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. That is, power transmission belts which have a V shape. They may be cogged (notched) or wrapped. They are run on V shaped pulleys which 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 you don’t know of.
If it is more convenient to measure the Outside Circumference, you can do that and would deduct the following for each size to find the Internal Circumference as the Outside Circumference is, of course, 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 section also discusses Narrow Section Imperial V Belts, which include 3V, 5V, 8V & 3VX, 5VX & 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 987mm.
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 – 13mm
SPA – 18mm
SPB – 22mm
SPC – 30mm
For Imperial belts it is different. 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. 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 V Belts without the machine 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 & 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 which 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!