Introduction: Dynamic Motor Balancing - With Sugru and an IPhone!

About: I'm an engineer in the renewable energy world, and help run a cooperative workshop makerspace in Boulder, CO called the Phoenix Asylum.

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:

  • A mini-pack of Sugru
  • An iPhone (or other smartphone)
  • Vibration App (or other vibration measuring app)
  • Notepad and writing instrument

The strategy:

  1. Measure the vibration of our motor.
  2. Add a little weight, and measure again.
  3. Find the spot that lowers the vibration and add more weight.
  4. Tune into the optimal position, and finish!

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).

Why Sugru?

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!)

Step 1: Setup the Motor

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).

Step 2: Take Your Measurements

Now we measure vibration! We will take a lot of measurements, and use that to determine the best location for adding weight.

We will take the following measurements:

  1. Baseline, no weight added.
  2. Sugru on Position 1
  3. Sugru on Position 2
  4. Sugru on Position 3
  5. Sugru on Position 4
  6. Sugru a little ahead of the best Position
  7. Sugru a little behind the best Position
  8. A little more Sugru on the best Position
  9. A little less Sugru on the best Position

As you take these measurements, you should see some getting better than your baseline, and some getting worse. You may even hear the difference in the form of better or worse motor noise.

Here is how you take each measurement:

  1. Place a gob of Sugru on your motor pulley, preferably in a spot with a little concavity so it doesn't want to get slung away. How much to use? Depends on how out of balance your motor is. I'd recommend starting with about 2g, or a little less than half a Sugru mini-pack.
  2. Fire up your motor and bringing it up to speed. This process will work best at 1000 to 2000 RPMs, but should work at anything up to the max measurement frequency of your app (half the max sampling frequency).
  3. Place the smartphone on a flat spot on the motor where it won't fall off during sampling. You may need to use a rubber band to secure it to the motor if you don't have a good flat spot. The orientation of the phone doesn't matter, but make sure to use the SAME orientation for all measurements.
  4. Hit the "Sample" button, and allow it to take a ~5 second sample.
  5. Stop the motor.
  6. View and record the results, remembering to take the RMS ("average") vibration in the X, Y and Z axis.
  7. Move your Sugru, and try again!

Tips and Tricks:
  • Start with a small amount of weight, but if you have a big pulley that is very out of balance and find yourself making improvements even up to 5g of additional weight (a whole Sugru mini-pack), you can use the Sugru to attach heavier items, such as lead shot, small nuts and bolts, or strong magnets.
  • You may need to secure your phone to the motor before starting it, or you may be able to place the phone on it while the motor is running. The one thing you probably DON'T want to do is leave the phone balanced on the motor and then start it - the initial vibrations are higher than the running vibrations and could send it flying! Be safe - don't place things where they could fall into the pulley and get launched!

Step 3: Analyze Your Data

The final step is simply to look over the data you collected, and hone in on the final balance for your motor. As you notice which position results in the lowest vibration numbers, you can experiment. Move the Sugru forward or backward a bit and see if the numbers improve. Add a little more or a little less and see if it improves. Just remember to only make one change at a time - position OR quantity - but never make more than one change between measurements.

The more measurements you do, the closer you'll get to the perfect solution. In my case, this WEG induction motor was already pretty well balanced, but after just 10 measurements and about 20 minutes, I had cut the vibration nearly in half, certainly a good improvement for the time. The motor was noticeably quieter after balancing.

Wrapping Up:

After balancing the motor with the pulley attached, you can kill power, reattach the belts, and set your machine back up for use. Give the Sugru a final squish to keep it attached to the best position, and allow to cure for 24 hours. If your Sugru has gotten particularly gross or dusty by multiple re-attachments, pull it off and replace with a fresh gob of the same size before the final cure.

If you want you can continue the balancing process by repeating the same steps, but by attaching weight to the next pulley down the line. The trick is to start at the motor, and work out one pulley or shaft at a time until they are all balanced. A balanced machine is a happy machine!

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