GB2272258A

GB2272258A - Rolling bearing

Info

Publication number: GB2272258A
Application number: GB9320966A
Authority: GB
Inventors: Yasuo Murakami; Kazuo Sekino
Original assignee: NSK Ltd
Current assignee: NSK Ltd
Priority date: 1992-10-13
Filing date: 1993-10-12
Publication date: 1994-05-11
Anticipated expiration: 2013-10-12
Also published as: JPH06129436A; GB9320966D0; GB2272258B

Abstract

In a rolling bearing which does not need a high level heat treatment, such as carburizing or carbonitriding, but which is sufficiently long in service life even when used with a lubricant containing foreign matter, (i) the amount of retained austenite ( gamma R vol %) in the surface layer of at least one of the inner and outer races is in the range of 25 to 40 vol %, (ii) the surface hardness (Hv) thereof meets the following condition: -4.7 x ( gamma R vol %) + 900 </= Hv </= -4.7 x ( gamma R vol %) + 1000 and (iii) the hardness of the rolling elements compared with that of the inner and outer races is higher by HR C 1 to 2.

Description

ROLLING BEARING This invention relates to rolling bearings, and more particularly to an improvement in service life of rolling bearings which are used, for instance, in the transmissions or engines of automobiles, agricultural machines, constructing machines or iron and steel machines.

One of the factors which shorten the service life of a rolling bearing is foreign matters mixed with the bearing lubricant. It is well known in the art that metal chips, shavings, burrs and/or powder are often mixed with the bearing lubricant. If the rolling bearing is used with the bearing lubricant containing such foreign matters, then the foreign matters damage the track rings, namely, the inner and outer races, and the rolling elements of the bearing, thus reducing the service life of the latter. At worst, the service life is reduced to 1/10 of that which the rolling bearing has when the bearing lubricant contains no foreign matters.

In order to overcome this difficulty, the present Applicant has proposed a rolling bearing under US-A5,137,375. In the rolling bearing, the relationships between the amount of retained austenite (YR vol %) and the hardness (Hv) of the surface layer of the rolling bearing are suitably determined that, even when the rolling bearing is used with the bearing lubricant containing foreign matters, concentration of stress at the edges of indentations formed by the foreign matters is lessened, and therefore the bearing is prevented from being cracked. That is, the rolling bearing is improved in service life.

The relations in hardness between the track rings and the rolling elements of a rolling bearing have been disclosed by E.V.ZARETSKEY teA study of residual stress in during rolling", transactions of the ASME, April 1969. That is, it has been known in the art that the service life of a rolling bearing is increased when the hardness of the rolling elements is made higher by HR C 1 to 2 (in a C-scale of Rockwell Hardness) than that of the track rings, because residual compressive stress is produced as the rolling elements roll.

The rolling bearing disclosed by the aforementioned US-A- 5,137,375 suffers from the following problems or difficulties: In the rolling bearing, the amounts of retained austenite (YR vol %) of the rolling surface layers of the track rings and the rolling elements are set in a range of 25 to 40 vol % by carburizing or carbonitriding them. In particular, as for the rolling elements, it is necessary to employ a high level heat treatment.

In addition, in order to suitably determine the relationships between the amount of retained austenite (YR vol %) and the hardness (Hv), it is necessary to use a special heat treatment technique, which takes a long period of time to achieve the heat. Hence, the rolling bearing is low in productivity.

The conventional rolling bearing, in which the hardness of the rolling elements is made higher by HR C 1 to 2 than that of the track rings, has been proposed for the purpose of increasing the service life with the lubricant containing no foreign matters. That is, in the case of the conventional rolling bearing, no attention is paid to increasing the service life when it is used with the lubricant containing foreign matters.

According to this invention a rolling bearing comprises track rings, namely, inner and outer races, and rolling elements set between the inner and outer races, in which, according to the invention the amount of retained austenite (YR vol %) in the surface layer of at least one of the inner and outer races is in a range of 25 to 40 vol %, and the surface hardness (Hv) thereof meets the following condition: -4.7 x (YR vol %) + 900 < Hv < -4.7 X (YR vol %) + 1000 and the hardness of the rolling elements compared with the inner and outer races is higher by HR C 1 to 2 (in a Cscale of Rockwell Hardness converted from the Vickers Hardness (Hv)) than that of at least the one of the inner and outer races.

