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Bearing friction and temperature rise

release time:2018-05-26
Bearing friction and temperature rise:
 
Bearing friction
One of the important properties of the bearing is the requirement of low friction. Under normal conditions of use, rolling bearings have less friction than sliding bearings, especially low starting friction. Mainly consists of: rolling contact friction, rolling friction and sliding friction between the rolling element and the cage in the contact area, friction of the lubricant, and sliding friction of the contact seal.
If the bearing load is close to 10% of the basic dynamic load rating, that is, P = 0.1C, the radial bearing bears a pure radial load, and the thrust bearing bears a pure axial load with a rotational speed of n≈0.5nj (nj is the limiting rotational speed). When fully lubricated, the viscosity of the oil is 30~35cst, and the operation is normal, the frictional moment M is calculated as follows:
M=0.5μF•d
In the formula
M = bearing friction torque, N • mm
μ = bearing friction coefficient,
F = bearing load, N
d= bearing inner diameter, mm
When the above working conditions are not the same, the total frictional moment of the bearing must be added by the frictional moment M0 that is irrelevant to the bearing load and the frictional moment M1 related to the load.
M= M0+ M1
Calculation of M0
Frictional moment M0 is mainly caused by the loss of bearing lubrication fluid power at high speed and light load, independent of the load. It depends on adequate lubrication, lubrication viscosity and bearing operating speed.
M0 is calculated as follows:
M0=f0×10-7(γn)2/3dm3
In the formula
Dm = average bearing diameter (=d+D2), mm
F0 = bearing type and lubrication factor,
n=bearing speed, r/min
γ = kinematic viscosity of the oil at operating temperature, mm2/s (oil lubrication should be calculated based on grease lubrication)
This formula is suitable for γn≥2000. If γn<2000, the following formula should be used:
M0=160×10-7 f0 dm3
Calculation of M1
The frictional moment M1 is mainly due to the fact that under the action of the load, the running speed of the bearing is low, and the elastic deformation of the contact surface is caused by a slight amount of sliding.
M1 is calculated as follows:
M1= f1p1 dm
In the formula
F1 = bearing type and load factor,
P1 = Calculated load to determine bearing frictional torque, N
Dm = average bearing diameter (= d+D2), mm
When there is a starting torque requirement, generally take 2 times M1 or higher.
 
The temperature rise of the bearing
The friction of the rolling bearing is a key factor in determining the bearing heating and operating temperature. Friction losses in rolling bearings are almost always converted into heat inside the bearing, which in turn causes the bearing temperature to rise. The heat generated by the friction torque can be expressed by the formula:
Q=0.105×10-6M•n
In the formula:
Q: Heat generation kW
M: friction torque N•mm
n: Bearing speed rpm
 
The balance between the amount of heat generated and the amount of heat emitted is that the bearing temperature is stable. At the early stage of normal operation, the temperature suddenly rises, but when it reaches a normal state, it is basically stable. The time to reach the steady state differs depending on the heat generated by the bearing, the heat capacity of the bearing housing, the cooling area, the amount of lubricating oil, and the ambient temperature.
Abnormal temperature rises can be caused by excessive clearance, excessive preloading, excessive or insufficient lubricant, foreign matter mixed in, and heat generated by the sealing device.