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Tailgate with gas springs
Two gas springs whose force depends on temperature and direction of motion, plus a parcel shelf on a cord. ASOM shows all manual forces for opening and closing in one diagram.
Knowledge · Counterbalancing
Gas spring design
A flap should open easily, stay open and close in a controlled way – in summer and winter alike. This page explains what matters when designing gas springs and how ASOM supports you.
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The task
Spring and weight must match over the entire travel
Tailgates, engine hoods, furniture flaps, lids, seats and beds: in all of them, gas springs counterbalance the weight of a component. Weight and spring act over the entire motion, each with a changing lever arm. Whether the flap is easy to operate therefore only becomes clear over the entire travel, not in a single position.
What a good design achieves
- The flap opens with little manual force.
- It stays securely in the open position.
- It can be closed with comfortable force and does not slam shut.
- This applies in cold and heat and over the entire service life.
- The spring fits the installation space, both compressed and extended.
Basics
How a gas spring behaves
Inside the pressure tube, nitrogen is under pressure. On the piston rod side, the piston area is smaller by the cross-section of the rod, so the pressure pushes the rod out. The filling pressure determines how strongly.
- Characteristic curve: flat and nearly linear. Even when extended, the spring pushes firmly, and somewhat more when compressed. F1 (extended) and F2 (compressed) are measured during extension. The ratio F2/F1 is called the progression.
- Friction: The force is greater during compression than during extension.
- Temperature: The force drops in the cold and rises in the heat. It is usually specified at 20 °C.
- Damping: Oil slows the motion shortly before the end position. For this and for lubricating the seal, the spring is preferably installed with the piston rod pointing down.
- End fittings: Articulated end fittings prevent the spring from binding or being loaded in bending.
Try it out
The mounting points make the difference
The gas spring force acts through its lever arm about the pivot. If its line of action passes close to the pivot, it contributes little to counterbalancing. Just a few centimeters at the mounting point change the entire manual force curve.
Mounting point on the flap
Simplified calculation example with assumed values: tailgate 18 kg, extension force F1 of both gas springs 560 N combined at 20 °C, without friction or tolerances. The spring length is adapted to each mounting point. Without friction, the curve shows the force needed to hold the flap: opening takes a little more upward force, closing a little more downward force.
More than one position
Why a rule of thumb is not enough
A rough calculation for the open position gives a first value for the spring force. In operation, further factors come into play, and only all of them together show whether the flap is easy to operate.
What else matters
- The curve between closed and open, not just the end positions.
- Cold and heat, because the spring force changes with temperature.
- Friction, which acts differently when opening and closing.
- Spring force tolerances.
- Inclination and additional loads, such as a vehicle on a slope or snow on the flap.
With ASOM
Gas springs in conceptual design
You move a mounting point, and ASOM immediately recalculates all forces and torques over the entire motion.
The gas spring as a component
You define lengths, forces and the characteristic curve yourself. ASOM takes temperature, friction for each direction of motion and tolerances into account, including for temperature-compensated gas springs.
All manual forces in one diagram
Opening and closing, summer and winter, vehicle on a slope: ASOM shows each scenario as a separate curve. More on force calculation
Finding the right spring
You specify which holding force should apply in which position. ASOM adjusts the force and lengths of the spring accordingly. You then use these values to check with your supplier whether the spring can be built this way. More on syntheses
Examples
All example videos
Springs and gas springs in ASOM and ASOMmini
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Kitchen cabinet flap
Set the tension springs on a six-bar hinge so that a 2–8 kg flap opens by itself from about 15–20°. ASOMmini shows the manual force curve, and you find the right attachment point in the model.
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Motor-driven car seat
An electric motor moves the seat to all extreme positions. ASOM shows torque and power for different seat occupancies – and instantly reveals what difference a torsion spring makes.
Questions and answers
Frequently asked questions about gas springs
How do I find the right force for a gas spring?
Not from the weight alone. What matters is how weight and spring interact over the entire opening angle. ASOM calculates this for every position and every scenario.
Do I need data for a specific gas spring?
No. In ASOM, you define which spring your mechanism needs: force, characteristic curve, lengths and stroke. You then use these values to ask your supplier whether the spring can be built this way. That way, the spring fits the flap – not the other way around.
Why does the flap no longer stay open in winter?
In the cold, the gas pressure drops and with it the spring force. If the spring is designed only for 20 °C, the force may not be enough in winter to hold the flap open. In summer, on the other hand, the flap is hard to close. In ASOM, you see the manual forces for cold and heat in one diagram and can spot this as early as the design stage.
One or two gas springs?
Wide flaps usually have two gas springs, left and right. The force is shared, and the flap is supported evenly. In ASOM, you calculate both springs together, including springs installed at an angle.
Does ASOM also handle tension, pressure and torsion springs?
Yes. In addition to gas springs, ASOM handles tension, pressure and torsion springs, as well as friction and inertia. For simple flaps with springs, often all you need is ASOMmini, as the kitchen cabinet flap example shows.
Designing a flap?
Tell us about your mechanism. In an online demo, we will show you ASOM using your own example.