1) metallic tube or lining, without moving parts, to reduce friction between mechanical parts such as a pulley sheave and an axle; also the nonrotating sheave used to increase the bend radius for the sling attachment (contrast with bearing).
2) a device, often made of rubber or plastic, to reduce vibration or wear between mechanical parts.
3) a fitting used to connect plumbing parts of different diameters (such as in spray equipment).