HVAC The Basics of V-Belts At MicroMetl

Abbie-Stancato-At-MicroMetl-Corporation-HeadshotUntil I began to specify belts for HVAC Accessory fans and motors, I thought a belt… was just a belt.

Here’s valuable information I’d like to share with anyone seeking to understand the basics and differences in V-Belts. There are so many belt types available. I’ll do my best to narrow them to what you’re most likely to encounter in the HVAC Industry.

Belts & Pulleys Have Several Uses;

  • To increase or reduce speed
  • To increase or reduce torque
  • To transfer power from one shaft to another

V-Belt Types Prefix Defined

Understanding the prefix of any belt part number is important. The prefix offers dimensional standards, and somewhat defines the purpose of the belt. This knowledge allows you to understand which belts can be substituted, and which cannot.

2L, 3L, 4L, and 5L prefix – The “L” stands for light duty, and is designed for use with fractional horsepower motors. Meaning less than 1 HP. “L” style belts are often designated as light duty industrial, or lawn & garden.

A, B, C, D, E, And AX, BX, CX, and DX size belts are more industrial. They’re sometimes referred to as “Conventional” belts. The “X” is a standard specification for inside perforations also known as notched or cogged belts. Although “Conventional” belts can be substituted for “L” style belts which appear to be the same, “L” style belts should never be used as replacements for “Conventional” belts.

My experience with “X” designation belts is that they last longer than those without it. Meaning a “BX” belts last longer than a “B” Belt. Especially in cases where one or more of the pulleys are small in diameter. Smaller diameter pulleys offer less stress on “X” style belts due to the notched under-bottom.

3V, 5V, 8V, AVX, BVX, CVX, DVX – IMPORTANT: Don’t Confuse “V” In The Part Number Prefix! I run this warning because common sense would lead you to believe a 3V belt is a comparable substitute for a 3L, or that an AVX belt is a suitable substitute for an AX belt. Common sense would be wrong. The “V” designation in the prefix is a difference in the profile of the belt. Never substitute or mix these belts. They do not fit properly into pulleys designed for 3L and AX belts. They do not make full contact against the bottom and walls, and therefore will fail to perform.

MicroMetl Instructor Robert Schram offers his thoughts on “V” & “VX” belt applications…

Robert Schram of MicroMetlV and VX belts are specifically designed for higher horsepower and longer center distance applications. For example, most applications up to 7.5 HP where a single belt is being utilized, an A or B belt is more than adequate to do the job. The V and VX belts, on the other hand, may be better for 10HP and larger.

Also, when multiple belts are used in industrial applications, the V and VX belts are narrower at the top to allow for a better fit. And since industrial applications are typically higher horsepower running at higher RPM, the belts need to be tightened more than non-industrial applications, which make the V and VX belts better.

The V and VX belts are not as flexible as the A, B, C belts; therefore they are typically used with larger sheaves, and cannot be used with Backside Tensioning Pulleys. That stiffer characteristic also makes them better for the long center distance span.

 How To Measure Belt Length

Let’s begin this section asking the question… Can belts stretch?

According to the website BeltsForAnything.com, “Belts don’t actually stretch in length, the sidewalls wear from pulling, and your belt gets narrower. That’s what makes your belt appear to have stretched.”

I don’t completely agree with this assessment. Be careful when measuring a worn belt, they can and do stretch as they wear. Therefore the measurements between the old and new may not be exact.

If you are unable to read the belt part number, you will need to measure. I could take the time to better explain, but instead will defer to a brief instructional video embedded below.


Discover Belt Length Without An Existing V-Belt

Discover Belt Length Without An Existing V-Belt At MicroMetl

  • Wrap a string around both pulleys.
  • Using a permanent marker, overlap the ends and mark any point between where the two overlapping end pieces.
  • Lay the marked string on a flat surface, and measure the distance between the two markings with a tape measure. This is the Inside Circumference (IC) for your replacement belt.

Interpreting Belt Part Numbers

Too often the belt you need to replace is either gone, the part number information is worn off, or the belt disintegrated when it broke. The standards for some belt part numbers reflect the Outside Circumference (OC) of the belt, and others the Inside Circumference (IC). Therefore, once you have obtained the part number, it will be easy to interpret and understand how to discover the replacement for an equal, or better quality belt.

The information below will allow you to cross reference each belt type (Some Manufactures Slightly Vary):
Standard V-Belt Measurement Profile At MicroMetl
2L Belt = 1/4 (.250) Inch Width x 5/32 (.15625) Height
3L Belt = 3/8 (.375) Inch Width x 7/32 (.21875) Height
4L Belt = 1/2 (.500) Inches Width x 5/16 (.3125) Height
5L Belt = 5/8 (.625) Inches Width x 3/8 (.375) Height
3V & 3VX Belts = 3/8 (.375) Inch Width x 5/16 (.3125) Height
5V & 5VX Belts = 21/32  (.65625) Inches Width x 17/32 (.53125) Height
8V & 8VX Belts = 1 (1.000) Inches Width x 7/8 (.875) Height
A & AX Belts =1/2″ (.500) Inches Width x 5/16 (.3125) Height
B & BX Belts =21/32 (.65625) Inches Width x 13/32 (.40625) Height
C & CX Belts =7/8″ (.875) Inches Width x 17/32 (.53125) Height
D & DX Belts =1-1/4″ (1.250) Inches Width x 3/4 (.750) Height
E Belt =1-1/2″ (1.500) Inches Width x 29/32 (.90625) Height

Part Numbers for Fractional Horsepower 2L, 3L, 4L & 5L belts all reflect their circumference as an Outside Circumference (OC) measurement. Example: 3L Section= (Part Number 3L450 = 45 Inch Outside Circumference)

Therefore, to determine the inside circumference measurement use the following simple formulas.

