Machine Design

Bearing Loads II

Operating most efficiently is a machine's first step to maintaining productivity.

Bearings permit smooth low-friction rotary or linear movement between two surfaces. Bearings employ either a sliding or rolling action. In both cases, there is a strong attempt to provide enough lubrication to keep the bearing surfaces separated by a film of oil or other lubricant. The absence of physical contact provides mostbearings with long service lives.

Bearings based on rolling action are called rolling-element bearings. Those based on sliding action are called plain bearings. Because there is more variation in the ways plain bearings are lubricated, they are also referred to by the lubricating principle involved. For example, comparisons often are made between roller bearings and "fluid-film bearings," a class of plain bearings.

Plain bearings are generally less costly than rolling-element bearings. This is especially true in mass-production quantities. In moderate and small quantities, roller bearings are more competitive from a price standpoint, especially if a plain bearing requires special lubrication or demands special designs that cannot be msupplied off-the-shelf.

Terminology differs somewhat for the two types of bearings. For example, a bearing can carry loads along its axis of rotation or perpendicular to its axis of rotation. For both types of bearings, those carrying loads along the axis of rotation are referred to as thrust bearings. Rolling-element bearings carrying loads perpendicular to the rotational axis are called radially loaded bearings. Plain bearings carrying such loads are usually called journal bearings or sleeve bearings.

Bearings are evaluated on the basis of how much load they can carry, at what speeds they can carry this load, and how long they will serve under the specified conditions. Friction, start-up torques or forces, ability to withstand impact or harsh environments, rigidity, size, cost, and complexity also are important design considerations.

Load and operating speed affect bearings. Service life is determined statistically from tests of numerous samples and is generally selected on the basis of a 10% failure rate (90% reliability). The result is the B10 or L10 life.

Hydrodynamic and hydrostatic bearings, however, operate with infinite service life below some critical value of load and speed. With a self-acting, oil-lubricated (hydrodynamic) sleeve bearing, load capacity increases linearly with speed since rotation builds the supporting lubricating film.

For the extremely pressurized (hydrostatic) sleeve bearing, there is a particular load capacity for infinite life, and this capacity is essentially unaffected by speed -- although at times there may be some hydrodynamic or speed-dependent contribution to load capacity.

If loads exceed the infinite-life value, the surfaces are assumed to come into rubbing contact, thereby setting up a condition of boundary lubrication. This condition often leads to accelerated wear and failure. However, bearings can be designed to operate satisfactorily in the boundary mode.

As a result of these interactions, rolling-element bearings are less sensitive than plain bearings to load variations. And because they do not rely on velocity effects to maintain a fluid film, they are well suited to supporting heavy load at low speeds. Fluid-film bearings tend to be a better choice if load increases with an increase in speed or if the load is dynamic. On the other hand, pulsating heavy loads on stationary rolling-element bearings can cause brinelling.

Both rolling-element and plain bearings can be vulnerable to severe impact loads. Often a case is presented that rolling-element types are more vulnerable because of their line or point contact and resulting high contact stress. The fluid film in plain bearings is said to provide a better "cushion" for impact. More realistically, however, resistance to impact for both types depends upon reserve capacity. Either type can be designed to sustain high impact.

TAGS: Bearings
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