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Thread: Stepper motor shaft shattered!

  1. #21
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    Default Re: Stepper motor shaft shattered!

    that was really nice..
    now your issue fixed.. hope you get time for some update, after you put onto the new motors..

    so other cncrp users can see it..

    still my opinion, independent axis drivers are better solution.. because like in his case, you can put larger motor example on Z axis without replacing the whole system..
    also in case an axis fault, you fix only one axis not the whole system..



  2. #22
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    Default Re: Stepper motor shaft shattered!

    I have seen this on belt drives that were over-constrained (hard-coupled) between the belt and the pulley. And it was designed by Parker Motion. After running these stages through reliability testing we saw more than a few failures and Nema23 shafts shearing in half because of cyclical loading fatigue. The pulley and shafts were supported on both ends with a large bearing(s) and the shaft mated to the motor shaft via a hard coupler - the bearings supporting the belt tension were on the shaft with the two bearings, not even on the stepper motor.

    It appears that your motor shaft failed in the same way as mentioned above. No clear defect or void can be seen that would cause premature failures from stress concentrations etc. The failure does appear to be close to a feature that reduces or necks the shaft down to a smaller size right at a bearing constraint. Your pulley makes a belt contact at the furthest point away from this bearing constraint along the length of the shaft - in other words, your belt is creating a large moment (due to this distance, and tension) around a feature that could be more subject to stress concentrations.

    It is always bad practice to hard-couple or put belt tension on your drive engine shaft. One can imagine a process that perfectly controls concentrities between mating shafts that would mate a motor to some other rotating component perfectly, but this won't readily apply to a good number of cases; processes are difficult to control while removing the failure mode all together by use of a flex coupling eliminates your problem.

    A larger diameter shaft may solve your problem, or just push off the failure mode to much later. You had a 1/4" diameter shaft fail within 10 hours, do you know how much time an extra .125" (3/8" shaft) diameter will buy you in terms of lifespan given all the other conditions are equal? That's about 2.25X the cross-sectional area. You should be able to calculate the life.



  3. #23
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    Default Re: Stepper motor shaft shattered!

    Quote Originally Posted by MechanoMan View Post
    Driving a motor rated for 570 oz-in at 5A with 3.5A is 399 oz-in of torque.

    Driving at a lower voltage reduces the high-speed performance. Well more specifically IF your voltage-to-inductance ratio is too low, you can't meet the target current at higher speeds. But adding excess voltage alone doesn't increase speed in the absence of a problem with high inductance, in fact it can make its current rise times excessively fast and make the drive unstable.
    I never heard of drives becoming unstable, unless the inductance was so low that the drive could not handle it. I tested 1.7 mH motors with 60v without issue. The main drawback is that the motor runs hotter, which can affect the longevity.



  4. #24
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    Default Re: Stepper motor shaft shattered!

    Quote Originally Posted by MechanoMan View Post
    Wow, I did not expect this to happen. I believe that's a CNCRouterParts 380oz-in on a CNCRouterParts timing belt drive, 48v G540, across the gantry axis.

    After running the router for 10 hrs or so we noticed some non-round holes. I found that gantry axis could move a bit when you push on it, the belt wasn't tight enough. Saw the larger timing pulley shift while the motor pinion didn't. I tightened it a bit but hardly gorilla'ed that tight though.

    Was running fine then not-running. Went to check and found this:

    http://www.cnczone.com/forums/attach...d=278376&stc=1

    Whole output shaft split right at the bearing! Ever seen anything like that??

    The same thing just happened to me. I was cutting a new spoilboard-- milling a grid of holes along the bottom, everything was going smoothly. I was watching it for awhile and then as it seemed to be cutting fine, I got bored and went into the other room. Awhile later I started smelling a scorched smell and went back out to see what was up and the room was full of (light) smoke and the machine was busy scorching the board. Upon examination it turns out that shaft on my X-axis motor snapped in two. Not sure why as it's a brand new motor from a reputable company. (Shrug) I looked at the pattern of the holes and it was just as you said. Everything started out fine, and from the evidence that I see, it seems that the problem started shortly after I left. The holes started getting less and less round and more and more "slotted". At this point the whole X-axis slides freely back and forth independently of the motor. When I attempt to move the motor with the pendant I can see it turning, but nothing happens at the pinion gear which I have confirmed is still securely anchored to the motor shaft. So the problem-- presumed break-- must be on the inside of the motor. I haven't taken it apart yet to see exactly what happened, but it's the only thing I can think of that makes sense and fits the evidence.



  5. #25
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    Default Re: Stepper motor shaft shattered!

    that picture shows the whole story
    belt is too far from the motor housing

    so the shaft otherwise might would be ok to holding up, this way becoming weak



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Stepper motor shaft shattered!

Stepper motor shaft shattered!