Have you ever spun an adjustable pulley and increased its diameter to squeeze a little more air out of a belt-driven fan? If so, there have probably been times when you’ve found that the old fan belt no longer fits. The first fix is usually to loosen the bolts on the motor mount and slide the motor as close to the fan as possible to see if the belt will slip over the edge of the pulley. If that doesn’t work, it’s time to surrender muscle to brainpower and calculate the size of a new belt.

While you’re at it, be sure to size and select the right new belt for the job. Your choice will be critical to the long-term performance of the system. Let’s take a look at how to resize a belt, and the steps it takes to be sure the job is done right the first time.

Inspection
Start by inspecting the existing belt, motor, fan and pulleys. Check to be sure the original belt’s groove size fits the pulleys, and make sure the bearings, shafts and set screws are in good condition and secure.

While you’re at it, inspect the motor mount, the fan and housings to be certain they will remain in good working order after the new airflow is set and that the equipment will be able to operate long term. We’ve all serviced equipment, only to have it die an awful death a short time later. Nobody’s happy when that happens, so look past the new belt.

Record the old belt number written on the exposed surface of the belt. This is your best reference to find the original size, type, and length of the belt. If you don’t have the replacement belt on your truck, these numbers come in handy for the inventory crew to verify your measurements by cross-referencing the belt number with the manufacturer’s catalog. A faded page in a manufacturer’s catalog is more accurate than field belt and pulley measurements any day.

Do the Math
Now it’s time to use the “other tool,” a math formula to determine the size of the replacement belt. Work the formula a few times before you try it in the field, just so you’re familiar with the math. Practice makes perfect.
Several measurements are required to complete the replacement belt formula. Measure and record the distance between the motor shaft and the pulley shaft, and then measure the diameter of the motor pulley (also called the drive pulley or sheave) and the fan pulley (called the driven pulley).

Apply the following formula to determine a new replacement belt length:

Belt Length = 2C + 1.57 x (D1 + D2) + 1-in.

Where:
C = The distance between the motor and fan shafts
D1 = The diameter of the small pulley
D2 = The diameter of the large pulley

The math is quite easy; it simply includes adding and multiplying. Here is a completed formula to walk you through the steps.

Example:
C = (12-in.) distance between shafts
D1 = (3-in.) diameter of the small pulley
D2 = (8-in.) diameter of the large pulley
New belt length = 2C + 1.57 x (D1 + D2) + 1-in.
2(12-in.) + 1.57 x ( 3”+ 8” ) + 1-in.
24-in. + (1.57 x 11-in.) + 1-in.
24-in. + 17.27-in. + 1-in. = 42.27-in.

So, select a new belt the next length above 43 inches.

Next, determine the thickness of the belt. This is determined by measuring its cross-section. Be careful; old belts may be so badly worn that the dimension may not be accurate. State the new belt size by stating its length and its cross section nomenclature (Click here to view a chart which shows typical HVAC belts and measurements.)

The key to success when selecting replacement belts is a huge inventory, or a great counter guy or gal at your power transmission distributor. They know the right questions to ask and can almost finish your sentences once they get to know you or your company. Each is worth their weight in adjustable pulleys.

Tips for Adjusting Belt Tension and Alignment

  • When increasing pulley size, always check amp draw immediately after making the change. Remember, amps increase at the cube of airflow, so a 10% increase in airflow may increase the power the motor draws by more than 30%. Pulley adjustments can be hazardous to the health of your motor. Do not leave a motor operating at more than full load amps.
  • Always measure fan RPM. Check the fan speed against the manufacturer’s fan data to be certain you don’t go over the fan curve. Airflow may decrease with an increase in fan speed under certain conditions.
  • Adjusting the motor mount is accomplished by loosening the bolts that hold the motor onto the motor mount. Next, slide the motor closer to or farther away from the fan until the correct belt tightness is achieved, then tighten the bolts on the motor mount to hold the motor in place and keep the belt tight.
  • Sometimes you’ll need to loosen the set screws on the pulley and slide them along the shaft until the pulleys line up with each other. Alignment increases efficiency and prolongs the life of the belt and pulleys.
  • How tight should the belt be adjusted? Too tight, and the power consumption may increase and equipment life could decrease. Too loose, and the belt squeals and slips. There are tools available to measure belt tension, and I bought one once. It was fun to use, but it only served to reinforce the practice of pushing on the installed belt half way between the pulleys with moderate pressure looking for 1/2-in. to 3/4-in. deflection in the belt.
  • Speaking of pushing on the belt, safety first. During our air balancing seminars, it’s not uncommon to find a technician waving half a finger, eager to share his near death experience with a blower motor. Lock off power at with a combination lock at the electrical disconnect box when working near belts or pulleys. A sudden start of a blower motor may end your service career in an instant.
  • When replacing belts on a multiple pulley, be sure to order a “matched set” of belts. This is a set of belts cut from the same piece of belt material. Each is exactly the same size as the other and will stretch and wear at the same rate as its brother. Insist on a matched set when multiple belts are being replaced.
  • With blower motors over 10 HP, it’s recommended that a good quality belt changeout include a second visit a day or two later to tighten the stretched belt by adjusting the motor mount. The belts do stretch and need to be tightened to perform their best.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a no cost one page Belt Sizing Procedure, contact Doc at robf@nationalcomfortinstitute.com or call him at 800/633-7058. NCI’s website is found at www.nationalcomfortinstitute.com.