Bearing with improved resistance to micropitting

Abstract

A bearing comprising a plurality of rolling elements arranged between an inner and outer raceway thereof. A rolling contact interface is between a first rolling contact surface on at least one rolling element and a second rolling contact surface formed by one of the inner and outer raceways. The first rolling contact surface has a first RMS roughness R.sub.q1 and a first roughness pattern .sub.1, expressed in terms of the Peklenik number . The second rolling contact surface has a second RMS roughness R.sub.q2 and a second roughness pattern .sub.2. To minimize micropitting in the bearing, the rolling contact interface has a surface topography wherein (a) the roughness pattern of the first and second rolling contact surfaces are oriented in the direction of rolling, whereby .sub.13.0 and .sub.210.0; and (b) the first and of the second rolling contact surfaces have substantially equal roughness heights, whereby 0.8R.sub.q1/R.sub.q21.25.

Claims

1. A bearing comprising: a plurality of rolling elements arranged between an inner raceway and an outer raceway of the bearing; and a rolling contact interface being defined between a first rolling contact surface on at least one rolling element and a second rolling contact surface formed by one of the inner and outer raceways, wherein the first rolling contact surface has a first RMS roughness R.sub.q1 and a first roughness pattern .sub.1, expressed in terms of the Peklenik number; and wherein the second rolling contact surface has a second RMS roughness R.sub.q2 and a second roughness pattern .sub.2, expressed in terms of the Peklenik number, wherein the rolling contact interface has a surface topography in which: (a) the first roughness pattern .sub.1 of the first rolling contact surface and the second roughness pattern .sub.2 of the second rolling contact surface are oriented in the direction of rolling (x), whereby .sub.13.0 and .sub.210.0; and (b) the first rolling contact surface and the second rolling contact surface have a substantially equal RMS roughness, whereby 0.8R.sub.q1/R.sub.q21.25.

2. The bearing of claim 1, wherein each of the first rolling contact surface and the second rolling contact surface has a roughness skewness, R.sub.SK, with a value of R.sub.SK0.1.

3. The bearing of claim 1, wherein the bearing has a mean diameter d.sub.m, and wherein an RMS slope R.sub.qx of a roughness profile, measured in the direction of rolling, and an RMS slope R.sub.qy of a roughness profile, measured in a direction transverse to the direction of rolling, of each of the first rolling contact surface and the second rolling contact surface, have the following values when d.sub.m300 mm: R.sub.qx15 mrad R.sub.qy45 mrad, and have the following values when d.sub.m>300 mm: R.sub.qx30 mrad R.sub.qy90 mrad.

4. The bearing of claim 3, wherein the first RMS roughness R.sub.q1 and the second RMS roughness R.sub.q2 have a value (in meters) of 810.sup.9 (1000 d.sub.m).sup.0.55.

5. The bearing of claim 1, wherein the bearing is one of: a cylindrical roller bearing, a spherical roller bearing, a tapered roller bearing, a toroidal roller bearing, or a needle roller bearing.

6. A method of manufacturing a bearing ring comprising a bearing raceway, the method comprising: subjecting the raceway to a grinding operation that creates asperities with a longitudinal directionality on the raceway, wherein the grinding operation is followed by a mechanical finishing operation in which asperity heights are reduced without modifying the directionality of the asperities to produce a finished raceway having a roughness pattern with a Peklenik number of 10.0.

Description

DESCRIPTION OF THE FIGURES

(1) In the following, the invention is described with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a sectional view of part of a rolling element bearing;

(3) FIG. 2 shows a surface topography with a longitudinal roughness lay.

DETAILED DESCRIPTION

(4) FIG. 1 shows a partial, sectional view of an example of a rolling element bearing. In the depicted example, the bearing is a tapered roller bearing comprising a plurality of tapered rollers disposed between an inner ring 10 and an outer ring 20. Each roller has an outer cylindrical surface 31 that is in rolling contact with an inner raceway 12 on the inner ring and an outer raceway 22 on the outer ring. A rolling contact interface is defined between the outer cylindrical surface 31 of each roller and the inner and outer raceways 12, 22. During bearing operation, the rollers will also experience some slip (sliding contact) in the direction of rolling, which will be designated as the x direction.

(5) In boundary or mixed-lubrication, when the lubricant film at a rolling contact interface has insufficient thickness to separate the contact, surface irregularities will influence the way that dry and lubricated spots are distributed within the contact. Discontinuities in surface traction and possible stress concentrations must also be considered. High roughness (or high roughness slopes) will promote local film collapse, high contact pressures and tractions. This will enhance stress concentrations in the critical areas of traction discontinuities. The presence of some sliding favors surface tractions, and the inventors have found that micropitting appears first in areas of pressure discontinuities (high pressure gradients) associated with roughness.

(6) The present invention defines a bearing in which the rolling contact surfaces that define a rolling contact interface have a surface topography that is adapted to minimize micropitting.

(7) Firstly, the roller surface 31 and the bearing raceways 12, 22 have a roughness pattern oriented in the longitudinal direction. An example of a longitudinal roughness pattern is depicted in FIG. 2. In the direction of rolling (x-axis), the asperities have a length greater than their width in the transverse direction (y-axis). The height of asperities is shown on the z-axis.

(8) Specifically, at least one of the bearing raceways, and preferably both raceways, has a roughness pattern, expressed in terms of the Peklenik number , whereby 10.0. Preferably, 12.0. The Peklenik number may be expressed by the following equation:

(9) $=\frac{{}_{0.5x}}{{}_{0.5y}},$
where .sub.0.5 is the length at which the autocorrelation function of a roughness profile reduces to half its initial value, at =0, whereby is the lag. The roughness profile is measured in the direction of rolling x and in the transverse direction y, whereby it is assumed that the x and y profiles are described by an essentially linear autocorrelation function.

