# Length-adjustable gas springs

I was uncertain wether to put this in the general physics forum, but I decided against it.

How do length adjustable gas springs(like for example in office chairs)work ?
I know how gas springs work and i even have an idea how length adjustable ones could work.

Since you hear a "gas flowing sound" when when you pull the lever on the chair and since the whole apperatus is pressurized and so an airflow out of the spring would be irreversible, I assume there is no air exchange between the inside and outside.
It has to work as a closed system.

My Idea:
If you could close the piston orifice in the upper picture, the gas spring would stop having a roughly constant force to extension profile and would behave like a normal mechanical spring(following an approximately linear curve with its equillibrium position on the locking position)
The gas springs could additionally apply forces much higher than in their "free" states.
That behaviour is necessary in chairs.You want the chair to be able to hold you up in its locked position but move down with you when you try to adjust the height.
There are however a few problems with this.
The spring hardness would be higher if you "locked" the spring in one of the more extreme (very short or long) positions due to the smaller volumes that get encompassed on one of the sides.

## Answers and Replies

Tom.G
Gold Member
I just tried bouncing on my chair at various set heights. It feels like there is a coil spring in the righthand chamber of your drawing that is never fully compressed. I suspect the piston orifice doesn't exist and there is an external, valve controlled, channel that connects the two chambers when you pull the lever.

Maybe someone else here knows how they are actually built (or has been foolish enough to try opening one).

I just tried bouncing on my chair at various set heights. It feels like there is a coil spring in the righthand chamber of your drawing that is never fully compressed. I suspect the piston orifice doesn't exist and there is an external, valve controlled, channel that connects the two chambers when you pull the lever.

Maybe someone else here knows how they are actually built (or has been foolish enough to try opening one).
That sounds scary. I did a little back of the envelope calculation.
I assume the gas spring in my chair has about 300 Newtons of force in the free mode.(I can just barely push it down with one foot on it)
And the rod diameter is 2.5 cm give or take a millimeter.So the necessary pressure to supply such a force on that small an area is roughly 5 atm
or 4800 hPa. How in the world would you vent that ?(I know there is a cap on the bottom of the spring but I don't want to imagine what happens when you pull it

I also thought about and "external gas channel".It could work like this. Then the "spring hardness" would be smaller because of the larger volumes and would vary less with the "extension of locking".

PS:I am quite sure there is no spring in there, it just feels like it.

I found this picture on the internet. Maybe you can figure it out ?

jack action
Gold Member
From http://www.explainthatstuff.com/gassprings.html:
For a gas-lift office chair, you need the spring to provide a little bit more force than the weight of the seat. In most chairs, the spring doesn't actually support the person's weight. Instead, it typically has a lever attached that grips and locks at a certain height, preventing the seat from moving up or down any further. The spring is simply designed to let the seat move up and down gently without your having to supply much force.

That is nice and all but I am specifically interested in how the locking works.
It probably is still the gas spring wich holds you up, not some secondary mechanism.
The "gripping and locking" sounds weird. That wouldn't explain the springlike and "bouncy" behavior of the locked spring.
There clearly is an equilibrium position around wich you get restoring forces.The locking is not just frictional!
And "you need the spring to provide a little bit more force than the weight..." is not how the designers of my chair did it.
A small child would have no way of shortening the gas spring in my chair. The extra force in its free state is easily a third or even half my weight.(about 60 kg)

I assume that isn't just a flaw of design. The higher pressure inside the gas spring would produce a higher spring hardness in its locked state if the locking works anything like I suspect.

jack action
Gold Member
Wow, I really appreciate that you found that diagram.
I at least got the basic principle right, but their design is somewhat more elegant.
I really like how the "venting" is done.It makes me appreciate my chair every time I hear the sound it makes when adjusting the height. :)
But it looks like this design would have the problem I came up with:
The spring stiffness would not be the same for all locking positions.
I tried bouncing on the chair a few times at different heights and that behavior seems to actually happen.
Maybe I am just imagining things

Thanks again