What's the issue with taking steppers apart?

[John_B]

Does anyone know what the harm would be in taking a stepper apart? I have noticed that all manufacturers seem to warn that this harms the motor and lowers torque levels after it's reassembled. I have several motors bought in a box and some were apart when I got them, and these do seem to be a bit lower in torque than the others that weren't pulled apart.

Also this is making me wonder if it's OK to tear apart the motors on the mill I'm using to drill the end caps for encoder mounts.

Thanks,
John B

[Al_The_Man]

It often depends on the quality/nature of the permanent magnet characteristics, the rotor can act as a 'Keeper' for the magnetic field, which some claim they have lost the P.M field when removing the rotor.
Some manufacturers of servo's state their can be dismantled with no problem.
I have dismantled several DC and AC servo's for service and so far have not had a problem.
It pays to check with the manuf.
On the safe side, you could always slide a iron bar in as you are removing the rotor.
Al.

[John_B]

Al,

So following that thought, would then these same folks be inferring that the longer the rotor was away from the stator the less PM field? One would then suppose that there would eventually be a point in time that there would be no remaining Pm field then right?

I'll try the iron bar suggestion on one of the five remaining together motors I have that aren't in use right now, run it and see what happens. I have 14 Slo-Syn motors that show a 450oz-in rating at 3.1 volts, and 6 were taken apart so I have several to compare against.

I guess if the 3 motors I'm using now do have some loss of field it's not a huge problem, because I'm still working on a Sherline rip-off copy at this point. When I graduate to a more capable machine it may really come into play though...


Thanks,
John

[Al_The_Man]

The high-end servo's use a powerful rare-earth magnet, neodymium.
A lesser used one is Samarium-cobalt.
But both are good at retaining PM without a keeper.
Cheaper motors used to use Ferrite or Alnico.
I have some servo magnets I have removed over time which I use in the shop for temporary work holders and they appear to be as strong as the day I removed them.
Al.

[steve323]

John B, can you mount your encoders using the screws holding the motor together? You could remove them one at a time to install brackets without actually taking the motor appart.

[phantomcow2]

Not to mention, these magnets will attract any and all steel chips nearby. Even if you don't think you have any on your work space, you may be surprised

[John_B]

Guys,

Thanks for the suggestions. As Al mentioned, I have tested the other motors and not really found any difference with the ones that were still together and the remainder of the ones that were apart.
I also tried to use an iron bar to maintain the mag field, again to no noticeable effect.
So far, I'm at a loss to explain the motors that were apart that I have bieng so noticeably less torquey. I personally think as long as mod's are made quickly that lasting damage can be averted.

Again thanks,

JB

[steve323]

It might depend on the particular motor. Some types of magnets are supposedly less tolerant of losing their flux. I have not seen any noticable difference on the cheap surplus motors that I have opened. I stopped opening them up when they became $50 motors for my CNC machine.