Experiment in home shop motor balancing
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    Default Experiment in home shop motor balancing

    I recently bought a surface/tool grinder from a fellow member here. I havenít been able to get a good finish with it. Shows some scalloping on the finish. It was pretty obvious there was a fair amount of vibration coming from the motor. It had, if not the original, the motor that was designed to go on it. It was a single phase Rockwell 3/4 hp motor.

    I decided to replace it. I got an 1800 RPM 3 phase WEG motor and matching WEG VFD. They arrived yesterday and I promptly wired up the motor on the bench. The literature for this grinder says that the original motor and pulley are balanced from the factory and that should the pulley be removed that it should be rebalanced.

    After searching the Internet for information on balancing I came across a site where someone had published an article and spreadsheet for a method of balancing using a real time analyzer, accelerometer, or displacement gauge. At the heart of it he says that the device used to measure the vibration can be almost anything that gives a quantifiable vibration measurement, even a speaker with a weight placed at the center of the cone to generate an electrical signal that can be measured. I got the feeling the article was written before smartphones were around.

    Being that I have an iPhone I looked and found a fairly sophisticated vibration measurement app. I decided to use his spreadsheet and the phone to attempt to take some measurements and improve the balance.

    Hereís the motor on the bench...



    Whatís interesting about the app is that it doesnít just show a composite vibration measurement, it gives values for vibration in 3 axis, x, y, and z. The way I placed the phone on the motor the x axis would be from the top to bottom of the picture above (lateral axis), the y axis (longitudinal) would be axial from the front to back of the motor shaft, and the z axis (vertical) would be up and down, normal to the bench.

    The app can give measurements in a variety of units but I chose to take readings in m/s/s. The values are so low that what Iím reporting below are the values times 1,000. So below where I say the value was .4 it was actually .0004.

    With the phone sitting on the bench, motor not running, it reports x .4, y .4, and z .5. This is the inherent noise in the sensor I believe. No matter where I place the phone I canít get a reading less than those values.

    Placing the phone on the motor while the motor sits directly on the bench, no pulley, key installed in shaft, motor running I was getting

    X 2.4, Y 8.3 and Z 1.8. It occurred to me that if the motor were rigidly mounted that the vibration values would go down since the motor wasnít free to move and thus generate any acceleration. So I then set a piece of semi-rigid insulation foam on the bench and set the motor on top of that. Again, no pulley. Values were

    X 29.7 Y 11.3 Z 27.5.

    The foam obviously gave the motor some freedom to move. My guess is that a softer piece of foam might work even better.

    As I would expect most of the vibration is in the X and Z axis which would be vibration up and down and side to side. The Y axis would be axial. Iíll leave out Y values from here on out.

    I then added the pulley and ran it again. Values were

    X 36.3 Z 37.9

    So the pulley increased vertical and lateral vibration which is what I would expect. Keep in mind that this thing FEELS dead smooth. If you rest a hand on the motor you can just perceptibly feel what Iíd characterize as the slightest hum through your hand.

    The process then calls for you to place a weight on the pulley at a zero reference, then a 120 degree reference, then a -120 reference and take measurements with the weight at each of those 3 positions. The amount of weight can supposedly be anything however it was obvious that too much weight would induce a vibration that was too harsh and I didnít figure would give good results. I ended up using a weight that was 1.06 grams. My measurements were as follows in X and Z.

    No weight
    36.3, 37.9

    Zero
    48.6, 47.9

    120
    33.6, 35.2

    -120
    77.5, 76.6

    You then plug these numbers into a spreadsheet and it spits out an amount of weight to add or remove and the position at which to add or remove it. The spreadsheet assumes a composite vibration number so I had to pick an axis to use. As you can see the X and Z axis moved almost the same amount and so I decided to use the Z axis measurements.

