ring10and the large end surfaces33of the tapered rollers30, such that the oil film forming capability at the sliding contact portions improves, thereby preventing a rise in the sliding velocity at the sliding contact portions. Thus, even if the tapered roller bearing is used under sever lubrication conditions, it is possible to prevent a sharp rise in temperature, and rotate the bearing smoothly.

Also, in this tapered roller bearing, since the relationship between the approach anglesaand b of the grinding undercut13of the inner ring10is set ata>b, and the relationship between the undercut widths A and B is set at A<B, it is possible to improve the turning machinability of the grinding undercut13, and, to alleviate the influence of any overshoot or undershoot of the ground amount of the large flange surface12a, on the amount of change in the width W of the large flange surface12a(influence on the undercut width A). Also, it is possible to easily form the large flange surface12aby grinding. Therefore, in this tapered roller bearing, it is possible to reduce the machining cost, and thus the entire cost of the bearing.

Also, in this tapered roller bearing, since the relationship between the depths c and d of the grinding undercut13of the inner ring10is set at c>d, it is possible to reduce the stress of the large flange12caused by loads applied from the large end surfaces33of the tapered rollers30to the large flange surface12aof the inner ring10, and to improve the strength of the large flange12. This is advantageous in reducing the inclination of the large flange12due to, e.g., disturbance, and in keeping a proper contact state between large flange surface12aand the large end surfaces33.

Also, in this tapered roller bearing, since the depth d of the grinding undercut13of the inner ring10is 0.3 mm or less, it is possible to reliably improve the strength of the large flange12.

Also, in this tapered roller bearing, since the approach angleaof the grinding undercut13of the inner ring10is set within the range of 20°≤a≤50°, it is possible to alleviate the influence of any overshoot or undershoot of the ground amount of the large flange surface12a, on the amount of change in the width W of the large flange surface12a(influence on the undercut width A). It is thus possible to easily control the width W of the large flange surface12a(undercut width A).

Also, in this tapered roller bearing, since the width W of the large flange surface12aof the inner ring10is set at a value that satisfies the above-shown Formula 1, it is possible to make the large flange surface12asufficiently opposed to the large end surfaces33. Therefore, even if the sliding contact portions of the large end surfaces33and the large flange surface12aare displaced radially outwardly of the large flange by disturbance, it is possible to keep a good contact state therebetween.

Also, in this tapered roller bearing, since the grain size number of old austenite crystal grains in the large flange surface12aof the inner ring10is No. 6 or more, it is possible to delay its surface damage due to metal contact with the large end surfaces33of the tapered rollers30.

Also, in this tapered roller bearing, since the large flange surface12aof the inner ring10is formed by a nitrided layer having a nitrogen content of 0.05 wt % or more, it is possible to delay its surface damage due to metal contact with the large end surfaces33of the tapered rollers30.

Also, in this tapered roller bearing, since the large flange surface12aof the inner ring10has a surface roughness of 0.1 μm Ra or less, and the large end surfaces33of the tapered rollers30have a surface roughness of 0.12 μm Ra or less, it is possible to improve oil film formation by improving the oil film parameter between the large flange surface12aand the large end surfaces33.

Also, even if the R/RBASEvalues are 0.70 or more and 0.95 or less, and the RACTUAL/R value for at least one of the plurality of tapered rollers30is 0.3 or more and less than 0.5, this tapered roller bearing can be used under sever lubrication conditions, and yet it is possible to improve the yield rate of the tapered rollers30, and thus to provide the bearing at a relatively low cost, compared to the tapered roller bearing of JP '027.

The tapered roller bearing of the present invention can be suitably used to support a rotary shaft of a transmission or a differential for an automobile in an environment where lubricating oil is supplied into the bearing from outside by splashing or oil bath lubrication. An example of use thereof is now described with reference toFIG.9.FIG.9illustrates a differential for an automobile.

The differential ofFIG.9includes a drive pinion104supported by two tapered roller bearings102and103so as to be rotatable relative to a housing101; a ring gear105in mesh with the drive pinion104; and a differential gear mechanism (not shown), and these components are received in the housing101, in which gear lubricating oil is sealed. This gear lubricating oil is used to lubricate the tapered roller bearings102and103, too, and is supplied to the side surfaces of the bearings by a splashing or oil bath lubrication method.

Another example of use of the tapered roller bearing according to the present invention is now described with reference toFIG.10.FIG.10illustrates a transmission for an automobile.

The transmission ofFIG.10is a multi-speed transmission, which changes the gear ratio in a stepwise manner, and includes tapered roller bearings202to205according to any one of the above-described embodiments which rotatably support its rotary shafts (e.g., an input shaft201to which the rotation of the engine is inputted). The shown transmission is configured such that, by selectively engaging any one of clutches (not shown), the rotation of the input shaft201is transmitted through the selected one of gear trains206and207to an output shaft in the selected gear ratio. Also, this transmission is configured such that lubricating oil (transmission lubricating oil) splashed due to the rotation of gears is supplied to the side surfaces of the tapered roller bearings202to205.

Since each of the tapered roller bearings102,103and202to205shown inFIGS.9and10corresponds to the tapered roller bearing of the present invention shown in, e.g.,FIG.1, even in a lubrication environment where oil is diluted to save fuel, it is possible to prevent a sharp rise in temperature due to the sliding contact between the large flange surface of the inner ring and the large end surfaces of the tapered rollers, by the initial lubrication when the bearing starts to operate. Also, even when the temperature of the bearing during its operation rises and the viscosity of lubricating oil decreases, it is possible to suitably form an oil film by keeping a stable sliding contact state therebetween, and thus to prevent damage to these surfaces.

The tapered roller bearing of the present invention is not limited to use in transmissions, and can be used under other extremely sever lubrication conditions, too. The above-described embodiments are mere examples in every respect, and the present invention is not limited thereto. Therefore, the scope of the present invention is indicated not by the above description but by the claims, and should be understood to include all modifications within the scope and meaning equivalent to the scope of the claims.

DESCRIPTION OF REFERENCE NUMERALS