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.