Help with sticking stepper motor

Hey there,

I have a Keling Nema 23 stepper motor for my McWire bot that is giving me trouble.

The problem happens even if the motor is not connected to the machine or control circuits. I can make it happen by holding it in my hand and spinning the shaft. After several rotations it just kind of jams, stops turning. If I work through it, or tap it, it seems to go away. A few more rotations and it comes back.

This does not happen with the other two Keling motors I have. I suspect this might be causing some of the printing issues I have where the printout suddenly shifts as if missing some steps.

Is this normal? I have several steppers here and none of them exhibit this behavior. I thought it might be induction building up in the coils as I spin the stepper but I don't know.

Any help is appreciated. I don't mind buying a new motor, I should probably have a spare anyway. I mainly want to know if this is normal so I can rule that out as contributing to print issues.

Thanks

Chris (Krafter)

You should be able to turn the shaft and feel the detent torque. If any of the wires as shorted it will be stiffer to turn, but it won't jam solid.

Sounds like the ball bearings are faulty or there is a lose body inside the motor.

If the motor is definitely faulty you would have nothing to lose by opening it up and having a look. I think the bearings are regular ball bearings so you might be able to replace one.

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I did open the motor and everything 'looks' fine on the inside. I spun the bearings a bit and they felt ok.

Its really odd. It will spin fine many times. I can spin the shaft 30+ times without any change and then suddenly it feels like it hit a huge resistance. Like the magnetism suddenly shot up.

If I set it down for a bit or tap on the body, then it goes away.

I will check the bearings again. I think I have some extras around.

Thanks for the reply.

Chris (Krafter)

I think if you open up some types of motors/generators, the magnetic circuit
[en.wikipedia.org]
can get mucked up, so you don't want to do it to working ones. ... Ok, I heard about it in the case of one particular antique bicycle generator, but it may be a general principal. Sometimes.

But if it's already broke, no worries.

sounds like you may have a bit of dirt in your motor, or perhaps one of the bearings

I had to undo one stepper that had suffered a hard jolt!

OK it fell off Mendel as I hadn't screwed the stepper on.

It felt like it was seized solid on loosening the screw it recovered.

I took it apart anyway as I was half way there the bearings both had two very springy washers mounted like this:

The case then washer springy parts at top and bottom then the second washer spring parts to left and right then the bearing. It had exactly the same 90 degree configuration of these washers at the other end.

As the stepper motors internal tolerances are very tight.

I can imagine If you do not replace these springy washers so they exert equal forces on the bearings IE top bottom left right.

You could end up with a tendency for the shaft and bearings to lean slightly to one side OT the other. Thus causing an interference fit IE The top or bottom of the rotor being to close to the stator just due to the springy washers being exactly on top of each other instead of at 90 degrees to each other.

So when re-assembling the case it must also be perfectly square for the very same reason.

With none of the wires connected it will turn easily you can feel the steps.

Short any of the windings and you will feel stepping resistance to tuning.

Short all of the windings together and you wont be able to turn the shaft with your fingers.

For the above reason only test movement of the Stepper with nothing connected to any of its windings.

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Ok, first post and have not been lurking for months, literally started yesterday.

Anyway, steppers will not be damaged by being dismantled and the rotor being handled gently. Drop the rotor = throw the stepper away. It will be unbalanced and will have lost part of its magnetism. It will most likely still work but will not be as strong in torque, or as even in movement steps. Both of the things you use a stepper for and both that can cause a lot of lost time trying to find and elusive fault. And only leave the motor dsmantled for as short a time as possible. Go so far as to temporarily refit the rotor into the chassis, along with the end plates, when leaving the motor for an hour or more. It helps by keeping the magnteic feild strength as high as possible. You dont have to have the bearings in, nor have the end plates done up tight, a pair of screws each end done up hand tight is fine.

