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Bill Adamsen
03-28-2017, 1:00 PM
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
03-28-2017, 1:16 PM
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
03-28-2017, 2:09 PM
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
03-28-2017, 2:22 PM
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
03-28-2017, 3:19 PM
^^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
03-28-2017, 5:33 PM
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
03-29-2017, 10:25 AM
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
04-01-2017, 9:18 AM
Check out the photos in this thread . http://www.sawmillcreek.org/showthread.php?252063-My-2X72-Belt-Grinder. Dave is using a lift to tension his belt grinder. Is that similar to your use?

Eric Keller
04-01-2017, 12:47 PM
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
04-10-2017, 8:15 AM
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
04-25-2017, 10:14 PM
Got a little bit of time to work on it over the weekend. Next the bracket.

Clark Magnuson
06-29-2017, 4:27 AM
My father used air as a spring. Israel still uses the M55 70 years later.
it was to balance the weight of an artillery barrel.
He had a problem that a spring is Hooke's law F = kx.
That was what he wanted. But he could not get a spring that big.
It is easy to get a cylinder full of air that can make that much force, but it follows the equation PV = nrt
So he had to trim out the difference hydraulically.

Peter Christensen
06-29-2017, 1:51 PM
If you were going to be using pneumatic cylinders from the start you could have just put the roller on the end of the piston and done away with the rest of the pivoting apparatus. Might just as well continue with the springs and keep it simple.

Art Mann
06-29-2017, 6:43 PM
Pneumatic cylinders behave very differently than a spring. The force a spring provides is directly proportional to how much it is stretched. It is not a constant (like Clark mentioned). A pneumatic cylinder provides a constant force that only depends on the pressure applied to the cylinder. In most cases, this doesn't make much difference. However, for long stroke motion, like you get with the Z axis on a CNC router, the inconsistency of the spring makes it undesirable as a counterbalance.

Bill Adamsen
06-30-2017, 3:08 PM
Art ... those "spring versus pneumatic" properties were what had initially piqued my interest. I felt the pneumatic would make the implementation easier since the pressure is constant. Advantage would be not having to adjust the base. The con would be having to learn how to use and size pneumatics, plus figuring out how to 'trip open" the gate (the pneumatic) to allow the first board in. Or it could work like some of my other pneumatics where there is a switch which allows the "spring" open and then activating the switch clamps it down. I generated a drawing showing how it would be modified.

Frankly it works great the way it is and I'm unlikely to do much in optimization for the small number of times I use it. For me, the electronics around sensing the gate opening and adjusting the pneumatics accordingly would simply have increased the learning curve to the point where I would have thrown in the towel. Clayton's design works remarkably well and I can tweak the adjustment mechanism to optimize it for my use.