Bearings catalog - UKF - #49

For greased bearings, a routine warm-up is recommended. A warm-up period has a significant effect on bearing life and efficiency-Warming up should proceed by gradually increasing speeds, with the first one set at about 20 %of the maximum allowable speed. The spread between beginning and maximum speed should then be divided into four steps, during the temperature should be monitored: subsequent, higher steps can follow as soon as the temperature has stabilized.

Alternatively, a brief moderate-speed run to distribute the grease, followed by a short ramp-up to speed and then off, again, culminating with a thermal equalization period may also be used. Although this more traditional warm-up is faster, it is not as conducive to long bearing life.

The working life of grease depends on various factors, including forces, vibration, contamination, humidity and temperature - and, above all, the operating speed. The following diagram gives an overview of possible lubricant lifetimes, dependent on the characteristic value of speed n • d_mas rpm • mm.

High precision, super-finished races reduce friction and heat, and with time, they reduce wear and tear. Compared with paired single-row bearings, the UKF double-row bearings have higher parallelism accuracy. The design creates a void between the rows, which can effectively function as a grease reservoir. A similar configuration can result by using UKF distance rings, for High Speed Bearings Types "UHS", "UHC".

Hybrid Bearings will run at lower temperatures than bearings with steel balls, for the comparable speeds (RPM).

The preceding remarks about accuracy and running characteristics, under Temperature" also apply to friction. Bearings have three kinds of friction:

1. rolling friction between the balls and raceways

2. sliding friction between balls and cages - and also between cage and its guidance on the Inner- or Outer Ring (rigid cage!)

3. viscous friction from the lubricant.

The inherent friction (resistance to rolling) in a rolling element bearing is quite low, and is insignificant when calculating f_L, L_h.

Determining an optimal preload, careful selection of lubricants, and proper care during installation may further reduce friction.

Noise can occur as the result of vibrations being trans­mitted through the components. Vibrations are usually caused by problems in the rotating parts, e. g., the natural frequency of the shaft with assembled bearings (-inner ring, cage, balls). Appropriate design analysis should prevent such occurrences.

0,1 0,5 1,0 1,5 xlO⁶

n xdm —- rpm X mm

Lifetime L_c of grease dependent on characteristic value of speed