In contrast to most other types of springs, gas springs have a built-in pre-tension force and a flat spring characteristic. This means that there is only a small difference in force between full extension and full compression.
As the piston and piston rod are pressed into the cylinder, volume reduces and pressure increases. This causes pushing force to increase. In conventional gas springs, this increase is normally around 30% at full compression. The diagram illustrates, in simple terms, forces F3, F4, F2 and F1 along the stroke when the gas spring is fully compressed and then released. F1 indicates the force just before full extension. It is this static F1 force we refer to when we talk about the force of a gas spring. The difference between force pairs F3/F1 and F4/F2 varies according to the amount of friction.
The pushing spring movement is slow and controlled. It is reliant on the gas flow between the piston sides being allowed to pass through channels in the piston during the stroke. Conventional gas springs use 'hydraulic damping', which involves a small amount of oil slowing down the speed of the stroke immediately before the spring reaches full extension. This gives the movement a braking character at the end position provided that the piston
rod is in the downward direction.