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Axel
07-24-2003, 10:50 AM
I'm in Sweden so exuse my English, I think you call them "Gas struts", they are springs powered by air. We see them every day, and they are used in very car on the planet, I think. A very sucessful invetion!

But what is the prime reason for the small dia shaft being crome plated? Is it for wear, dirt prevetion, friction?

How round would the sliding, small diameter shaft, need to be to prevevent pressure loss? (My guess is that the large diameter part seals on the smaller shaft outside diameter)

I like to belive that the faster they move the better roundness has to be? But how good? Is "lathe turned round" ok?

Any experinces?

Evan
07-24-2003, 11:12 AM
Parts like that will most likely be produced by centreless grinding, both for accuracy of finish and for production speed. It would be very difficult to produce a part like that on a lathe considering the small diameter and long length. The part would spring from the tool and produce an uneven diameter ensuring a leak. I am guessing, but I imagine that consistent diameter is more important than absolute roundness.

Forrest Addy
07-24-2003, 11:21 AM
Hey there, Axel.

I've long admired gas struts myself for their low cost, utility, simplicity, etc.

The rod is chrome plated for hardness and resistance to abrasion. The seal is some phenominal elastomer like polyurethane, maybe. I understand gas struts are under hundreds of pounds internal pressure yet they somehow hold pressure for years with a sliding seal with a life of thousands of cycles.

It would be difficult to make a gas strut but with perseverance it could be done with the right materials and processes. Unless the challenge of making a gas strut is irresistable I suggest you purchase what you need. Gas struts come in an incredible range of sizes, capacities, and service suitability from arctic to furnace heat.

Here in the US we have an on line industrial catalog called Thomas Register. I'm sure that Sweden with its extensive and efficient industrial base has something similar.

Here's a link:

http://www.avminc.com/technical/1.1.html

[This message has been edited by Forrest Addy (edited 07-24-2003).]

Axel
07-24-2003, 12:11 PM
Wow! I've heard of TR but never knew what it realy was. Many thanks!

Well thing is I need to make something similar to a gas-spring...and all input is valued highly!

Bill Cook
07-24-2003, 12:13 PM
One of the suprising things about these struts is that the "piston area" is the rod diameter. What looks like a piston on the end of the rod is a damper to slow the extension of the unit down, and a rod guide.

This arangement also allows enough volume in the unit that the pressure doesn't build too quickly or too high when the rod is pushed in. This makes for a pretty flat spring rate when compared to a spring occupying the same space.

bc

Evan
07-24-2003, 12:35 PM
You could probably use drill rod (aka silver steel) and polish it to use as a piston. Don't know about the seals though.

lunkenheimer
07-24-2003, 02:18 PM
Axel,
The rod is very similar to what is used on hydraulic dampers, except for dimensions. If you want to make a single prototype, you might want to look at salvaging what you need from a similar part. The chrome plating is for wear resistance and also for corrosion resistance. Any corrosion on the rod will destroy the seal.

The gas in the struts is under extremely high pressure (thousands of PSI for some designs), which means that you should avoid cutting open a gas strut.

Cass
07-25-2003, 11:54 PM
The key to gas springs is extraordinarily smooth finish on the rod. The chrome is there to provide a hard scratch resistent material. A simple O-ring will hold extreme pressure on a near perfect surface. You can make a very nice and repeatable hydraulic adjuster by using a micrometer head and an O-ring as the actuator if you polish the rod. I am sure the gas springs have more sophisiticated sealing than plain O rings but in principle it is the same. If you want to cut a gas spring open you might want to set it up at some distance and shoot it to let off the pressure. It would probably be dangerou to hack saw or drill even one that had lost a lot of pressure.

CCWKen
07-26-2003, 07:30 PM
Yes, they are under pressure. No, it's not "thousands of pounds". If you need a gas strut, why not just buy one. You can also get just the cartridge (shock). On a smaller scale, there's gas charged "springs" that hold car hatches and hoods open. You can get these in forces from 10lbs. to 150lbs.
I cut the rods off old shocks and struts. They make great tool and fixture stock. I also used some to restore a couple of old (1940's) automotive knee shocks.

Evan
07-26-2003, 07:36 PM
CCWKen,

Gas spring pressures go up to 2500 lbs.