KR0157067B1 - Rolling element - Google Patents

Abstract

Rolling member with improved strength, allowing stable formation of oil film between rolling surface and joining surface and long life under poor lubrication. The workpiece is heat nitrided. This aggregate is treated to form microrecesses in the surface such that the surface has a surface roughness parameter RMS and a negative SK value of at least 0.10 μm. Therefore, the surface strength of the rolling element is increased and the oil film formation ratio is improved. The surface of the rolling member has less peeling breakage and wear.

Description

Rolling member

1 is a cross-sectional view of a rolling contact bearing having a rolling element such as a rolling member.

2a is a longitudinal front view of a cam follower having an outer ring such as a rolling member.

Figure 2b is a longitudinal side view of this cam follower.

FIG. 3 is a graph showing Weibull distribution showing cumulative failure ratio and life of needle roller with heat treated surface.

FIG. 4 is a graph showing the Weibull distribution showing the cumulative failure rate and lifetime of a needle roller with a superfinishing surface.

FIG. 5 is a graph showing a Weibull distribution showing cumulative failure ratio and lifetime of a roller of a needle roller having a surface with a fine recess;

6 is a graph showing a Weibull distribution showing the cumulative failure ratio and life of the needle roller according to the present invention having a heat treated crucible and a surface formed with a fine recess.

* Explanation of symbols for main parts of the drawings

1, 7: outer ring 2: cylindrical roller

3: cage 4: housing

5, 6: Axis 8: Needle Roller

The present invention relates to a rolling member such as a cylindrical rolling element of a rolling contact bearing or an outer ring of a roller follower or cam follower.

Two important factors that affect the life of rollers and rolling members of rolling bearings are fatigue strength and lubrication.

Known methods for extending the service life of rolling bearings include bearing parts made of bearing steel and heat-treating these parts to increase the strength on the rolling surface to HRC58 or higher, and superfinishing the rolling surface to produce a surface of about 0.1 µm Ra. There is a method of providing surface roughness.

This surface structure has a problem that if it is difficult to provide a sufficient amount of lubricant on the rolling surface, a low viscosity lubricant must be used. Moreover, the oil film will be sufficient in situations where the ambient temperature is so high that during operation the viscosity of the lubricant will be lower than 12 mm2 / s and the mating surface of the cam follower with rolling rings or cylindrical rollers has a roughness value higher than 1.6 µm Pmax. It cannot be formed. As a result, the outer ring or cylindrical roller of the cam follower is flaking or peeling on the cloud surface in a very short period compared to the estimated life.

In order to solve these problems, when quenching a rolling member made of steel such as bearing steel, when nitrogen is penetrated into the rolling surface and formed into a finished product, the residual austenite in the surface area is increased by about 30% to improve the fatigue strength. A method of heat treatment has been proposed. In addition, a method of forming a fine concave portion having a depth of about 1 μm in the rolling surface of the rolling member is proposed to improve the oil film forming ability in the rolling member cloud and increase its life.

However, although the heat treatment furnace can improve the fatigue strength of the rolling member in the surface layer, it cannot improve the oil film formation ratio. Therefore, under the above-mentioned operating conditions, this method cannot sufficiently extend the life of the rolling member.

In the latter method, in which the rolling surface is finely roughened, the oil film formation ratio is increased to a sufficient level under the above-mentioned operating conditions. However, the fatigue strength of the rolling member itself is kept low, and despite the high oil film forming ability, it is difficult to sufficiently extend the bearing life.

An object of the present invention is to provide a rolling member having a long life even if the surface roughness or lubrication conditions of the mating surface for both the fatigue strength and the oil film formation ratio is good.

According to the present invention, there is provided a rolling member having a surface subjected to nitriding heat treatment, the surface having a fine concave portion formed such that its surface has an RMS value and a negative SK value not smaller than 0.10 mu m, and a residual compressive stress not smaller than 600 MPa in the surface layer. . By heat treatment of the rolling member to increase the residual compressive stress in the surface layer, high fatigue strength appears in the surface area. Furthermore, by forming the fine recesses in the surface such that the surfaces have roughness parameters RMS and negative SK values not smaller than 0.10 mu m, the rate of oil film formation between the rolling surfaces is improved. These advantages work in concert to reduce the occurrence of flaking and delamination, extending the life of the rolling members even if the lubrication conditions are poor or the mating surface is rough.

Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings. 1 shows a rolling contact bearing with rolling elements such as rolling elements in an exemplary manner. The rolling contact bearing includes an outer ring 1, a plurality of cylindrical rollers 2 mounted along the inner circumferential surface of the outer ring 1, and a cage 3 for holding the rollers 2 spaced apart from each other at equal intervals. have. The outer ring 1 is mounted in the housing 4. The shaft 5 is inserted in the rolling contact bearing and supported by the cylindrical roller 2.

2A and 2B show examples of chem followers. The cam follower includes a shaft 6, an outer ring 7 mounted to the shaft dole, and a plurality of needle rollers 8 disposed therebetween. The cam 9 is in contact with the outer circumference of the outer ring 7.

The rolling member of the rolling contact bearing is a cylindrical roller 2 and the rolling member of the cam follower is the outer ring 7 and the needle roller 8. This rolling member is made of high carbon steel such as bearing steel and heat treated to increase the residual compressive stress in the surface layer.

In the heat treatment, the workpiece is cured at a temperature of 820 to 860 ° C. for 4 hours. During curing, the workpiece is nitrided such that nitrogen permeates the surface of the workpiece, increasing the content of carbon and nitrogen in the surface layer of the workpiece. The workpiece is tempered at a temperature of 150 to 200 ° C. to increase the residual austenite content in the surface layer by about 30% and to increase the residual compressive stress in the surface layer, thereby increasing fatigue strength and rolling fatigue life.

The surface of the rolling member becomes rough after heat treatment, so that fine or minute recesses are formed in the surface in an irregular direction. After roughening, the rolling member has a residual compressive stress in the surface layer not less than 600MPa. This surface roughness has an RMS value and a negative SK value not smaller than 0.10 mu m, preferably the negative SK value is -1.6 in the axial and circumferential directions.

The roughness of the surface on which the microrecesses are formed is such that when expressed in terms of the parameter RMS, the RMS is not less than 0.10 μm and the SK value, another parameter of surface roughness, is negative, preferably -1.6 or less. .

In order to achieve this surface roughness on the rolling surface, barrel grinding can be used to obtain the desired finished surface.

The above-mentioned SK value represents the distortion degree of the surface roughness distribution curve. If the distribution curve is symmetrical as in the Gaussian distribution, the SK value is zero. In the present invention, if the surface roughness has a SK value of -1.6 in the axial and circumferential directions, the recesses in the surface are formed and distributed in an advantageous manner in which an oil film can be formed.

The present inventors prepared needle rollers having surfaces treated in different ways and tested their life using a rolling contact fatigue life tester, the results of which are described below.

Eight rollers were prepared for each type, each roller having a diameter (D) of 5 mm and a length (L) of 13 mm. The outer ring equipped with the needle roller has an outer diameter Dr of 38 mm and an inner diameter dr of 28 mm. Four different types of rollers were prepared with different types of surface treatments on the rollers, namely 1) super finishing, 2) heat treatment, 3) micro recesses, and 4) micro recesses after heat treatment according to the present invention. It became.

3 through 6 show the lifetime and cumulative failure ratios of each type of needle roller tested. Table 1 shows the experimental crystallization.

As is apparent from the life test results, the life of the needle roller surface treated according to the present invention is several times longer than the life of the roller treated with other surface treatments.

This long life seems to be due to improved fatigue strength due to increased residual compressive stress at the surface area and improved oil film formation ratio due to the formation of fine recesses.

Claims (1)

The rolling member is a nitriding heat treatment, the rolling member having a surface on which the fine concave portion is formed such that the surface has an RMS value and a negative SK value not smaller than 0.10 μm, and has a residual compressive stress of at least 600 MPa in the surface layer.