Making a pneumatic cylinder?

[loply]

Hi folks,

I suddenly find myself interested in making a pneumatic cylinder for a project. The sort of thing which, when pressurised at one side, moves a piston by 15mm or so, and if pressurised on the other side it moves it back again.

I know you can buy these fairly cheap but I want one with a large bore and short stroke and would love to make it myself.

Trouble is I don't have one to reference off, so I'm guessing about how they're usually made.

I gather the piston is going to need a seal similar to that found on a compressor piston, and I'll need one of those 'cup' type seals on the output shaft which seals harder when it gets pressurised. Is it pretty much as simple as that?

I'd hate to spend hours producing it to discover it just blows air past every seal!

I'm hoping to run this around 100psi. Bore will be somewhere in the region of 60-80mm.

Would appreciate any input!

Cheers,
Rich

[BillTodd]

With such a short stroke, it might be possible/easier to use a diaphragm or 'rolling rubber' seal  rather than a typical sliding seal




Diaphragm seals:
http://www.diaphragmseals.net/diaphragm_products/diaphragms.htm

lots
of info in here: http://www.diaphragmseals.net/assets/pdfdownload/DiaCom_Catalog.pdf



Piston seals:

http://www.claron.co.uk/pistonse.htm.

[David Jupp]

Particularly for the piston seal, a simple O is a possibility - some commercial short stroke actuators use O rings.

[cfellows]

As David says, simple o-rings will probably work just fine for you.  One o-ring on the piston and one around the output shaft.  I've done a couple of projects with these and they are air tight.

Chuck

[loply]

Hmm, I presumed an O-ring wouldn't make an air tight seal. Interesting.

Am I correct in assuming that if I pressurise a cylinder to 100psi, and the piston has a surface area of 10 inches, the piston is going to push with a force of around 1000lbs? Sounds obvious but just want to make sure.

Presumeably therefore the bigger the bore/piston is, the bigger the force is?

[andyf]

Hmm, I presumed an O-ring wouldn't make an air tight seal. Interesting.

Am I correct in assuming that if I pressurise a cylinder to 100psi, and the piston has a surface area of 10 inches, the piston is going to push with a force of around 1000lbs? Sounds obvious but just want to make sure.

Presumeably therefore the bigger the bore/piston is, the bigger the force is?

Yup, The force is proportional to the surface area. Think of a hydraulic jack, where you use comparatively little manual force to pump oil out of a small-bore cylinder into a big-bore one whose piston exerts a lot of push.

Andy
 

[David Jupp]

As long as the 'nip' on the O ring is correct and the extrusion gap isn't too large it will seal well.  100 psi is at the low end of what O rings can tolerate.

[Lew_Merrick_PE]

Rich,

If you go to http://www.scribd.com/Lew%20Merrick/shelf you will find a document O-Ring Seal Application that will walk you through sizing the gland for your o-ring.  It is written using inch-based measure, but that should not be too much of a problem.  The biggest problem I see in this application is sizing the piston for stability.  With large bore piston cylinders you generally need to make the piston long enough to prevent "tipping" as it moves.  If you get this wrong, you have stick-slip stick-slip stick-slip out the kazoo!

General purpose air cylinders are normally designed with a nominal .001 inch (0.025 mm) diametral allowance (clearance).  The piston would be toleranced as +.000/-.002 inches (+0/-0.05 mm) from the nominal (clearance allowed) diameter.  The cylinder bore would be toleranced +.002/-.000 inches (+0.05/-0 mm) from the nominal diameter.  The piston OD and sidewalls of the gland(s) could have a finish as coarse as 32 micro-inches (80 micron).  The cylinder bore and all the bottom face of the gland(s) should have a finish of 16 micro-inches (40 micron) to insure sealing.  You would probably want to design the o-ring groove to provide 12%-15% compression of the o-ring (assuming nitrile o-ring material).

If you use aluminum for either or both your piston and cylinder, you will want them anodized (sulfuric anodize is just fine, chromic anodize should be avoided).  If your operating pressure is 100 psi, then your design pressure should be 150 psi.  If you are using 6061-T6 aluminum for the cylinder, the allowable yield stress is 40,000 psi (MIL-HNDBK-5J).  This would set your minimum wall thickness (at the thinnest part of the cylinder) to: t = Pr/s = (150 psi)(inside radius in inches)/(40,000 psi).  If you use some other material for the cylinder, substitute in the maximum allowable yield stress for that material (you can e-mail me at tangent@olympus.net if you need such values) for the "s" in the equation.

Does that help?

[bry1975]

If you need o-rings www.onlineorings.com are good.

You making something cool like a punching or shearing machine? one of my old firms use to make flying shears for the metal section industry use to be great fun seeing them fly along and punch or shear!

Bry

[John Stevenson]

You can buy service kits for air cylinders dependant on bore, i.e. all one makers 63mm bore cylinders will be the same. You can also buy cylinder tube.

I made a stamping cylinder for a pencil stamper that was 6" diameter but only 1" stroke, seals were bought as was the tube, rest was made from block and bar alloy.

John S.

[David Jupp]

Following on from John's comments above - maybe you could buy (or scrounge) an existing cylinder, then shorten tube, tie rods and piston rod...

[krv3000]

HI well you may be in luck as i have sum smoal pneumatic cilenders i will dig them out and if ther any good for you you can have them at the cost of postige

[loply]

Hi folks,

Thanks for all the excellent input, particularly from Lew, that document was excellent.

krv3000, I'd be interested in seeing dimensions etc for sure. Let me know.

[Lew_Merrick_PE]

lopy,

I was a member of the team that wrote the o-ring section for the 1976 edition of the SAE Handbook.  The material was removed from the next edition -- supposedly at the behest of Parker-Hannifen.  Most of the material was kept available in the American O-Ring catalog for a number of years.  When that finally disappeared, I put that document together as one area in which I regularly work is pyrotechnic actuators.  Being "almost right" when sizing seals that see 65,000 psi pressure impulses is just not good enough.  You have to find texts that have been out of print since the 1960's to find some of that information.

And no, such information would not have saved the Challenger.  It was flexure of the Solid Rocket Booster (attach) Beams (SRB Beams) that tore the tanks and caused the LH2 and LOX to "leak" from the External Tank.  Had that mixture not been ignited from the "leakage" at the o-ring joints (caused by poor assembly rather than poor design), it would have been ignited by the main plume 5-7 seconds later.

[krv3000]

HI right dont no if thes will do i only have two the over all lenthe is 49 mm  with a di of 5 mm the center shaft cums out by 17 mm with a di of 2 mm the cilinders is threded at the top  any way sum pics o they have riten on them
S M C  4-10

[krv3000]

must be no good for him

[loply]

Hi mate,

Sorry, I thought I had replied!   

Unfortunately I need a much bigger bore for the extra power, but many thanks for your offer and help.

Cheers,
Rich

[krv3000]

ok no problem

[Scuba1]

I may be a bit late on this one, but I do have some cylinder extrusion tube in 63mm diameter out of anodised alu. It has 4 shaped hollow profiles on the outside, you can tap in m8 to fix your endplates to. I can cut you some of that stuff and post it to you if you are interested. We make air cylinders for our capping machines out of that stuff.


ATB

Michael