I have recently replaced a motor for my attic fan because the original one burned out. I did measurements for the height of the fan blades in relation to the supports and ensured that when I re-attached the blades that they fit.
In the picture below, the fan is located in the angled ceiling of my attic.
After putting everything back together, I spun the blades and realized that they were making contact with 1 of the 3 support struts, suggesting that my replacement was not balanced. In order to fix this, I used several washers between the motor and the support strut to push the motor up and angle it in a manner that did not permit contact between the fan blades and the strut.
Afterwards, I reconnected the wires and let the fan run for about 5 minutes straight without issue. On the day of the work and test run, the attic was maybe only 80° F and I had to trigger the fan by lowering the thermostat down.
Satisfied that things were fine, I closed everything up. A few days later was much hotter and presumably the attic reached a temperature of 100° F and tripped the thermostat switch causing the fan to turn on. I heard a clanging downstairs, realized the blades were striking and shut the whole thing down.
Right now, I've just cut the electricity to the fan for now, but when I go to fix this, I want to ensure that however I balance the fan it will stay balanced through both temperature fluctuations as well as vibrations of the fan rotating. I'm pretty sure my washer-spacer solution was the wrong thing to do. What is the correct way to do this?
If it helps, I situated the fan blades at the same elevation as they were previously, but there is room on the spindle to move them further up if needed.
Best Answer
A few suggestions.
None of these include removing material from the blades (because after doing so you would indeed have a balance problem).
Forgive this one, but are you certain the blades are installed correctly? The fan indeed exhausts, when it's running?
In my experience, "replacement" motors, even under identical part numbers, might be subtly different from their originals. If this new one is slightly shallower, then you've done nothing wrong. Maybe check them against each other, and then cut yourself some slack.
(The fact that your fan isn't mounted dead vertically could cause you to have a problem that the manufacturer's quick & dirty testing failed to detect. Because gravity.)
Those bent-sheet brackets are a primitive thing. They're likely to have gone asymmetrical. Maybe put the palm of your hand against the motor's base, and press firmly to bend them, just a little.
Unless, if I understand you, the position on the blade assembly on the motor shaft is indeed adjustable. If so, then why did you bother with the washers? Side it on out!
None of the above explains why the problem didn't appear when you tested it. Only with a hot attic. Difference in air density? Behavior of the motor's lubrication? Or (as already suggested) a loose mounting screw?
Good luck. Let us know what works for you!