How to Measure a Power Transmission V Belt

Refer to these handy tips the next time you try to measure your power transmission V belts without a measuring machine.
how to measure a v belt

At this point, we are assuming that you have identified the belt but need to ensure you get the right length.

(In another article, we discussed how you can identify and measure synchronous (timing) belts.)

We are dealing here with V belts, which are power transmission belts that have a V shape. They may be cogged (notched) or wrapped. They are run on V shaped pulleys that do not have teeth. If your pulley has teeth, then you have a synchronous timing belt.

We’ve updated this article for timeliness, clarity and depth. It may contain technical details, terms and jargon. You may jump past the nitty-gritty and go straight to how you can measure classic V belts and narrow section belts

 

Different lengths defined

According to Gates, in their GatesFacts Technical Information Library, figuring out the belt length can be “confusing” especially when people don’t always refer to the same terms. For clarity, here are the various lengths and how they are described (directly taken from Gates):

  • Outside Belt Length: The outside length of a belt is usually measured with a tape under no tension. Its validity is questionable since some belts have an arched top whereas others are flat. This would give different readings for the two belts. It can be used only as an approximation.
  • Inside Belt Length: The inside length is measured with flat pulleys (not v-pulleys) and a tape measure. As with the outside length, the inside length varies with the manufacturer. This length should not be used.
    • Note: As clarified with Gates, if you know the diameter of the flat pulleys and the center distance between them, then you can calculate belt length. This will be equal to half the circumference of each pulley plus double the length between the shaft centres.
  • Belt Pitch (Datum) Length: The pitch length of a belt is the length at the pitch diameter of the sheaves being used. Classical industrial belts are specified in terms of pitch length. The pitch length is obtained by adding the pitch circumference of one sheave (pulley) to twice the center distance between two equal diameter sheaves (pulleys) at a specified tension.
    • The belt pitch length is normally the length at the belt pitch line. This line is generally located at the neutral axis near the cord line and varies with cross section and construction. The pitch of the belt and sheave is actually a theoretical point that relates to the more accurate, reliable, and useable effective diameter and effective length.
    • This datum system defines specific sheave (pulley) and belt dimensions previously known as the pitch system for classical belts and sheaves. What were previously identified as pitch diameter or pitch length are now known as datum diameter or datum length. These are the new catalog identifiers which allow all belt and sheave manufacturers and users to use the same nomenclature.
  • Belt Effective Length: The effective length of a belt is the length about the effective outside diameter of a sheave at a specified tension. The effective outside diameter of a sheave is measured where the groove top width is a dimension as specified by RMA, ASAE or SAE standards. The effective length is obtained by adding the effective outside circumference of one sheave (pulley) to twice the center distance between the two standard measuring sheaves (pulleys) at the standard measuring tension.
  • Nominal Belt Length: The nominal length is used to refer to the length and cross section of a specific belt. For example, an A38 has a nominal length of 38 inches; a 4L440 has a nominal length of 44.0 inches; and a 3V300 has a nominal length of 30.0 inches. The nominal length is used for designation purposes and exists in name only.

Related post: Belt Length Acronyms (La, LE, Ld, Lp, Lw and Li)

 

When to use which diameter

The formulas have all remained the same, but this table from Gates makes it easier for everyone trying to figure out which belt diameters to measure:

To Calculate Previously Used Now Use
Speed Ratio Pitch Diameters Pitch Diameters
Belt Speed Pitch Diameters Pitch Diameters
Horsepower Rating Pitch Diameters Pitch Diameters
Rim Speed Outside Diameters Outside Diameters
Center Distance Pitch Diameter and Pitch Length Datum Diameter and Datum Length
Belt Length (Datum) Pitch Diameters Datum Diameters
Center Distance Factor "h" Pitch Diameters Datum Diameters
Arc of Contact correction Factor "G" Pitch or Outside Diameters Datum, Pitch or Outside Diameters
Span Length Pitch or Outside Diameters Datum, Pitch or Outside Diameters
 
Source: GatesFactsTM Technical Information Library, Datum Diameters: A New Industry Standard

 

How to measure classical V belts

This refers to the original form of power transmission belts with a belt profile of M, Z, A, B, C, D & E.

Classical Belt Section Sizes

It also refers to the cogged (notched) version of these belts, referred to as ZX, AX, BX & CX.

These belts are dimensionally the same as the wrapped belt but have cogging running perpendicular to the length, as per this image.

cogged belt
Source: Optibelt Technical Manual V-Belt Drives, page 15

Classical belts are traditionally measured by internal circumference in inches.

Therefore, an A26 belt has an internal circumference of 26 inches.

A note of caution: Although rare, we have started to see occurrences of classical belts being marked by measures other than internal circumference, so it is worth double-checking the measurement of your belt if it is from a manufacturer that you don’t know.

Quality brands such as Gates, Carlisle and Opti are all measured by internal circumference.

If it is more convenient to measure the outside circumference, you can do that. Just DEDUCT the following for each size to find the internal circumference since, as you know, the outside circumference is greater than the internal.

  • Z: 1”
  • A: 2”
  • B: 3”
  • C: 4”
  • D: 5”
  • E: 6”

The above applies whether the belt is the wrapped or cogged version.

M section belts have a different profile with a very small difference between the internal and external circumference.

 

How to measure narrow section V belts

Narrow section belts include SPZ, SPA, SPB & SPC belts. It also includes their cogged equivalents, XPZ, XPA, XPB & XPC belts.  (Some manufacturers refer to these as SPZX, SPAX, SPBX & SPCX.)

These belts are metric belts, but this section also discusses narrow section imperial V belts, which include 3V, 5V, 8V & 3VX, 5VX and 8VX belts.

Dimensions for the wrapped and cogged narrow section belt are as per this image:

Narrow section V belts are identified by their pitch length.

For example, an XPZ987 has a pitch length of 987 mm.

In order to calculate the pitch length of the belt, you must measure the inside length (circumference) and then ADD a figure, depending on section size, as listed below:

  • SPZ: 13 mm
  • SPA: 18 mm
  • SPB: 22 mm
  • SPC: 30 mm

It's different for imperial belts. As with the classical belts, these are measured and identified in inches according to their inside circumference. For example, a 3V400 has an internal circumference of 40.0 inches, and a 3V450 has an internal circumference of 40.5 inches.

 

Conclusion

Chances are you don’t have a belt measuring device so we find the handiest way to measure them is to clearly mark a starting point on the belt with a text and then use a dressmaker’s tape to measure slowly and carefully around the belt.

And we like to do it at least twice. To borrow from the carpenter’s golden rule: Measure twice and buy once!

Hopefully, this has helped you to identify the length of belt you need but, of course, if you’re still having a challenge, give us a call or send us an email and we’ll work with you to figure it out.

If you would like to read more about the various types of power transmission belts, Wikipedia has an excellent page that summarizes the various types of belts which have been using across the decades.

To borrow from the carpenter’s golden rule: Measure twice and buy once!