GAS SPRING ANATOMY AND FUNCTION

Gas springs, as the industry calls them, consist of a pressure tube (heavy gauge steel cylinder), a piston rod and a piston (hardened steel chromium-plated precision ground and highly polished), and a sealing system. The unit is sealed to the outside atmosphere and contains 250 to 2,500 pounds-per-square-inch (psi) of compressed nitrogen. The energy for the spring effect comes from the compression of the gas sealed in the unit by the piston. When the rod and piston are pushed against the gas inside the sealed cylinder, a spring-like force is generated against the direction of the push.

Gas springs are replacing coil springs as counter balances on pull-down attic stairs. Industry Web sites (attic stair and gas spring, i.e., strut manufacturers) state that these gas springs provide a better, easier, smoother operation with less effort and are not as noisy as coil springs. Obviously, you will find them in new residential construction and in replacement pull-down attic stairs, as it was with the main focus of the case history that follows. They are cheaper topurchase and install than standard springs.

These units are filled with nitrogen and a small amount of lubricating oil. If they were filled with air when the piston rapidly compresses the air, there will be a dramatic rise in temperature and, as such, could ignite the oil; this is much like how a diesel engine operates. Nitrogen is inert and will not combust.

Oil is required to be in the cylinder to keep the seals lubricated and improve the seal life and capacity, hence the functionality of the gas spring assembly. The oil also has a dampening effect on the last few fractions of an inch in the stroke length.

The seals can be made from several materials: nitrile, teflon, rubber, or viton. The seals will wear out over time, ending the useful life of the gas spring. Also, the seals play a key role in the strut/gas spring not being able to withstand the high temperatures of fires.

Gas springs are anchored on each end by a wide variety of end fittings made of everything from stainless steel to nylon to plastic composites reinforced with fiberglass. The nylon and plastic end fittings also play a key role in their dangerousreaction to fire conditions.

USE OF GAS SPRINGS

As previously mentioned, these devices are being used in many areas of residential occupancies. We are familiar with their use on our car hatchbacks, rear SUV windows and, of course on the infamous exploding car bumpers. Their proliferation in residential occupancies is relatively new, especially in attic stair assemblies. They are not designed to be used in any areas where high temperatures can be reached. Gas springs are closed/sealed cylinders with high-pressure gas inside with no relief valve. Manufacturers do not recommend their use in areas that will exceed 150°F, although “high temperature” units are available to withstand 350°F. We know from National Institute of Safety and Technology and Underwriters Laboratories data that a simple room-and-contents fire with adequate ventilation will reach 2,000°F+. Typical temperature ranges for application of gas springs is from -4°F to -150°F because of the nature of the sealed unit, seals, and high pressures of compressed gas.