"anorton" wrote in message
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"Ed Huntress" wrote in message
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On Sun, 24 Mar 2013 17:09:03 -0400, "Existential Angst"
wrote:

"Ed Huntress" wrote in message
...
On Sun, 24 Mar 2013 16:35:24 -0400, "Existential Angst"
wrote:

wrote in message
...
On Mar 24, 1:11 pm, Ed Huntress wrote:



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

http://en.wikipedia.org/wiki/Ultra-h...t_polyethylene

You shouldn't need to lubricate it.

--
Ed Huntress

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.

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.


Hey Ed, is yer email workin?

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

--
Ed Huntress

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

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.


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.

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.
--
EA