Other types of bearing
There are several other types of bearing which have been developed for special applications, unsuited to the more standardised types of sleeve or rolling element bearings. Because of their unique features they are only briefly described to give an indication of what is available should the need arise.
Where the fan/motor combination cannot be adequately sealed against the escape of oil, water has been used as a lubricant. This can mean a much lower film thickness because of the lower viscosity of water. However satisfactory bearings for certain applications have been designed.
Air may also be used as a lubricant in sleeve bearings if supplied under pressure. It produces little friction loss but is really only suitable for small high speed bearings running in excess of about 6000 rev/min.
Sleeve bearings may be manufactured with porous bushes impregnated with substances such as PTFE. This produces a reasonably low coefficient of friction such that they can be used in small fans where the radial and thrust loads are low and the rotational speeds are not too high.
Magnetic bearings have been used in large units operating at high radial loads and high rotational speeds. As there is no physical contact of lubricant, frictional power losses are virtually zero. However, power circuits, position sensors and controls are all needed to keep the shaft central within the housing. Provided that the fan duty remains fairly constant and, therefore, that the power absorbed also remains steady, successful bearings can and have been designed. At the present time development continues in an endeavour to reduce the very high cost.
Posted in Fans Ventilation A Practical Guide