GASKET:

Gasket is a mechanical seal between two objects, usually used to prevent the two objects from being subject to pressure, corrosion, and pipeline natural thermal expansion and contraction of leakage. As the machined surface can not be perfect, use the gasket to fill the irregularity.

A beater (usually in the middle) is usually used to distribute the load thread fastener. Other uses are used as spacing, spring ( wave washer), wear pad, display device, lock device. Rubber washers are also used in faucets (valves) to cut off flowing liquids or gases. Rubber or silicon gaskets can also be used to reduce fan vibration. Usually the outer diameter of the gasket is about twice the inner diameter.