In the transmission, the role of the needle bearing is to support the floating gear 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, the needle roller and the cage together form a complete needle roller bearing. . Fretting wear is one of the main failure modes of needle roller bearings. Generally, the hardness of the needle roller is slightly higher than the hardness of the 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 that is in contact with the needle. Or the outer circle part of the shaft.
1. The mechanism of fretting wear of needle roller bearings
The traveling gear in the transmission is usually a helical gear. During the torque transmission operation, the force analysis is shown in Figure 3. Among them, the normal force Fn is the comprehensive force of the gear; the tangential force Ft causes the gear to transmit torque; the radial force Fr causes the gear inner hole to exert pressure on the roller needle; due to the existence of the gear axial and radial clearance, the axial force Fn causes the gear to tilt in the axial direction and swings during operation. When the gear is swinging, the inner hole of the gear produces a small displacement axial reciprocating movement under the action of radial pressure with respect to the roller needle. This movement is called "micromotion", so the wear caused is called "micromotion" Wear".
Fretting wear is a molecular abrasion process, that is, two contact surfaces perform a reciprocating movement with a slight displacement under a vertical load, so that the contact surface is close enough to the extent that the van der Waals force works, causing the material to break away from the matrix and then be oxidized. It can be seen that the material loss caused by fretting wear is the result of the combined effect of chemical (oxidation) and mechanical (loaded motion). The mechanical action scrapes off the oxide layer and the adsorption layer, exposing a clean and lively fresh metal surface. The fresh surface quickly adsorbs the surrounding gas and undergoes an oxidation reaction, which is a chemical action. The alternation of mechanical and chemical action causes material loss.
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2. Influencing factors and prevention of fretting wear
From the analysis of the mechanism of needle roller bearing fretting wear, 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 analysis of tribological mechanism, the degree of wear is also affected by factors such as material hardness, surface roughness, and lubrication. The following China Bearing Network (Huazhou Network) share one by one.
2.1 Radial vertical load
The radial vertical load of the needle roller bearing mainly comes from the radial force of the gear. The greater the radial force, the greater the pressure (less than 4000MPa) acting on the contact part, and the easier it is to increase the wear during relative movement. The size of the radial force of the gear is determined by the transmitted torque, and usually cannot be changed, and the pressure at the contact part can be reduced by optimizing the needle bearing. The optional optimization plan is: reduce local pressure, improve wear resistance, and prevent fretting wear by increasing the length of the needle, the number of needles, and the diameter of the needle.
2.2 Displacement of reciprocating motion
The displacement of the reciprocating motion 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 displacement of the reciprocating motion, the faster the speed, and the higher the friction work, which is more likely to cause local wear. There are two optional optimization schemes: one is to reduce the axial clearance (0.1～0.35mm) by improving the axial positioning accuracy of the gear and the shaft; the other is to reduce the diameter tolerance of the shaft and the hole or the shaft and the hole Diameter tolerance grouping matching to reduce radial clearance (0.009～.048mm). Through the optimization of the gap, the displacement of the reciprocating motion can be significantly reduced, the friction work is reduced, and the fretting wear is prevented.
2.3 Number of cycles
During 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 is determined by the vehicle's mileage (3×105 km) and speed ratio (varies with the 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. The mileage represents the life of the vehicle and is usually unchangeable. The speed ratio is related to the power and fuel consumption of the vehicle. Under the premise of obtaining the agreement of the OEM, minor adjustments can sometimes be made to appropriately reduce the risk of fretting wear.
2.4 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 with slightly higher hardness than gears and shafts, so fretting wear usually occurs on the inner hole of the gear or the outer diameter of the shaft. The material hardness of gears and shafts depends on the material type (20CrMnTiH, 20CrMo, 18MnCr5, etc.) and heat treatment conditions (usually carburizing and quenching). The optional optimization plan is: appropriately lower the tempering temperature to obtain higher surface hardness (above HV700) and improve wear resistance.
2.5 Surface roughness
To improve the surface roughness level, the friction factor can be reduced and the friction work can be reduced, thereby achieving the effect of inhibiting fretting wear. The roughness of the inner and outer raceways of the grinding process can reach Ra0. 4 μm, which can basically meet the requirements of use. When other influencing factors are not ideal, fretting wear sometimes occurs. The optional optimization plan is to add a fine grinding process to the process flow. Appropriately increasing the roughness level (Ra0. 2 μm) of the inner hole of the gear and the outer diameter of the shaft can effectively reduce the risk of fretting wear.
The lubrication of needle roller bearings is usually realized by introducing the oil from the shaft center into the raceway through the oil hole. Good lubrication can ensure the formation of an oil film between the needle roller and the raceway, reduce the friction factor, reduce friction work, and prevent wear. The optional optimization scheme is: First, by improving the depth and angle of the oil guide groove, guide sufficient lubricating oil to the shaft center. Secondly, by enlarging the diameter of the oil holes on the shaft or increasing the number of oil holes, a sufficient amount of oil is guided from the shaft center to the bearing raceway. Finally, through the adjustment of additives, the extreme pressure performance of the lubricating oil and the pressure stability of the oil film are improved. From the analysis of the occurrence, countermeasures and test results of fretting wear in the product development process, adequate lubrication is one of the most effective ways to avoid fretting wear.