Bearing it all

A brief overview of temperature’s effects on mounted sleeve and rolling-element bearings

“I can’t lay my hand on the bearing because it’s operating too hot!”

While this is a commonly heard complaint, most sleeve and rolling-element bearings can operate successfully at temperatures well above the pain threshold of a human hand.

The stabilized operating temperature of a bearing is the result of many factors. The key influences on operational temperature are bearing style, lubrication type, operational factors, environmental conditions and level of maintenance. The particular bearing style (ball, roller, sleeve, etc.), the shaft mounting style (slip fit, adapter mounted, press fit, etc.) and auxiliary items (housings, seals, shields, flingers, etc.) all contribute to a final operating temperature.

High heat cuts service life
For a given set of application conditions, a particular bearing type generates friction given off as heat. A typical bearing temperature rise range is 40 to 80 degrees F (4 to 27 C) for most industrial applications. However, a bearing temperature rise over ambient of up to 120 F (49 C) is possible under extreme conditions.

Note that when bearings operate at higher-than-normal temperatures, service life may suffer due to a deterioration of the lubricant oil’s film thickness and quality. Therefore, when making the initial bearing selection, you should choose with either:
• adequate design life hours to compensate for the anticipated reduction in service life;
• or, the appropriate lubricant for high-temperature operation.

Heat-reducing ideas
Bearings may be exposed to abnormally high ambient temperatures or elevated temperatures in equipment such as furnaces, fans, ovens, blowers, steel mill/foundry casters, rollout tables, dryers, electric motors and generators, to name a few. In many of these cases, bearings are expected to operate above the limits for standard bearing products.

Preferred means to achieve optimum bearing service life performance include locating bearings out of the immediate heat zones or providing provisions to reduce bearing heat. You can accomplish this through insulation procedures to reduce radiant heat. Also, shaft heat flingers or cooling wheels coupled with heat-resistant shaft materials can reduce bearing temperatures. Using water- or air-cooled bearing units is another method to reduce bearing temperatures to a more manageable range.

Though these steps incur higher installation costs, you gain long-term benefits by reducing lubrication and maintenance problems often encountered with high-temperature bearing applications.

When there is no way to avoid heat exposure, you can specially modify bearings to accommodate high-temperature applications. Bearings with optional component materials, special internal radial clearances, high-temperature lubricants and special heat treatments (if necessary) can operate successfully at very high temperatures, as shown in the chart below.

The above-maximum operating temperatures are limited by either the standard bearing unit features, component materials or the lubrication provided.

Achieving long service life
Bearings can provide years of service while operating at temperatures well above ambient and at levels well above what’s thought of as “too hot.”  As with any equipment component, proper bearing selection, correct lubrication and adequate maintenance procedures are vital to satisfactory service life.

This article is provided by the Power Transmission Distributors Association, a trade organization representing the PT/motion control distribution channel. To learn more, visit www.ptda.org.

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