Gas spring

A gas spring is a type of spring that, unlike a typical metal spring, uses a compressed gas, contained in a cylinder and compressed by a piston, to exert a force. Common applications include automobiles (where they are incorporated into the design of struts that support the weight of hatchback doors while they are open) and office chairs. They are also used in furniture, medical and aerospace applications. Much larger gas springs are found in machines that are used in industrial manufacturing (the press tooling industry), where the forces they are required to exert often range from 2500N to 400,000N (Forty tonnes). To properly use gas springs in even the simplest application, users should understand how they work and how a gas spring can be modified to meet specific operating requirements.

How Gas Springs Work

"In physics terms, Force = Pressure x Area".

Gas springs work by building up pressure inside the area of the cylinder encasing the spring - making the internal pressure much greater than that of its surroundings.

Therefore, Pressure x Area = an outward force which encourages the gas spring to expand.

Types

Gas springs are manufactured in many varieties including:

Standard cylinder
Fixed-height cylinder
Spindle only
Cable cylinder
Stage cylinder
Non-rotating cylinder
Return cylinder (no height adjustment)
Auto-return cylinder with height adjustment
Bouncing cylinder
Dual-mode cylinder
Heavy duty cylinder

Common applications

If the internal plunger has a diaphragm which extends to the side of the gas tube, it will cease to move once an applied force is constant and will support a weight, like a normal spring. If a fine hole exists in the plunger, however, it is termed a "slow-dampened spring" and can be used on heavy doors and windows. A gas spring designed for fast operation is termed a "quick gas spring" and is used in the manufacture of air guns and recoil buffers. Reducing the gas volume and hence increasing its internal pressure by means of a movable end stop, or allowing one tube to slide over another, can allow the characteristics of a gas spring to be adjusted during operation. The rod may be hollow by use of clever seals or may consist of multiple small-diameter rods. A small amount of oil is normally present. The gas may be introduced by a Schrader-type valve, using a lip seal around the rod and forcing it to allow gas in by external overpressure or a shuttling O-ring system. Gas springs of high pressure contain a very large amount of energy and can be used as a power pack. In emergency use the gas may be introduced via a gas generator cell, similar to those used in airbags. Gas springs are used to operate the main valve on Formula 1 racing cars. Passive heave compensators features large gas springs. The stroke lengths can exceed 6 meter.

History

It was in 1962 that the company STABILUS was the world's first to standard the production of gas springs.

In the same year, the danish furniture designer Vermund Larsen got the innovative idea to use a gas spring, on an office chair. This was the world's first use of a gas spring on a chair.

Features

A variety of features are available from various manufacturers:

Adjustable push-in force via a local knob or remote via a bowden wire.
Single touch release to allow full extension or the ability to lock it out.
Extended stroke through telescoping mechanisms, usually composed of one rod and two cylinders (the smaller of the two cylinders actually acts as a second rod extending in and out of the larger cylinder).
Vari-lift - intended for short production runs and prototypes, in applications where the exact force is important but hard to estimate in advance, such as lifting a lid slowly in a known time. In this case the cylinder is supplied filled to maximum design pressure, but equipped with a bleed port, to allow gas to be released once installed. The intention is that the design can be over-sprung, and then the pressure reduced in stages, to optimise behaviour. If too much gas is released, a new spring must be installed, and for large quantities, it is better to order a bespoke pressure setting from the maker.

References