On Tue, 29 Jan 2019 12:58:33 -0500, Ralph Mowery
wrote:

http://www.casablancafanrepair.com/H...nuals/Chapter1
4/SM_WobblingFan_P1.htm
Dynamic balance is a funny thing. Weight distribution comes i nto
play. If all the blades are exactly the same weight, you are not
NECESSARILY in ballance. The weight distribution also comes into play.
If one blade is tip heavy it will balance statically, but not
dynamically (at speed). Dynamic unbalance can change from speed to
speed depending on weight distribution. At a given speed the blade may
be unballanced dynamically enough to excite it's resonance - while at
a higher speed it may be damped enough to not be noticed, and at a
lower speed the rotational inbalance may not be enough to excite it.

You also noted these were reconditioned fans.
The blades are MATCHED from the factory, and if in the reconditioning
process one or more blades from a different "set" were mixed in you
could well have a "missmatch".

Another thing - you checked tracking, and you static balanced - but
did you check the angle of incidence of each blade? If one has a
slightly different pitch it WILL shake - and possibly onl;y at ons
speed setting due to resonance. I've adjusted incidence on fans by
inseting paper shims or washers under one screw, between the blade and
the mounting bracket, or between the bracket and the fan
motor/housing.