    The spreadsheet said to add 1.08 grams at 72 degrees. Now, I didnít go mark the degree positions with any high degree of accuracy. I just eyeballed them. I took my original 1.06 weight and added it at approximately the 70 degree position. I then took a final measurement to see if it had reduced the vibration. My results were

    21.7, 15.5

    Compared to the original measurement with the pulley and no weight of

    36.3, 37.9

    Seems to be a significant improvement.


    A couple of things to note, the app is very interesting. It can not only spit out a graph and values of the realtime vibration going on it can do a FFT to isolate the frequencies at which the vibrations are occurring. It then plots those on a graph marked in either Hz or RPM (same thing, different expression). As youíd expect hereís what that looks like



    Itís got a resonant vibration at exactly the motor rotation speed of 1805 RPM. If there were other things such as ball bearings, etc. causing resonance at other frequencies it would show those as well.

    And hereís a screen shot of one of the measurement screens. I did use the RMS as opposed to peak values.



    This can also be done without a spreadsheet just using trial and error. As opposed to taking measurements at 0, 120, and -120 degrees you could take them at 0, 90, 180, and 270. No weight initially, then each of those positions, and see which position makes the most improvement. Then take 2 more measurements either side of the most improved position, etc, until you hone in on the best reading.

    Just FYI the iPhone is only good up to around 3000 RPM for doing this.

    I also want to qualify this post by saying I no little to nothing about balancing. I also realize this is only a static balance. Since the pulley itself is the only thing Iím actually balancing here yes, it could be done by putting it on a static balancing fixture but I donít have one and havenít made one. Additionally the title should have been changed but I canít change it now. I thought this was an interesting experiment at the least.

    If nothing else it gives a way to quantify the vibration of one machine or system compared to another. The sensor in the phone is incredibly sensitive.

    Below is another screen shot. Notice the spike in the Z axis (orange) right in the middle. For this I set the phone on my concrete shop floor, walked back 6í, and stomped my foot moderately.




    Sent from my iPad using Tapatalk

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    Work from the inside out. Remove the rotor in the new motor and sit the two bearings on some wood V blocks. Spin and balance it without pulley.

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    I thought the screen capture below was pretty interesting. After installing the motor back on the grinder I set the phone on top of the mag chuck.

    You can clearly see the up and down vibration coming from the motor at 1800 RPM. The big peak however is coming from the spindle spinning at 2600 RPM and is a left to right vibration. Iím not sure what the other smaller resonances are.




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    Quote Originally Posted by TravisR100 View Post
    I also want to qualify this post by saying I no little to nothing about balancing.
    How about I "know" little to nothing about balancing.

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    For my own curiosity, is the 1805 measurement with a tachometer? Unless you are using a VFD, you are above synchronous speed. Normal speed for a 60 hz 4 pole motor is 1800-slip rpm which will get you to 1780 or so. Unloaded I don't know the speed should be. Was the die cast pulley balanced?

    Tom

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    Quote Originally Posted by TDegenhart View Post
    For my own curiosity, is the 1805 measurement with a tachometer? Unless you are using a VFD, you are above synchronous speed. Normal speed for a 60 hz 4 pole motor is 1800-slip rpm which will get you to 1780 or so. Unloaded I don't know the speed should be. Was the die cast pulley balanced?

    Tom
    No, itís not with a tachometer. Itís derived from the resonant vibration. It was balanced on the motor with the app, but not off the motor on a balancing stand.


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    Quote Originally Posted by jabberwoki View Post
    I did!


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    Quote Originally Posted by TravisR100 View Post
    After searching the Internet for information on balancing I came across a site where someone had published an article and spreadsheet for a method of balancing using a real time analyzer, accelerometer, or displacement gauge. At the heart of it he says that the device used to measure the vibration can be almost anything that gives a quantifiable vibration measurement, even a speaker with a weight placed at the center of the cone to generate an electrical signal that can be measured. I got the feeling the article was written before smartphones were around.

    Being that I have an iPhone I looked and found a fairly sophisticated vibration measurement app. I decided to use his spreadsheet and the phone to attempt to take some measurements and improve the balance.
    I can no longer grant "likes" due to software, but yah EARNED easily a double one, thanks!


