Machine Design

Bearing Cost

Hardware costs: In high-volume lots, sleeve bearings and bushings are considerably less costly than rolling-element bearings. In mid-range volumes, prices are comparable. For special designs in small quantities, sliding bearings are more costly than rolling-element bearings. Dry-film and boundary-lubricated bearings usually use expensive proprietary materials. Powdered-metal bearings, however, are inexpensive.

Design costs: Rolling-element and dry-lubricated bearings normally require the least cost for end user. Manufacturers of rolling-element bearings can provide considerable cost-saving assistance by virtue of well-documented design manuals. Self-acting sleeve bearings, however, may require considerable end-user design effort except for light-duty applications experience. The behavior of externally pressurized bearings usually are predicted by calculations, but considerable design effort may be required to verify the design completely.

Shop costs: Rolling-element bearings normally require precise housings and shafts and require fairly costly machining for products in which they are used. Sleeve bearings, in contrast, generally operate well with less finely prepared machine finishes. Many plain bearings operate satisfactorily with lathe-turned journals.

Maintenance costs: When the bearing lubrication is self-contained, maintenance costs are determined by sealing requirements. If there is full-pressure lubrication, costs may be determined by the amount of filtration needed. Generally, rolling-element bearings have the lowest maintenance costs because of lower lubrication requirements. The very minimum maintenance cost is associated with self-lubricating bearings -- provided they deliver sufficient service life.

Replacement costs: These costs depend more on the specific design than on the type of bearing. In general, however, sliding bearings are replaced more easily than rolling-element types. Both types can be damaged during installation if not handled properly. Sliding bearings can often be replaced quickly by machining from bar stock or by altering available stock sizes.

Cost of failure: Rolling-element bearings give ample warning that they are approaching failure by virtue of increasingly noisy operation and usually fail from fatigue. Sliding bearings, on the other hand, usually perform well up to moments before a violent failure.

If a rolling-element bearing fails at high speed, it is usually total and catastrophic. With a journal bearing, the effect is normally less drastic. Often, only a bit of polishing puts it back into service. However, sliding bearings can suffer catastrophic failure.

TAGS: Bearings
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