Pneumatic Springs?

[Bill Adamsen]

I am building tool where I had intended to use (trampoline) springs to provide the required tension. I saw a listing on ebay for some pneumatic cylinders listed at a price actually cheaper than the springs. I couldn't help but wonder if pneumatics can work like a spring? Conceptually they would likely provide an easier approach to adjustment (self-adjusting). It is just that in all my prior uses they are either on, or off.

[Malcolm McLeod]

Not sure how you intend to use them, but do you mean spring dampeners (like on a hatch-back)? Or, true pneumatic cylinders? Suspect the cylinders are the cheap option you see, because they need so much other stuff to make them useful.

If true pneumatic cylinders, then are they single-acting, or double-acting? Single-acting have a spring built in and are usually 'spring return, pressure to extend', but you can order them as 'spring extend, pressure to return'. All will need your 'on/off' control of some type (a manual- or solenoid-actuated valve). You can even get a cam-actuated valve, so the tool could reach a certain point (or you open a door) and the cylinder actuates. And don't forget a compressed air supply.

[William Shelley]

A really basic pneumatic cylinder with just a threaded plug installed in the air inlet that would be used for extending could work. Install the plug with the piston fully extended. When you push on the piston, you'll essentially be compressing the air that is now trapped inside the bore of the cylinder. This should work like a "spring".

[Malcolm McLeod]

A really basic pneumatic cylinder with just a threaded plug installed in the air inlet that would be used for extending could work. Install the plug with the piston fully extended. When you push on the piston, you'll essentially be compressing the air that is now trapped inside the bore of the cylinder. This should work like a "spring".

^^It will do so, but be careful sizing this setup for installation. At full extension, internal pressure drops back to zero (gauge) and its no longer a 'spring'. You probably have to design some amount of pre-load into the cylinder length and mounting points.

Edit: I might use a valve (flow control or even schrader-style) instead of a plug. Some (small?) amount of air will bleed off while the cylinder is compressed. When you then extend the cylinder, it will be more of a vacuum than a spring. The valve will let you easily do a 'reset' and equalize the pressure when extended.

[William Shelley]

^^It will do so, but be careful sizing this setup for installation. At full extension, internal pressure drops back to zero (gauge) and its no longer a 'spring'. You probably have to design some amount of pre-load into the cylinder length and mounting points.

Edit: I might use a valve (flow control or even schrader-style) instead of a plug. Some (small?) amount of air will bleed off while the cylinder is compressed. When you then extend the cylinder, it will be more of a vacuum than a spring. The valve will let you easily do a 'reset' and equalize the pressure when extended.

A tee with a check valve that allows air in easily but when flowing out is restricted by the flow control, that might work. Good idea.

[Bill Adamsen]

I've attached a simple drawing (top view) which shows the spring mechanism. There are four arms that need to be "sprung" to provide tension.

[Clint Bach]

Springs tend to break in a spectacular sort of way. They also wear on the ends where they connect. I have used auto hatch back lift cylinders as push springs. Levers with weights will work as springs also if your application doesn't require high speed. I like levers with weights for their reliability, adjustability, economy and ease of use.

i don't know what your application is...

c

[Walter Plummer]

Check out the photos in this thread . http://www.sawmillcreek.org/showthre...2-Belt-Grinder. Dave is using a lift to tension his belt grinder. Is that similar to your use?

[Eric Keller]

a lot of people that convert manual knee mills to cnc use pneumatic springs on the knee as a counterbalance. I'm sure there is a layman's discussion of using them as springs on the web somewhere. My mill actually has a pneumatic cylinder built into the knee as a counterbalance because it's so heavy.

You can buy a pneumatic spring from McMaster-Carr. Been a while since I looked at them. I don't think there is any significant difference between a cylinder with ports plugged and a spring.

[Bill Adamsen]

Hopefully this provides a better visualization of how the "pneumatic" spring might work (same force direction as the steel spring in photo).

Yesterday fabricated one of what will eventually be four stacked "arms" to apply pressure to a board during the process of cutting veneer. Now starting to really think about the use and how to optimize the carriage that would hold these. Ideally, it would move in the X Y axis on the table, definitely towards and away from the blade to accommodate different thickness wood stock. The spring will need the ability to fasten on the other end and adjust pressure. The bronze bushing is 3/4"od and 5/8"id. I'd calculated that it would require 15 tons to punch the 3/4" hole in the 1/4" cold rolled steel, but I just couldn't punch it with the 20 ton manual unit to which I had access. My local steel shop had a 50 ton Edwards Ironworker (what a machine) and that did the job beautifully. The wheels have an 8mm bore and I had bought drill and tap for that on Ebay.

[Bill Adamsen]

Got a little bit of time to work on it over the weekend. Next the bracket.