A gas spring is an energy-storage device similar in function to mechanical coil springs. Mechanical coil springs store energy by straining the material composing the spring. A gas spring stores energy by compressing the nitrogen gas within the gas spring. As a mechanical coil spring is compressed, additional strain is placed within the spring, which adds to the spring’s stored energy. Likewise, as a gas spring is compressed, the gas chamber volume is reduced due to the intrusion of the shaft into the gas spring tube; thereby causing the gas pressure to rise, storing more energy.
The goal is the same with either type of spring; to move or resist the movement of some object. Typically for gas springs, the object to be moved is an engine cover, access panel or even a hospital bed. Gas springs are not limited to just these uses, and in fact can be used in many applications where mechanical springs are applied.
One of the significant differences between mechanical springs and gas springs is the force provided at their free length. Gas springs always require some initial force to begin compression, while mechanical springs have a characteristic known as free length. This is the length of the spring with no force applied. The force required to move the spring begins at zero and increases according to the spring rate. Gas springs in their “free length” require some initial force before any movement takes place. After the full initial force is applied the gas spring will begin to compress. This force can range from 20 to 450 pounds. In mechanical springs this initial force is called pre-load and requires additional hardware to achieve.
Another significant difference is the spring rate. Gas springs can be designed with a very low spring rate utilizing a small package. A similar mechanical spring would require as much as twice the package space.
The ability to have a controlled rate of extension is another major difference. Gas springs can provide a rate of extension (controlled release of the stored energy) that can be set to a prescribed velocity. Mechanical springs do not have this ability. In fact, gas springs can have multiple extension rates within the same gas spring (typically two: one through the majority of the extension stroke, another at the end of the extension stroke to provide dampening).
When designing a gas spring one must allow for between 1% and 2% force loss per year, and understand the force variation due to temperature, we discuss this further in the gas spring product pages.