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, and not a great material, like alum... no flashing.
I see two possibilities: o-ring material itself with a square xsection, in a groove around the 3/4 material. But the corners would have a radius, unless I could find o-ring stuff with a square xsection and also for square/rect rod.
Square xcection o-ring material can be found herebut round bar/rod, not sq or rect. bar.
Or, on the END of the 3/4 bar, take a nominal 3/4 sq pc of some compressible (but hopefully low-frcition) material, and screw it to the end end of the3/4 bar, the tension of the screw (and washer/plate) squeezing out the wiper material for a suitable "fit" on the inner wall of the tube.
Compressibility (rubber) and low friction *seem* to be mutually exclusive properties, but not sure. Some kind of leaf spring steel/brass wiper could do as well, altho I wouldn't want stuff too mechanically fragile.
Or just screw some felt/cloth to the end of the 3/4 bar???
The logic, rationale behind this is that the 3/4" bar needs to slide freely but be reasonably low tolerance (.005 to .010), which could allow grit to jam it in the tube. Ergo the wipers. A square fuzzy o-ring for rect. mat'l would be ideal.