Basics
1.1 The Basics
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 springs 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 250 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).
A gas spring is typically comprised of the following parts:
| Cylinder | Heavy gauge steel body, painted and cured to a glossy finish. |  |
| Piston Rod | Chromium-plated, hardened steel, precision-ground and highly polished. | |
| Piston Assembly | Self-cleaning design automatically opens during each stroke to keep the piston area free of contaminants. Not offered by all manufacturers. | |
| Sealing System | This is the area where most manufacturers differ in their approach. AVM uses a patented Triple-Lobe Rubber Seal, as well as a Rubber O-Ring Piston Seal. | |
| Seal Backup System | Teflon ring, functions as a backup to the seal system, unique to AVM. Prevents seal wear. | |
| Temperature Compensation | Optional feature, this module provides for an increase in the force when the temperature drops below approximately 40°F enabling the use of lower forces at room temperatures to provide easier closing efforts. | |
| Nitrogen Gas Charge | Gas springs are charged with nitrogen up to 2500 psi. Nitrogen does not react with any of the internal components. The amount of charge varies from 1/3 gram in the smallest springs to about 24 grams in the largest. Nitrogen is inert and is not flammable. | |
| Glycol Fluid | Lubricant for internal components. Also provides dampening to slow down movement of liftgate just prior to full open. This is a high viscosity index synthetic oil with a pour point of -70°F. |
 Figure 1 |
How a Gas Spring Works.
In its simplest form: the compression of the rod/piston into the tube/cylinder reduces the volume of the tube as it compresses. When the cylinder is filled with gas, this constitutes the spring like force or action associated with gas springs.
At first glance many people think a gas spring's force is generated the same way an air cylinder's force is generated. This is not true. An air cylinder creates force by the air pressure inside the cylinder pushing on one side of the piston with a higher pressure than from the opposite side of the piston. This creates a net force difference which is exerted back through the air cylinders shaft. In other words, the internal pressure times piston area equals the output force exerted by the shaft.
 Figure 2 |
Gas springs create force in a different manner. The gas pressure on both sides of the piston are equal. However, there is the small area of the shaft where the internal gas pressure does not exert any pressure. Therefore, the internal presssure times shaft cross-sectional area equals the output force exerted by the shaft.
