As hybrid and electric cars have developed rapidly, environmental requirements demand that electric motors be smaller and lighter. Smaller electric motors will reduce output power, but to ensure that they have the same or higher output power as current engines, higher motor speeds are required. This article analyzes and optimizes deep groove ball bearings used in electric motors to withstand speeds exceeding 30,000 rpm, which is 1.5 times the current limit speed of bearings.
Analysis of influencing factors:
1) Lubrication
Oil lubrication is used to ensure sufficient lubrication for the internal parts of the bearings. However, due to the high-speed rotation of the rolling elements and cage, a large amount of lubricating oil will move towards the outer diameter direction of the bearing under the action of centrifugal force, and the lubrication between the inner ring and rolling elements or between rolling elements and cage may not be sufficient. Insufficient lubrication can cause the internal parts of the bearing to stick and cause the bearing to seize. Increasing the amount of lubricating oil will increase the resistance caused by the stirring of the lubricating oil, resulting in an increase in torque.
Internal state of bearings under high-speed operation:
2) Friction
Friction between the ball and the raceway surface
When the deep groove ball bearing rotates, the sliding friction between the ball and the raceway surface is proportional to the speed of the bearing. Under normal speed conditions, the influence of sliding friction can be ignored. However, controlling the friction between the ball and the raceway surface is very important under ultra-high-speed conditions, because the sudden increase in frictional force can cause an increase in heat generation of the bearing.
Friction between the ball and the cage
Deep groove ball bearings are commonly used with stamped steel wave-shaped cages and plastic crown-shaped cages. The stamped steel cage is made by riveting two half wave-shaped semi-cages with a ball pocket structure. The plastic crown-shaped cage is made of nylon resin reinforced with glass fiber, and both cages have ball-shaped pockets with locking grooves. The frictional force generated between the ball and the cage pocket can be ignored under normal speed conditions, as the force acting on the ball and cage is very small. However, under ultra-high-speed conditions of 30,000 rpm, the circumferential speed of the ball can reach 40 m/s or even higher, and the point stress between the ball and the cage will increase. The plastic cage has self-lubricating properties and is lighter in weight, making it more suitable for high-speed operation. Under ultra-high-speed operation, if the lubrication is insufficient, the frictional force between the ball and the cage pocket will increase, causing wear on the pocket surface and even the possibility of the cage coming out of the bearing.
3) Cage strength
Plastic crown-shaped cages are used to withstand high-speed rotation. Under the action of centrifugal force during high-speed rotation, the outer surface of the cage will deform and contact with the outer ring of the bearing. When the cage contacts the outer ring of the bearing, the temperature of the bearing will increase. In addition, a cage with large deformation can fracture due to excessive stress on the circumferential surface of the cage pocket. Therefore, an enhanced cage structure is required to withstand the deformation and damage caused by centrifugal force. Increasing the wall thickness of the cage can improve the strength of the cage, but the width of the bearing cannot be increased.
Optimization measures:
1) Install a baffle to control the oil flow rate
Installing a baffle can prevent lubricating oil from overflowing onto the end face of the bearing, ensuring sufficient internal lubrication of the bearing and solving the problem of insufficient lubrication under ultra-high-speed operation.
2) Reduce friction
Optimizing the size of the groove diameter and ball diameter can reduce the sliding between the ball and the bearing raceway. Improving the lubricity of the pocket surface and reducing the impact force between the ball and the cage can effectively avoid wear on the pocket surface. Installing a baffle can increase the flow rate of lubricating oil, and adjusting the clearance between the ball and the cage pocket and the clearance between the inner ring outer diameter and the cage inner diameter can reduce the impact force and wear between the ball and the cage.
3) Increase the strength of the cage
Increasing the wall thickness of the cage can increase its rigidity, reduce the deformation of the cage under ultra-high-speed rotation, avoid excessive stress on the circumferential surface of the cage pocket, and prevent fracture. The optimized bearing design can meet the requirements of ultra-high-speed operation of 30,000 rpm through temperature rise tests and high-speed durability tests.
In summary, the above optimization measures can be comprehensively applied to design deep groove ball bearings that can withstand speeds exceeding 30,000 rpm.
