Ferrous metals, typically ordinary hot rolled steel. Of course stainless is also common, but a manufacturers catalog or 2 will really give you the answers you're loooking for.
Please let us all know how this works out for you! and don't forget faq731-376
I was under the impression that ferritic materials that are magnetic are not desireable due to the effect they have on magenetic flux within the motor, hence the use of non-magnetic (300 series) stainless steels. Is that impression wrong?
The vast majority of motors coming into our shop have magnetic shafts. A change to non-magnetic shafts in some of these units can cause poor performance.
Why might 303 stainless steel be used in a motor design? What are the advantages (besides the obvious corrosion resistance)? What are the disadvantages?
303 might be used due to pretty good corrosion resistance and if stainless is called for, fairly good machining characteristics.
Cons, at least from a repair standpoint, include:
Hard to take a deep cut while machining without changing properties, high initial cost as compared to some other products, welding problems due to changes in characteristics and the fear of hexavalent chromium by the welders. The material also reacts differently than non stainless materials when heat straightening which can cause miscalculations and excess repair times.
Thanks for the reply oftenlost, but I am looking for reasons possibly more inherent to motor design, if there are any good reasons as to why 303 is used. It could be that there aren't any good reasons to use it in regards to motor function/interaction. Maybe there is. Maybe the rotor shaft material does not have a significant impact on motor performance (I doubt that). Maybe there are materials much more suited as far as motor performance. Maybe those other material advantages don't outweigh the advantages that 303 may have. I don't know. Can anyone offer up more info on rotor shaft material directed at motor function/performance? Thanks for the help so far.
A ferromagnetic rotor shaft material becomes part of the overall magnetic circuit. Properly designed, it helps the magnets operate more efficiently by transfering flux to adjacent magnets.
Using 303 stainless would be the equivalent of air as far as the magnetic flux lines are concerned. This low permeability path means the magnets have to work harder.
This isn't to say that 303 stainless or some other non-ferromagnetic material couldn't be used, it would just be much less efficient from a magnetic standpoint.
In the hybrid stepper designs I've seen, you don't want the center shaft as part of the magnetic circuit. Everything magnetic should be happening "outside" the center shaft, between the outer part of the rotor (where the permanent magnets are) and the stator.
An old Parker Compumotor catalog states, "Because the shaft of the motor passes through the center of the permanent magnet, a non-magnetic material must be used in order to avoid a magnetic short-circuit. Stepper shafts are therefore made of stainless steel, and this should be borne in mind when handling the motor. Small-diameter motors are particularly vulnerable if they are dropped on the shaft end, as this will invariably bend the shaft."
