A sleeve bearing is a kind of cylindrical bearing. It got its name from having a single internal rotating cylinder inside it. Sleeve bearings are porous, so they draw up the oil applied on the outer sleeve.
Sleeve bearings are a kind of plain bearing--that is, bearings with few moving parts. Many spherical ball bearings have an internal ring, which is lined with smaller balls inside. In contrast to regular ball bearings, a sleeve bearing only has two moving parts; the outer sleeve and the inner rotating cylinder. They are also known as journal bearings, after the technical term for the outer sleeve. The outer journey of a sleeve bearing may be whole, split, or clenched between the two halves.
Sleeve bearings can be made of compressed powdered metal, such as bronze or copper. Because of the material from which they are made, the metal is microscopically porous. When they are oiled on the outside, the oil will be drawn up through the pores to lubricate the inner cylinder.
A sleeve bearing can be lubricated in a number of ways besides oiling. Sometimes, molten metal or graphite is used. Some man-made polymers can lubricate moving parts without seizing up in extremely cold temperatures. Other sleeve bearings are surfaced with porous, oiled hardwood so that the oil will be drawn up into them more readily.
Though they are self-lubricating, sleeve bearings often fail through lack of lubrication. The inner bearing might wear away at the sleeve until the space is no longer perfectly cylindrical. This may cause the bearing to shake when it moves, adversely affecting the movement of the mechanism. Other times, there may not be enough lubricant, or the lubricant may become viscous through adverse temperature conditions. When there is not enough lubrication, the bearing will cease to move.
Due to these issues, sleeve bearings are often elaborately shielded from dust and dirt by seals. A designer or mechanic needs to carefully consider where a sleeve bearing will be located in a machine before using it. They are criticized for being more finicky than ball bearings in that inadequate lubricant will cause them to come to a complete stop rather than wear down gradually over time.
A sleeve bearing is an integral part of many machines used in everyday life. Automobiles, home appliances, fans and office machinery all may use sleeve bearings.