a typical plain bearing is also known as sleeve
bearing, plain 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.