Bearing friction
One of the main functions required of a bearing is that it must have low friction. Under normal operating conditions rolling bearings have a much smaller friction coefficient than the slide bearings, especially starting friction.
The friction coefficient for rolling bearings is expressed by formula
μ = 2M / Pd
where,
μ:
Friction coefficient
M:
Friction moment, N・mm {kgf・fmm}
P:
Load, N {kgf}
d:
Bearing bore diameter, mm
Although the dynamic friction coefficient for rolling bearings varies with the type of bearings, load, lubrication, speed, and other factors; for normal operating conditions, the approximate friction coefficients for various bearing types are listed in Table below,
| Bearing type | Friction coefficient μ |
|---|---|
| Deep groove ball bearing | 0.0010 ~ 0.0015 |
| Angular contact ball bearing | 0.0012 ~ 0.0020 |
| Self-aligning ball bearing | 0.0008 ~ 0.0012 |
| Cylindrical roller bearing | 0.0008 ~ 0.0012 |
| Full complement type needle roller bearing | 0.0025 ~ 0.0035 |
| Needle roller and cage assembly | 0.0020 ~ 0.0030 |
| Tapered roller bearing | 0.0017 ~ 0.0025 |
| Spherical roller bearing | 0.0020 ~ 0.0025 |
| Thrust ball bearing | 0.0010 ~ 0.0015 |
| Spherical thrust roller bearing | 0.0020 ~ 0.0025 |
Temperature rise
Almost all friction loss in a bearing is transformed into heat within the bearing itself and causes the temperature of the bearing to rise. The amount of thermal generation caused by friction moment can be calculated using formula
Q = 0.105 x 10-6Mn N
Q = 1.03 x 10-6Mn {kgf}
where,
Q:
Thermal value, kW
M:
Friction moment, N・mm {kgf・fmm}
n:
Rotational speed, min-1
Bearing operating temperature is determined by the equilibrium or balance between the amount of heat generated by the bearing and the amount of heat conducted away from the bearing. In most cases the temperature rises sharply during initial operation, then increases slowly until it reaches a stable condition and then remains constant. The time it takes to reach this stable state depends on the amount of heat produced, heat capacity/diffusion of the shaft and bearing housing, amount of lubricant and method of lubrication. If the temperature continues to rise and does not become constant, it must be assumed that there is some improper function.
Possible causes of abnormal temperature include bearing misalignment (due to moment load or incorrect installation), insufficient internal clearance, excessive preload, too much or too little lubricant, or heat produced from sealed units. Check the mechanical equipment, and if necessary, remove and inspect the bearing.