Plain bearing

A plain bearing is a bearing (mechanical) which carries load by sliding. A plain bearings is often called a bushing or a babbit or journal bearing. Plain bearings are very widely used and appear in most kinds of equipment.

A typical plain bearing is made of two parts. For example a rotary plain bearing can be just a shaft running through a hole. 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 have the advantage over most other bearing types that they are simple and thus inexpensive. They are also often compact,light weight, and easier to repair without specialized equipment. Plain bearings can have high load-carrying capacity for their size and cost, although sometimes these have rapid wear. Plain bearings have the disadvantages of lower accuracy, faster wear, and higher friction than most other kinds of bearings.

A common plain bearing design is to use a hardened and polished steel shaft and a soft bronze bushing. The general idea is the bronze portion wears away and is periodically renewed. The actual operation of such a bearing, however, is more complex. For example, good lubrication will carry away steel shaft wear debris, but some gets embedded in the bronze, leading to steel-on-steel rubbing and faster wear.

Sometimes the bronze is made porous and a lubricant embedded in the pores. The result is a self-lubricating bearing. As bearing operates and lubricant is displaced from the bearing surface, more is carried in from non-wear parts of the bearing.

Plain bearings are simple in idea, and some plain bearings are of very simple construction. However, some plain bearings are very sophisticated. For example, a shaft may be one part of a bearing, and the design and materials of the shaft may be very complex. Some plain bearings more closely resemble fluid bearings during normal operation and do the majority of their rubbing at equipment startup and shut-down. Some plain bearings are ceramic rubbing on ceramic without oil or grease; the ceramic is very hard, and sand and other grit which enter the bearing are simply ground to a fine powder which lubricates the bearing.