The surface accuracy (roughness) of the bearing mounting position does not affect the bearing performance as much as its dimensional tolerance and form and position tolerance. However, the accuracy of the mating surface affects the smoothness, thereby reducing the interference in the fit. Controlling the surface accuracy (roughness) can ensure the required fit.

For ultra-precision bearings, the highest rotational accuracy, high speed and low operating temperature can only be achieved when the mating parts and other related parts are manufactured with the same accuracy as the bearing. Therefore, when machining the mating parts, the geometric deviation of the relevant mounting surfaces and shoulders should be kept to a minimum.

一: The concept of roughness

After the machining of the mating surface of the shaft (or bearing seat) and the bearing, the tool, built-up edge and scale burrs cause large or small peaks and troughs on the workpiece surface. This microscopic geometric feature is called surface roughness.

二：Roughness evaluation parameters

2.1：Roughness is expressed as Ra, Rz or other codes plus numbers.

Arithmetic mean deviation of profile Ra: the arithmetic mean of the absolute value of the profile deviation within the sampling length L.

Ten-point height of micro-roughness Rz: the sum of the average value of the five largest profile peak heights and the average value of the five largest profile valley depths within the sampling length l.

Rz has few measuring points, and its characteristics in reflecting the height of micro-geometry are not as sufficient as Ra, but it is more convenient to measure than Ra. Ra has more measuring points, and its parameters can more fully reflect the height of micro-geometry, with higher accuracy. The Ra value is usually used for the surface of the bearing installation.

2.2：Roughness comparison and conversion

三：Effect of surface roughness on bearing performance

Surface roughness has a great influence on the quality of bearing use, mainly focusing on wear resistance, matching properties, fatigue resistance, workpiece precision and corrosion resistance.

3.1 Effect on friction and wear.

The effect of surface roughness on part wear is mainly reflected in the peaks and peaks. The contact between two parts is actually the contact of part of the peaks. The pressure at the contact point is very high, which can cause the material to flow plastically. The rougher the surface, the more serious the wear.

3.2 Effect on matching properties.

Bearing installation matching mainly has two forms, interference fit and clearance fit. For interference fit, the peaks of the surface are squeezed flat during assembly, resulting in a reduction in interference and a reduction in the connection strength of the component; for clearance fit, as the peaks are continuously smoothed, the clearance will increase. Therefore, surface roughness affects the stability of matching properties.

3.3 Effect on fatigue resistance.

The rougher the surface, the deeper the dent, the smaller the radius of curvature of the trough, and the more sensitive to stress concentration. Therefore, the greater the surface roughness, the more sensitive it is to stress concentration, and the lower its fatigue resistance.

3.4 Impact on corrosion resistance.

The greater the surface roughness, the deeper the trough. In this way, dust, deteriorated lubricating oil, acidic and alkaline corrosive substances are easily accumulated in these dents and penetrate into the inner layer of the material, aggravating the corrosion of the parts. Therefore, reducing the surface roughness can enhance the corrosion resistance of the parts.

四：How to obtain appropriate roughness through processing

五：Roughness requirements for bearing mounting surfaces

Note:

The above recommendations are from the FAG catalog;

When the bearing is installed by hydraulic oil injection, Ra≤1.6 μm

For example: a bearing NU 260 ECM, ordinary precision bearing, mating surface roughness design.

NU 260 ECM, bearing inner diameter = 300 mm, bearing outer diameter D = 540 mm, shaft size tolerance grade IT6, corresponding roughness Ra = 1.6; bearing seat size tolerance grade IT7, corresponding roughness grade Ra = 3.2 can be selected.