All self-aligning linear bearings aren't created equal

April 18, 2001
From left: sharply crowned, gradually crowned, and radiused-ring bearing plates.

From left: sharply crowned, gradually crowned, and radiused-ring bearing plates. New Excel self-aligning linear bearings from Nook Industries, Cleveland, use gradually crowned bearing plates.

This open-style pillow block from Nook Industries, Cleveland, contains an Excel open-style, self-aligning linear bearing.

Shaft hardness dramatically affects equivalent load and device life. For example, a self-aligning linear bearing equipped with a Rockwell 30 C shaft subjected to a 10-lb load would, for lifetime calculations, have an equivalent load of 100 lb.

Although most self-aligning linear bearings look similar, each design can have different operating characteristics. Take bearing plates, for example. The bearing plate or outer race surrounds a plastic ball-bearing retainer. The plate transfers load from the bearing bore — through the bearing balls — to the shaft or "inner" race.

There are three basic bearing plate types: sharply crowned, gradually crowned, and radiused. Bearings with a sharply crowned, segmented bearing plate can typically handle several times the load of bearings without the feature. Here, the crowned portion of this independent outer race pivots or rocks against the bearing bore to tolerate misalignment (in any direction) to ±0.5°.

These independent races also make possible zero clearance between the ball bearings and shaft. However, the sharply crowned shape can raise contact stress at bearing bores. Under higher loads, plates may embed into bores which, over time, loosens bearing-to-shaft fit.

The gradually crowned bearing plate functions much the same way as the sharply crowned design, but puts more material in contact with the bearing bore surface. And more contact area reduces contact stress so the bearing maintains proper bearing-to-shaft fit longer than sharply crowned versions.

The third design combines a flat, segmented outer bearing plate with a radiused outer ring, hence the name. Radiused linear bearings self align when bearing plates rock or pivot against the curved ring surface. This effectively boosts contact area with the bearinghousing bore and minimizes the bearing outer race into the housing. However, this design requires a greater number of smaller bearing balls to function properly. The result is a bearing more susceptible to failure from contamination than designs that use larger balls.

Also consider how the outer race is made. Bearing plates are either stamped, drawn, or ground. While all three methods produce functional bearings, only bearings with a ground bearing plate can provide the highest degree of smoothness. This is because stamping and drawing operations tend to distort plates, and this distortion negatively impacts smoothness of operation. The true and straight rolling surface of a ground bearing plate provides better control of bearing plate/ball spacing.

Also critically important is the hardened and ground shaft that forms the inner race. Bearing life depends directly on shaft hardness. Shafts should have a minimum hardness of Rockwell C 58. They should also have a surface finish of between 10 to 16- in. Ra to ensure smooth operation and proper lubricant retention.

Finally, don't neglect seal design. Seals should keep dirt out and lubricant. They should also tolerate misalignment without losing contact with the shaft. Add-on or external-type seals do the job but with substantially more drag or friction. Floating type, double-lip integral seals made from nitrile rubber are a better choice. The seal's floating design athe description and alt textllows it to re-main in constant contact with the shaft while minimizing friction.

Information for this article comes from Rick Christyson, chief engineer, Nook Industries Inc., Cleveland. Motors Default: Motors Default Description

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