# Gas spring

A gas spring (also known as a gas spring for short ) is a pneumatic spring that uses high pressure gas to provide the spring force. Advantages over coil springs are the force almost independent of the spring travel, the small space requirement and the possibility of integrating a damping mechanism into the spring.

3D representation of a gas spring in section:
1) Plunger piston
3) Plunger piston wiper
4)
Guide bush 5 ) Support ring (to avoid gap extrusion for the O-ring (6))
6) O-ring (seal)
7) Plunger piston seal Shaft sealing ring (RWDR)
8) cylinder
9) damping
piston 10) opening (in piston (9))
11) guide bush
12) valve
13) screw plug

## Applications

Gas pressure springs are often used for weight compensation and can be found, for example, in office chairs , as a supporting opening and holding device for flaps in vehicles ( trunk flaps , engine hoods ) or in the luggage flaps in the passenger compartment of aircraft. In tool making, gas pressure springs replace conventional steel pressure springs due to their high power density.

Under no circumstances should they be opened by laypeople, as there is very high pressure inside.

## Function and design

The gas pressure spring is a hydropneumatic adjustment element and consists of the pressure tube and the piston as well as suitable connections. It is filled with a compressed gas, the amount of which determines the preset internal pressure. The piston fulfills two tasks: The longer part has the smaller diameter and runs in a seal on the pressure tube (plunger piston) The pressure acts on its cross-sectional area. This results in a force in the direction of extension. The larger part of the piston with its overflow opening is used for damping and guidance. The extension force can be precisely determined within physical limits through the appropriate choice of the filling pressure and the diameter of the pressure tube and plunger.

Gas pressure springs always consist of a cylinder and a piston that moves in it . More complex constructions are provided with a separating piston that separates two spaces filled with gas and oil . Then the damping piston runs in oil.

In the simplest and most frequently used design, the entire interior is  filled with gas - mostly nitrogen . The damping piston has a small opening through which the gas can flow and whose cross-section determines the degree of damping. The component also contains a small amount of oil, which is used for damping and lubrication. The seal against gas loss is located on the guide of the plunger.

The penetration of the piston rod into the interior reduces the volume there (with the piston rod extended) . The damping piston (because of the overflow opening 10) does not contribute to the change in volume. Because no gas can escape, the amount of substance in the gas remains constant. ${\ displaystyle V_ {from}}$${\ displaystyle V_ {a}}$

The operating situation must be taken into account when calculating the forces or pressures. In principle, both compression and rebound can be viewed as an adiabatic change in state .

In the case of occasional actuation, for example on the hoods of motor vehicles, the compression is adiabatic, but because the spring cools down, the output pressure for the adiabatic rebound must first be calculated from the ideal gas equation . This pressure is lower because the internal pressure has also decreased as the spring cools down.

The pressure after an adiabatic change of state from state 1 (extended or retracted) to state 2 (reversed to state 1) is:

${\ displaystyle p_ {2} = p_ {1} \ cdot {\ bigg (} {\ frac {V_ {1}} {V_ {2}}} {\ bigg)} ^ {\ kappa}}$,

where the isentropic exponent κ for diatomic gases (air, nitrogen and others) is 1.4 and for noble gas (monatomic) it is 1.6.

If the gas can cool down, the internal pressure drops and can be calculated from the ideal gas equation (changes in ambient temperature are neglected):

${\ displaystyle p_ {on} = p_ {off} \ cdot {\ frac {V_ {off}} {V_ {off} -V_ {on}}}}$,

Because of the high friction in the seals, the real restoring force is smaller than the pressure in the gas spring. In addition, elements for end position damping can also be present.