Design Q: O-rings as wipers? Fuzzy O-rings?

A square on the end of your rod, made of UHMWP:

You shouldn't need to lubricate it.

That is exactly what I would do. You can get UHMW poly from either MSC or Grainger. I got a little from MSC to have some around.

Dan

That is exactly what I would do. You can get UHMW poly from either MSC or Grainger. I got a little from MSC to have some around. ===================================================

Haven't read ed's link yet, but does it have any "give" to it, is it compressible at all?

I'll let Dan comment on that, since he has some. Its measured hardness is pretty high, though, like typical thermoplastics: Shore D 80.

Hmmm, perty hard.

Hey Ed, is yer email workin?

It tends to "cold flow" when compressed, so I would say that using it in compression might require replacing it often.

BobH

Yes, but as Dan knows, it's slippery as a greased pig -- slipperier, actually. It makes a great unlubed bearing surface -- like Teflon on steroids.

As long as you don't have a lot of interference, it shouldn't have any trouble with roughness. It's pretty exceptional material. It won't handle much heat, though: 180 deg. F for continuous duty.

Try it, if you can get your hands on a sample piece.

Yup. Delete the "3" from the phony one. 'Just one "s" in the "huntres" part.

The UHMW could be a thin sheet so it bends more than it compresses.

But I am confused about one thing; if you can screw this wiper to the end of the inside rod, couldn't you just seal up that end of the outside tube? Afer all, if the end of the inside rod ever protrudes, the wiper will not seal anything.

Ed Huntress fired this volley in news: snipped-for-privacy@4ax.com:

UHMWPE is both compressible and elastic -- A LOT compressible, but not a lot elastic. FWIW, you can usually let the compression-set between fixed-spacing parts determine the elastic point for you, since it will deform under pressure, then stop with some elasticity left to recover back a few tenths.

LLoyd

Since you want to keep dirt out I think your application would benefit from a dense felt wiper. McMaster Carr used to sell felt in various densities and probably still does. Cut a square of this stuff and use a slightly smaller square of some type of stiff material held against it with screws threaded into the end of the 3/4 bar. If the sliding element is going to operate without lubrication, especially if the tubular component is aluminum, you may run into problems with galling. If you have room I would suggest making several plastic short cylindrical pieces from acetal or UHMW. Press these plastic parts into shallow flat bottom bores in the 3/4 bar. UHMW, while not as hard as acetal, is an excellent material for this application. Delrin AF, a teflon filled acetal plastic, would work better than UHMW, and is easier to machine because it is a little harder. I know McMaster Carr sells Delrin AF. Good Luck, ERic

It has some give. Milk jugs are made of polyethelene, and UHMW poly is similar. Maybe slightly harder , but not much harder. We used UHMW poly tape on the missile to prevent fretting of the graphite epoxy where the seals contact the missile skin.

Dan

I've only run my fingers over it and watched it perform as a bearing, but the reports on its properties suggest it's pretty unusual stuff, in terms of lubricity and other physical and mechanical properties. How does it cut?

Not O-Ring - but Washer, being square with a round hole.

Mart> I know o-rings are used in hydraulics as seals, wipers.

Ed Huntress fired this volley in news: snipped-for-privacy@4ax.com:

Like butter on a saw. Like strings of spiderweb on a lathe!

It's important to take as heavy a cut as you can afford to without springing the work out of the chuck. This, only because if you take light cuts, you end up with a rat's nest of 'stringies' that will rapidly interfere with the work. The heavy cut will produce a chip than can be readily cleared.

Also, don't expect to take your last cut just moving swarf. All the polyolefins have a tendency to spring away from the tool tip -- even if it's razor-sharp of the correct rake. So you must take a deep enough cut to overcome that resiliancy, and get the tool tip into the work and cutting. Once in and cutting, the stuff cuts cleanly and easily.

Lloyd

Hmm. I'll have to try some. I have a possible application, too.

Thanks, Lloyd.

That's a good insight. A block of UHMW poly would seem ok, but as a wiper, you'd then have to machine the size pretty accurately. And then how long would it last at that dim?

So give, springiness, flexibility would seem to be a requirement in any kind of wiper -- even literally a spring pushing the wiper mat'l to the surface, if the wiper mat'l itself is not flexible.

So a thin sheet has promise, in principle, altho it would remain to be seen if the "concavity" of the bending sheet would accommodate both pushing and pulling, or if the concacity could change with pushing/pulling. Also, it would seem that two sheets would be necessary, to accommodate the wiping of opposite faces, independently. Which can be finagled, with a spacer between the two sheets.

This is food for thought.

This was a simpliification of the app, for illustration of the problem. The actual geometry is a little more complicated, but not by much: a rect slider in a channel (actually, a tube with a long wide-ish slot in one face), but the jamming issue is very similar.

Alum is one pita when it comes to sliding. I have indeed made what you described, in various forms, and the ultimate "sliding solution", esp. inside alum tube, may be to make the slider itself out of some plastic/polymer (nylon, delrin, some of the stuff mentioned here), which would certainly solve the galling/friction issue, but jamming could *still* occur, ergo proly still the need for some kind of wiper.

I mentioned felt/cloth, altho that seems sort of a brute-ish force solution, but mebbe ultimately the only solution. I like anorton's idea, in part because I think such a material would have a much longer lifespan than felt or cloth. But felt is def'ly a consideration, I'll check out McM.

Sounds like a place for an oversize leather washer. It'll conform, and if you get it wet it'll seal. And if you run it with a charge of carbide grit or metal polish, the inside of that tube stops being unpolished after a while.

If I wanted a sliding fit, I'd consider drilling sockets in the bar, and gluing buttons of delrin into 'em. For big loads, screw delrin plates onto the bar faces.