plain bearing

a typical plain bearing is also known as journal bearing, sleeve bearing or babbitt bearing and is made of two parts. A rotary plain bearing can be as simple as a shaft running through a hole, or a simple linear bearing can be a pair of flat surfaces designed to allow motion, for example a drawer and the slides it rests on.

Plain bearings may carry load in one of several ways depending on their operating conditions, load, relative surface speed (shaft to journal), clearance within the bearing, quality and quantity of lubricant and temperature (affecting lubricant viscosity). If full-film conditions apply, the bearing's load is carried solely by a film of fluid lubricant (hydrostatic wedge), there being no contact between the two bearing surfaces. In this condition they are known as fluid bearings. In mix or boundary conditions, load is carried partly by direct surface contacts and partly by a film forming between the two. In a dry condition, the full load is carried by surface to surface contact.

A common plain bearing design is to use a hardened and polished steel shaft and a softer sacrificial layering such as tin (Sn) - antimony (Sb); tin (Sn) - aluminum (Al) or bronze bushing. Originally, the lead-tin metallurgy was specifically babbitting.  However, today babbitting is used in such designs the softer bronze portion can be allowed to wear away, to be periodically renewed.

Plain 'self-lubricating' bearings utilize porous journals within which a lubricant is held. As the bearing operates and lubricant is displaced from the bearing surface, more is carried in from non-wear parts of the bearing. Dry plain bearings can be made of a variety of materials including PTFE (Teflon), graphite, graphite/metal (Graphalloy) and ceramic. The ceramic is very hard, and sand and other grit which enter the bearing are simply ground to a fine powder which does not inhibit the operation of the bearing.