Ecnerwal wrote:



Well, the actual systems in use at the rock shop were a bit different,
but still a lot simpler than what's been talked about here previously.
One problem with what you describe above might be excessive grit
transfer to the drive system, but it's heading the right direction.

A more detailed description of a system which was in use for decades at
a rock shop near where I grew up, and may yet be in use (I haven't
visited recently).

A motor drives one, or better two shafts through some sort of chain or
belt or gear reduction. If only one is driven, the other is an idler.
The shafts are as long as you like, and may have intermediate bearings
or support wheels (psuedo-bearings) as needed. The bearings should be
well-sealed, and you might want to arrange covers for them, too. You may
want to texture the shafts a bit between bearings, but probably don't
need to. The shafts should not be too small, at least on the
tire-contact area. 2" would be a lot better than 5/8", though I can't
say where, exactly, it would stop working. 1-1/2" strikes me as about
what the rock shop used, might have been pipe with smaller shafts welded
or pressed into the ends for the bearings.

On the shafts sit one or more tires. Just tires, no seals, no plywood
covers. I forget, there might have been some arrangement to help keep
any drips inside the tire, but if so, it was a fixed arrangement (like a
reverse fender), not something attached to the turning tire. The turning
of the shaft(s) turns the tire(s). I vaguely recall that there might be
a particular SFPM range that works pretty well for this (rock tumbling
gets a lot of its science from ball-milling), so the same set of shafts
can turn larger or smaller tires at the appropriate speed (same SFPM,
different RPM for each size tire). Inside the tires are rocks and grit
and water. In multi-tire systems, those can be different grits in each
tire. If you want to check progress, you reach in and pull out a handful
of rocks. If the rocks (or if you care to sort them, some of the rocks)
are ready for the next grit, you wash them carefully and move them to
the tire with the next grit.

That's the idea I like..

John