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
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
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
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.