Adjustable pulleys are the most commonly used method of changing fan speed in light commercial balancing. Let’s take a look at how adjustable pulleys work on a fan and blower motor and learn how to adjust them. We’ll also learn how to use Fan Law One to calculate how to change the dimensions of a pulley. This will result in a predetermined change in the fan speed.

Since adjustable pulleys are more expensive than fixed pulleys and since motor pulleys are smaller than fan pulleys, adjustable pulleys are usually used on the motor shaft, not on the fan shaft.

How They Work

Adjustable pulleys are manufactured in two tapered sections. The two sections are connected as one half is screwed (turned) onto the other. This allows the diameter or pitch of the belt to vary, as the two halves of the pulley are turned together or apart. (Fig. 1)

As the two sections are turned closer to each other, the belt is forced to the outside of the pulley causing the belt to travel a greater distance around the pulley. This increases the speed of the fan.

Once the pulley, also called a sheave, is adjusted to the proper size, the set screw is tightened usually with an Allen wrench, to lock the pulley. Be sure to tighten the set screw onto the flat surface of the shaft. This assures it will not slip off the shaft during operation. (Fig. 2)

Be certain to shut off and lock out power before working near fans and pulleys. No measurement or adjustment is worth the loss of a finger. Always use extreme caution when working around air moving equipment.

Pulley Measurement

To effectively measure the adjustable belt or pitch diameter (where the belt rides) on a pulley, remove the pulley and belt from the motor and grasp by the belt. Using a caliper, measure the outside diameter of the belt, while it’s wrapped around the pulley. The diameter of the pulley is the physical outside dimension of the pulley. These are usually two different diameters.

Open or close the pulley to increase or decrease the diameter to the calculated size. Then tightly wrap the belt around the pulley again to measure the diameter to assure the right pitch has been achieved. After you’ve measured and adjusted the diameter to the required size, replace the pulley on the shaft. Start the system and measure to verify expected airflow is being delivered.

Align the Motor and Fan Pulleys

When attaching pulleys to the shaft, care must be taken to align both pulleys in a straight line to each other. This will increase power transmission and reduce wear and tear on the belts and the pulleys, extending the life of both. Also make sure the shaft of the motor and the shaft of the fan are aligned with each other.

Alignment can be done on smaller fans by simply laying a ruler or straightedge flat on the face of each pulley to assure both pulleys are in line with each other (Fig. 3 & 4). More advanced technology such as laser belt pulley alignment instruments may be required to accurately align larger systems.

Belt tension is normally adjusted by loosening the motor mount bolts and sliding the motor closer or farther away from the fan. Ideally when a 10 PSI pressure is placed between the pulleys on the belt, the belt deflection should equal the distance between the centerline of the pulleys divided by 64. For more detailed procedures see the manufacturer instructions. Use a belt tensioning tool to measure the 10 psi of pressure.

Example: Two pulleys have a 24-in. center-line distance. Twenty-four inches divided by 64 equals an ideal deflection of .375-in. or .4 in. (Fig. 5)

Fan Law One

Fan laws are formulas that enable you to determine the outcome of the belt and pulley adjustment before the adjustment is actually made. Basically, they allow you to peek into the future and see what is going to happen before happens. Using Fan Law One, you can calculate the change in pulley diameter needed to increase or decrease fan speed until the fan is delivering the required airflow needed.

Begin by measuring the airflow at the fan. This can be done by traversing airflow near the fan. Fan airflow can also be plotted by measuring the fan speed in RPM and the fan operating total external static pressure, and by plotting the fan airflow in the manufacturer’s fan performance tables.

Measure the outside diameter of the belt riding on the pulley as described above. Also measure the physical outside diameter of the pulley just to be sure the pulley is large enough to adjust to the required diameter.

The math required for Fan Law One is quite simple. Simply divide once, and then multiply once to find the new pulley size needed for the fan to move the required airflow.

Example: Suppose we have a 5.5-in. adjustable motor pulley and the fan is currently delivering 5100 CFM on a 15-ton system that requires 6000 CFM. Here’s what the raw formula looks like: (PD represents Pulley Diameter)

Here’s the formula with the actual fan numbers poured into it:

Divide the 6000 CFM by 5100 CFM to find the ratio of airflow increase. Notice the airflow should increase 17% for the fan to deliver the required airflow. Then multiply the pulley belt diameter of 5.5-in. times 1.17 to find the new belt diameter of 6.44-in.

Adjust the motor pulley to 6.44-in. (or as close as you can get it, 6.5-in. will do) to take the airflow up to 6000 CFM. Of course, this will only work if the pulley diameter is at least 6.5-in. or so. Also the fan and motor capacity must be at least 6000 CFM. Notice the airflow increased 17% and the pulley size also increased 17%. The idea that pulley diameter and airflow increase at the same rate is what we learn from Fan Law One.

Learn to measure airflow and fan properties, and then use the fan laws and fan engineering data to calculate what the change in fan performance will be before you ever pick up a wrench. Fan laws can also calculate the changes in fan RPM, static pressure and motor amp draw. Give these measurements and calculations a try the next time you need to change fan airflow.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free fan law calculation report, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.