On Sunday, March 24, 2013 9:53:38 AM UTC-7, Existential Angst wrote:
I know o-rings are used in hydraulics as seals, wipers.

But what about in a dry environment?

Imagine a 3/4" sq bar freely sliding inside a square tube, nominal 3/4" id,
with the id smooth but unpolished...

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.