DC servomotor maintenance - can you disassemble them? How do you clean commutators?

I have always been told to never disassemble a step motor as it ruins them - reduces their output torque significantly by demagnetizing them.

So, is there a problem if you disassemble a permanent magnet DC servo motor? I am thinking that this is not a problem with DC motors in general because I have disassembled many DC starting motors without problems. However, servos may be different - so I ask:

Is it OK to disassemble a DC servomotor to clean it up inside, to replace a bearing, clean the commutators, or etc.?

My servos show very erratic ohms readings across the motor terminals. Turning the motor shaft to different rotational positions gives ohms readings from 3 to over 120 depending on where the shaft stops. I am thinking that this will cause erratic operation as a servo. Am I correct to be concerned? The commutator does not appear to be burned up or out of round or grooved, just very badly blackened by use. There was carbon dust in the brush holder as well. The brushes are nearly full length and in good condition and move freely.

If I can disassemble these motors without reducing their performance, how should I go about cleaning up a dirty commutator? Is it OK to polish them with fine emery paper? Will polishing the bars with abrasives embed grit in the copper and cause excessive brush wear? Do they have to be turned in a lathe to clean them up? How do you guys handle servo maintenance and what should I do to bring these motors up to their peak performance before I install them?

I have a used set of Siemens 1HU3056-OAC01-0ZZ9 and a used set of Baldor 4203623 motors.

I wold appreciate any tips for maintaining these motors.

The resistance will change as you rotate it, thats normal to a fair degree.

IMHO i would not sweat commutator condition, unless there real uggly there not that detrimental, what is is a badly sanded one or far more commonly sticking - worn brush gear. Some wear and the odd grove won't matter, black arcing + pitting yeah you have a problem you have to fix!! Nearly all comutator daamge is caused by brush problems though, so make sure you fix that!! Or its all in vane.

Some motors will lose field strength if disassembled, hence its really best not too unless your forced too.

Commutators want to be turned not sanded. sanding generally reduces brush contact and makes things worse on a motor you actually want to make power and have great control on.

By all means blow - clean out all the brush dust make sure the brushes slide nicely and freely in there holders and replace them if there over worn! More than a few brush types swell slightly with age and won't slide freely, don't oil em, just give there sides a few swipes on some clean abrasive paper - a file, If they don't slide freely they arc - make intermittent contact and you will damage the motor.

Good reply.
Magnets can get demagnitised by excess heat and physical shock, so if you want to disassemble a stepping motor or anything, don't bang or jerk the magnets.

Just be careful if you take it apart as said before the magnets are usally glued on. Also their are special stones to clean commutators i used them all the time with my job and the ones i have i don't need to take the motor apart. It fits down the brush slid.