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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Single-row angular-contact ball bearings and tapered roller bearings only can accommodate axial loads acting in one direction. In applications with axial loads of alternating direction, these bearings must be combined with a second bearing capable of supporting axial loading.

A combined load results when radial and axial loads act simultaneously. The most common bearing solutions in these cases are single- and double-row angular-contact and single-row tapered ball bearings (although deep-groove ball bearings may also be appropriate, depending upon the ratio of axial to radial loading).

Further factors

Other primary factors for bearing selection include:

Connection type. The nature of the device placed between the drive and the driven unit (coupling, belt or gear drive) influences the loads on motor bearings and resulting performance.

Good alignment is important in both flexible and rigid coupling drives because misalignment can induce additional forces into the bearing system and reduce service life. Proper alignment is particularly crucial with a rigid coupling where typically three bearings are on a shaft (two in the motor and a third in the coupled device). When rigid couplings are aligned very accurately, the drive-end bearing might become relatively unloaded. In these cases, a deep-groove ball bearing would be recommended at the drive end.

A belt or gear drive frequently will load the motor bearings more heavily than a coupling drive; cylindrical roller bearings most often are used at the drive end where significant loads are encountered. In applications with heavy loads and the possibility of misalignment or shaft deflection, spherical and toroidal roller bearings offer solutions.

Speed. The rpm influences operating temperature and, in turn, bearing and lubricant life. So, cage, lubricant, running accuracy and clearance of the bearings, the resonance frequency of the system, and the balancing of the rotating components are significant factors in bearing selection.

For high-speed applications, ball bearings generally prove more appropriate than roller bearings. In very-high-speed applications, precision or hybrid bearings may offer benefits.

Temperature. The permissible bearing operating temperature in a motor application will limit the speed at which rolling bearings can perform. Bearing types with low friction and corresponding low heat generation inside the bearing will work well in high speed operation. The highest speeds can be achieved with deep-groove ball bearings when loads are purely radial and with angular-contact ball bearings for combined loads. This holds especially true for such bearings with ceramic rolling elements.

Type of lubrication. Under normal speed and temperature conditions, the bearings in electric motors usually are lubricated with grease. Grease allows for simpler more-cost-effective housing and sealing designs, offers better adhesion of lubricant to critical surfaces, and provides more reliable protection against contaminants than oil.

The life expectancy of grease depends upon several factors, including the type of bearing, the type of grease, the orientation and speed of the motor, and the operating temperature of the bearings.

Small ball bearings in standard electric motors usually are fitted with seals or shields and lubricated for life — they aren’t intended to be relubricated but replaced at normal motor repair intervals. Severe-duty motors, regardless of size, often are supplied with open bearings and provisions for regreasing. (If the grease life is shorter than the expected bearing life, the bearings obviously will need to be relubricated while the grease is still performing as intended.)

Sometimes, rotational speeds or operating temperatures make it impractical or impossible to use grease because the grease life or relubrication times are too short. These cases demand oil lubrication. In general, only large electric motors are oil-lubricated, in part due to the need for more sophisticated seals and the potential risk of leakage from the systems.

Tips for longer life

When motors fail, bearings may be the culprit but possible non-bearing causes also abound. These encompass windings, wiring, grease or seal failures, which, in turn, may result in bearing failures (although bearings are not the root cause). Improper motor use and inadequate maintenance can add to potential problems and premature bearing failure.

Here are some potential problems along with pointers on how to avoid or address them:

Electric arcing. In variable-speed motors, stray currents from arcing may cause bearing damage. Although arcing typically tends to be isolated and localized, the effect on a bearing is almost like a series of small lightning strikes that melt and retemper internal bearing surfaces. The result is that some surface material flakes away and spalls out to create noise in the bearing and potentially shortened service life.

Users can be sure that bearing damage from electric arcing has occurred when they notice characteristic “fluting” patterns (Figure 1).

Figure 1. Characteristic fluting patterns provide a telltale indication of arcing problems.
Figure 1. Characteristic fluting patterns provide a telltale indication of arcing problems.

Fluting is caused by the dynamic effect of the rolling elements continually moving over the microcraters and etching a rhythmic pattern into the running surfaces of a bearing’s races. Noise and vibration from the bearing increases and, eventually, the deterioration will lead to complete bearing failure.

Even if a bearing isn’t affected by these discharges, its lubrication could be. Grease composition can degrade rapidly due to the high localized temperatures generated by current discharges.

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