"Lloyd E. Sponenburgh" wrote in message
...

wrote in message
ups.com...
a pair of circular pipes exists, running in parallel.

I wish to use them as tracks for a small railcar.

where to buy wheels, whose "tread" surface is a semi-circle of the
right size to fit over the pipes? very low speeds in this cargo
application, friction-caused efficiencies should not be an issue.

Or must they be made up by a shop on a custom order?

I don't recall your saying what size the pipes were. As was mentioned by
another poster here, wire rope pulleys are available in that
cross-section. Lighter construction of the same type is available in
fibrous rope pulleys. Some have integral bearings, others are just bushed.

McMaster-Carr carries them in sizes to accommodate up to 1-1/4" rope.

LLoyd

I used to have a horizontal photographic copy bench, and the carriage on it
that held the camera ran on a pair of 2" diameter tubular rails. The
carriage did NOT have wheels in the ordinary sense, but four sets of triple
balls races, each set had its three races at 120 degrees to each other, so
the contacting surfaces were the outer surface of the tubes and the flat
surface of each outer race, thus they made line contact.

The use of sets of three races surrounding the rails may not be suitable for
your purpose, but you could use four PAIRS with the members of a pair set at
120 degrees to each, running on the upper part of the rails . This would
give a very low friction running system, better than trying to use wheels
with semicircular circumferences, which would have quite high wiping
friction where the outer edges of each wheel wipe past the rails. Wheels
with outer surfaces like vee pulleys would be better, but still have higher
wiping friction than the pure line contacts of the outer surface of standard
ball races.

Dave.(uk)