How to Measure a Power Transmission 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 will look at how to identify your belt if you don’t know what it is and, in another, go into detail about how to 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.
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.
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.
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.
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.
Chances are you don’t have a belt measuring machine 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 & buy once!