In addition, in the present invention, the rolling elements may be made of a high-carbon chromium steel.

In the rolling bearing of the invention, the amount of retained austenite (YR vol %) in the surface layer of at least one of the inner and outer races is in a range of 25 to 40 vol %, and the surface hardness (Hv) thereof meets the following condition: -4.7 X (YR vol %) + 900 < Hv < -4.7 X (YR vol 96) + 1000 and the hardness of the rolling elements compared with the inner and outer races is higher by HR C 1 to 2 than that of the one of the inner and outer races. Hence, as for the rolling elements, it is unnecessary to employ a high level of heat treatment such as carburizing or carbonitriding which requires a long period of time to achieve. The rolling elements are sufficiently long in service life even when used with a lubricant containing foreign matters.

The reasons for these features will be described in detail.

It is well known in the art that foreign matters mixed in the lubricant form indentations in the rolling surface layer. Cracks which reduce the service life of the rolling bearing are liable to be formed at the edges of the indentations. The cracks are related closely to the amount of austenite retained in the rolling surface layer. The retained austenite is normally soft and viscous depending on the content of carbon in the material. Therefore, if the retained austenite is present in a predetermined rate in the rolling surface, then concentration of stress at the edges of the indentations can be lessened; that is, the formation of cracks there can be suppressed.

On the other hand, when the frequency of relative passage of the member which passes over the indentations during rolling (for instance the track rings with respect to the rolling elements) exceeds a predetermined value, then the retained austenite in the rolling surface layer is transformed into martensite by deformation energy applied to the rolling surface layer, so that the rolling surface layer is hardened. Thus, the service life of the rolling bearing is increased.

In order to utilize those two characteristics optimumly, the amount of retained austenite ( R vol %) in the rolling surface layer should be set to 20 to 45 vol %, preferably 25 to 40 vol %.

If the amount of retained austenite (YR vol %) is smaller than 20 vol %, it is difficult to sufficiently lessen the concentration of stress at the edges of the indentations.

If, on the other hand, the amount of retained austenite ( YR vol %) exceeds 45 vol %, then the effect of lessening the concentration of stress is saturated, and the hardness of the rolling surface layer is decreased; that is, the fatigue resistance is lowered (U.S.P. 5,137,375).

It is known in the art that, in the case where the rolling bearing is used with the lubricant containing foreign matters, the highest in durability of its components is the inner race, the next highest component is the rolling element, and the outer race comes last. and therefore, in order to increase the service life of the rolling bearing, it is essential to increase the service life of the inner race.

Hence, in the invention, the amount of retained austenite (YR vol %) in the rolling surface layer of the inner race or in the rolling surface layers of the inner race and the outer race is set in a range of 25 to .40 vol %.

In the invention, the term "surface layer" as used herein is intended to mean a layer having a predetermined depth from the surface, the depth being, for instance, 2% of the average diameter of the rolling elements where the shearing stress is maximum.

Another factor for determining the service life of the rolling bearing is the hardness (Hv) of the rolling surface layer. In the invention, the rolling surface layer of the inner race, or the rolling surfaces of the inner race and the outer race have a hardness (Hv) which satisfies the following condition: -4.7 x (YR vol t) + 900 < Hv < -4.7 x (YR vol %) + 1000 The condition is determined as follows: If the hardness (Hv) of the rolling surface layer is smaller than the lower limit of the above-described condition, then the fatigue resistance is decreased, and the service life of the rolling bearing is short irrespective of the lubricant (that is, it is short even when the lubricant contains no foreign matters). It is difficult to make the hardness (Hv) larger than the upper limit.

Furthermore, in the invention, the hardness of the rolling elements is made suitable for the inner race or the inner race and the outer race which have the above-described most suitable amount of retained austenite (YR vol %) and hardness (Hv). Hence, by combining the rolling elements with the inner race or the inner race and the outer race, formation of indentations in the rolling surfaces of the rolling elements can be lessened which is due to the foreign matters in the lubricant being nipped.

