thetyreman
Established Member
I can also highly recommend herman miller second hand chairs, I got one because I had problems with my back, it's definitely made my life infinitely better.
Office chair gas cylinder....
- Thread starter Cozzer
- Start date
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When Cozzer first wrote about his captains chair i pictured in my mind the chair in The captain and Mrs Muir now that was a chair dont think they had gast cylinders fitted tho (vision blown)
I recently spent a long time getting to understand these gas springs and deciding how to incorporate one into 'My Chair' (see recent posting)
The unit has 3 functions , 1 the free rotation of the chair, 2. the height adjustment and thirdly the slight dampening as your weight goes on to the chair.
The 50mm diam. housing simply holds the plastic bushes that the cylinder turns and goes up and down in. there is no gas or oil in this outer casing. the lower end has the slight taper to form the fixing into the chair base, similar to a Morse taper. The inner functioning cylinder is 28 mm diam. and has a taper at the top to fix to the under side of the seat.
The damping aspect is achieved by a piston in the cylinder filled with a gas at high pressure .So when you sit on the chair the gas is compressed a little bit more.
The clever bit on these cylinders is the height adjustment . The 28mm cylinder is twin walled with a port linking the chamber above and below the piston. When the height adjustment lever is depressed it opens a valve in the port . The gas pressure is uniform throughout, but one side of the piston has an effective area of its diameter but the other side has an effective area of its diameter minus the diameter of the piston rod connected to it; so there is more force acting one side of the piston to the other so the chair rises, with the gas transferring from one side of the piston to the other via the twin wall gap.
The engineering quality to achieve the leak free system must be of such a high standard but yet these springs are available for as little as £10.
So in my situation the 50 OD was too large so I took the inner cylinder out of the outer, machined the plastic bushes thinner and reinstalled in a 38mm tube. .... Straight forward so far ....
The piston rod was also too short but to my surprise I found I could cut a thread on it with a standard die, so I could screw on an extension piece.
The amount of movement for the damping effect to feel right I decided needed to be about 25mm . As the unit I had to use needed to be a short one I found the damping was only about 10mm for my weight, meaning the pressure in the unit must be too high for what I wanted. So I needed to bleed off some of the gas.... after a lot of thinking about it, the solution was simple. At the extreme end of the piston rod I ground a tiny flat. Then using a pillar drill to compress the unit I compressed it to the end of travel so the tiny flat area just passed through the rubber seal, it was really easy to ease it lower and hear the gas gently hiss out just like pressing the middle pin in a car tyre valve. After several tweaks I achieved exactly the amount of dampening movement I wanted. Adding a spacing washer then prevented the piston traveling that far when in use.
There was nothing risky or dangerous in any of these modifications and I ended up with the gas spring I required specific to my requirements.
I'm currently on the lookout for a 2nd hand reproduction Eames lounge chair and ottoman. Needs to be as cheap as poss as i want to saw it into bits.
Tris
What am I doing here?
SGS supply gas struts that allow you to reduce the pressure using an Allen key in a grub screw valve. We modified the roof platform in a Mazda Bongo and needed shorter struts, they were able to supply just the right size and I'diot proof instructions. I wouldn't have wanted to try adapting them myself
Glad you managed to modify your Bongo with readily available adjustable gas struts. But an office chair gas spring lift is not a ‘gas strut’ and is totally different in the way it works as I explained. If you used a gas strut on a chair, depending on its rating it would either not move when you sat on it or slowly sink the whole length of the strut.
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