Bearing Types and Selection

A bearing supports a rotating shaft, carries the load imposed on it, and locates the shaft in position — all while keeping friction low enough that the machine does not destroy itself.

Rolling element bearings

• Deep groove ball — the general-purpose bearing. Moderate radial load, some axial load in both directions, high speed capability, low friction.
• Angular contact ball — designed for combined radial and axial load in one direction. Normally fitted in pairs so that thrust is carried in both directions.
• Cylindrical roller — high radial capacity, little or no axial capacity. Often used as the free bearing that allows the shaft to expand.
• Spherical roller — high load capacity and self-aligning, tolerating shaft deflection and housing misalignment. Common on heavy, slow equipment.
• Tapered roller — carries heavy combined radial and axial load. Used in pairs and set with a defined clearance or preload.
• Thrust bearings — purely axial load, as in a vertical pump.

Plain (journal) bearings

A shaft rotating inside a soft-lined shell, separated by a pressurised film of oil. There is no rolling element and, in normal running, no metal-to-metal contact at all. They carry very high loads, tolerate shock, run quietly and — provided the oil film is never lost — have effectively unlimited life. They are the standard on large turbines, compressors and high-speed machines. Their weakness is that they depend entirely on the lubrication system: start-up, shutdown and any loss of oil pressure are where the damage happens.

Fixed and free

A shaft supported by two bearings must be located axially by one of them. The other must be free to move as the shaft grows with temperature. Fixing both is a common and destructive error: thermal growth then loads the bearings axially until something fails.