Calculate a gas spring — sizing step by step

What determines the design of a gas spring?

The right gas spring is always the interplay of three factors: force, stroke and geometry of the pivot points. Only when all three match do you get a pleasant motion and a safe end position.

In practice the design starts with weight and centre of gravity of the flap to be moved. From that you get the torque around the pivot. From this torque and the geometry between pivot, attachment on the flap and fixed point on the frame, the required spring force is calculated.

Which values do I need to know?

For a reliable design we typically need this information:

• Weight of the flap in kg
• Position of the centre of gravity (ideally with a sketch)
• Position of the pivot
• Open and closed position
• Desired holding force at the end stop
• Installation space and maximum spring dimensions

Force, stroke and safety factor

The calculated extension force is the minimum force to safely hold the flap in the open position. In practice a safety margin of 10–20% is added to compensate for tolerances in geometry, friction and temperature.

The stroke must match the opening motion — it should be about 1–2 mm shorter than the theoretically required stroke so the spring does not run into the mechanical end stop.

Choose pivot points wisely

The choice of pivot points largely determines the force behaviour. If the attachment is far from the pivot, the required force decreases — but the spring needs a larger stroke. Close to the pivot, the required force increases.

It is important that the spring is guided over a so-called dead point when opening, so it safely holds the flap in the end position and does not fall back.

Stobo recommendation

Send us your sketch, flap weight and the key dimensions — we'll calculate the matching gas spring free of charge and recommend stroke, force and end fittings.