Blower Belt Installation
It takes just a little more attention to properly install a motor and pulley on a furnace blower.
First, be sure to mount the variable-pitch motor pulley on the motor shaft with of the movable face of the pulley mounted away from the motor. Be sure to tighten the set screws on the flat or key of the motor shaft.
Don't push the pulley all the way to the motor. Leave at least 1/8 of an inch clearance between the pulley and the motor bearing. Install of the Belt in the pulley grooves by adjusting the tension on the motor bracket so that the belt slides over the pulley easily. Then re-tighten the belt by adjusting the motor-mount. You shouldn't roll or snap the belt on to a pulley because it causes the belt to strain and may cut it.
A quick and easy way to check pulley alignment is to sight down the top of the belt (with the motor shut off, of course) like you would sight down a rifle barrel. You can easily see then if the belt angles off the pulley.
The best rule of thumb for adjusting a blower belt is to tighten the belt tension until you can push on the belt at midpoint between pulleys and it will deflect just the the width of your thumb.