Before the fighting starts about term definitions, let's just settle on using “adjustable drive pulley” and “sheave” interchangeably to describe the belt-driven power transmission device shown above.

Second, for the newer tech, you shouldn't be altering these sheaves so that the belt rides lower or higher in them unless you know EXACTLY what you are doing. You can mess up the air balance, change sensible heat ratios, and impact fresh air intake by adjusting the sheaves.

If you want to adjust belt tension, you can generally do that by adjusting the motor mount, using the tension adjustments made for that purpose. Belt tensioning and belt replacement is a part of regular maintenance; adjusting sheave halves isn't.

In some cases, you may find yourself in the position where you are replacing a sheave or where you need to adjust a sheave to increase or decrease blower RPM or airflow.

When replacing a sheave

Sheaves, like all pulleys, do tend to wear over time. When they wear, it is better to replace them rather than attempting to adjust them.

Make sure to use the same diameter and count the number of turns “in” from the edge or edges on the old one and do the same on the new.

Only tighten the set screw on the “key” or flat to keep from damaging the sheave threads.

When making a sheave change

See the image above. The drive sheave will obviously rotate at the same RPM that the motor rotates. The RPM of the driven pulley on the blower is dictated by the size ratio of the drive to the driven pulley.

As you can see above, if the drive pulley has 1/2 the diameter of the driven pulley, the driven pulley will rotate at 1/2 the speed of the motor.

In the scenario shown above, the drive pulley is 6″ and turning at 1000 RPM, and the driven pulley is 12″ and turning at 500 RPM.

Let's say we wanted the driven pulley to spin at 600 RPM rather than 500. All we would need to do is divide 600 by 500 to come up with 1.2.

You then multiply the drive pulley diameter x 1.2 to get 7.2″ (or about 7 3/16″) to get to that RPM.

In practice, adjusting a sheave to get a belt to ride lower (closer to the shaft) or higher (further from the shaft) won't usually result in huge airflow changes, but it will have an impact.

The main thing to know when adjusting an adjustable drive pulley is:

Riding Lower/Halves Further = Less Air

Riding Higher/Halves Closer = More Air

When you make an adjustment, you will need to re-tension the belt, and it's a good practice to measure before and after blower amperage to see the impact of the change on the motor.

Never make a sheave change that results in a blower overload.

—Bryan