What Factors Are Related To The Fretting Wear Of Needle Roller Bearings?

In a transmission, the purpose of the needle bearing is to support the traveling 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, needle rollers and cage together form a complete needle roller bearing. .
Fretting wear is one of the main failure modes of needle roller bearings. Usually the hardness of the needle roller is slightly higher than the hardness of the matching shaft outer circle and gear inner hole, so fretting wear usually occurs in the inner hole of the gear that is in contact with the needle roller. Or the outer circumference of the shaft.

1. The mechanism of fretting wear of needle roller bearings

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 inner hole of the gear to 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 is tilted in the axial direction, causing oscillation during operation. During the swinging process 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 movement is called “fretting”, and the wear caused by it is called “fretting”. wear and tear”.
Fretting wear is a molecular wear process, that is, the two contact surfaces undergo reciprocating motion with small displacements under vertical load, making the contact surface close enough for the van der Waals force to act, causing the material to separate from the matrix and then be oxidized. It can be seen that material loss caused by fretting wear is the result of the combined action of chemistry (oxidation) and mechanics (loaded movement). Mechanical action scrapes off the oxide layer and adsorption layer, exposing a clean and lively fresh metal surface. The fresh surface quickly absorbs the surrounding gas and undergoes an oxidation reaction, which is a chemical effect. Material loss occurs as a result of alternating mechanical and chemical actions.

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, reciprocating displacement, 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.

2.1 Radial vertical load

The radial vertical load of needle roller bearings 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 easier it is to intensify wear during relative motion. The magnitude of the radial force of the gear is determined by the transmitted torque and usually cannot be changed, while the pressure at the contact part can be reduced by optimizing the needle bearing. Optional optimization solutions are: reducing local pressure, improving wear resistance, and preventing fretting wear by increasing the length, number, and diameter of needle rollers.

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 roller bearing. The larger the gap, the larger the reciprocating displacement, the faster the speed, the higher the friction work, and the easier it is to cause local wear. There are two optional optimization solutions: one is to reduce the axial clearance (0.1~0.35mm) by improving the axial positioning accuracy of the gear and shaft; the other is to reduce the diameter tolerance of the shaft and the hole or to adjust the shaft and the hole. Diameter tolerance group 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 can be reduced, and the fretting wear can be prevented.

2.3 Number of cycles

During the operation 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 the speed ratio (which changes with the vehicle and gear). The longer the driving mileage, the more cycles; if the traveling gear is a driving wheel and the mileage is constant, the smaller the speed ratio, the fewer the cycles. Both 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. With the consent of the OEM, minor adjustments can sometimes be made to appropriately reduce the risk of fretting wear.

2. 4 Material surface hardness

All other things being equal, the higher the surface hardness of the material, the higher its ability to prevent fretting wear. Needle rollers are mostly made of bearing steel, and their 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 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 solution is to appropriately lower the tempering temperature to obtain higher surface hardness (above HV700) and improve wear resistance.

2.5 Surface roughness

Improving the surface roughness level can inhibit fretting wear by reducing the friction factor and friction work. The roughness of the ground inner and outer raceways can reach Ra0. 4 μm, which can basically meet the usage requirements. Fretting wear sometimes occurs when other influencing factors are not ideal. The optional optimization solution is to add a fine grinding process to the process flow. Appropriately increasing the roughness level of the gear inner hole and shaft outer diameter (Ra0. 2 μm) can effectively reduce the risk of fretting wear.

2. 6 Lubrication

The lubrication of needle roller bearings is usually achieved by introducing 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. Optional optimization solutions are: first, guide sufficient lubricating oil to the shaft center by improving the depth and angle of the oil guide groove. Secondly, by enlarging the diameter of the oil hole 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 is improved and the pressure resistance stability of the oil film is improved. From the analysis of the occurrence, countermeasures and test results of fretting wear during the product development process, sufficient lubrication is one of the most effective ways to avoid fretting wear.

3. Analysis of other factors that aggravate fretting wear
   
   3.1There is no relative rotation between the needle roller and the inner and outer raceways.

The fretting wear marks of needle roller bearings are usually equidistant indentations. This is because when the transmission is running in gear, the needle rollers revolve with the inner hole of the gear and the shaft. The needle rollers do not rotate, and the needle rollers and the inner hole of the gear are in contact with each other. There is no relative rotation of the outer diameter of the shaft. At this time, the relative position of the needle roller and the inner hole of the gear or the outer diameter of the shaft in the circumferential direction remains unchanged, and only moves back and forth in the axial direction, so equidistant indentations appear. Through the opening of the needle roller bearing cage or the method of splitting the cage into two halves, the radial asymmetrical force and the imbalance of gravity and centrifugal force are achieved, thereby forcing the needle roller to rotate relative to the inner hole of the gear or the outer diameter of the shaft. , to avoid fretting wear caused by axial friction at the same position for a long time. This method is also one of the measures commonly used in engineering to solve the fretting wear of needle roller bearings.

3. 2 Resonance with the shaft’s own vibration

Due to machining errors and assembly errors, the shaft itself will produce eccentric vibration during work. If the frequency of this vibration is the same as or close to the vibration frequency of the fretting process, the two will easily resonate. Once resonance occurs, the fretting wear of the needle roller bearing will be significantly aggravated. By improving the straightness machining accuracy and assembly coaxiality accuracy of the shaft, the amplitude of the shaft itself can be effectively reduced, thereby weakening the resonance.

Fretting wear is one of the common failure modes of needle roller bearings. Through the analysis of the fretting wear mechanism of needle roller bearings, the main influencing factors of fretting wear were found. Through the analysis of the working principles of various influencing factors of fretting wear, a number of preventive measures to avoid fretting wear are proposed from the design aspect. Among these preventive measures, improving the lubrication structure to achieve sufficient lubrication and using open or split cages to achieve relative rotation between the needle roller and the inner and outer raceways are the most commonly used and most effective measures in engineering. Reducing the fretting displacement of needle roller bearings is a new measure proposed based on the study of fretting wear mechanisms, which provides new ideas for subsequent design and analysis and solution of market problems. With more in-depth research on the fretting wear mechanism and more engineering practices, more and better optimization measures will be discovered and proposed to completely avoid the occurrence of fretting wear problems.