(10) The problem may be reduced to the numerical calculation of an autocorrelation matrix A comprising rows (x) and columns (y), so that one can look in the rows and columns to find the location where the value of A becomes A/2. The x-location will give .sub.0.5x and the y-location will give .sub.0.5x.

(11) The autocorrelation matrix for the x and y profiles are respectively given by:

(12) $A\left(x-\right)={}_{-}^{}{}_{-}^{}z\left(x,y\right)z\left(x-,y\right)xy$$A\left(y-\right)={}_{-}^{}{}_{-}^{}z\left(x,y\right)z\left(x,y-\right)xy$

(13) whereby z (x, y) is the function that describes the surface.

(14) Then:

(15) For .sub.0.5x, find A such that A(x)=0.5 A(x0), and

(16) For .sub.0.5y, find A such that A(y)=0.5 A(y0).

(17) Using a numerical routine to evaluate the autocorrelation function, can be found by iteration. The Peklenik number may then be calculated.

(18) The Peklenik number is a measure of surface isotropy and may be visualized as the length-to-width ratio of a representative asperity. Thus, =1 for a perfectly isotropic surface; =0 for a surface with a purely transverse roughness pattern and = for a surface with a purely longitudinal roughness pattern.

(19) With reference to FIG. 1, the rolling elements in a bearing according to the invention have a rolling contact surface 31 with a Peklenik number of 3.0. In other words, the roller surfaces 31 may possess greater isotropy raceways 12, 22. This is because, in practice, the surface of the rollers is smoother than the raceways and the present inventors have found thatin the presence of poor lubrication conditions and some slidingmicropitting first appears on the smoother surface. The rougher surface imposes a stress history (pressure amplitudes) upon the smoother one. The smoother surface sees a fluctuation in pressures (load micro cycles), while all points on the rougher surface always feel the same stresses (which are higher in the contact areas and lower in the non-contact areas). In other words, the rougher raceway surface 12, 22 imposes load micro cycles on the smoother surface 31 of each roller, which are therefore more prone to micropitting in the presence of some sliding.

(20) Although the roller surface 31 may have an RMS roughness R.sub.q1 that is smaller than an RMS roughness R.sub.q2 of the bearing raceways 12, 22, the difference between the two RMS roughnesses may not be too great. Specifically, in a bearing according to the invention:

(21) $0.8\frac{R_{q1}}{R_{q2}}1.25.$

(22) Furthermore, the magnitude of the RMS roughnesses in a rolling contact interface has been found to play a role in the minimization of micropitting. Suitably, the surface 31 of the roller and the inner and outer raceways 12, 22 have a maximum RMS roughness that is related to the mean diameter d.sub.m of the bearing as follows:
R.sub.q1 and R.sub.q2810.sup.9(1000d.sub.m).sup.0.55 [meters].

(23) The present inventors have found that other roughness parameters play a role in minimizing micropitting. Suitably, the surface 31 of the roller and the inner and outer raceways 12, 22 have a roughness skewness R.sub.sk0.1.

(24) The slope of the roughness profile in the rolling direction (x) and in the transverse direction (y) is also of importance, particularly in bearings with a mean diameter d.sub.m smaller than 300 mm. Suitably, the surface 31 of the roller and the inner and outer raceways 12, 22 have a slope parameter, R.sub.qx, in the direction of rolling x, and a slope parameter, R.sub.qy, in the transverse direction y, with the following values when d.sub.m300 mm:

(25) R.sub.qx15 mrad

(26) R.sub.qy45 mrad,

(27) In large-size bearings having a mean diameter d.sub.m>300 mm:

(28) R.sub.qx30 mrad

(29) R.sub.qy90 mrad,

Examples

(30) Three calculation examples were evaluated using a micropitting model as described in Micropitting Modeling in Rolling-Sliding Contacts: Application to Rolling Bearings, developed by the present inventors and published in Tribology Transactions, vol. 54, 2011.

(31) Example A represents a rolling contact interface with roughness parameters as prescribed by the invention. Examples B and C represent comparative rolling contact interfaces with certain roughness parameters which deviate from the prescribed ranges. The corresponding values of the roughness parameters are shown in Table 1 below.

(32) In each example, a maximum Hertzian contact pressure of 1.16 GPa and standard lubrications conditions were.

(33) After 1 million loading cycles, the results for the rolling contact interface of example A showed barely any micropitting; the results for the rolling contact interface of example B showed severe micropitting; and the results for the rolling contact interface of example C showed moderate micropitting. The problematic values of the roughness parameters which are thought to be responsible for the increased micropitting in examples B and C are underlined in Table 1.

(34) TABLE-US-00001 TABLE 1 Example (A) (B) (C) Surface I II I II I II R.sub.q, nm 107.7 107.7 107.7 323 107.7 107.7 R.sub.qI/R.sub.qII 1 3 1 R.sub.qx, mrad 12.29 7.256 12.29 21.77 44.41 26.2 R.sub.qy, mrad 44.41 26.2 44.41 78.6 12.29 7.256 R.sub.sk 0.3579 0.5811 0.3579 0.5811 0.3579 0.5811 14.14 13.20 14.14 13.20 0.071 0.076

(35) A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. Moreover the invention is not restricted to the described embodiments, but may be varied within the scope of the accompanying patent claims.