by Drew Troyer

The bearing configuration you select will significantly influence the reliability of industrial electric motors. Open bearings, shielded bearings and sealed bearings all offer advantages and disadvantages. Here are thoughts about the lubrication-related aspects of each, including statistics from a survey conducted at Noria Corporation.

Open bearings
Open bearings have no seals or shields. The open bearing configuration runs cooler than either sealed or shielded bearings, poses no risk for shield collapse into the motor and is easy to regrease. However, open bearings pose the greatest risk for grease churning, contaminants and hardened thickener entering into the bearing, and grease entering the motor’s windings, which compromise heat dissipation and decrease the life of motor insulation.

A recent study revealed that among industrial respondents stating a preference, 9 percent prefer an open bearing configuration for their industrial motors.

Shielded bearings
Shielded bearings are equipped with overhung metal shields affixed to the outer race section of the bearing. At the inner race, a gap, or annulus, of approximately 125 to 375 microns allows grease to enter and exit the bearing cavity. Bearings may be equipped with a single shield facing the grease supply, a single shield facing the motor, or double shields, one on each side of the bearing.

Advantages of shielded bearings include reduced grease churning, regulated grease flow to the bearings, restricted entrance of contaminants and hardened thickener and reduced risk of grease entering the motor’s windings. However, shields cause the bearings to run hotter than open bearings. Shields facing the grease supply risk pushing the overhung shield into the bearing, so use extreme caution when regreasing shielded bearings.

Approximately 64 percent of industrial respondents indicated a preference for shielded bearings, particularly single-shielded bearings with the shield facing the motor.

Sealed bearings
Sealed bearings come with elastomer seals that completely enclose the bearing. Sealed bearings must be prelubricated before being sealed and may not be relubricated. They’re often called “lubed for life” bearings.

Sealed bearings will not last as long as properly maintained motor bearings that enable relubrication because the limiting factor of the bearing’s life is the oxidative life of the base oil in the grease.

Sealed bearings tend to run hotter than both open and shielded bearings.

Twenty-seven percent of industrial users in our survey stated a preference for sealed bearings, primarily because they require less routine maintenance for relubrication and there is no risk of overlubrication. Sealed, lubed-for-life bearings are typically limited to small motors, usually less than 10 horsepower. However, they have been employed on larger motors.

Ponder this
Carefully consider your maintenance plan, including lubrication, and the operating objectives for the asset when specifying the bearing configuration for industrial electric motors. In particular, make the decision about where to specify sealed bearings very carefully. Too often as a consultant, I observe identical machine trains (often side-by-side) equipped with identical motors — save for the bearing configuration. Using different bearing configurations that require different lubrication procedures in otherwise identical machine trains providing the same service unnecessarily complicates lubrication maintenance, which can lead to mistakes.

Rationalize your choice of bearing configuration in motor bearings, capture it into design and procurement standards and stick with it.

References
Troyer, D. (2005) “Lubricating Industrial Motors,” chapter from the CRC/STLE Handbook of Tribology and Lubrication Engineering, G. Totten, Editor (Forthcoming).

Drew Troyer is the senior editor of Machinery Lubrication Magazine. If you have a lubrication or oil analysis question, contact Coach Troyer at 800-597-5460 or e-mail dtroyer@noria.com.

This article appeared in the April/May 2005 issue of MRO Today magazine. Copyright 2005.

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