Balancing Fans

Why isn't there an app for that? 

I've tried to balance fans a few times with no success. 

I've got one in my living area that is atrociously out of balance, and as a result makes a knocking sound. 

What's the secret?

I second this. 
I recall some procedures involving clay that you could add to the fan blades but not sure in the details 

If it's that far out, swap the blades. Once I get past gross imbalance, I use painters tape to try out weights on each blade. If you have a mail scale, you can be more scientific and weigh the blades. 

Is it possible or realistic to set up a dead-level fixture to hold the blade on a shaft and let gravity tell you which blade is the heaviest?  I did this with a blower motor armature once, using two parallel pieces of 5/16" brass rod that the armature could roll across.  Probably would have worked great if the bushings weren't augered out and the fan shaped like a football...

I've had fans come with a balancing kit in the past, but I don't recall the brand. 

They had a plastic weight that could be placed on the blade and slid along the length of the blade to determine the right location and then double sided tape with weights to stick to the top of the blade. 

I did the same once. Dimes work great. 

Amazon kit

Weigh the blades prior to install.  Get them to the nearest gram, then assemble.  

Too late for that?  Use a chip clip on the blades 1 at a time, once you figure out the heavy blade and the approx location, then superglue a coin to the back of the offending blade.  

I have duct taped change to the top in the past to great success. Once you have found your spot by trial and error, glue the quarter(s) or whatever in place.

To find the light side, measure from the tip of the blade to the ceiling. The shortest distance is the light side-assuming your ceiling is level

grover said:

I did the same once. Dimes work great. 

Or pennies, depending on the weight needed.  There are several cents' worth on the one in my living room.

Thanks all. 

I've been looking for an excuse to get a small digital scale outside of weighing bike parts and food for a diet (like who does that?) - so being able to weigh fan blades with one would be a handy third reason to own one.