Considerations of coupling selection, installation and alignment: effects on motor mechanical performance

70% to 80% of motor field problems are mechanical in nature. A significant proportion of these mechanical problems can be attributed to bearing problems which arise when external loads are exerted on bearings not designed to accommodate any extraneous loading. When a motor is intended to be used in a direct connected mode, it is assumed that the only loads which the motor bearings have to support are the motor´s rotor weight, one-half the coupling weight, and possibly some internally unbalanced magnetic forces. Unfortunately, if misaligned, it is possible for the couplings to exert a sustained combination of axial, radial, angular load on the motor bearings, with a subsequent reduction in motor bearing life, or in extreme cases, failure.