After all the effort to get the weight exactly the same on
all the blades and the fan is still out of balance. Do this
simple test. Run the fan at low, medium, and high speed.
Does the balance get worse with higher speed? I thought so ;-)
Now, fix the real problem. The pitch of each blade must be
identical! The pitch is the angle of twist from the leading
edge to the trailing edge of the blade. Most blades are
mounted with an "H" shaped bracket and can be adjusted by
slightly twisting the bracket at the short section between
the two outside bars of the H.
You can use the reflection of a light bulb off the flat of
the top or bottom of the mounted blades to adjust the twist
so the bulb appears at exactly the same place on all the
blades as you hand rotate them. I can almost tell you, only
one of the blades will be off when you do the first
reflection test. Tweak it back into position and your fan
will run true at any speed.
I spent 4 hours moving quarters, nickels, and dimes,
swapping brackets, blades (yes, weighing was the first thing
I tried-they were identical) and it ran so well, until I
switched to medium speed ;-) I spotted that the reflection
of a nearby lamp on the blade surfaces "wobbled" as I turned
the blades. It's the pitch! One blade was really "off". In
a few minutes I had the same lamp reflection on all the
blades. Fan then ran smooth and quiet at all speeds.
It is very easy for a blade to get hit and the bracket
-larry / dallas