If the difference between the hardness of the rolling elements and the hardness of the inner race or the inner race and the outer race {(the hardness of the rolling element) (the hardness of the inner race or the inner race and the outer race)} is smaller than AHa C 1, then the resultant indentations are large in diameter. The concentration of stress at the edge of the indentation is closely related to the ratio (r/c) of the curvature (r) of the edge (FIG. 2) to the radius (c) of the indentation. That is, it is known in the art that the concentration of stress at the edge of the indentation is lessened as the ratio rlc increases (Japanese Patent Application (OPI) No. 55432/1989).

The aforementioned value (r) is proportional to the amount of retained austenite (YR vol %). Therefore, in the case where the amount of retained austenite (YR vol %) is the same, the ratio r/c is increased as the value c decreases.

Hence, in order to lessen the concentration of stress at the edge of the indentation, the diameter of the indentation should be decreased as much as possible. Therefore, in the invention, the hardness of the rolling elements compared with the inner race or the inner race and the outer race is made higher by at least HR 1 C than the hardness of the inner race or the outer race. If the hardness of the rolling elements is made higher by HR C 3 or more than that of the inner race or the outer race, then the inner race or the outer race suffers from flaking. Therefore, the upper limit is set to HR C 2.

Particular examples of rolling bearings in accordance with this invention will now be described and contrasted with the prior art with respect to the accompanying drawings, in which: FIG. 1 is an explanatory diagram for a description of a test method of forming an indentation in a test piece; FIG. 2 is a sectional view showing an indentation formed in a steel material employed in the invention.

Concrete ExamPles As conducive to a full understanding of the invention, concrete examples of the rolling bearing according to the invention and comparison examples will be described.

A steel material having a composition as indicated in the following Table 1 was subjected to carbonitriding, and then to hardening and tempering, to form a test piece having an amount of retained austenite (YR vol t) of 32 to 36%, and a hardness (Hv) of 753 to 760 (hereinafter referred to as "an "A" steel", when applicable).

TABLE 1

Composition C Si Mn P 1 S Ni Cr Mo Cu utZ 0.42 0.41 0.77 0.012 0.007 0.08 1.53 0.9 0.11 Composition Ti (O) ppm 30 7 Next, inner races, outer races, and rolling elements, which were made of steels indicated in the following Table 2 were combined as shown in Table 2 to form deep groove ball bearings;i.e., first through third concrete examples, and first through eighth comparison examples.The service lives of the deep groove ball bearings thus formed were tested with a service life testing machine of a ball bearing lubricant immersing type produced by NSK LTD under the following test conditions: Test Conditions: P/C = 0.32 (P is the equivalent load, and C is the fundamental load rating) Speed N = 3000 rpm Foreign matters mixed in, Hardness = Hv 540 Diameter = 74 to 147 zm Mixing rate = 1000 ppm Oil bath lubrication (Lubricant; turbine VG68) The bearing lives were represented by the periods of time which were obtained as follows: Ten bearings were tested per test condition, and the "10% life (a period of time in which 10% of the bearings are broken, on the short life side) thereof was obtained according to Weibull distribution function.

The data AHa C were obtained for the roller bearings in each of which the amount of retained austenite ( YR vol %) of the inner or outer race was in a range of from 25 to 40 vol %, and the surface hardness (Hv) thereof satisfied the following condition: -4.7 X (YR vol %) + 900 # Hv # -4.7 x (8R vol %) + 1000 The data AHa C were calculated from the following expression: (hardness of the rolling element) - (hardness of the inner and outer race made of the "A" steel) The results of life tests are as indicated in Table 2.

