Web Exclusive: Extend the life of motors and gearboxes

By |  May 27, 2011

Bearings operating in harsh environments such as mining must perform well at higher speeds with heavier loads. Traditional ball bearings, depending on their position and load, may not offer optimal performance in extreme applications like this. Sometimes, reliability can be increased by replacing them with cylindrical roller bearings.

Large horsepower electric motors illustrate the shift away from ball bearings. Often, the drive-end bearing is a ball bearing. However, many applications require the bearing on this end to carry a high radial load, via shaft-mounted pulleys or gears in certain cases.

Ball bearings may not have the required capacity to yield a satisfactory life. Substituting a cylindrical roller bearing could allow the motor to perform more reliably under high loads.

Like the electric motor, many high-speed applications combine a cylindrical roller bearing with a ball bearing on the shaft, since there are key similarities between the two bearings:

• Dimensionally, ball bearings and cylindrical roller bearings are interchangeable. For example, an NU310 cylindrical roller bearing shares the same bore, O.D. and width as a 6310 ball bearing.

• Both bearing types have similar limiting speed capabilities, allowing the cylindrical roller bearing to operate at maximum speeds virtually identical to the ball bearing.

Variety of designs

Cylindrical roller bearings are available in a variety of designs, allowing you to customize for specific applications. Choosing the right design for the job at hand depends on the mounting arrangement and whether or not the bearing must carry an axial (thrust), as well as a radial load.
Tolerances for bearing bore, O.D. and width normally adhere to ISO specifications. While Class Normal is the standard tolerance, C3 and higher clearances are common to applications with increased loads and vibrations. Tolerance and class designations are identical to those used for ball and spherical roller bearings.

Several different cage variations are available for cylindrical roller bearings. Stamped steel is common for small sizes, most often the one-piece window type. Some small sizes use high-strength polyamide materials. Larger bearings call for machine brass cages, either two-piece riveted or one-piece designs. Multi-row bearings usually employ a one-piece finger type cage made of machined brass.

Design considerations

High radial loads affecting both bearings on a shaft will lead some engineers to consider cylindrical rollers in both positions. Thus, the bearings may carry some axial (thrust) loads, as well. The engineer may choose to specify a pair of NJ- or NF-type cylindricals in these situations.
Typical applications are gearboxes or a pulley jack shaft, where small axial loads result from gear loads or belt alignment. The thrust load such bearings can carry depends on the series, normal operating speed and lubricant type. As a rule of thumb, when bearing speed increases, thrust load capability decreases. If oil is used as the lubricant instead of grease, the bearing can carry a higher axial load.

Information for this article courtesy of NSK Corp., www.nskamericas.com.

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