Hydraulic cylinder used with air (actually compressed air in PVC)

Dear BobK207:

Something like that.

Don't. PVC isn't rated for use with compressed air, because it fails by shattering. There are polymer systems that are rated for compressed air. They fail in plastic deformation (over the range of temperature they are rated for), which keeps the little bits all together.

URL:

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If you are bound and determined to do this, no one can stop you. It isn't safe, and if you provide this "mechanism" to someone else (in a half dozen different ways), you could be sued.

David A. Smith

Dave-

Thanks for the reality check. No, I'm not bound & determined to do this.

I fully understand the issues with PVC shattering & would never use PVC for something like shop air.

But I wanted to "fudge" because it would be so easy to throw this thing together & the pressures were so low (in the 10 to 20 psi range).

Kinda like........glass is ok for a coffee or kitichen table but a entire floor diaphragm, maybe not.

I was hoping to get a vote of confidence from the ng members.

I checked out the link & since this stuff exists there is really no reason for me to use the PVC.

cheers Bob

Dear BobK207:

...

Not trying to help you spend your money. Think "water hammer", and how this naively cannot happen in low pressure systems. It is too easy to get sued these days.

David A. Smith

Dave-

Thanks for you time on this issue.

And don't think I'm trying to wiggle back to a "wrong" design, at this point I'm a little concerned about my original reasoning "if the stresses are really low it'll be ok"..

(it's not my money but it would be my ass if something goes wrong)

So switching to the "ductile" plastic is hte right thing to do.

I'm making a small capacity (reseach project) cementious grout dispenser.

My design has about a two quart capacity, about 14" overall height (length). An end cap on each end & a union at the 2/3 point.

The lower (larger) section would be filled with grout, the upper section just provides a way to seal the chamber and provide air input through a threaded fitting.

The upper end cap has three 1/4" threaded penetrations:

0 to 15 psi gage input fitting pop saftey valve (6 to 15 psi setable)

The unit would be pressurized thru a 0 to 20 psi regualtor (with gage)

This whole setup seemed safe to me, basically idiot ressitant but I've done dumb things before........................(& luckily lived to tell the tale)

cheers Bob

You REALLY should actually be safe at the low pressures you're talking about, but you can easily ensure your own safety with a cheap method of buttressing the outside of the pipe. Simply coat it with a good quality FLEXIBLE glue (not Elmer's, except for their Polyurethane glue which is a bit hard to find) and wind it with a strong cord. Coat the cord with more glue after you've wound the pipe completely. Even if the PVC shatters (extremely unlikely especially with the cord wound about it) it is very unlikely that a shard will come flying.

But no, even these measures are not good enough for a consumer product.

PVC is a bad choice for higher pressures and especially sudden pressure differentials, but I know one (crazy, idiot) fellow who actually made a backyard mortar from PCV pipe. It took a quite a large number of launches before the thing finally shattered with a larger load (the payload went 1/4 mile). The guy narrowly escaped one razor-sharp shard that went singing by his head. Needless to say, he was a fool, but he learned his lesson (because he's still alive). It did, however, illustrate that the pipe could take quite a lot of internal pressure before shattering.

'Sporky'

BobK207 wrote:

Dear Sporkman:

"Sporkman" wrote in message news: snipped-for-privacy@bigfootDOT.com... ...

Working pressure < burst pressure

Also, the ability to yield plastically is a strong function of temperature. Something tells me "your acquaintance" had PVC that was pretty warm...

David A. Smith

Can't say. Don't know if it was summer or winter, but if it was summer odds are you are correct. Even white plastic that sits out in the sun for a few minutes gets pretty warm. Seems highly probable that stress fractures -- perhaps even microcracks -- occurred over multiple launches, then one "hot load" did the trick. Even still, as I recall the story, it was really only a couple of chunks that flew off -- one of 'em past this guy's head, close enough to shave some hairs if he had moved a little. Regardless, it really doesn't sound much like BobK has a lot to worry about if he's only putting 10 or 20 psi in the tube. Doubtless the "mortar" sustained many, many times that pressure repeatedly.

'Sporky'