[Aegeon]

If anyone can validate my logic here, it would be appreciated (newbie alert)...

I'm considering a timing belt/pulley setup for the X on a large table. I'm trying to work out general motor requirements so I can keep an eye out. Here's how I understand things...

For example, a certain NEMA 34 stepper is listed with 793 oz-in holding torque. The motor is bipolar, 4.68V, 3A, 1.8deg/step. Looking at its torque chart, it looks like it will do 550oz-in at 2000 half-steps/sec.

My assumptions:
1.8deg/step = 0.9deg/half-step, so 400 half-steps/revolution.
2000 half-steps/sec = 5 revolutions/sec = 300RPM @ 550oz-in.

If you have this setup mounted in a gantry:
Pulley A: 1.00" dia, 3.14" circ, attached directly to motor spindle.
Pulley B: 3.00" dia, 9.42" circ, attached to pulley A via belt.
Pulley C: 1.00" dia, 3.14" circ, attached to same axis as Pulley B.
Pulley C is coupled to the timing belt, to move the gantry.
So, the motor drives pulley A, A turns B at 3:1 reduction, B turns C at 1:1, C rides along the belt to move the gantry.

If the motor turns pulley A at 300RPM @ 550oz-in, pulley A turns Pulley B at 100RPM @ 1650oz-in (3:1 reduction). Pulley B turns pulley C at 100RPM @ 1650 oz-in also (1:1). So, if the circumference of pulley C is 3.14", at 100RPM, the gantry would travel 314" per minute with a force of 1650oz-in.

Since the motor does 400 half-steps/rev, each half-step moves pulley A 1/400th of it circumference. Therefore, pulley B moves 1/1200th of its circumference. Pulley C (and the gantry itself) also moves 1/1200th of its circumference, or 0.0026" with each half-step.

I know all this ignores friction, efficiency, belt stretching, etc., but I'm looking at the basic logic for now. Tell me if I'm way off here...

Thanks,
Aegeon