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How do gas springs work ?

A gas spring is basically a system consisting of a pressure tube, rod and piston. The energy for the spring is provided by gas at high pressure and the whole system is self contained and sealed against loss.

The force F exerted by the spring arises as a result of the out of balance forces acting on the piston which can be calculated as follow:

F + [P * (A - a)] = P * A

which can be reduced to:

F = P * a

F = Expressed in Newtons (N)
P = Gas pressure in Pascals (Pa) (Newtons per square metre)
a = Rod cross section in square metres (m^2)
A = Piston cross section in square metres (m^2)

Compression ratio - "K Factor"

Because the gas spring is a closed system, as the rod is pushed into its body, the gas has nowhere to go and therefore the gas in the spring compresses and its pressure increases. If P1 is the force of a fully extended gas spring and P2 the force of the same spring when compressed then:

K = P2 / P1

If there was no friction in the gas spring, the graph of the force expressed in function of the stroke would be linear and would look like the following:

The importance of Nitrogen

Compressed air should never come in contact with oil, otherwise the mixture will explode in the same way diesel does when it is injected in a diesel engine.

Nitrogen is a neutral gas that is economical to obtain and use. Because it is neutral and used pure without oxygen, it will not react with oil when put under a very high pressure. It is therefore the best choice for gas springs.

Why should springs be used rod down ?

In order to keep the rod seal lubricated, a small amount of oil is used in every gas spring. When used rod down, the oil is kept in contact with the rod seal which improves the sealing properties and ensure the seal will never become dry.

For this reason, one of the basic recommendations when incorporating gas springs in your design, is to make sure they are used at an angle of less than 60 degrees from the vertical.

The oil used in the gas spring is also useful to obtain a high damping effect on the last few millimeters of the stroke.
If the gas spring was not used rod down, the piston would not have to go through the oil at the end of the extension and no damping effect would occur.

When this is not possible and the gas spring has to be used either rod up or near horizontal, we specify an oil chamber or a fully damped gas spring.
While these solutions are not as economical as standard gas springs, they will ensure your gas springs will last as long as a standard gas springs used with the rod facing down.

Example of a fully damped gas spring:

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