The spring is metal bellows filled with compressed air used as an alternative to the metal wire springs used to close valves in high flexibility. Pneumatic valve technology replaced steel springs with light weight compressed air bellows. Pressurized air (nitrogen) is pumped into this cylinder which then causes the piston/retainer to rise to the top of cylinder, causing the valve to form an airtight seal with the seat. It is basically a column of air confined within a rubber-and-fabric container shaped like a bellows. The spring action results from the compression and expansion of the air. When used on tapping arm, spring is fitted between both ends of arm and retainer, this air springs can keep the up-arm at a constant standing height regardless of load, so that when the user is position arm at a required position, pneumatic spring ensures that the arm will not fall down.
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