TABLE 2

Sample Steel YR (volZ) Surface aEaC Bearing No. hardness life (time) Inner race "A" steel 32 750 1 1 outer race SUJ2 9 746 - 141 Rolling element ~ SUJ2 11 737 E Inner race "A" steel 32 760 2 X X 2 Outer race SUJ2 9 746 ~ 136 u 12 816 o, Rol1ing elemen t: !1 32 760 1 C Inner race "A" steel 32 760 1 o 0 3 Outer race "A" seel 36 753 1 145 Rolling element SUJZ 11 787 1 Outer race Rolling element SUJ2 7 731 Inner race "A" steel 32 760 0 2 Outer race "A" steel 36 753 0 64 Rolling element SUJ2 9 756 Inner race SUJ2 9 746 3 Outer race "A" steel 36 753 1 20 Rolling element SUJ2 7 731 cu Inner race SUJ2 9 746 X 4 Outer race SUJ2 9 ~ 746 - 40 x Rolling element "A" steel 33 758 a .

Inner race SUJ2 9 746 5 5 Outer race nA" steel 36 753 0 38 g Rolling element "A" steel 33 758 Q Inner race / SUJ2 9 746 6 Outer race SUJZ 9 746 - 16 Rolling element i SUJZ 12 800 Inner race "A" stcel 32 760 0 7 Outer t;;lcc SILT2 9 746 151 Rolling elemenr "A" sael 33 758 Inner race A seel 32 760 0 8 Outer race "A" steel 36 753 0 155 Rolling element "A" steel 33 758 As is apparent from Table 2, in each of the first through third concrete examples, the amount of retained austenite (YR vol %) of the inner race or the inner and outer races was in the range of 25 to 40 vol %, and the surface hardness (Hv) satisfied the condition -4.7 x (YR vol %) + 900 < Hv S -4.7 x (YR vol %) + 1000, and the hardness of the rolling element was higher by at least Ha C 1 than the hardness of the inner and outer races. That is, the first through third concrete examples were in the scope of claim of this invention. The first through third concrete examples were high in service life, although the steel SUJ2 was employed which was heat-treated normally. Thus, the first through third concrete examples satisfied the above-described conditions, and the rolling elements which are rather difficult to subject to a high level heat treatment such as carburizing or carbonitriding were obtained through a simple heat treatment. Hence, the first through third concrete examples are advantageous in that the bearings can be manufactured at low cost and are high in service life.

In each of the first and second comparison, the amount of retained austenite (YR vol %) of the inner race or the inner and outer races was in the range of 25 to 40 vol %, and the surface hardness (Hv) thereof satisfied the condition -4.7 x (YR vol %) + 900 < Hv < -4.7 x (YR vol %) + 1000].

However, the hardness of the rolling elements combined with the inner race or the inner and outer races was smaller than or equal to the hardness of the inner race or the inner and outer races. Therefore, the rolling elements were not so improved in service life as expected. That is,, the first and second comparison examples were not so improved in bearing life as the first through third concrete examples.

In the third comparison example, the amount of retained austenite (YR vol %) of the outer race was in the range of 25 to 40 vol %, and the surface hardness (Hv) thereof satisfied the condition -4.7 x (YR vol %) + 900 < Hv < -4.7 x (YR vol %) + 1000], and the hardness of the rolling elements was higher at least by HR C 1. However, the inner race is smaller than the outer race in a diameter and is higher than the outer race in a contact pressure, so that the inner race is lowest in durability. The inner race did not satisfy the conditions, and therefore it was impossible to increase the durability of the inner race. Thus, the bearing life was short.

In the fourth comparison example, the amount of retained austenite (YR vol %) of the rolling elements was in the range of 25 to 40 vol %, and the surface hardness (Hv) thereof satisfied the condition [-4.7 x (YR vol %) + 900 < Hv ' -4.7 x (YR vol %) + 1000]. However, the inner race which is lowest in durability did not meet the conditions, and therefore it was impossible to increase the durability of the inner race. Thus, the bearing life was relatively short.

In the fifth comparison example, the amounts of retained austenite (YR vol %) of the outer races and the rolling elements were in the range of 25 to 40 vol %, and the surface hardness (Hv) thereof satisfied the condition -4.7 x (YR vol %) + 900 < Hv < -4.7 x (YR vol %) + 1000. However, the inner race which is lowest in durability did not meet the conditions, and therefore it was impossible to increase the durability of the inner race. Thus, the bearing life was relatively short.

The sixth comparison example was a conventional bearing in which the inner race, the outer race and the rolling elements were made of steel SUJ2. The sixth comparison example was short in service life.

