Gas Spring Design Guidelinesby Pauline Gill
Gas springs provide a number of benefits in automotive, marine, industrial and domestic springing applications, especially wherever controlled damping is required. Whether displaced in tension or compression, gas springs provide a nearly constant force through their entire stroke as compared to coil or leaf metallic or composite springs. They can also be constructed with special features such as variable damping and blocking. When applied properly, gas springs are reliable and long-lasting alternatives to coil and leaf springs in linear situations.
Gas Spring Principles
A simple gas spring consists of a sealed tubular metallic cylinder with a smooth polished bore, a tight-fitting piston with a metered bore hole, a micro-finished piston rod, a specified amount of oil fill, and a gaseous high-pressure charge, usually nitrogen. Since the actual spring force is provided by the high pressure of the charge gas acting on the difference in areas between the top and the bottom of the piston, the force is basically determined by the high gas pressure acting against the cross-sectional area of the protruding piston rod itself. These rods are usually between 1/4 and 1/2 inch in diameter. So to develop a force of 100 pounds with a piston rod diameter of about 1/3 inch requires a pressure of about 1400 pounds per square inch. Damping is provided by oil and or gas being displaced to the opposite side of the piston through a tiny hole in the piston. When a compressed spring is released, it extends slowly rather than snapping back.
Gas springs can be very long compared to compression coil springs because of the inherent rigidity of the piston-rod-cylinder design. The vast majority of gas springs supplied are compression, meaning they are extended at rest, and the rod is pushed into the cylinder as the spring is compressed. Most gas springs can be compressed to about 60 percent of their extended length.
Adjustable-height office chairs use gas springs to raise or lower the seat, and then provide a lock to keep it at one height, using the control lever. Gas springs are also commonly used in automotive and marine applications to hold up engine hoods, luggage compartments and hatches. They are also used in reclining chairs, positioning fixtures and industrial machines.
Application Design Guidelines
Observe the following design criteria. First, determine the effective stroke of the spring and the desired spring force. The force should always be in a direct line with the centerline of the gas spring to prevent buckling of the slender device. Then, the cylinder and piston rod both need to be protected from hits, dents and dings, damage or abrasion. Finally, special features such as external locking and variable damping should be considered.
Being under extremely high pressure, gas springs should never be opened, cut or bent. Piston rods need to be kept clean, and never sanded, filed or painted.
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