What would cause a SEM DC servo to stop free spinning


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Thread: What would cause a SEM DC servo to stop free spinning

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    Default What would cause a SEM DC servo to stop free spinning

    From what I understand SEM DC servo motors are robust and built very well. I tested a motor with DC power (in the appropriate voltage range), then shipped the motor with the shaft protected from damage, packaged properly, etc. Now the buyer claims the motor will not spin free, but the box wasn't overly banged up by USPS, no disaster just typical stuff. What could cause the motor to stop free spinning by hand. The motor is an MT30M4-59 SEM motor.

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    Community Moderator Jim Dawson's Avatar
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    Default Re: What would cause a SEM DC servo to stop free spinning

    Maybe ask the buyer what ''does not spin free'' means. I have some on the shelf and they are are a bit stiff, but will turn by fingers with some force. If he expects to flip the shaft with his fingers and the motor freewheels for awhile it ain't gonna happen. On the other hand I have a router spindle that takes a bit to stop after giving it a spin by hand.

    Jim Dawson
    Sandy, Oregon, USA


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    Community Moderator Al_The_Man's Avatar
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    Default Re: What would cause a SEM DC servo to stop free spinning

    A couple of instances, one is the fact that a motor with the leads shorted together when spun, is self braking.
    But more serious is if a PM pole piece came loose in shipping, they are often attached by an epoxy adhesive to the outer case.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

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


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    Default Re: What would cause a SEM DC servo to stop free spinning

    He sent me a picture with a pair of vice grips on the shaft, he is claiming that it will not spin with vise grips on it. The motor was sent in a large USPS box with a large tube over the shaft to protect it from damage. All the motors were tested as working and checked for other issues before being sent. The buyer is claiming that the motor shaft was bent during shipping. I find it HARD to believe that the motor shaft was bent on on one of these motors in shipping, unless there was some disastrous event. In which case there would have been evidence, destroyed box, signs of damage on the housing, etc. Is it possible to bend one of these shafts (that is protected) in shipping? Is there any other means of stopping these motors from spinning, if it was dropped and the shaft lodged itself backward, shorting out the brushes on the comutator, anything like that?



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    Community Moderator Jim Dawson's Avatar
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    Default Re: What would cause a SEM DC servo to stop free spinning

    If it was dropped it out of an airplane from 30,000 feet, then that could be a problem. Normal shipping and handling shouldn't bother a well packaged motor. If it was dropped exactly on one end or the other and there was no cushion room between the box sides and the motor I suppose the armature could be pushed into the brushes if the thrust bearing had enough give in it. Or maybe the armature could actually move on the shaft with a hard enough hit.

    In any case, there should be some evidence of a violent blow to the box. I normally use quite a bit of bubble wrap for packaging stuff like that.

    The only other things I can think of are what Al_The_Man said above.

    On the other hand, I could send you a picture of a SEM motor with vice grips on the shaft, might even be able to make it look like it wouldn't turn.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: What would cause a SEM DC servo to stop free spinning

    I had a SEM motor lock up like that for no obvious reason, when I opened it up one of the magnets had detached itself from the frame and stuck itself to the rotor, it had not been dropped or abused in any way. The magnets appear to be merely glued into place. I cleaned it up and used Araldite to fix it back on and it worked fine afterwards. I suspect if they get too hot then it may weaken the bond. I have heard that removing the rotor weakens the magnets but there was no obvious loss of torque afterwards. Dave



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    Default Re: What would cause a SEM DC servo to stop free spinning

    The shaft taking a hit was a concern so I used plenty of heavy shipping packaging around the motor and a cardboard shipping tube cut longer than the shaft to protect the shaft from impact. The shipping tube was longer than the shaft by over an inch and was taped directly to the motor face so any direct impact would hit the tube face instead of the shaft face. I had considered the magnet coming loose, I even offered to have the motor shipped back so I can inspect it. Would it be a huge surprise that today I just looked back through the initial messages and the buyer said in an message that he only needed one for sure but possibly more of the three motors in the lot, this was before he received them. It was only when he got the other two motors installed that one doesn't spin and he wants a refund of %30. I originally offered a full refund, but the buyer won't take it.
    Does anyone want to take a guess as to which website this sale occurred through...



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    Community Moderator Al_The_Man's Avatar
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    Default Re: What would cause a SEM DC servo to stop free spinning

    If you have ever visited a UPS loading dock, you shouldn't be surprised, I once received a motor that had obviously been dropped on the shaft end.
    After that, when ever I shipped a motor I used a short piece of rubber hose slid over the shaft and extending out an inch or two.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

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


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    Community Moderator Jim Dawson's Avatar
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    Default Re: What would cause a SEM DC servo to stop free spinning

    I don't know what to say. I think all you can do is try to get him to ship the ''defective'' motor back, and give him his 30%, but only after you get the motor. One of the hazards of doing business on the Bay.

    Jim Dawson
    Sandy, Oregon, USA


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What would cause a SEM DC servo to stop free spinning

What would cause a SEM DC servo to stop free spinning

What would cause a SEM DC servo to stop free spinning