Tolerance for pneumatic cylinder and piston

[etherist]

What is the tolerance for pneumatic cylinder and piston operating about 2 psi, at room temperature, so that the piston can move freely without leakage? The piston diameter is 20 mm.

 

[Q_Goest]

Hi etherist, A piston won't seal against a cylinder simply by being close fitting. There will always be a gap between the piston and cylinder which allows some leakage. If there isn't, the piston will rub on the cylinder, create heat, wear, etc...

Seals function because the contact stress between the sealing faces is higher than the pressure differential, so without this contact stress, you won't form a seal. You can limit the leakage and that leakage can be calculated by modeling the gap between your piston and cylinder but it will leak nevertheless as long as there's a gap.

 

[etherist]

Thanks Q_Guest, is it i need a very close tolerance so the i can lessen the leakage? but what is the of it in friction? Is the friction depends on the gap? I wanted to produce an air spring, which if i applied a blow it will vibrate freely.

 

[Baluncore]

There will be thermal variations due to materials. You must allow for that.
Depending on speed and lubrication, it will need a seal such as an O-ring in a big groove, that will be pushed against the piston cylinder gap by pressure.