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    Quote Originally Posted by TDegenhart View Post
    For my own curiosity, is the 1805 measurement with a tachometer? Unless you are using a VFD, you are above synchronous speed. Normal speed for a 60 hz 4 pole motor is 1800-slip rpm which will get you to 1780 or so. Unloaded I don't know the speed should be. Was the die cast pulley balanced?

    Tom
    AFAIK rotating part vibration would be at slip speed but magnetic (frame) vibration is at exactly line frequency or multiples of it ie 60hz, 120hz, or 180hz or.. .
    Some motors are worse than others, 2-pole motors(frame experiences higher magnetic forces) and single-phase motors(auxilary winding/capacitor is perfect match only for certain load and anything else causes "phase" imbalance. Lucky for OP he has 4-pole 3-phase motor.

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    Quote Originally Posted by MattiJ View Post
    AFAIK rotating part vibration would be at slip speed but magnetic (frame) vibration is at exactly line frequency or multiples of it ie 60hz, 120hz, or 180hz or.. .

    ..and a device with a tad under 3/10 of one percent time-base-error.

    I say "device" rather than "line", given that major utilities are borderline insane as to holding cycles per second, even if nothing else they do is as close to spec.

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    Quote Originally Posted by thermite View Post
    I can no longer grant "likes" due to software, but yah EARNED easily a double one, thanks!

    Thanks. Still playing around with this trying to eliminate as much vibration as I can from the drive/belt. It uses a standard 4L belt with a 5Ē pulley on the drive motor and I believe a 3.5Ē pulley on the driven end.

    There is no way to tension the belt or align the pulley other than loosening the 4 motor mount bolts, moving it around, then tighten. Not conducive to alignment or tensioning.

    Is there a better drive method than a rubber v belt?


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    Travis - thanks for the interesting post.

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    For anyone interested this is the link to the app I used. No affiliation.


    Vibration analysis on the App Store


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    Quote Originally Posted by TravisR100 View Post
    Thanks. Still playing around with this trying to eliminate as much vibration as I can from the drive/belt. It uses a standard 4L belt with a 5” pulley on the drive motor and I believe a 3.5” pulley on the driven end.

    There is no way to tension the belt or align the pulley other than loosening the 4 motor mount bolts, moving it around, then tighten. Not conducive to alignment or tensioning.

    Is there a better drive method than a rubber v belt?
    The belt may very well be a source of trouble.

    ISTR a "4L" will fi where an "A" section was meant to be.

    "A" series are one of a few legacy Vee's built by intention to a very high standard. They do not need to be bought in "matched" sets, for example. The QC spec is so tight all you need is same-age. Eg: purchase at the same time, and they will be INHERENTLY "matched". That you have no gross-motion adjustment ALSO sez "predictable length" or "A" section.

    Same again with not being "lumpy". They are the smoothest-running of the Vee belt tribe.

    A 10EE uses two. Some larger lathes can use six or even eight. That instead of heavier-section Vee belts, and fewer of them is because of the "A" smoothness. Plus is that an array of belts also cancels-out each other's faults.

    Even so.. modern PolyVee and MicroVee are better yet. Lesser vertical height, tension-member to gripping faces = less stress, load-carrying band to sheave chord line, lower loss into heat, smoother running.

    They are manufactured in metal molds over a meter wide, then "slit" by whatever rib-count the market demands. Among the common PolyVee goods, figure about point-four HP transfer per each ONE tiny rib. A one-ribber won't live long, Four and more do.

    Better grinders, Old Sklew, if belted at all, rather than direct-drive, often used FLAT belts, BTW.

    Same reason studio-grade audio record-player turntbles did. Linen or "composite" flat belt isolated a dirty-little shaded-pole split-phase 50 or 60 Hz motor from discerning human ear that could detect its acoustic signature. Much as a grinding wheel "writes" a "watermark" of its own vibrations for the Mark one eyeball to detect.


    But.. all that sadi? We don't know squat about yer spindle or its bearings, do we?

