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Thread: Opened stepper / loss of magnetism, pressure?

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    Default Opened stepper / loss of magnetism, pressure?

    Hey all, Im going crazy trying to figure this out. I have a proprietary 4 wire stepper in a printer, that comes pre-pressed with a lead-screw. The stepper has "gone bad", which Ive come to see is that theres play on the pressed area of the shaft, under the bearing it rolls on. The shaft looks to be lightly worn in that area. so I researched a replacement motor, found an idential one in size and specs, from the same company that makes it, but without the pressed lead-screw.

    Before anything I attached the new stepper and it worked. I noticed the tight and proper stepping feel when rotating the shaft by hand. I then opened the new stepper to attempt to swap armatures, from this new stock one to the pressed leadscrew one from my old motor. Big mistake, not sure why I did that since the leadscrew was bad, but I wanted to test it out. After the swap I notices a drastic change in how the shaft rotates by hand, very smooth instead of a tight step. I fired it up and it works but I still have that initial issue with the leadscrew stepper. so I swapped the armatures back, exactly how they were.

    Now back to normal, the new stepper rotates smooth by hand, without that stepping feel. I made sure to use the exact same pressure washers it came with, same screws to close the case, same direction the case was seated at. The only 2 things I can think of is that I handled the armature with leadscrew grease on my hands, creating a film on the contacts.... and or, and this might sound amature, but that steppers are magnetically sealed, that would require a "recharge" after opening???

    I fired up the new stepper once again, and it seems to have less torque, where Im using less pressure with my grasp to stop the shaft. I re-opened the motor, cleaned everything of any grease, used an pencil eraser to clean the contacts, and can air sprayed everything clean again. I reassembled it and its still not as it was when I first fired it up. I can buy another one and have it shipped, but once again im under alot of deadlines. I still have to have my machinist turn down a shaft and create a spacer to retro-fit this to the system.

    Here are my questions, and I really do appreciate any input possible...

    Does anyone know why the motor would lose its strength, simply by removining then re-installing the armature?

    Are steppers extra pre-magnetized durring assembly, and how would I return this force once oepned?

    At 2.7 Amps, 2.30 VDC, 200 Steps (unsure of its torque in oz)... Should I be able to stop its rotation by hand?

    Thanks.

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    Community Moderator Al_The_Man's Avatar
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    Some magnetic materials require what is known a 'Keeper' to complete the magnetic field, if the armature is removed, they can lose magnetism, probably what happened to you.
    Many servo motors use rare earth magnetic materials now that some manuf. claim do not need a keeper when servicing.
    It is seemingly more common with steppers to lose the magnetic strength if no metal 'keeper' is used.
    A common example is a simple horse shoe magnet, they generally have a metal bar across the poles to help retain their strength when stored.
    For $25.00 you should have bought a couple while you were at it.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design (Skype Avail).

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    Q Does anyone know why the motor would lose its strength, simply by removining then re-installing the armature?

    R The rotor probably had a multi pole magnetised core. Removing the rotor from its stack has caused the loss of magnetism as the magnetic circuit via the stator was not there to act as a magnetic keeper. As you have discovered opening a stepper motor of the pm or hybrid type spells the end of that motors useful life.

    Q Are steppers extra pre-magnetized durring assembly, and how would I return this force once opened?

    R Yes some motors of the stacked sort are flash magnetised after assembly.

    It night be possible to re-magnetise but this will require a very high pulse of current through the windings to produce an excessive magnetising force pulse but with out over heating the coils. The current would have to be balanced with the three phases of the coil so is not a DIY task.

    Q At 2.7 Amps, 2.30 VDC, 200 Steps (unsure of its torque in oz)... Should I be able to stop its rotation by hand?

    R Probably not but a better test is to check the resistance to turning with the coils shorted this will be next to impossible to twist by hand or with pliers.. A demagnetised rotor will offer much less resistance to a twisting of the shaft with the coils shorted. Similarly a motor that has over heated and has shorted turns will exhibit a locked rotor.

    There are repairers who have the necessary equipment to repair motors but this could be expensive and may well not be economic.

    Sad but IMO you now have two dead stepper motors. Regards - Pat



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    Community Moderator ger21's Avatar
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    A good rule of thumb is never take a stepper motor apart. It'll almost always lose a lot of it's power.

    Gerry

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    Quote Originally Posted by wildwestpat View Post
    Q Does anyone know why the motor would lose its strength, simply by removining then re-installing the armature?

    R The rotor probably had a multi pole magnetised core. Removing the rotor from its stack has caused the loss of magnetism as the magnetic circuit via the stator was not there to act as a magnetic keeper. As you have discovered opening a stepper motor of the pm or hybrid type spells the end of that motors useful life.

    Q Are steppers extra pre-magnetized durring assembly, and how would I return this force once opened?

    R Yes some motors of the stacked sort are flash magnetised after assembly.

    It night be possible to re-magnetise but this will require a very high pulse of current through the windings to produce an excessive magnetising force pulse but with out over heating the coils. The current would have to be balanced with the three phases of the coil so is not a DIY task.

    Q At 2.7 Amps, 2.30 VDC, 200 Steps (unsure of its torque in oz)... Should I be able to stop its rotation by hand?

    R Probably not but a better test is to check the resistance to turning with the coils shorted this will be next to impossible to twist by hand or with pliers.. A demagnetised rotor will offer much less resistance to a twisting of the shaft with the coils shorted. Similarly a motor that has over heated and has shorted turns will exhibit a locked rotor.

    There are repairers who have the necessary equipment to repair motors but this could be expensive and may well not be economic.

    Sad but IMO you now have two dead stepper motors. Regards - Pat
    Agreed, to buy a new motor is more economic

    Last edited by Al_The_Man; 06-03-2011 at 06:48 PM. Reason: advirtising


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    Opening a stepper motor that has a multicoil stack (that's most modern steppers) will destroy the alignment of the coils in the stack, the stepper is useless after that.

    I don't know of any techniques to realign the coil stack.



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    Thanks guys, Im so used to modifying R/C motors (swapping armatures) that I would never suspect this issue. A new motor is in route for 25$, free priority shipping too Ill save the lesser torque, effected stepper for something else. My machinist was able to salvage the pressed lead-screw from the original motors armature, and turn it down to fit a coupling. I created a few different thickness motor spacers to compensate for the coupling. Now just waiting on the coupling to test the modified system, then waiting on the new motor to finalize and return to printing. If all else fails, I'll budge and purchase the direct replacement stepper (475$) for the printer. Thanks for all the help, input and advice. I came here for knowledge and I appreciate the lesson

    Next project... My first CNC router build!

    Attached Thumbnails Attached Thumbnails -1-jpg  


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    Also, heres a quick 3D model of what the printer carriage or gantry looks like. Sorry if im unfamiliar with tems, still have alot to learn

    First 2 images are factory, pressed lead-screw stepper.
    Second 2 images are modified to fit a steel coupling, 4 screw clamp style, modified by adding 2 set screws, with both shafts flat filed only in where the set screws meet the shafts.
    Like this... ------__------ | ------__------

    Attached Thumbnails Attached Thumbnails -edgefx-current1-jpg   -edgefx-current2-jpg   -edgefx-mod1-jpg   -edgefx-mod2-jpg  



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