The information below will not return an exact measurement as implied by the belt part number. For those interested in the mathematical equation… Outside Circumference, Divided by Pi, which equals the Diameter, Subtracted by the belt Height, Multiplied by Two, Multiplied By Pi.

Example: A 4L Belt (0.3125 Inch Height when viewing the profile) with a 25 Inch Outside Circumference.

25 (Inch Outside Circumference) divided by 3.14 (PI) = 7.962 (Inch Outside Diameter). 7.962 (Inch Outside Diameter) – 0.625 (Profile Height of 0.3125 Inch * 2) = 7.337 (Inch Inside Diameter). 7.337 Inches (Inch Inside Diameter) * 3.14 (PI) = 23.04 (Inch Inside Circumference). 25 (Inch Outside Circumference) – 23.04 (Inch Inside Circumference) = 1.96 Inch Reduction From Outside Circumference to Inside Circumference

2L Belt = The Outside Circumference (length) (OC) – Minus 1 Inch. (Example 3L250 = 24 Inch Outside Circumference)
3L Belt = The Outside Circumference (OC) – Minus 1-1/2 (1.5) Inches. (Example 3L250 = 23.5 Inch Inside Circumference (IC)
4L Belt = The Outside Circumference (OC) – Minus 2 Inches. (Example 4L320 = 30 Inch Inside Circumference)
5L Belt = The Outside Circumference (OC) – Minus 3 Inches. (Example 5L380 = 35 Inch Inside Circumference)

Conventional Belts Part Numbers Reflect Inside Circumference (IC) Measurements in Inches. To convert the measurement to an Outside Circumference (OC), use the following information.

A & AX Belts = Add 2 Inches to the Belt Number (Example A80 = 82″ Outside Circumference)
B & BX Belts = Add 3 Inches to the Belt Number (Example B80 = 83″ Outside Circumference)
C & CX Belts = Add 4 Inches to the Belt Number (Example C80 = 84″ Outside Circumference)
D & DX Belts = Add 5 Inches to the Belt Number (Example D80 = 85″ Outside Circumference)
E  Belts = Add 6 Inches to the Belt Number (Example E80 = 86″ Outside Circumference)

“V” Belts Part Numbers Reflect Outside Circumference (OC) Measurements in Inches. To convert the measurement to an Inside Circumference (IC), use the following information.

Example: 3V Section= (Example 3V450 = 45 Inch Outside Diameter)
Therefore, to determine the inside measurement use the following simple rule of thumb.

3V & 3VX Belts = The Outside Circumference (OC) – Minus 1 Inch. (Example 3V250 = 24 Inch Inside Circumference)
5V & 5VX Belts = The Outside Circumference (OC) – Minus 4 Inches. (Example 3V500 = 46 Inch Inside Circumference)
8V & 8VX Belts = The Outside Circumference (OC) – Minus 5.5 Inches. (Example 3V1020 = 96.5 Inch Inside Circumference)

The Importance of Proper V-Belt Tension

V-belts operate on the friction, multiplied by the mechanical advantage of the wedging principle (The majority surface area of the belt seated upon the pulley). Proper tension and pulley to pulley alignment is critical, and necessary for long, satisfactory operation.

Too little tension results in slippage, causing premature belt & pulley wear. Too much tension results in excessive stress on belts, bearings, and (motor & fan) shafts. However, there is still a wide range of tension which a belt will operate satisfactorily. The intent is to find this proper range for any V-Belt drive. Always consult an owners guide for the proper tension range.

Proper V-Belt Tension At MicroMetl

You should always use a Belt Tension Tool and refer to the manufacturer’s specs for each belt. Now for reality, you don’t have one… A good “Rule of Thumb,” seek approximately ½ Inch of deflection for a V-belt. To be more precise… The “Span Length” is the distance from where the belt is leaving one pulley, and is completely seated on the second. Seek 1/64 (0.015625) of an inch deflection for every 1 inch of belt span length. For example, if the span length is 50 inches, the desired belt deflection is 0.015625 (1/64 of an Inch) X 50 = 0.78125, or 25/32 of an inch.

The more you tension a belt, the faster it will wear out. Your belt should spring back when you press down on the backside.

Proper Pulley Alignment

VBelt And Pulley Angularity At MicroMetlTwo misaligned pulleys will quickly wear out belts and bearings as well as cause belts to fall off.

Pulley misalignment can occur as horizontal angularity, vertical angularity and/or axial offset as illustrated.

Alignment should be performed with the use of lasers. However, if you don’t have one in your arsenal of tools, a straight edge as shown in the video below demonstrates the process.

Stop The Squeak

Everyone hates this noise. If your fan system has developed a squeak the root cause could be due to the belt wearing out, dirty pulleys, belt is too loose, incorrect belt type, poor belt quality, oil on the belts, a bad bearing in a pulley, worn idler pulley, motor going bad, or belt misalignment. The belt should always be seating and aligned to the center of each pulley.

Final Words

This journey allowed me to discover one of the best pdf printable reference guides on the subject of pulleys and belts I’ve ever encountered. It is from Browning and is available for free. I recommend you follow this link and print as a reference guide.