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Influencing factors of needle roller bearings and prevention of fretting wear

Through the analysis of the fretting wear mechanism of needle roller bearings, it can be seen that there are three main influencing factors. They are the radial vertical load, the displacement of the reciprocating motion, and the number of cycles. From the analysis of the friction mechanism, the degree of wear is also affected by factors such as material hardness, surface roughness and lubricity.
1. Radial vertical load
The radial vertical load of the needle bearing mainly comes from the radial force of the gear. The greater the radial force, the greater the pressure acting on the contact parts, and the easier it is to aggravate wear during relative movement. The magnitude of the radial force of the gear is determined by the transmitted torque, and usually cannot be changed, and the pressure of the contact part can be reduced by optimizing the needle bearing.
2. Displacement of reciprocating motion
The reciprocating displacement is caused by the axial clearance (0.15-0.45mm) and radial clearance (0.015-0.058mm) of the needle bearing. The larger the gap, the greater the reciprocating displacement, the faster the speed, and the higher the friction work, which is more likely to cause local wear. By optimizing the gap, the reciprocating displacement can be significantly reduced, friction work can be reduced, and fretting wear can be prevented.

3. Number of cycles
In the working process of needle roller bearings, the higher the number of cycles, the more obvious the degree of fretting wear. The number of cycles depends on the vehicle's mileage (3×105 km) and speed ratio (varies with vehicle and gear). The longer the mileage, the more cycles; if the traveling gear is the driving wheel, the mileage is constant, and the smaller the speed ratio, the fewer cycles. Both of these parameters are determined by the OEM. Mileage represents the life of the vehicle and is usually constant. The speed ratio is related to the power and fuel consumption of the vehicle.
4. Material surface hardness
Under the same other conditions, the higher the surface hardness of the material, the higher the ability to prevent fretting wear. Needle rollers are mostly bearing steel, whose hardness is slightly higher than that of gears and shafts, so fretting wear usually occurs on the inner hole of the gear or the outer diameter of the shaft. The optional optimization plan is to appropriately lower the tempering temperature to obtain higher surface hardness (higher than HV700) and improve wear resistance.
5. Surface roughness
By reducing the friction coefficient and reducing the friction work to improve the surface roughness, so as to achieve the effect of inhibiting fretting wear. When other influencing factors are not ideal, fretting wear sometimes occurs. The optional optimization plan is to add the fine grinding process to the process flow. Appropriately increasing the roughness grade (Ra0.2μm) of the gear inner hole and shaft outer diameter can effectively reduce the risk of fretting wear.

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