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Carl A
(@carl-a)
Estimable Member
magnets and extruder motor

I recently had an extruder motor fail and was surprised that it did. In the past I put a magnet with a pointer on the motor to track the movement of the motor as it printed. The magnet was a very strong magnet and when I removed it I had very fine black dust on it. Now I think the magnet may have been the cause of the failure as it effected the freedom required for the bearings to turn as they should. As I know that putting such indicators on the motor seems to be a regular practice, it may cause problems in the future for other hobbiests. May be an upper limit on magnet strength used. Any thoughts on this?

Posted : 08/06/2019 2:52 am
toaf
 toaf
(@toaf)
Noble Member
RE: magnets and extruder motor

ive often wondered about that, but I think the magnet inside the motor is stronger.

 

or I should say the magnets alignment in the motor is stronger.

This post was modified 3 years ago by toaf

I have a Prusa,therefore I research.

Posted : 08/06/2019 5:32 am
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(@)
Illustrious Member
RE: magnets and extruder motor

Depends on why and how to motor failed.  It could be a frozen bearing due to shaft loads; winding failure to to heat build up; clearance issue in manufacturing; to name a few. Or it could be magnetization failure for a variety of reasons (poor manufacturing, poor material, a crack developing during use) ...

Posted : 08/06/2019 4:28 pm
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: magnets and extruder motor

First it started skipping extrusion and then stopped all together. Replaced with new motor.

Posted : 08/06/2019 7:12 pm
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(@)
Illustrious Member
RE: magnets and extruder motor

Kind of a swag, but if the dead motor has an uneven jagged / scratchy feel when spinning it by hand, rather than a nice smooth rotation, it's likely a mechanical failure: bearings most likely.  The spin will have the "notchy" feel of the rotor/stator fields, but should be smooth and even. 

The Prusa motors are only rated around 11 lbf 1/2" out the shaft; and the Bondtech gear may be exceeding this if the idler is over tightened. Especially on the MK3S where the motor is already biased a bit tight.

If the motor feels smooth, you can ohm the windings to check for differences: they should measure near identical values.  A difference above 5% indicates a good chance of a winding failure. You can also short the windings (one at a time) and spin the motor to get a better sense of the fields - and shorting one winding should feel the same as the other. If they feel different, another sign it's likely the windings have failed. 

 

Posted : 08/06/2019 9:01 pm
Nigel
(@nigel)
Honorable Member
RE: magnets and extruder motor

Extruder motor and skipping is an age old problem. High torque and the motor can get hot. I have come across this in many different printers. Printing slower helps.  The extruder seems under strain with the MK3S and does get hot. I suggest a better motor is needed.

Nigel
Life is keeping interested and excited by knowledge and new things.

Posted : 10/06/2019 3:25 am
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: magnets and extruder motor

I have a Mk2.5. I thought of a heating problem but used a scope and it was no hotter than other motors when it was failing. Reading about 33C. I think PRUSA uses LDO MOTORS. My guess that it is a 1.8 Hybrid Stepper Motor  LDO-42STH40-1004ASR #181120. I thought LDO made some of the best motors but if you know of a better motor replacement than I would be interested.

Posted : 11/06/2019 12:04 am
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(@)
Illustrious Member
RE: magnets and extruder motor

Check the thread Guy started regarding better motors for reducing surface artifacts... he mentions a few 0.9 degree replacements, has good words for some.

ps:

https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-user-mods-octoprint-enclosures-nozzles-.../stepper-motor-upgrades-to-eliminate-vfa-s-vertical-fine-artifacts/

This post was modified 3 years ago by --
Posted : 11/06/2019 12:09 am
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