Revolution speed is always significant with plastic bushings. The absolute rotational speed is not decisive, instead it's the relative speed between the shaft and the bushing.
The surface speed is expressed in feet per minute (fpm) and calculated from the rotational speed (rpm) with the following formula:
Rotations: v = rpm x d1 x π/12 = fpm
Oscillating motion: v = 4ab x πd / 360 x 12 = fpm
|d1||Shaft diameter (inch)|
|a||Angle of motion either side of the mean position (°)|
|b||Frequency (cycle per min)|
|d||Inner diameter (inch)|
iglide® plastic bushings were primarily developed for low to medium surface speeds in continuous operation.
Table 1.1 shows the permitted surface speeds of iglide® plastic bushings for rotating, pivoting and linear motion.
These surface speeds are limit values under the assumed minimum pressure loads of the bearing. In practice, these limit values are rarely reached due to an inverse relationship between load and speed. Each increase of the pressure load inevitably leads to a reduction of the permissible surface speeds and vice versa.
The speed limit is measured by the bearing temperature. This is also the reason why running speeds differ depending on the type of movement. For linear movements, more heat can be dissipated over the shaft, since the bushing uses a longer surface area.
Considerations about the maximum permissible surface speeds should also include the wear resistance of the plastic bushing. High surface speeds automatically bring correspondingly high wear rates with them.
The coefficient of friction for plastic bushings depends on the surface speed. High surface speeds have a higher coefficient of friction than low surface speeds. Figure 1.8 illustrates this relationship in the example of a cold rolled steel shaft with a load of 102 psi with 30 and 59 fpm.
Picture 1.7: Wear and friction tests with the example of an igubal® pillow block bearing
Picture 1.8: Test to determine the maximum surface speeds of an igubal® rod end bearing at high rotational speeds
Figure 1.8: Coefficients of friction of iglide® plastic bushing materials at various surface speeds (Cf53 shaft)