How to choose a gas strut

---

Every gas strut we supply is a Bansbach gas spring, made to order to the force and size the job needs. Bansbach's standard designs run from 7N to 12,000N with a stroke of 10mm to 1000mm, so you're matching numbers rather than picking a model off a shelf.

The three Bansbach examples in the Gas Springs collection show what a finished part looks like: a 110N strut with a clevis, a 300N damped strut with an elbow joint and a 370N spring with a hinge eye. The diagrams further down are those same three, and all three are made to order.

There are five figures that define a strut: the force in newtons, the stroke in mm, the extended length in mm, the rod and tube diameter, and the fitting at each end.

1.What a gas strut is, and the types

A gas strut is a sealed cylinder of nitrogen that pushes the piston rod out. Bansbach fills its gas springs with pure nitrogen at up to about 300 bar, depending on the series and the force, and the gas is inert, doesn't burn and isn't toxic. There's oil inside too, and the oil's viscosity and the nozzle bore set a strut's extension speed and damping. End damping is the strut slowing itself at the end of its travel, and you can have normal, increased or none, and a fast, normal or slow extension speed. A standard strut works at ambient temperatures from -30°C to +80°C, and outside that it needs special seals, which take it down to -55°C or up to +200°C.

2.The five things that define it

To make a strut, we need five numbers. Get these and the rest follows.

The one that trips people up is extended length. It's the fully-open, centre-to-centre (eye-to-eye) measurement, and it equals the closed length plus the stroke. A strut that closes to 400 mm with a 300 mm stroke opens to 700 mm.

3.Replacing one? How to measure it

To replace a strut, read the label on the old one and take four measurements off it.

1. Read the body first. Bansbach labels every gas spring with a high-pressure warning, the part number and the production date. On a Bansbach part code the force can sit at the end, after the slash, so C5A1B42-290-689--001/370N is a 370N strut. Not every brand marks the force that way. If the part number is readable, send it, because it's the quickest match.
2. Extended length, eye to eye. Open it fully and measure centre-to-centre between the two mounting points, not tip to tip.
3. Stroke. Stroke is the extended length minus the fully closed length, both measured centre to centre. So measure the strut closed as well as extended, then take one from the other.
4. Diameters. Callipers on the shiny rod and on the painted tube.
5. End fittings. Note what's on each end, and the thread if you want to reuse them.

Send those measurements through the form below, and we'll match it.

4.Getting the force right

On a new job, force is the figure that's hardest to get right. Too little and the lid won't stay up; too much and it's hard to pull shut. As well as the weight, it depends on where the lid's centre of gravity sits, where the strut mounts relative to the hinge and how many struts share the lid. Those are the same numbers Bansbach asks for when it sizes a gas spring.

For a top-hinged lid, the estimate per strut is F = (W × L) ÷ (n × d). F is the force per strut in newtons. W is the lid's weight in newtons, which is its mass in kg times 9.81. L is the horizontal distance from the hinge to the lid's centre of gravity. Then d is the perpendicular distance from the hinge to the strut's line of action (the line the strut pushes along). That isn't the distance along the lid to the mount, because the strut's angle matters. And n is the number of struts, so the formula already shares the load between them; don't halve the answer again. Take a 25kg lid, which weighs 245N, with its centre of gravity 400mm from the hinge, d at 100mm and two struts, and each strut needs about 490N. That balances the lid at one position only. Bansbach's sizing also takes the closed and open angles, so treat the result as a starting figure.

If you'd rather not work it through, send the lid's weight, where its centre of gravity sits, where it hinges and where a strut could mount, how many struts, and the closed and open angles through the sizing form below.

5.Fitting it: rod down

Fit the strut so it sits rod down, because that's the position that gives the best damping. Bansbach gas springs have a built-in grease chamber, so they can still be fitted in other directions.

Bansbach says a gas strut shouldn't take tilting or side loads in use. Where the mounting can't avoid them, check the installation and use connecting parts that suit. For a permanent load, or where the strut itself would take high forces at the end of its stroke, Bansbach recommends an additional mechanical limit stop. That's a fixed stop on the lid or frame, separate from the strut. Some connectors, elbow joints among them, may not suit high stop forces.

Protect the piston rod from knocks, scratches and paint, because damage to its surface destroys the seals, and don't let the cylinder get dented either. The strut is maintenance-free, so don't grease or oil the rod.

6.Common jobs

Bansbach builds gas springs for vehicles, furniture and machinery, and the option worth asking for changes with the job.

• Ute canopies, toolboxes and anything in the dirt - a scraper for rough dirt, or a cover tube for knocks and scratches.
• Trailers, tippers and heavy ramps or doors - a thicker rod and tube, because Bansbach advises against a small diameter on a long stroke at high force.
• Boats and anything outdoors - 316L stainless, which Bansbach lists for outdoor use.
• Food vans, food processing and washdown - the same 316L stainless, with food-grade (FDA) oil as an option.
• Cabinets, hatches and doors that should open gently - slow extension speed or increased end damping, as on the 300N damped strut.
• Anything that has to hold part-way, like adjustable seating or a console - a lockable gas spring, or one with increased friction.

One to keep in mind: nitrogen pressure moves with temperature, about 3.3% for every 10°C from a 20°C base, so a strut set for a cold Melbourne morning pushes a bit harder in a Pilbara summer. We spec to your normal operating conditions, so tell us if it lives somewhere extreme.

At end of life a gas strut is still charged, at up to about 300 bar, so it has to be depressurised before it goes for scrap. The same goes for one that's been damaged, with a bent rod or a dented cylinder, before anyone takes it apart. Follow Bansbach's disposal instructions. Keep everyone out of line with the extended rod, don't cut the ends and never put the strut in a lathe. Wear eye and face protection and cover the drill area before you start, because oil and shavings can spurt out. Then drill the extended strut slowly with a 2mm to 3mm bit at the point Bansbach's instructions show for that size. Move the rod slightly to check there's no pressure left, drain the oil and dispose of it separately, and the empty strut goes with iron scrap.

The form below needs just your name and your email.