Plain bearings are the simplest form of bearing available as they have no moving parts. They are, quite often, simply a cylinder, though the design of the bearing does differ depending on the intended motion. The three designs include: journal, linear and thrust.
Journal style bearings are designed to support radial motion where a shaft rotates within the bearing. Linear bearings are often used in applications requiring slide plates, as these bearings are designed to permit motion in, as their name suggests, a linear motion. Finally, a plain thrust bearing is designed to do the same job as its roller bearing counterpart, but instead of using cone shaped rolling elements, the bearing uses pads arranged around in a circle around the cylinder. These pads create wedge-shaped regions of oil inside the bearing between the pads and a rotating disk, which supports applied thrust and eliminates metal-on-metal contact.
Out of all the bearing types available, plain bearings tend to the be the least expensive. They can be made from a variety of materials including bronze, graphite and plastics such as Nylon, PTFE and polyacetal. Improvements in material characteristics has made plastic plain bearings increasingly popular in recent years. Plain bearings of all types, however, are lightweight, compact and can carry a substantial load.
As far as lubrication is concerned, some plain bearings require outside lubrication while others are self lubricating. Plain bearings made of bronze or polyacetal, for example, contain lubricant within the walls of the bearing, but require some outside lubrication to maximize performance. For other plain bearings, the material itself acts as the lubricant. Such is the case with bearings made from PTFE or metalized graphite.