A sliding bearing is a machine element that transmits reaction forces to a shaft. A journal bearing is cylindrical (either continuous or segmented) and serves when the force on it is essentially radial. A thrust bearing is washer-shaped (either continuous or segmented) and serves where the force is axial relative to the shaft. Sliding bearings are also called plain bearings. (Some sources prefer to call them “plane” bearings.) This article deals mostly with metallic sliding bearings, though parts apply also to the growing class of nonmetallic bearings such as nylon, PTFE, and polymer composites.

A flange bearing can accommodate both radial and axial force. It is a journal bearing constructed with one or two integral thrust-bearing surfaces. Sliding of the shaft or thrust-collar surface relative to the bearing surface is characteristic of all plain bearings. In many applications, plain bearings offer advantages over rolling-element bearings, such as lower cost, smaller space needs, ability to run with marginal lubricants, corrosion resistance, and capacity for high specific loads.

The first use of a metal alloy for its special properties was probably Isaac Babbitt’s adaptation of a pewter composition in 1839. Since the 1930s, developments have proliferated on all counts for metallic plain bearings. Single-metal, cast-inplace bearings have been mostly supplanted by replaceable-insert bearings of multilayer laminated metals. Bearing mechanical design is now a much more refined engineering specialty. And manufacturing technology has evolved into a complex matrix of metallurgical, chemical, and mechanical processes.

Size, configuration, manufacturing method

Sliding bearings are often classified as thin-wall (less than about 0.2-in. wall thickness) or heavy-wall. In general, bearings of diameter greater than about 6 in. are in the heavy-wall class.

Configurations are also described as half round, full round, flanged, or washer. Society of Automotive Engineers (SAE) standards classify thin-wall bearings as sleeve-type half bearings, split-type bushings, and thrust washers. Most such bearings are made by high-speed forming and machining processes from flat strip. You may sometimes hear them called “striptype” or “sheet-metal” bearings.

Heavy-wall bearings are produced in small lots, say, no more than a few thousand pieces, by more conventional machine- shop processes. Starting materials may be in flat slab or tubular form. They are sometimes called “slab” or “shellcast” bearings. Structural characteristics Sliding bearings are also often classified according to material construction. They can be solid (single-metal), bimetal (two-layer), or trimetal (three-layer) bearings. The terms refer to the number of principal layers used. Each construction is in wide use. Separate layers allow combinations of properties unobtainable with single metals.

Environments & material requirements

In bearing materials design and selection, at least some of these wear damage mechanisms must be considered:
• Surface fatigue wear.
• Abrasive wear.
• Adhesive wear.
• Erosive wear.
• Corrosion.

Figure 1 shows examples of damage these processes cause.

Wear-damage potentials are linked intimately to interactions among system characteristics that include:
• Magnitude of bearing loads.
• Nature of loads (cyclic or steady, unidirectional or reversing).
• Speeds.
• Lubricants and lubrication system characteristics.
• Operating temperature.
• Counterface (shaft) material and finish.
• Alignment and rigidity.
• Life expectancy.

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