Large Induction Motor Anti-Friction Bearing Selection

The majority of motor specifications utilize L10 life as the sole selection criterion for anti-friction bearings. At times, L10 lives in excess of 100,000 hours are specified in an attempt to prevent recurring field problems when the failures are originating from non-L10 mechanisms. In some cases these failures occur indirectly from over-specifying L10 life. Some general examples of this are as follows: □ An oversized bearing is utilized to obtain a very high L10 life, minimum load is not maintained, and the bearing fails rapidly from rolling element tracking problems. □ The bearing selection necessary to meet the L10 criteria is closer to its speed limit, demand on lubrication increases, and marginal lubrication, possibly due to poor grease selection or lack of relubrication, results in net loss of service life. □ A spherical roller bearing is utilized to improve L10 life, small design thrust load is actually much greater due to a combination of axial magnetic force, coupling thrusts, and misalignment, the axial to radial load ratio requirements are not met, and bearing lives achieved are relatively short. □ The bearing type is changed from ball to roller to increase the L10 life. As a result, the relubrication interval drops, this increased maintenance requirement is neglected, and no life improvements are obtained. A successful bearing design requires the consideration of many equally important factors. These are discussed in the following text.