What’s in a gas spring?

Apr 13, 2012 | Springs & shock absorbers

Gas springs offer many benefits and are therefore used in applications we see every day. But how are they constructed and what features do they offer to design engineers? Phil Burge, communication manager at SKF, explains

Gas springs are used in applications we see every day – for a car boot or hatchback opening, on the guarding of machine tools, in office chairs, and in engine covers on commercial and agricultural vehicles. But how is a typical gas spring constructed and what are the benefits of these devices for design engineers?

Gas springs are generally used as a counterbalance, for force assistance, as an end-stop or to provide damping.  In each case the basic construction is the same, being based on the potential energy of a compressed gas. These are closed systems, manufactured from a pressure tube normally filled with nitrogen and containing a piston and piston rod assembly. The rod exits the pressure tube through an axial locating guide and seal unit, which prevents both the escape of gas and the ingress of contamination. The piston incorporates a specially designed nozzle that controls the flow of gas to either end of the pressure tube, as the piston assembly moves in and out.

As the piston rod is pushed into the tube the gas is compressed. This increases the gas pressure – or the resistance to the force applied to the piston rod – with the piston nozzle providing a gradual and controlled balancing of gas as the rod depresses, with flow resistance being dependent on actuation speed. In effect, the compressed gas acts as a spring.

A self-contained solution

This technology offers a number of benefits, including being compact and self-contained, making a gas spring easy to manufacture and install, with almost no maintenance required. In addition, the spring rate or force generally remains constant throughout each stroke and is therefore predictable. It is also easy to apply damped motion control, which can be defined at specific positions over the full stroke length. Finally, features such as piston position locking and end stops can be incorporated during manufacture to extend the functionality of gas springs still further.

The spring force for an individual device is based on the filling pressure, the force ratio and the cross sectional dimensions of the pressure tube and piston. This calculation can, however, be affected by other factors, including the ambient temperature, which can cause the gas volume to change, and the effect of friction between the piston sealing ring and tube, and in the flow of gas through the piston nozzle.

In practice, these calculations will be performed by the manufacturer, which generally provides datasheets and technical support to help designers select the best variant for each application. This can often be considerable – for example, in automotive applications in excess of 30,000 full stroke cycles is common, with only minimal loss in spring force due to normal wear and permeability of seal materials.

Adapting to suit demand

The basic gas spring configuration has over the years been adapted to offer designers a wide range of options. In applications such as vehicle tailgates, for example, where the spring needs only to have two end positions but has to provide force support with controlled extension speed or damping, an oil chamber is incorporated at the base of the pressure tube to increase the damping effect over the final part of the piston stroke and lubricate the piston rod seal. These gas springs also use labyrinth pistons to produce a predetermined flow for controlling extension and retraction speeds, and enhance the damping effect still further as the piston moves from the gas to the oil phase. Early versions of these gas springs could only be installed with the piston rod pointing downwards to prevent oil draining away from the base of the seal. The latest devices, however, have either a double sealed chamber, for springs that move vertically, or use a specially modified oil chamber that allows the spring to move in any orientation.

Where springs have to be locked in place an internal cam sleeve is used within the pressure tube.  This works on a similar principle to a ballpoint pen, with the cam unit locking the piston in place and being released by a compression load. Similarly, when variable adjustment of stroke position is required, a pressure valve and coil spring located at the back of the piston prevents gas exchange, securing the piston in place. A refinement of this system, for applications with pre-set piston positions, features a user-operated switch on the piston rod, which opens and closes a valve in the piston; a groove machined in the pressure tube over only part of the stroke length ensures that when the switch is closed the piston rod can only extend to a predetermined point.

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