A plain bearing, or more commonly sliding contact bearing and slide bearing, is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore, the journal slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a lathe.
Plain bearing on a 1906 S-Motor locomotive showing the axle, bearing, oil supply and oiling pad
A wheelset from a Great Western Railway (GWR) wagon showing a plain, or journal, bearing end
Split bi-material bushings: a metal exterior with an inner plastic coating
Archbar type truck with journal bearings in journal boxes as used on some steam locomotive tenders. A version of the archbar truck was at one time also used on US freight cars
A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or the directions of the loads (forces) applied to the parts.
A ball bearing
Drawing of Leonardo da Vinci (1452–1519) Study of a ball bearing
Early Timken tapered roller bearing with notched rollers