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

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

Micro motion wear is one of the main failure modes of needle roller bearings. Usually, the hardness of the needle roller is slightly higher than that of the mating shaft outer circle and gear inner hole. Therefore, micro motion wear usually occurs in the gear inner hole or shaft outer circle in contact with the needle roller

1. The mechanism of micro motion wear of needle roller bearings

The traveling gear in the gearbox is usually a helical gear, and the force analysis during torque transmission is shown in Figure 3. Among them, the normal force Fn is the comprehensive force acting on the gear; The tangential force Ft causes the gear to transmit torque; The radial force Fr applies pressure to the needle roller through the gear bore; Due to the presence of axial and radial clearances between the gears, the axial force Fn causes the gears to tilt along the axial direction, resulting in oscillation during operation (as shown in Figure 4). During the swinging process of gears, the inner hole of the gear undergoes small displacement axial reciprocating motion under radial pressure relative to the needle roller. This motion is called "micro motion", and the resulting wear is called "micro motion wear".

Micro motion wear is a molecular wear process in which two contact surfaces undergo small reciprocating movements under vertical loads, causing the contact surfaces to approach the level where van der Waals forces act, resulting in material detachment from the matrix and subsequent oxidation. It can be seen that the material loss caused by micro motion wear is the result of the combined action of chemical (oxidation) and mechanical (loaded motion). Mechanical action scrapes off the oxide layer and adsorption layer, exposing a clean and lively fresh metal surface. Fresh surfaces quickly adsorb surrounding gases and undergo oxidation reactions, which is a chemical process. The alternation of mechanical and chemical reactions causes material loss.

2. Influencing factors and prevention of micro motion wear

From the mechanism analysis of micro motion wear of needle roller bearings, it can be seen that there are three main influencing factors. They are: radial vertical load, displacement of reciprocating motion, and number of cycles. From the perspective of frictional mechanism analysis, the degree of wear is also influenced by factors such as material hardness, surface roughness, lubrication, etc. Below, China Bearing Network (Huazhou Network) will share them one by one.

2.1 Radial Vertical Load

The radial vertical load of needle roller bearings mainly comes from the radial force of gears. The greater the radial force, the greater the pressure acting on the contact area (less than 4000MPa), which is more likely to exacerbate wear during relative motion. The magnitude of the radial force on gears is determined by the transmitted torque and cannot be altered, while the pressure at the contact area can be reduced through optimization of needle roller bearings. The optional optimization plan is to reduce local pressure, improve wear resistance, and prevent micro motion wear by increasing the length, number, and diameter of the needle rollers.

2.2 Displacement of reciprocating motion

The displacement of reciprocating motion is caused by the axial clearance (0.15-0.45mm) and radial clearance (0.015-0.058mm) of the needle roller bearing. The larger the gap, the greater the displacement of reciprocating motion, the faster the speed, the higher the frictional work, and the more likely it is to cause local wear. There are two optimization options available: one is to reduce the axial clearance (0.1-0.35mm) by improving the axial positioning accuracy of gears and shafts; Another approach is to reduce the radial clearance (0.009-0.48mm) by reducing the diameter tolerance of the shaft and hole or grouping and matching the diameter tolerance of the shaft and hole. By optimizing the gap, the displacement of reciprocating motion can be significantly reduced, friction work can be reduced, and micro motion wear can be prevented.

2.3 Cycle times

During the working process of needle roller bearings, the higher the number of cycles, the more obvious the degree of micro motion wear. The number of cycles is determined by the vehicle's mileage (3 × 105 km) and speed ratio (varying with the vehicle and gear). The longer the driving distance, the more cycles there will be; If the traveling gear is the driving wheel, the mileage is constant, and the smaller the speed ratio, the fewer the number of cycles. Both of these parameters are determined by the OEM, and the mileage represents the lifespan of the vehicle and is usually unchangeable. The speed ratio is related to the power performance and fuel consumption of the entire vehicle. With the consent of the host factory, sometimes minor adjustments can be made to appropriately reduce the risk of micro motion wear.