Gas spring

A gas spring, also known as a gas strut or gas damper, is a type of spring that, unlike a typical mechanical spring that relies on elastic deformation, uses compressed gas contained within an enclosed cylinder. They rely on a sliding piston to pneumatically store potential energy and withstand external force applied parallel to the direction of the piston shaft (loosely analogous similarly to a bicycle pump without a gas outlet).

Simplified constant-force pneumatic gas spring with sectional view:

1. Piston rod
2. Head cap
3. Guide bushing
4. Seal
5. Cylinder
6. Piston
7. Flow-restriction orifice

Gas springs are used in automobiles to support hatches, hoods, and covers. They are also used in furniture and doors, as well as in medical beds. They are used industrially in machine tool presses. Fast-acting gas springs are used in aerospace design and weapons applications, and large, extended gas springs are used in passive heave compensators, which stabilize drilling operations against waves.

Gas springs are usually implemented in one of two ways. A pneumatic suspension gas spring directly compresses a chamber of air with the piston. A hydro-pneumatic suspension gas spring instead compresses a chamber of oil linked to an accumulator in which the pressure of the oil compresses the gas. Nitrogen is a common gas in gas springs because it is inert and nonflammable.

Principle of operation

A gas spring consists of a sealed cylinder filled with a charge of high-pressure gas, a piston rod attached to a piston with a sliding seal, and some oil. The piston (or the cylinder wall) contains a number of channels that allow the gas to transfer between the lower chamber (between the piston and the closed end of the cylinder) and the upper chamber (between the piston and the head cap). This causes the pressure in both chambers to equilibrate no matter how far the piston is pushed down the tube.

In the ideal case of a piston moving with zero friction through a fully sealed cylinder, there are two key phenomena which can be thought of as acting simultaneously inside the spring as it is compressed. Firstly, as the spring is compressed and the piston rod is pushed into the cylinder, the effective volume of the cylinder decreases due to the extra space now being occupied by the solid piston rod. This would be the case even if there were no piston attached to the rod, as the only changing factor is the cylindrical volume of the rod itself. Second, the bottom of the piston always experiences a greater force opposite to the compressing action than the top. Even though the compressed gas exerts an equal amount of force on all the inner surfaces of the cylinder and piston rod, the force contribution in the direction of travel is practically zero for the sides of the rod, but nearly total for the bottom of the rod. The addition of a piston to the end of the rod thus does not alter the net forces involved, but instead partially separates the cylinder into two volumes with restricted flow of gas and oil between them, through flow-restriction channels in either the piston or the cylinder wall.

Gas springs whose pistons have fine holes in them for damping are called slow-damper springs and are common on safety gates and doors.

Other details

According to Hooke's Law, 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. Some gas springs have fine holes in the plunger for additional damping: these are called "slow-damper springs" and are common on safety gates and doors. 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.

Variations

A gas spring can be given adjustable push-in force via a local knob or remote via a Bowden wire.

Extended stroke is usually acquired through telescoping mechanisms, composed of one rod and multiple 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 is also possible to make degressive gas springs, where the spring becomes more, not less, powerful as the main cylinder expands.

Consequences

Poor Manufacturing

Gas springs are often used in office chairs, truck tailgates, and wheelchairs, just to name a few. A rare concern with gas springs is that if it is made cheaply, using oxygen as its internal gas, the oxygen will combust given enough force. This is because oxygen is a combustible gas and nitrogen is not. Oxygen is not flammable its self however, it does oxidize the internal parts in high heat. The internal parts of the gas spring, in great tension and heat, then explodes with the oil and grease within the gas spring. A reported incident in 2008 involved an elderly man suffered sever injuries due to a 150mm rod forcefully ejected from the explosion.

There are a few common signs of failure for gas springs. To list a few: Leakage of gasses, failure to support its weight compacity, unusual noises, excessive bouncing, and sagging.

See also

• Shock absorber