Piston seals are dynamically active on the outside diameter. (View rod seals here)

Single acting:
The single-acting seal is designed to function where pressure is only applied from one side.

Double acting:
A double acting seal has the benefit that it is the most compact way to design a reliable sealing system, when pressure is applied from two sides.

For applications requiring eg. high thermal or chemical resistance, our ASL-seals might be the right solution.