When tightening down a blower wheel or a fan blade on a motor shaft, ONLY tighten it on the flat of the shaft.

That flat is machined into the shaft on purpose. It gives the set screw a face to bite into instead of a curve to slide off of. On most residential motors, it runs from the end of the shaft back an inch or two, so when the wheel is on, you can usually see it in the bore.

Line the set screw up with the flat before you snug anything down. If the screw lands on the round part of the shaft, it has almost nothing to hold, and the wheel will eventually spin on the shaft no matter how hard you crank it.

If you have more than one screw but only one flat surface on the shaft, only tighten the one set screw.

Refrain from over-tightening set screws; they need to bite into the shaft, but you don't need to mangle the poor thing. Setting it on the curve and over-tightening can make it hard or impossible to remove the blade or wheel later. I've seen this several times in high-wall ductless systems: an overtightened or improperly set wheel can make it impossible to remove later.

A little grease on the shaft and a carefully tightened screw can make it a lot easier the next time.

Simple, but important.

—Bryan

Illustrated by JD Kelly