For motor products with different speed requirements, the supporting bearing performance and parameters will inevitably be involved. In the previous article, we talked about the motor operating conditions for the bearing insulation performance, temperature, lubrication and other requirements. Our today's Focus on rolling bearing speed.
01The rotational speed that rolling bearings can achieve is limited!
There is a limit to the rotational speed that a rolling bearing can achieve. The limiting speed is usually determined by the lubricant used and the operating temperature that can be achieved by the material of the bearing components. The speed at which the bearing reaches the operating temperature limit depends on the heat generated by friction as the bearing rolls, and of course any other external heat that affects the bearing temperature, and how much heat can ultimately be removed from the bearing. This involves a problem of heat generation and heat dissipation. The related factors include the characteristics of the bearing itself, as well as the bearing system components, bearing operating environment and working conditions.
The type and size of the bearing, the internal structure, the load it bears, the lubrication and cooling conditions, the structure of the cage, the manufacturing accuracy and clearance, etc., all have a certain influence on the speed that the bearing can achieve. In most bearing parameters, two different definitions of rated speed are usually listed, namely reference speed and limit speed, both of which are parameters based on different considerations. The reference speed is related to heat and represents a reference value used to determine the allowable operating speed of the bearing under a given load and lubricant viscosity; the limit speed is related to the physical structure.
The reference speed of the bearing is specified in the product standard requirements of the bearing. The reference speed of the bearing refers to the speed of or fat), a balance is reached between the two.
02How to determine the heat balance state of an open bearing?
According to the standard, the reference working conditions to achieve thermal balance include bearing temperature rise, load conditions and other specific contents.
(1) The bearing temperature rise does not exceed 50K. For example, at an ambient temperature of 20 °C, the temperature rise of the bearing is 50 °C, that is, the bearing temperature measured in the non-rotating outer ring or bearing chamber does not exceed 70 °C.
(2) Radial and axial load conditions of open bearings for common set clearance. For radial bearings, the radial load is stabilized at 5% of the basic static load rating; for thrust bearings, the axial load is stabilized at 2% of the basic static load rating.
(3) Lubricant conditions: For mineral oil, oil lubricated bearings without extreme pressure (EP) additives, the kinematic viscosity at 70 °C is: radial bearing v=12mm2/s; thrust roller bearing v=24mm2/s. For oil bath lubrication, the oil level should be controlled at the center of the rolling elements at the lowest position.
For bearings lubricated with lithium soap mineral oil grease, v=100-200mm2/s when the base oil viscosity is 40℃. And the grease filling amount is about 30% of the free space in the bearing, and the temperature of the grease-lubricated bearing may reach a certain peak in the initial stage of operation. Therefore, it may take 10 to 20 hours for the bearing to reach normal operating temperature.
Under the above specific conditions, the reference speed is the same for oil-lubricated and grease-lubricated bearings. When the outer ring rotates, the achievable speed will be lower. In addition, the rotational speed that some bearings can achieve is not limited by the heat generated by the contact between the rolling elements and the raceway, such as bearings with contact seals.
