On Mon, 08 Mar 2010 13:08:45 -0600, Jon Elson
wrote:

Don Foreman wrote:

I used the example to keep my post simple, but I really did do the
math here. I guessed at 4" dia capstan with a mass of 1 lb,
accelerating and decelerating to have peripheral travel of 3" in 0.25
seconds starting and stopping at zero velocity. Peak torque was
trivial compared to the capability of even a small servomotor driving
direct -- no gears or anything. Torque required was 0.056
Newton-meters, trivial for even a very small coreless servomotor.
Accurate design calculation? Of course not, it's a quick sanity check
that clearly shows feasibility.
Well, those vacuum-column tape drives had capstans weighing around 10
grams, and accelerating at insane rates, so the gaps in recorded info on
the tape could be kept short. To maintain a 0.6" gap, you had 0.3" to
start, and 0.3" to stop. The acceleration had to be performed in a
sub-millisecond interval. I've got some of the motors used. One is a
Yaskawa minertia motor with an ironless rotor and ceramic shaft, the
capstan was made of magnesium and fiber composite materials. Typical
capstans are about 1.5" diameter.

Jon

I'm starting to regret having mentioned tape.

I'm looking at a servomotor with stall (all day) torque of 0.734 N-m,
peak torque of 5.5 N-m. The whole damned motor weighs about 2.4 kg,
the rotor mass is surely less than half of that. Motor OD is about
80mm or 3.125". Do you think this motor could accomplish Tom's task?
I think it could do it easily and for a very long time.