Bearing metrics for budget-minded projects

Oct. 24, 2002
Standard ball bearings give engineers plenty of design options.

Radial ball bearings from NMB Technologies Corp. come in a variety of sizes and precision levels, with or without shields and seals.

Off-the-shelf radial ball bearings handle a variety of applications and are widely available. These are important considerations, especially for budgetconscious OEMs. Such "standard" bearings are good for designs with light-tomoderate radial and light axial loads and those requiring low noise and long life such as power tool and vacuum cleaner motors.

To take full advantage of these attributes, however, engineers should think about bearing selection early in the design process, says bearing maker NMB Technologies Corp., Chatworth, Calif. ( A product built around standard ball bearings can be economical yet reliable, though it requires the right mix of bearing metrics for the job. Metrics include bearing materials, lubrication and seals, cage type, noise level, and lifetime, all within the context of anticipated loads, speeds, and operating environment.

First, look at bearing materials. SAE 52100 chrome alloy steel and its equivalents are used in most standard ball bearings. A fine microstructure and the ability to be ground to an extremely smooth finish make these materials ideal for bearings in noise-sensitive applications. Another choice, DD400, is suited for corrosive environments. DD400 is a 400 Series martensitic stainless steel prepared by a heat-treat process developed at NMB's parent company, Minebea Group Co. ( Bearings built with DD400 are said to meet or exceed performance specs of those made of AISI 440C martensitic stainless steel in terms of hardness, low noise, and corrosion resistance.

Bearing cage selection is next. Cages separate and position balls at approximately equal intervals around a bearing raceway. A two-piece, closed-pocket metal ribbon retainer can handle most jobs. It provides a loose contact between components for a low-running torque. High-speed applications may require stainless-steel or plastic-crown retainers.

Another factor is bearing noise. NMB uses an Anderon meter to measure noise and vibration levels of completed bearings. The test effectively checks raceways and balls for any defects and verifies that each bearing will meet an overall noise rating. In general, quieter ball bearings cost more than noisier versions because the former require rounder raceways and better surface finishes.

Bearing precision also impacts cost. No surprise, bearings built to tighter tolerances cost more. The ABMA (American Bearing Manufacturers Assn.) and its associated ball bearing technical committee ABEC (Annular Bearing Engineer's Committee) publish tolerance specs for bearing manufacturers. These tolerances are reviewed regularly by the ABMA and pertain to the manufacturing specs of the inner and outer rings. The ABEC controls bearing dimensional tolerances, key to how bearings physically fit a housing or shaft.

For example, a standard radial play spec is good for normal press fits, moderate temperature differentials, and operating speeds. Applications with heavy press fits require bearings with a larger radial play. Shafts or housings that accept these bearings should be held to close tolerances to lower press-fit variations. Interference fits in applications that experience large temperature swings are calculated at the temperature extremes to determine ideal radial play.

Operating temperatures influence lubricant choice as well. Greases are typically chosen for their temperature rating, low noise, load capacity, or electrical conductivity. Oils, on the other hand, are noted for giving low running torque. A variety of available shields and seals keep lubricant in and contaminants out, a key to long bearing life.

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