Many shop or machine tools have big motors, and many big motors VIBRATE. In some cases this is a good thing, but usually it is not. Smooth running motors last longer, make less noise, and in the case of saws or other shop tools, make better quality cuts.
The best way to make your motor perform better is to do a dynamic balance, which just means "balance it while it is moving". The old school way of balancing a motor would be to take it apart, balance each part on a scale individually, and then reassemble. This is static balancing and it certainly has its place.
But not today!
Dynamically balancing a motor involves adding small amounts of weight to the motor in order to counterbalance any parts that are heavier on one side than the other. That imbalance is the major source of vibration, and if you correct it, you can have a much smoother running machine. There are lots of ways to dynamically balance a motor, but this one is intended to be quick and simple, for machines that are already setup (without taking them apart).
You will need:
Why the iPhone?
Dynamic balancing used to require specialized instruments. Fortunately, we carry these specialized instruments around in our pockets! The accelerometers in modern smartphones are quite precise, and best of all, are with you all the time. You will need a phone and an app that can sample the accelerometer with at least 100Hz frequency. 100Hz allows reliable measurement of a 50Hz signal, which is about the fundamental vibration node of a motor spinning 3000RPM (because 50Hz * 60 = Revolutions per Minute!). There is a lot of science behind that, but trust me on this one. You can use almost any smartphone and any number of vibration measuring applications, just make sure that it gives you RMS ("average") g-force acceleration in X, Y and Z axis separately, and can measure in hundredths of a G (0.01 Gs).
People have been dynamically balancing for a long time, and there are a lot of tools for doing it. I like Sugru because it has the perfect balance of permanent (it won't dissolve or fall apart like tape or blu-tac) and removable (you can scrape it off with a razor, unlike epoxy or drilling/welding). It is also dispensed in 5 gram packets, making it easy to divvy out a known quantity without a scale. It is also just plain easy to work with, especially on a motor that is assembled into a machine, rather than out on a bench.
Note - this process is for performing a calculation-free single-plane balance, which means the vibration we are trying to eliminate is only on a single plane (the one parallel to the rotor), and that we are using many measurements instead of fewer measurements and some vector math to identify the optimal location. For thin disks, like most workshop flywheels or pulleys, this is fine, and eliminates the dominate source of vibrations without much fuss. It should be noted that very thick pulleys, long shafts, or really wonky pulleys may require multi-plane balancing, which can include out of balance motion in the "in-out" direction as well as the "up-down" direction. That is out of the scope of this Instructable, though (if you write one, leave a link in the comments!)
I am balancing the 3hp motor to a CNC lathe, but it could be nearly any motor, big or small. To prep, I've removed the belt, cleared any obstructions away from the motor, and set up the control so I can turn it on and off with ease. I also wiped down the sheave (pulley) with some degreaser so the Sugru will stick.
Make sure you aren't wearing any loose clothes, and that your hair is tied back securely. Throw on some safety glasses since we will be making things spin!
Label your motor pulley in four places. I labeled mine "1, 2, 3, and 4" at each quadrant, though you could use A, B, C, D, or 12:00, 3:00, 6:00 and 9:00. Just make a sketch, make the same markings on your pulley with a sharpie, and keep track.
On your notepad, make a table with rows for each of your positions, 1, 2, 3, and 4, and a column for X, Y and Z vibrations (side to side, front to back, and up and down).
Setup your iPhone by loading the Vibration app. This is a paid app, but I use it a lot and like its simple, precise interface. It also allows for exporting vibration data, changing sample rates, viewing frequency data, etc. That said, use whatever smartphone technology platform and vibration app you prefer. If you have a favorite, leave it in the comments!
In your app, make sure that sample rate is at least 100Hz, that you have a sample time of at least 5 seconds, and an easy way to start the measurement without bumping the phone (or a start delay like Vibration).