- How a spring works
A gas spring consists of a piston attached to a shaft moving within a sealed cylinder charged with nitrogen. The output forces are the result of the differential between the pressure in the cylinder and the atmospheric pressure outside the cylinder acting on the cross section of the piston/shaft. As the piston/shaft is compressed into the cylinder the internal pressure increases according to the volume of gas displaced by the rod. This increase in force is called the K-Factor. Because they operate on simple pressure differentials, gas springs will perform as well in the vacuum of space as they do on land.
Gas spring force is often designated as P1 which is the force measured 1 inch from full extension. Force is a function of the charge pressure in the cylinder acting on the cross section of the rod. The smaller the diameter of the piston/rod the lower the force at the same pressure. For example, a gas spring with a 9/16 (14mm) rod charged to 1000 psi will have a P1 force of 200 pounds while a spring with a 5/16 (8mm) rod charged to the same pressure will have a P1 force of 65 pounds.Compressed force is referred to as P2. This force is measured .2” (5mm) from full compression. The P2 force will always be greater than the P1 force.
K-factor is the ratio of the compressed force (P2) and the extended force (P1). As governed by Boyle’s Law, P2 force is always greater than the P1 force. During compression, the volume of the piston/shaft introduced in the cylinder displaces an equal volume of gas, increasing the pressure in the cylinder which increase the force of the spring.
- How temperature affects gas spring force.
The force produced by a gas spring varies linearly by .19% for each degree F change from ambient temperature of 70 degree F. For example, a 30 degree change in temperature results in a 5.7% change in spring force (30 x .19% =5.7%).
- Mounting orientation
Shaft down is the preferred orientation for mounting a gas spring. Optimum design permits the support to be oriented shaft down through its entire actuation. To achieve damping or cushion at the end of the stroke, the piston assembly inside the gas spring must travel through the damping fluid and the end of stroke. Additionally the rod seal has maximum life when it is constantly lubricated by the internal fluid.