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Gas strut forces and spring rates

Static force (P1) and spring rates and how to calculate.

Forces and Spring Rates

Static Force (P1)

A gas spring is defined by its force output in the extending direction; this is referred to as the P1 force.

The P1 force is the static force measurement taken 5mm from full extension in the extending direction, the units of measurement are Newtons. This is the industry standard measurement for force output.

Force output is the result of the charge pressure acting on the cross-sectional area of the rod, mathematically this is described by the equation;


As a result, the larger the diameter of the rod, and therefore rod area, the greater the force output will be for the same charge pressure.

Spring Rates

The force output of a spring is greater in its compressed state than in its extended state.

whitepaper 1.1 graph 300x238 - Gas Strut Forces and Spring Rates

The reason being that a gas spring is a sealed unit and the available gas volume is reduced as it’s displaced by the volume of the rod as it’s inserted. As a consequence, the internal pressure (and therefore force output) increases the further the rod is pushed into the tube. This is in accordance with Boyle’s Law, a basic law of chemistry, which states that at a fixed temperature, the volume of gas is inversely proportional to the pressure exerted by the gas.

The rate of climb in force as the rod is inserted is described as the force ratio, spring rate, progression rate, P2/P1 ratio or ‘k’ factor.

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