In the transmission, the needle roller bearing is used to support the floating gear on the shaft. The outer diameter of the shaft is equivalent to the inner raceway of the needle roller bearing, the inner hole of the gear is equivalent to the outer raceway of the needle roller bearing, and the inner and outer raceways together with the needle roller and cage form a complete needle roller bearing.
Fig. 1 Needle bearing
Fretting wear is one of the main failure forms of needle roller bearings. Generally, the hardness of the needle roller is slightly higher than that of the matching shaft excircle and gear bore, so the fretting wear usually occurs in the gear bore or shaft excircle that contacts the needle roller
1. Mechanism of fretting wear of needle roller bearings
The floating gear in the transmission is usually helical gear, and the stress analysis during the torque transmission operation is shown in Figure 3. Wherein, the normal force Fn is the comprehensive force of the gear; 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 swing during operation (as shown in Figure 4). During the swing of the gear, the inner hole of the gear produces small displacement axial reciprocating motion under the radial pressure with respect to the roller needle. This motion is called "fretting", and the resulting wear is called "fretting wear".
Fretting wear is a molecular wear process, that is, the two contact surfaces move back and forth with small displacement under vertical load, so that the contact surface is close enough to the extent that van der Waals force acts, resulting in material separation from the parent body and then being oxidized. It can be seen that the material loss caused by fretting wear is the result of the combined action of chemistry (oxidation) and machinery (loaded movement). The mechanical action scrapes off the oxide layer and adsorption layer, exposing the clean and active fresh metal surface. The fresh surface quickly absorbs the surrounding gas and oxidizes, which is a chemical action. Material loss is caused by alternating mechanical and chemical actions.
2. Influence Factors and Prevention of Fretting Wear
From the analysis of fretting wear mechanism of needle roller bearings, it can be seen that there are three main influencing factors. They are: radial vertical load, displacement of reciprocating motion, number of cycles. From the analysis of tribological mechanism, the wear degree is also affected by material hardness, surface roughness, lubrication and other factors. Next, we will share them one by one.
2.1 Radial vertical load
The radial and vertical load of needle roller bearing mainly comes from the radial force of the gear. The greater the radial force, the greater the pressure acting on the contact part (less than 4000MPa), and the more likely it is to aggravate the wear during relative movement. The size of the radial force of the gear is determined by the torque transmitted, which cannot be changed generally. The pressure of the contact part can be reduced through the optimization of the needle roller bearing. The optional optimization scheme is to reduce local pressure, improve wear resistance and prevent fretting wear by increasing needle length, needle number, needle diameter, etc.
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 needle roller bearing. The larger the clearance is, the larger the displacement of reciprocating motion is, the faster the speed is, and the higher the friction work is, the more likely it will 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 gears and shafts; The other is to reduce the radial clearance (0.009~. 048mm) by reducing the diameter tolerance of the shaft and hole or grouping and matching the diameter tolerance of the shaft and hole. Through the optimization of clearance, the displacement of reciprocating motion can be significantly reduced, the friction work can be reduced, and the fretting wear can be prevented.
2.3 Number of cycles
During the working process of needle bearing, the higher the number of cycles, the more obvious the fretting wear. The number of cycles is determined by the vehicle's mileage (3 × 105 km) and speed ratio (varying with vehicle and gear). The longer the mileage, the more cycles; If the traveling gear is the driving gear, the mileage is fixed, and the smaller the speed ratio, the fewer cycles. These two parameters are determined by the main engine factory, and the mileage represents the life of the vehicle, which is usually unalterable. The speed ratio is related to the power performance and fuel consumption of the whole vehicle. With the consent of the main engine manufacturer, minor adjustments can sometimes be made to properly reduce the risk of fretting wear.
2.4 Surface hardness of materials
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, and their hardness is slightly higher than that of gears and shafts, so fretting wear usually occurs in the inner bore of gears or the outer diameter of shafts. The material hardness of gears and shafts depends on the material model (20CrMnTiH, 20CrMo, 18MnCr5, etc.) and heat treatment conditions (usually carburizing and quenching). The optional optimization scheme is to reduce the tempering temperature appropriately to obtain higher surface hardness (above HV700) and improve wear resistance.
2.5 Surface roughness
To improve the surface roughness level, the friction coefficient can be reduced and the friction work can be reduced, so as to achieve the function of restraining fretting wear. The roughness of internal and external raceways for grinding can reach Ra0.4 μ m. It can basically meet the use requirements. When other influencing factors are not ideal, fretting wear may occur sometimes. The optional optimization scheme is to add fine grinding process to the process flow. Properly improve the roughness level of gear inner hole and shaft outer diameter (Ra0. 2 μ m) It can effectively reduce the risk of fretting wear.
2.6 Lubrication
Lubrication of needle roller bearings is usually achieved by guiding the oil in 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 coefficient, reduce the friction work, and prevent wear. The optional optimization scheme is: firstly, sufficient lubricating oil is guided to the shaft center by improving the depth and angle of the oil guide groove. Secondly, by enlarging the diameter of oil holes on the shaft or increasing the number of oil holes, sufficient oil volume is guided from the shaft center to the bearing raceway. Finally, through the adjustment of additives, the extreme pressure performance of lubricating oil is improved, and the compression stability of oil film is improved. According to the occurrence, countermeasures and test results of fretting wear during product development, adequate lubrication is one of the most effective ways to avoid fretting wear.
3. Analysis of other factors that aggravate fretting wear
3.1 There is no relative rotation between the needle roller and the inner and outer raceways
The fretting wear trace of the needle bearing is usually an equidistant indentation (as shown in Figure 2), because when the transmission is in gear, the needle rotates with the gear bore and shaft, the needle does not rotate, and the needle does not rotate with the gear bore and shaft outside diameter. At this time, the relative position of the needle roller and the outer diameter of the gear bore or shaft in the circumferential direction remains unchanged, and only the axial reciprocating movement occurs, so the equidistant indentation appears. Through the method of opening the needle bearing cage or dividing the cage into two halves, the radial asymmetric force and the imbalance of gravity and centrifugal force are realized, thus forcing the needle to rotate relative to the inner hole of the gear or the outer diameter of the shaft, so as to avoid fretting wear caused by axial friction at the same position for a long time. This method is also one of the measures often used to solve the fretting wear of needle roller bearings in engineering.
3.2 Resonance with shaft vibration
Due to machining error and assembly error, the shaft itself will produce eccentric vibration in the working process. If the frequency of this vibration is the same as or close to the vibration frequency of the fretting process, the two will easily have resonance. Once resonance occurs, fretting wear of needle roller bearings will be significantly increased. The amplitude of the shaft itself can be effectively reduced by improving the straightness processing accuracy and assembly coaxiality accuracy of the shaft, so as to weaken the resonance.
Fretting wear is one of the common failure forms of needle roller bearings. by analyzing the fretting wear mechanism of needle roller bearings, the main influencing factors of fretting wear are found out. Through the analysis of the working principle of various influencing factors of fretting wear, several preventive measures to avoid fretting wear are proposed from the design aspect. Among these preventive measures, improving the lubrication structure to achieve full lubrication and using an open or split cage to achieve relative rotation between the needle roller and the inner and outer raceways are the most commonly used and effective measures in engineering. Reducing the fretting displacement of needle roller bearing is a new measure based on the study of fretting wear mechanism, which provides a new idea for the subsequent design and analysis of market problems. With more in-depth research on fretting wear mechanism and more engineering practices, more and better optimization measures will be found and proposed to completely avoid fretting wear problems.
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