Bearings and Lubricants

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 most bearings with long service lives.

Bearings based on rolling action are called rolling-element bearings. Those bearings 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 supplied 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.


Bearings and Lubricants

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