    Pope, Parker-Majestic (now Penn Tool?) SMALL handful of others?

    Worth chasing further.

    Not all others ARE worth it.

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    Itís a Delta Milwaukee Toolmaker grinder. 5Ē drive pulley, 3,5Ē driven. My understanding is spindle has ball bearing in back and tapered bronze bearing in front. Iím definitely getting vibration from the 4L rubber belt.


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    Quote Originally Posted by TravisR100 View Post
    It’s a Delta Milwaukee Toolmaker grinder. 5” drive pulley, 3,5” driven. My understanding is spindle has ball bearing in back and tapered bronze bearing in front. I’m definitely getting vibration from the 4L rubber belt.


    Sent from my iPhone using Tapatalk
    It can be useful. Those half-way grinders were beloved for their small mass and small footprint where a body otherwise had no SG of any sort.

    But they are a full order of magnitude SHORT. Work to ONE thou, not to splitting one TENTH of a thou, and routinely, as their bigger brethren do.

    So... not a "real" surface grinder, so to speak.

    Lightest SG's are Reid and such.

    GOOD ones? At the small end Taft-Pierce, Parker-Majestic, Gardner... etc.

    Mediums - Check the mass on a Grand Rapids/Abrasive Machine or a Gallmeyer & Livingston. HEAVY buggers!

    High end - yah don't even want to know what those WEIGH!



    That - and usually a legendary "Pope" spindle, no matter what horse it was astride of - is what it takes to split "tenths" all day, every day, day after day, and for long years.

    BTW.. 4L belts are for lawnmowers, mostly!

    Cheap seats. Lumpy joints, indifferent QC, low-cost materials in general. Disposables, y'see.

    IF you can get an "A" on there, it should help. So would a PolyVee or flat. Also balanced Cast-Iron, not light-metal pulleys.

    But you just don't have much of a spindle, nor the rest of the stiffness, high-grade precision finished support structure, axis of movement and rate-of-movement control.

    Chase too much lipstick for this pig, it is no longer a value for money, generally useful deal.

    Run what yah got. Acquire better, later.

    Or farm out critical work. MUCH wiser.

    It takes a major investment in time to become a dab hand as a grinder-guru, and serious money in choices of wheels, tools, and fixtures to put it all to best use.

    I am not he, don't want to be.

    We have members on PM who ARE those experts. HAVE those large collections of wheels, HAVE more than one grinder, HAVE lots of fixtures.

    Did I forget to mention? "Know their s**t?"

    UPS and FedEx still exist?


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    Iím aware of what it is and its limitations but figure why not try to get the best performance out of it that I can?


    And at the moment it definitely has some vibration issues most of which I believe are related to the belt.

    So youíd recommend an A belt of MicroVee? Havenít heard of a MicroVee but Iíll look it up.


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    Ah, I see. A micro ribbed belt. Have to see if I can find pulleys and belts that will fit.


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    Quote Originally Posted by TravisR100 View Post
    Ah, I see. A micro ribbed belt. Have to see if I can find pulleys and belts that will fit.


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    General replacement for legacy Vee belt, many multiple tens of millions by now, and as little as ten bucks, nearest discount auto parts outlet;

    Automotive Micro-V Belts | null | null | Gates Corporation

    Poly V(R) belt : Power transmission components | Hutchinson Belt Drive Systems

    The resilient ribs don't actually require the sheaves be grooved, will run on plain smooth surfaces as if they were ribbed flat belts as well.

    Plenty of cone-head lathes running them. Some of them "inside out" on their plain backside.

    I do DO NOT recommend glue splice or lacing or metal gripper splice where vibration is an issue!



    Get the next-closest-smaller size. Shim-up the motor 'til it is about the right tension to haul the load, not so taut as to overload the bearings. Spring loaded idlers are cheap - MOPAR van V6 one for example, but yah don't really need the complication, this light of a load. Belt won't wear or stretch that fast.
    Last edited by thermite; 11-30-2019 at 11:46 PM.

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