What factors are related to the fretting wear of needle roller bearings?

What factors are related to the fretting wear of needle roller bearings?

In the transmission, the role of needle roller bearing is to support the floating gear on the shaft. The outer diameter of the shaft is equivalent to the inner raceway of the needle bearing, the inner hole of the gear is equivalent to the outer raceway of the needle bearing, and the inner and outer raceways, together with the needle and cage, form a complete needle bearing.

The fretting wear is one of the main failure forms of the needle roller bearing. Generally, the hardness of the needle roller is slightly higher than that of the matched outer circle of the shaft and the inner hole of the gear, so the fretting wear usually occurs in the inner hole of the gear or the outer circle of the shaft in contact with the needle roller

1. Mechanism of fretting wear of needle roller bearing

The traveling gear in the transmission is usually a helical gear. In the process of torque transmission, the stress analysis is shown in Figure 3. Where, the normal force Fn is the comprehensive force of the gear; The tangential force Ft makes the gear transmit torque; The radial force Fr makes the inner hole of the gear exert pressure on the needle roller; Due to the existence of the axial and radial clearance of the gear, the axial force Fn causes the gear to tilt along the axial direction, and the swing is formed during the operation (as shown in Figure 4). During the swing of the gear, the inner hole of the gear produces a small displacement axial reciprocating motion under the action of radial pressure relative to the needle roller. This motion is called "fretting", so the resulting wear is called "fretting wear".

The fretting wear is a molecular wear process, that is, the two contact surfaces carry out the reciprocating movement of small displacement under the vertical load, so that the contact surface is close enough to the extent that the van der Waals force acts, thus causing the material to separate from the parent body and then be oxidized. It can be seen that the material loss caused by fretting wear is the result of the joint action of chemical (oxidation) and mechanical (loaded movement). The mechanical action scrapes off the oxide layer and the adsorption layer, exposing the clean and active fresh metal surface. The fresh surface quickly absorbs the surrounding gas and undergoes oxidation reaction, which is chemical action. Alternate mechanical and chemical actions cause material loss.