Get your proper bearings

Don’t head off in the wrong direction when replacing motors. The type of service and the orientation of the motor determine which bearings it should have. Using the wrong bearings can lead to significant problems.

By Daniel R. Snyder, SKF

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For electric motors, there’s no universal “one-size-fits-all” solution for anti-friction bearings. Every bearing type incorporates specific performance characteristics and so suits particular operating conditions. A mismatch between bearings and motor application can lead to significant problems.

Consider, for example:

  • Using motors larger than 125 hp, faster than 1,200 rpm and having deep-groove ball bearings optimized for direct-connect duty with belts may lead to mechanical overload and premature failure.
  • Putting a motor designed for heavy belting on a coupled load may not give enough radial loading for the bearing’s rolling elements to roll. They will skate or skid on the race, causing high temperatures and potentially rapid and complete lubrication failure.
  • Underloading a roller bearing, even with a belted load, may result in premature failure from the same skidding phenomenon.
  • Installing a motor with bearings selected for horizontal mounting in a vertical application adds an unplanned axial bearing loading and may cause loss of grease in the bearings.
  • Not addressing stray currents passing across bearings (often encountered in large variable-speed electric motors) may prompt electrical erosion damage leading to premature bearing failure.

While there’s always the possibility that a replacement motor may perform differently from the original or, even worse, fail, selecting the bearings inside a motor consistent with application and conditions greatly improves the chances for success.

Bearings and motor design

Rolling bearings (ball and roller) in electric motors serve to support and locate the rotor, keep the air gap small and consistent, and transfer loads from the shaft to the motor frame. The bearings should enable high- and low-speed operation, minimize friction, exhibit low noise, promote long life and save power.

Ultimately, the application will drive bearing selection. Appropriate bearing types for electric motors include deep-groove ball bearings, angular-contact ball bearings, cylindrical roller bearings, tapered roller bearings, spherical roller bearings, toroidal roller bearings and spherical roller thrust bearings. Each has its own configurations and performance characteristics.

Electric motors typically incorporate a locating and non-locating bearing arrangement to support the rotor radially and locate the rotor axially relative to the stator. Locating bearings position the shaft and support axial loads while non-locating bearings permit shaft movement in the axial direction and compensate for overload conditions when thermal expansion of the shaft occurs.

For smaller motors used in horizontal equipment, the most common bearing arrangement consists of two deep-groove ball bearings mounted on a short shaft in a cross-locating bearing arrangement to control shaft movement. In most medium and large electric motors for horizontal machines, deep-groove ball bearings act as the locating bearing while the bearing in the non-locating position may be a ball bearing or cylindrical or toroidal roller bearing, depending upon the loads, speeds, temperature and environment of the application. The non-locating bearing accommodates any axial expansion due to heat or machine tolerances.

Vertical machines typically rely upon deep-groove or angular-contact ball bearings or spherical roller thrust bearings, depending upon the vertical load, shaft and rotor weights, speeds, temperature and operating environment.

The impact of load

The nature of the load is a primary factor in bearing selection. Bearings always require a given minimum load for proper rolling-element rotation and enhanced lubricant film formation in rolling contact areas. Otherwise, skidding can occur, resulting in higher operating temperatures and lubricant degradation.

The bearings, of course, must provide a sufficient load capacity to yield satisfactory service life in the application. All loads must be considered — not just the weights involved and the forces derived from the power transmitted but also coupled and belted loads from connection to the driven load.

With a coupled load, the motor shaft usually is connected via a flexible coupling to the driven load’s shaft. This type of load presents no axial or radial load to the motor bearings except for the weight of the motor’s rotors and shaft assembly. (Misalignment from mounting errors, however, can add radial load.) Ball bearings are typical choices for coupled loads.

The most common type of belted load is when a V-groove pulley is mounted on the motor shaft in an overhung arrangement and is connected to another pulley on the driven load by one or more belts held in tension. This can generate high radial loads on the motor shaft and the drive-end bearing (because it’s closest to the applied external loading). Depending upon the magnitude of radial load, either ball or roller bearings can be employed.

On smaller motors under normal conditions, a ball bearing may serve in applications with either coupled or belted loads.

In general, heavy loads are carried by roller bearings and lighter loads by ball bearings. Loads can be radial, axial or a combination of the two. Certain bearings, such as cylindrical roller types, usually are used for radial loads only; others, such as angular-contact ball bearings, mainly suit axial loads.

Cylindrical and toroidal roller bearings only can support pure radial loads with minimal axial loads. Other radial bearings (such as tapered and spherical roller bearings) can accommodate axial loads in addition to radial loads but also have minimum load considerations.

Angular-contact ball bearings can support moderate axial loads at relatively high speeds. For moderate and heavy axial loads acting in one direction, specify spherical roller thrust bearings. A bearing’s ability to carry an axial load will be determined by the angle of contact or load action internal to the bearing (the greater the angle the more suitable the bearing for axial loads).

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