The spring washers either side of the bearings are there to allow the rotor to self centre in the feild coil's magnetic feild when pulsed. If they dont you end up with a magnetic pulse end load on the bearings which will eventually peen dents into the races and cause inaccurate stepping (the balls will want to slip into/be reluctant to go out of the dents). Your stepper should have a flex coupling that allows a few millimtres of end play for the stepper motor shaft so that the spring washers can do their job.


On the original post - Yes the change of movement freedom you have described will cause random errors to appear in your jobs, and also be near impossible to find. change the bearings on both ends if after checking the case poles ( the metal bits the feild windings are wound around) have no shiny/bare metal areas. Look closely as the scrape may be only a hair width wide. If there are scrape marks, give the case a blast of air and test again. If the lock up is still there, look on the rotor very carefully for a flake or splinter of metal that is probably just hiding in a gap. It will be magnetic.
No scrapes means bad bearing - usually a bit of dirt or a little bit of feild insulation that has cracked off and wormed its way into the bearing. It will typically come and go out of the interference zone much as you have described. You can clean and reoil the bearings after taking the side sheilds out, but just getting new bearings is easier. Price wise though, a new stepper will be about the same cost as a pair of 'intrument bearings'!

I appreciate all the suggestions. RonP I will take a look inside and see if I see any of the items you noted.

I did end up getting a spare from Keling. They had the same model on-hand and shipping from Chicago so I ended up getting it next day, I live just north of Chicago.

My plan is to install the new one in my McWire and then take my time researching the old one to see if I can find the source of the trouble.

I figure this way the worst I can do is wreck the stepper and learn something in the process.

Chris (Krafter)

another thing to note is if the stepper was running in micro step mode or 1/4,1/8,1/16 mode. just realize that a motor has a lot less torque in those modes, and also at various speeds. slow the motor down or do only micro (1/2) stepping and see if that fixes the issue. i think that people have responded correctly 1) it may be damaged somehow, and 2) just replace it rather that spend more timer troubleshoot it.

i dropped a stepper motor once that had a dual shaft .the bottom side hit the concrete floor when i moved it. it only dropped 8 inches, and i though it would be fine. it worked for only 1 week before the bearings locked up. any signs of damage with moving parts, get rid of, the locked up motor cost me more in damaged materials from a failed cut of a part, that the motor itself cost.

I have this same problem on a new motor. Thanks for the information I think I will just replace it.

Why stepper motor is sticky at places when not powered up?

I also have the same problem as OP is seeing.

I have the motor disconnected and am turning the spindle by hand. When it sticks it appears to be held magnetically though it is not possible manually turn through the magnetic resistance. By turning slightly in the opposite direction and back again it turns freely again. It sticks in either direction.

I have been sent pre-emptive replacement motor and the new one appears to be fine.

Steve

Keep in mind that the OP's comments are now 4 years old. I suspect they have worked out their issues by now ....

Hi all,
this is my first reprap forum post, but I just fixed my 'sticked' stepper motor - works as new, so I want to share it with you!.
At first, this is caused by magnetic short between two rings with magnets.Once you unscrew backplate, just gently pull out rotor with bearings.There will be two rings of magnets angulary shifted, split with non magnetic gap. If you look closely, you would see iron dust or tiny chips making shortcut between rings of magnets. I used toothbrush to get rid of most of them and then two times thick sticky tape to remove all dust from 'teeth' of magnetic 'gears'. At the and I carefully checked gap and rings for tape leftovers. Finally I assembled motor - slowly pushing body against backplate on flat surface, and holding rotor shaft to avoid touching rotor with stator (and making iron dust again). Screws should be tighten evenly and starting in opposite corners (like changing wheel on car). Don't forget on spring which pushes rear bearing against backplate!
I cleaned all of my cheap 'heacent' stepper motors and now they have much lower 'breaking' torque and run smoother.
Btw. fixing this 'electronically' as proposed by james villeneuve is just ....
Ivan

Edited 2 time(s). Last edit at 11/07/2014 04:34AM by igi79.