With the rapid development of hybrid electric vehicles and electric vehicles, the requirements for environmental protection and other requirements are that electric motors are smaller and lighter. The smaller the size of the motor, the output power will be reduced, but in order to ensure the same or higher output power with the current engine, a higher speed motor is required. The following introduces the analysis and optimization of deep groove ball bearings for motors in order to be able to withstand speeds exceeding 30000 r/min (1.5 times the limit speed of the bearings currently used).
Analysis of influencing factors
Use oil lubrication. Due to the high-speed rotation of the rolling elements and the cage, a large amount of lubricating oil will move to the outer diameter of the bearing under the action of centrifugal force, and the lubrication between the inner ring and the rolling elements or between the rolling elements and the cage is insufficient. Insufficient lubrication will cause the internal parts of the bearing to stick and cause the bearing to seize. The solution of adding lubricating oil will increase the resistance and torque caused by lubricating oil stirring.
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Friction between ball and channel surface
When a deep groove ball bearing rotates, the sliding friction between the ball and the raceway surface is proportional to the speed of the bearing. The influence of sliding friction at normal speed is negligible. However, it is very important to control the friction between the ball and the raceway surface under ultra-high speed conditions, because a sharp increase in friction will lead to an increase in the heat generation of the bearing.
Friction between ball and cage
Deep groove ball bearings commonly used stamped steel wave cage and plastic crown cage. The stamped steel cage is riveted by two halves stamped into a corrugated half cage with a ball pocket structure; the plastic crown cage is made of glass fiber reinforced nylon resin. Both cages are provided with spherical pockets with locks; friction will occur between the ball and the cage pockets. Under normal speed, the force acting on the ball and the cage is very small, and the friction force at the cage pocket is the same as the friction between the ball and the bearing ring, which can be ignored. But under the ultra-high speed running condition of 30 000 r/min, the circumferential speed of the ball can reach 40 m/s or even higher, and the point stress between the ball and the cage increases. Plastic cages are self-lubricating and light in weight, so they are more suitable for high-speed operation than stamped steel cages. If the lubrication of the bearing is insufficient under ultra-high speed operation, the friction between the ball and the cage pocket will increase the wear of the pocket surface, and the cage may even fall out of the bearing.
3) Strength of cage
Use plastic crown-shaped cage. When the plastic crown-shaped cage rotates at a high speed, the outer surface of the cage will be deformed under the action of centrifugal force, and the cage will contact the outer ring of the bearing. When the cage contacts the outer ring of the bearing, the temperature rise of the bearing increases. In addition, the pocket cage that has undergone large deformation will break due to excessive stress on the circumferential surface of the pocket. It is necessary to develop an enhanced cage structure that can withstand the deformation and damage of the cage caused by centrifugal force. The wall thickness of the cage improves the strength of the cage, but it cannot increase the width of the bearing.
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Optimization and improvement
1) Install the cover to control the oil flow
By installing the baffle cover to prevent the lubricating oil from overflowing to the bearing end surface, the internal lubrication of the bearing can be fully lubricated, and the problem of insufficient internal lubrication of the bearing under the action of the centrifugal force of ultra-high-speed operation can be solved.
2) Reduce friction
Reduce the sliding between the ball and the bearing ring by optimizing the groove diameter and the ball diameter. By improving the lubricity of the pocket surface and reducing the impact force between the ball and the cage, the abrasion of the pocket surface can be effectively avoided. Install the stop cover to increase the lubricating oil flow; adjust the gap between the ball and the cage pocket and the gap between the outer diameter of the inner ring and the inner diameter of the cage to reduce the impact and wear between the ball and the cage.
3) Improve the strength of the cage
Increase the wall thickness of the existing cage, improve the rigidity of the cage, reduce the deformation of the cage under ultra-high-speed operation, and avoid excessive stress on the surface of the cage pocket, which may cause fracture.
The optimized design of the bearing can meet the ultra-high-speed running demand of 30000r/min through temperature rise test and high-speed endurance test.
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