A gas spring is a hydro-pneumatic device, uses a compressed gas, contained in a cylinder and compressed by a plunger and piston rod. The amount of gas determined the exerted force.
The diameter of the plunger works into the pressure of the gas and applies a force in the push our pull direction. This force can be determent inside the physic borders by the choice of the right pressure
At the most used type of spring the whole cylinder is filled with Nitrogen. The plunger has a fine hole where the gas can flow true, these and the diameter of the piston rod determent the amount of damping.
A small amount of oil is normally present for end damping and lubrication.
If the rod with plunger is pressed in, the cylinder, the volume inside the cylinder is decreasing. Because no gas can escape the amount of gas stays the same. The result of this is a increasing of pressure. (Progression)
The restoring force results from different pressure on both sides oft he plunger.
The effective cross-section of the gas on the side of the piston rod is reduced by the cross-sectional area of ??the piston rod. This results in an initial return of the piston rod, together with the plunger in the initial position.