Gas spring

A gas spring, also known as a gas strut, is a type of spring that, unlike a typical mechanical spring that relies on elastic deformation, uses compressed gas contained within an enclosed cylinder. This cylinder is sealed by a sliding piston to pneumatically store potential energy and withstand external force applied parallel to the direction of the piston shaft.

Gas springs are used in automobiles to make struts that can support the weight of hatchbacks, tailgates and hoods. They are also used in furniture and doors, as well as in some medical applications and aerospace applications. Larger gas springs are found in machines that are used in industrial manufacturing, specifically in the press tooling industry, where the forces they can exert can range from 2500 N to 400,000 N (forty tonnes).

Gas springs rely solely on pressure to function, and this results in a smooth extending and retracting motion.

Forms
If the internal plunger features a diaphragm that extends to the side of the gas tube, it will stop moving once the applied force becomes 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(s) is termed a "quick gas spring" and is used in the manufacture of air guns and recoil buffers.

It is possible to reduce the gas volume and increase its internal pressure by means of a movable end stop, or by allowing one tube to slide over another, allowing the characteristics of a gas spring to be adjusted during operation. The rod may be hollow by use of clever seals, or it 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 with high caliber 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. Passive heave compensators feature large gas springs, and their stroke lengths can exceed 6 meters.

Variations
A gas spring can be given adjustable push-in force via a local knob or remote via a Bowden wire. Extended stroke is sometimes acquired through telescoping mechanisms, usually composed of one rod and two cylinders, where the smaller of the two cylinders actually acts as a second rod extending in and out of the larger cylinder.

There are also techniques to make variable-lift gas springs. These are intended for short production runs and prototypes, and 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 is reduced in stages to optimize behavior. If too much gas is released, a new spring must be installed.

It's also possible to make degressive gas springs, where the spring becomes more, not less, powerful as the main cylinder expands.