Tapered roller bearings are widely used in automobile chassis, especially in the main reduction, differential, hub parts. This paper analyzes the tapered roller bearing used in the hub.
The transmission system of the chassis of a car:
Engine → Clutch → transmission → drive shaft → main reducer → differential → half shaft → hub.
1.1 Hub structure
After the main reducer deceleration and torque increase, differential adjustment of the hub speed, half shaft through one end of the spline and half shaft tooth rolling connection, half shaft through the other end of the bolt and hub connection, the torque transfer to the hub, hub and half shaft rotation at the same time.
1.2 Wheel block bearing
In this paper, the drive axle hub bearing is analyzed, the steering axle bearing is discussed separately.
Hub bearings mainly bear radial load, so the contact Angle should not be too large, choose the outer ring contact Angle α :12~17°. According to the work span needs to adopt two sets of tapered roller bearings back-to-back (two inner ring narrow end faces opposite) installation. The bearing outer ring is pressed into the shell hole with interference fit. Bearing outer ring and shell hole - rotation. The inner ring of the bearing is pushed (pressed) into the half-shaft bushing with a transitional fit. Bearing inner ring and half shaft sleeve do not turn to support. Before assembling the two bearings, lithium base grease or compound lithium base grease should be injected, and the grease injection amount should not exceed 2/3 of the effective space of the bearing, so as to avoid the bearing temperature rise and discoloration caused by poor heat dissipation. The bearing is preloaded by adjusting the nut, so that the inner component is in close contact with the outer ring, and the elastic deformation of -constant is generated at the contact point. The contact area increases, the number of rolling bodies involved in the force increases, and the rolling bodies within the range of more than 180° or even 360° are stressed.
Then loosen the adjusting nut 1/8~1/4 pitch and adjust the axial clearance. The adjustment of axial clearance is very crucial. Too much clearance will cause vibration and loud noise, leading to early fatigue failure of bearings. Too small clearance will cause the temperature to rise sharply, destroy the oil film, and even the phenomenon of bearing lock. In order to prevent grease leakage, inner and outer bearings must be oiled and sealed.
1.3 Hub bearing failure analysis
Due to design, manufacturing, installation, lubrication, debugging and other reasons, hub bearings will appear different forms of failure mode during work. The following is an analysis of the frequently occurring failure modes.
1.3.1 Hub bearing locked
Hub bearing lock is a malignant accident, the consequences are very serious. The axial clearance adjustment is too small, resulting in the clearance of the hub bearing disappearing during operation, or even interference, and the bearing being locked.
During the installation process, the cage is deformed or the inclination of the cage window hole is too large for some reason, resulting in the roller inclination during the operation of the hub bearing. The roller tilt causes sliding friction, the temperature rises sharply, the oil film is damaged, the cage is twisted and broken, and the bearing is locked.
Poor lubrication or grease deterioration will also cause a sharp rise in temperature, the above phenomenon.
The coaxiality between hub shell hole and half shaft sleeve causes the change of clearance and roller tilt.
Bearing parts have cracks, in the process of operation under the action of repeated alternating stress, cracks will expand until fracture, bearing locked.
1.3.2 Hub bearing discoloration
Hub bearings in the running process due to the change of temperature caused by bearing color according to light yellow, yellow, purple blue, blue black change. In general, the bearing is light yellow and can continue to be used. The grease injection amount of hub bearing in high-speed operation generally does not exceed 1/3 of the effective space of bearing; Hub bearing grease in medium speed operation - generally not more than 2/3 of the effective bearing space; Hub bearing at low speed can fill the effective space of the bearing. Hub bearing high and medium speed operation grease is too easy to heat poor, temperature rise, bearing discoloration. If the use of inferior grease will cause grease deterioration, oil and water separation, corrosion of bearings. The axial clearance is small and the temperature increases (the normal operating temperature of the bearing does not exceed 90°C).
1.3.3 Hub bearing preload cannot be adjusted
Truck hub bearings according to the design requirements - generally using two sets of tapered roller bearings "back to back" installation. The outer bearing inner ring is not assembled chamfer (bearing design requirements this chamfer can be 45°), which becomes the assembly chamfer. In the assembly site of the main engine factory, there will be a phenomenon that the inner ring of the outer bearing is pressed into and cannot come out, that is, the preloading force cannot be adjusted (commonly known as the preloading force cannot be adjusted). The reasons for this phenomenon are as follows:
Incorrect assembly method; The sleeve assembly method is adopted, the end face of the sleeve is flat, the force is uneven during hammering, and the bearing enters the half-shaft sleeve in a inclined shape.
The fit tolerance occurs at two limit positions: the hub bearing outer ring rotates and the inner ring is stationary. The inner ring is a transitional fit with the shaft.
The maximum interference is reached when the shaft is the maximum limit size and the hole is the minimum limit size. The bearing inner ring is easy to scratch the shaft when the chamfering Angle is not assembled into 45°, resulting in burr and increasing interference.
Solution: the bearing inner ring is designed as R Angle without assembly chamfer; Improve the process capacity, matching tolerance as far as possible not limit position; Assembly conditions can be hot loading or oil pressure, sleeve assembly can be designed into a ball end of the sleeve.
1.3.4 Abnormal sound occurs during operation
Excessive axial clearance causes vibration and loud noise. Bearing working surface knock, bump, scratch damage oil film, cause sliding friction abnormal sound. Roller tilt and cage deformation cause abnormal noise caused by sliding friction. Poor cleanliness, the existence of debris and particles, affect the normal operation of bearings, aggravate the wear of bearings, and produce at the same time
Make noise.
1.3.5 Early fatigue
Fatigue is the main form of bearing failure, and there are many influencing factors.
The purity of steel is one of the main causes of early fatigue. Especially, the nonmetallic inclusions and oxygen content in steel seriously affect the bearing life.
Stress concentration is one of the main causes of early fatigue. The stress concentration mainly occurs on the surface of raceway and rolling body and the big retaining edge of inner ring.
Poor lubrication is one of the main causes of early fatigue. Poor lubrication can not form oil film, produce sliding friction, temperature rise leads to bearing fatigue.
Rough surface is not conducive to the preservation of oil film, greater friction.
The cleanliness difference is easy to spalling the working surface of the bearing.
Solution: The steel meets the requirements of GB/T 18254; Inner and outer ring raceway, rolling body surface only convex not concave, convex value in line with the process requirements; Control the Angle of the big guard, and the contact position between the roller ball base and the big guard is correct; Good lubrication, the use of grease in line with the requirements; The surface roughness and cleanliness meet the technical and standard requirements.