Choosing a step motor


I've just bought arduino last week and I'm still learning.

My goal is to make a decoiler machine powered with a step motor and controled by arduino.

Here's my newbie drawing

The wheel weights 25kg (~55lbs) and I need it to reach 5rpm.

Can anyone explain how do I choose a step motor that will do the job?

Thanks in advance!

You guys rock!

In short: Calculate the torques involved. Look at the speed/torque graphs for candidate motors.

Torque is needed for three reasons, to overcome inertia as the wheel speeds up or slows down, and to
give power to the load and to overcome friction.

You've said nothing about the load, but a back-of-the-envelope calculation suggests the moment-of-inertia
(MoI) of the wheel will be about 1.0 kg-m^2 (using formula for MoI of solid disk radius 0.2m and 25kg mass).

The friction depends on the bearings principally.

The eccentricity of the mass on the wheel will also create a load.

But the actual load is presumably due to decoiling (and may be negative if the motor has to resist pulling?)

Any reason why a stepper is selected for this task?

My guess is that this is for a 3D printer and the stepper motor will be used to rotate the spool at a speed (slow) that is compatible with the rate at which the filament is being used. If so that seems like a job a stepper motor could do.
But so could a small geared DC motor.

Oops, I did not see that it weighs 25kg


IMO better get a DC wiper motor and magnetic switch for indexing.