This method is easy, fast, no special tools are required and it achieves an excellent balance. This only works on free spinning, direct drive fans that can be placed horizontally at least for the balance procedure. It works for metal blades, though it should work for plastic blades too.
Step 1: Align the Blades
With the fan UNPLUGGED and one side of the grill removed, stick a nerf dart into the cracks of the grill until it starts to touch the edge of a blade as in the picture. Spin the blades slowly with your hand and note where each blade is hitting the dart. If any blades are out of alignment, gently bend it a little at a time until it matches the other blades. On my fan one blade was about 1 centimeter out of alignment.
Safety Tip: If the fan is operated by a thermostat, for example an ac condenser fan, make sure the power is turned off at the breaker because they can start at any time.
Step 2: Compare the Blades So They Can Be Ranked Heaviest to Lightest
With the fan UNPLUGGED remove one side of the safety cage. Clean the blades. Label the blades 1, 2 and 3. Use clear tape to attach a dime 'Helper Weight' to the tip of blade 2. (See picture) Hold blade 1 at the top center and let go and note how far blade 2 rotates down. Repeat a few times and note about where blade 2 stops each time. If it doesn't rotate, try something heavier like a penny or nickel. If it rotates too far and swings back, try a smaller weight like a paper clip.
Remove the dime from blade 2 and attach it to the tip of blade 3. Hold blade 1 at the top center and let go and note how far blade 3 rotates. Repeat a few times, noting where blade 3 stops.
The blade that rotates further is heavier, so you will now know which blade is heavier, 2 or 3.
Repeat this process to compare all the blades until you are able to rank the blades from heaviest to lightest.
Step 3: Make the Blade Weights Equal
Now you will adjust the lightest blade until it is equal to the heaviest blade. Start out by lightly sticking a 6" piece of matching color duct tape on the center of the lightest blade. Using the comparison method in step 2, compare the lightest blade to the heaviest blade, adjusting the length of tape on the lightest blade until the distance both blades travel during the test are equal which indicates they are balanced.
Repeat this method to adjust the weight of the middle blade to equal the heaviest blade. After the weight of the lighter blades have been adjusted, tape the dime to each blade in turn and note how far it rotates, the stop points should be close for each test. You can see the duct tape on 2 blades in the picture, the lightest blade has about 5" and the middle blade about 4".
For a 4 bladed fan make sure opposite blades are equal, 1=3 and 2=4.
Try taping the dime to each blade in turn and release from the 3 o'clock position. Measure how far each blade rotates, ranking them heaviest to lightest. Start adding tape to the lightest blade first and keep retesting until they all rotate the same distance.
Step 4: Move the Blades As Far From the Motor As Possible
If your fan allows the blades to be moved up and down the shaft, move them as far from the motor as possible. My fan was tested with the blades close to the motor as in picture 1, and as far from the motor as possible as in picture 2. Having the blades far from the motor made a large decrease in vibration. With my Air King I was able to flip the blade assembly over to get the blades an extra 2 centimeters away from the motor. Note in picture 2 the set screw collar is not visible because the blades have been flipped around.
Tip: The Air King set screw needs a 1/8" hex key, make sure it is really tight.
Put the safety grill back on and test!
My Air King 20" Whole House Fan (which is a fantastic fan) had irritating vibrations on high speed, this balancing method made a huge difference. I tried 3 other methods before coming up with this method, they were more difficult and didn't work as well.
Balance Fail #1
I took the fan off the motor shaft, mounted it on a shaft sized drill bit which was then propped up between two level chairs. It was then balanced by comparing the blades 2 at a time and adding tape to the lighter blades similar to the method above. The balance was not quite as good and this method was a pain.
Balance Fail #2
I took the fan off the motor and hung it from a table, just above the floor, by a rope going through the center hole. As the rope was raised and lowered off the floor the light blades were noted and adjusted. This method was also a pain and the resulting balance was poor.
Balance Fail #3
A vibration sensing app 'VibSensor' was used on an android phone to measure the vibrations. A piece of duct tape was attached to each blade in turn and then the fan was run so the vibration intensity could be noted. The app was great but adding the right amount of tape was a guessing game. Hours were wasted.