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    Default Electric motor question

    Does anyone know how accurate the base or mounting bolts are located on an electric motor in regards to the centerline of the motor shaft? Not concerned with the elevation centerline, not sure of the proper term but the left to right parallelism if that makes sense. The ones I'm currently dealing with are 25 HP. Good quality name brand. (Can't recall off the top of my head) Not being an electrician but these appear to be well built American made. I can only assume the parallelism must be close because the base bolts are are close to size telling me that there is enough wiggle room to do a precision alignment with the pump in this case. This particular motor uses a three piece drive coupler, two hubs, and a center shaft with flex plates.

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    The mounting bolts should be at least 1/16 smaller than the base holes and likely more. Either the base holes or the mount plate holes will have to allow adjustment horizontally ( assuming the shaft is horizontal). You have to allow for the stacking of tolerance in both sets of holes and the base plate is often crudely done on site.

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    The only motor that you can figure is decently accurate on mount vs shaft is a C-face type having a locating ring machined-in....(I think there is a designation for that, dunno) All others are going to be "off" somewhat, and the mount needs to adjust to get it in line.

    The housings are cast, and are usually bored etc with respect to the OD, and not to the feet.

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    Ive set up lots of drives ,and you simply shim the motor mounts until the shafts are co axial......or use one of the very flexible drives like the ones thsat use a small "tire" clamped in flanges,or the well known "airflex" type made by GM years ago in very large sizes for marine drives.

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    Quote Originally Posted by gmach10 View Post
    Does anyone know how accurate the base or mounting bolts are located on an electric motor in regards to the centerline of the motor shaft? Not concerned with the elevation centerline, not sure of the proper term but the left to right parallelism if that makes sense. The ones I'm currently dealing with are 25 HP. Good quality name brand. (Can't recall off the top of my head) Not being an electrician but these appear to be well built American made. I can only assume the parallelism must be close because the base bolts are are close to size telling me that there is enough wiggle room to do a precision alignment with the pump in this case. This particular motor uses a three piece drive coupler, two hubs, and a center shaft with flex plates.
    Like any other manufactured item, the tolerances are called-out, right on the print. Which means.. you need to know the makers and at least the "family" of each motor so you can go and GET the matching print and/or specifications in a published table.

    "Foot" mounts, as others have said, are expected to be shifted left-right / in-out, and shimmed up-down at the time of installation.

    "Face" mounts are made to a closer fit, those with a locating ring ledge most of all. Even so, just "how" close is, again, in the callouts on the maker's drawings or tabular published specs.

    Many motors are "both", of course.

    Industry standards for "frame" types differ, US or Euro/IEC, practice, but standards they are. No mystery about the specs. Only about "which motor" have we, here?

    Absent data plates, yah just have to do the detective work on an unknown motor to find what it "seems to be" at least as far as frame type or equivalent so as to find what is already published.

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    There are so many motors and motor manufacturers out there that the best you can say is some are better than others.

    So, yes, check the specs for your particular motor. A 25 HP should have a spec sheet available.

    If you can not find the OEM's spec sheet, then assume the worst and make your design adjustable.

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    Quote Originally Posted by john.k View Post
    Ive set up lots of drives ,and you simply shim the motor mounts until the shafts are co axial......or use one of the very flexible drives like the ones thsat use a small "tire" clamped in flanges,or the well known "airflex" type made by GM years ago in very large sizes for marine drives.

    A couplings ability to flex is not a substitute for alignment. The life of the coupling and bearings on either side of it are drastically affected by the alignment or lack thereof.

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    Align the shafts properly or the coupling will wear out in short order. There will be enough wiggle room in the bolt holes for final alignment but they should be fairly close.

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    Thanks for the replies. I hear what all of you are saying. I was more wondering if there was an industry standard for hole location/centerline. We typically used two dial indicators to get it in the ballpark then the electricians put their laser aligner for the final alignment. Sucks when they send guys over who aren't familiar with the laser though.

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    There are industry standards..NEMA, IEEE, and API come to mind.

    You don't say what size or speed motor...that makes a big difference. But assuming under 600 volts and 500HP, NEMA has standard frame sizes which include footholes and shaft heights. Most motors made by non-Chinese countries are very close to spec.

    That said, it is always assumed the installer will properly align any motor. There's no way a motor maker can manufacture a motor that will 'know' how flat and in-spec your motor base is. Keep in mind, on a 3600RPM motor just a couple thousandths will make a big impact. On slower motors, it's almost as critical.

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    He said it is a 25 HP. Not something I am going to pick up, but not overly large either.



    Quote Originally Posted by GregSY View Post
    There are industry standards..NEMA, IEEE, and API come to mind.

    You don't say what size or speed motor...that makes a big difference. But assuming under 600 volts and 500HP, NEMA has standard frame sizes which include footholes and shaft heights. Most motors made by non-Chinese countries are very close to spec.

    That said, it is always assumed the installer will properly align any motor. There's no way a motor maker can manufacture a motor that will 'know' how flat and in-spec your motor base is. Keep in mind, on a 3600RPM motor just a couple thousandths will make a big impact. On slower motors, it's almost as critical.

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    Right you are.

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    Ive seen dozens of drives on augers ,conveyor shafting,etc where the motor wobbles all around,then either the keyway flogs out or the mounting cracks .....seems to be standard practice with manufacturers of dust collectors to have blower motors dancing about.....

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    Quote Originally Posted by gmach10 View Post
    Thanks for the replies. I hear what all of you are saying. I was more wondering if there was an industry standard for hole location/centerline. We typically used two dial indicators to get it in the ballpark then the electricians put their laser aligner for the final alignment. Sucks when they send guys over who aren't familiar with the laser though.

    Alignment is usually a millwright job. You should be good with indicators. The lasers are great UNLESS the machine is in the sun or other variable heat source. But they are the cat's meow for Cardan shafts.


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