Simple Solution- Drive 1" Ball Screw Shaft with NEMA-23-Size Stepper Motor Shaft

[Jonathan]

Why would we cut the shaft-retaining bore hole so inaccurately that run-out would cause interference problems at ballscrew rotation speeds?

We would because the mounting tolerances required for a solid coupling are generally too high. Lack of concentricity places a cyclical load on the bearings and the load rating for that type of load on a bearing is significantly smaller. This is one of the numerous advantages of using a timing belt drive.

I have an air-driven dental drill that spins over 100,000 rpm, and its removable tools seem to stay pretty concentric with the motor's shaft.

Tool is only supported on one end there, so different situation entirely.

Oldham couplings do impose radial forces on non-concentrically-connected shafts. That radial force is zero only at zero rpm,

Yes they do, but only due to friction between the sliding disk and aluminium end pieces. It will be negligible. Again if you're worried about radial forces use a timing belt as that applies a constant radial load which deep groove bearings tolerate better than cyclical loads. Still, that's analysing it too much . . much better reasons to use pulleys, such as matching the motor torque/rpm characteristic and resonance supression.

One big issue with permanent magnet motors is gradual flux loss from heating those magnets.

See here:

http://www.electricmotors.machinedes.../bdeee3_5.aspx

Ceramic or ferrite magnets lose about 0.13% / °C of their remanence above 25°C, while rare earth and AlNiCo may lose only 0.03% / °C. But this loss is generally reversible if the temperature is kept within the motor rating. Colder temperatures are seldom a problem. Since the coefficient curve is linear, magnets are stronger at lower temperatures.

So the temperature rating is a threshold, which if you don't exceed that the magnets will not suffer permanent damage. The figure often quoted for stepper motors is 80°C case temperature, which is below the temperature rating of the magnets to account for the thermal resistance between the case and rotor.

As these magnet's strength gradually degrades from heat-cool cycling

That's thermal fatigue, which is a different problem not addressed by shaft heat rejection.

A heat sink on an expensive machine doesnt mean it's necessary, many machines add aesthetic and unnecessary features to appear expensive. The manufacturer's ultimate objective isn't a top notch machine but money. Also the motors to which you're referring are now obsolete - the design and materials have improved significantly since then so if heatsinks were required then, that does not imply they are required now.


As long as the motor is bolted to an aluminium mount I'm sure it will be fine, unless you're running them at a high voltage. Either way just make it, measure the temperature and think about adding heatsinks if you need to. My hottest motor (Z-axis) never goes above 50°C and that's just on 3mm thick mild steel.