In each of the seventh and eighth comparison examples, the amounts of retained austenite ( YR vol %) of the inner race, and outer races, and the rolling elements were in the range of 25 to 40 vol %, and the surface hardness (Hv) thereof satisfied the condition [-4.7 x (TR vol %) + 900 < Hv < -4.7 x (YR vol %) + 1000, and the service life was relatively long. However, the seventh and eighth comparison examples, when compared with the first through third concrete examples, are disadvantageous in that, since the rolling elements were made of the "A" steel heat-treated specially, the manufacture was intricate, and accordingly the resultant bearing was high in manufacturing cost.

Next, test pieces A having surface hardness as indicated in the following Table 3 were combined with test pieces B having amounts of retained austenite (Ya vol %) and surface hardness indicated in the same Table 3. More specifically, as shown in FIG. 1, each test piece A 10 was set -over each test piece B 20 with metal powder 30 (corresponding to the foreign matters) therebetween, and the diameter of an indentation formed in the test piece A 10 was measured under the following test conditions: Test conditions Metal powder; Hv 490 Diameter = 100 gm Material = SUS420JI Load = 500 kgf The results of test are as indicated in Table 3, in which AHa = (hardness of test piece A) - (hardness of test piece B), and 2CA is the diameter ( > m) of an indentation in the test piece A.

TABLE 3

Test Piece B yu-9vol% ya=32vol% Hay62 HRC 63 Test Piece A Hv 742 HV 773 HRC61 dHRC=-1 AHRC=-2 (Hv 722) 2CA=18311m 2CA=175m HRC62 AHRC=0 bHRC=-1 (Hv 745) 2CA=l54pin 2CA=165m HRC63 AHRC=1 AHRC= 0 ( HV 770) 2CA=148 I1D 2CA=152ss1R HRC64 AHRC=2 AHRC=1 (HV 802) 2CA=146Rm 2CA=101ss1E HRC65 AHRC=3 AHRC=2 ( HV 834) 2CA=148Rm 2CA=9811X In the Table 3, a value of the C-scale of Rockwell Hardness (HRC) is calucurated and converted from the Vickers Hardness (Hv).

It was confirmed from Table 3 that, in the combinations of the test pieces A and B in each of which the hardness of the test piece A was larger by to HR C 1 or more and smaller by HR C 2 or less than the hardness of the test piece B, and the amount of retained austenite (YR vol %) was in the range of 25 to 40 vol %, the diameter of the indentation was small, and the concentration of stress at the edge of the indentation was lessened, and the service live was high.

However, in the present invention, a high-carbon chromium steel can be utilized as a material of which the rolling element are made.

As was described above, in the rolling bearing of the invention, the amount of retained austenite (YR vol %) in the surface layer of at least one of the inner and outer races is in a range of 25 to 40 vol %, and the surface hardness (Hv) thereof meets the following condition: -4.7 x (YR vol %) + 900 s Hv < -4.7 x (YR vol %) + 1000 and the hardness of the rolling elements combined with the inner and outer races is higher by HR C 1 to 2 than that of the one of the inner and outer races. Therefore, as for the rolling elements, it is unnecessary to employ a high level of heat treatment such as carburizing or carbonitriding which requires a long period of time to achieve. The rolling element is sufficiently long in service life even when used with a lubricant containing foreign matters. Thus, the rolling bearing of the invention is low in manufacturing cost and long in service life.

Claims

CLAIM;B

1. A rolling bearing comprising track rings, namely, inner and outer races, and rolling elements set between said inner and outer races, in which: the amount of retained austenite (7R vol %) in the surface layer of at least one of said inner and outer races is in a range of 25 to 40 vol %, and the surface hardness (Hv) thereof meets the following condition: - 4.7 X (TR vol %) + 900 < Hv < -4.7 x (eR vol %) + 1000 and the hardness of said rolling elements compared with said inner and outer races is higher by HR C 1 to 2 than that of said one of the inner and outer races.

2. A rolling bearing according to claim 1, characterised in that said rolling elements are made of a high carbon chromium steel.

3. A rolling bearing substantially as described.