A mechanical assembly for sealing the point where a rotating part (usually a shaft) passes through a wall (mostly a machine housing). Characteristically, mechanical seals have two faces sliding one on top of the other – usually at right angles to the axis of rotation – which are pressed together by axial forces. Between the sliding faces there is a liquid or gaseous lubrication film.

Each mechanical seal contains five basic elements: 

• a ring (mostly the seal face) that circulates in synchronization with the rotating part (shaft)
• a secondary seal between these parts
• a stationary ring (usually the stationary seat) in the stationary part (housing)
• a secondary seal between these parts
• a spring backing for the seal face.