BuildingAfloat

[Robin Hewitt]

I forgot to explain

A heavy pulley is a flywheel, flywheels have long been used to add inertia to systems, and if you look at the maths you will find they do it well. It takes surprisingly little effort to accelerate a gantry, if it weighs 20 lbs then a 20 pound force will accelerate it at 1G, same as if you dropped it. If you add unnecesary inertia to the system you have to watch out for that moment when you hurl the gantry in to reverse. That is the moment you risk losing steps and blowing your position. Unnecessary inertia is to be avoided for that reason.

As you like explanations I should also warn you about choosing stepper motors by holding torque alone. For some obscure reason the hobby market has settled on stepper motors with an arbitrary 200 full steps per rev, so the field coils have to turn the rotor 1.8 degrees. If you use a large rotor you get more leverage on the shaft but the pull in distance for 1.8 degrees is larger than for a small motor. Unfortunately as the distance increases the magnetic field falls away by the inverse square law so you tend to lose more than you gain.

Next problem is that the suppliers have found that nobody seems to care a fig for the actual motor performance, they only look at the holding/detente torque for the motor. Obviously the holding torque is measured with the minimum gap so big numbers are easy achieve so that is exactly what they do. What you are really interested in is the pull in torque and the inductance of the field coils.

You can "fix" a large 200 step motor by increasing the volts but that is expensive in the driver/PSU department. When choosing a motor you want to view holding torque as a simple indicator so you gett in the right ball park, what you really want is a graph of torque against speed and see how it goes with what you are trying to achive in accuracy while still giving a good rapid which gets more important as the size of the machine bed increases.

Edit: There is also a tendency to sell motors in sets of 3 for XYZ, why anyone would want to use the same motor for 3 completely different tasks eludes me.