Have you ever stood in front of a broken-down mower or machine with a worn-out belt and no part number in sight? Here’s how to figure out what you’re dealing with:
A V-belt is a wedge-shaped belt made from rubber and reinforced with high strength cords running through them. The have a vast range of applications from use in ride on lawn mowers, pumps and in industrial machinery. They are available in a range of sizes and materials to meet the specific need of the application.
There are several possible types of V-belts available:
- Classical Heavy Duty - Wide range of sizes and the most common.
- Cogged - Cogs allow the use of smaller diameter pulleys and provide heat dissipation. Raw edge sidewalls prevent slippage.
- Wedge - Narrower, deeper profile with higher power capacity than classical v-belt; allows for smaller, more compact drives.
- Wedge Cogged - Same properties as wedge, but cogged for greater flexibility and heat dissipation.
- Banded – Two or more v-belts joined to form a single belt allowing the belts to function as a single belt with equal load distribution and wear.
Ideally the belt you have in your hand still has the markings of the part number as that saves us all a lot of time. If we have this, we can then hunt down the specific size e.g. A50, B25 or 3L300. But highly likely you are replacing the belt because it’s worn, and those part numbers were rubbed off in the early 80s!
Step 1. Cross Section
With no visible markings our starting point then becomes to first determine the belts cross section. V-belts are categorised based on their top width and depth measurements. Each width/depth combination or section is labelled using a letter of the alphabet. For example, v-belts with a 13mm (1/2”) width and an 8mm (5/16”) depth are labelled an "A" belt. There are many different v-belt sizes. Some common sizes are standard (Z or M, A, B, C, D, E), narrow (3V, 5V, 8V), wedge (SPZ, SPA, SPB, SPC), hexagonal (AA, BB, CC), and fractional horsepower (2L, 3L, 4L, 5L). See the Gates Powerated Belt Sizes below as an example:
Gates Corporation has an excellent resource that helps identify the different belt sections. Click to download this here.
Step 2: Belt Construction
V-belts are available in either wrapped or raw edge styles. A conventional wrapped v-belt will have fabric covering the belt. A raw edge cogged v-belt will have exposed rubber sidewalls and moulded cogs or notches on the bottom that resemble teeth. The following characteristics may also be present, depending on the construction type:
- Rubberised fabric cover – Cover envelopes the entire belt and protects the belt core.
- Top fabric – Provides heat and oil resistance.
- Load carrying section – Cords (also known as tensile members) give the ability to transmit power and ensure uniform load distribution.
- Compression section – Elastomer resists compression fatigue and dissipates internal heat build-up, providing firm lateral pressure against the sheave/pulley sidewall and distributing the load to the cords.
- Precision moulded cogs – Provide additional flexibility, allowing for the use of smaller pulley diameters, and aiding in heat dissipation.
We now should know the type of v belt we have in hand.
Step 3: Measuring Length
To measure a V-belt, you’ll need a flexible cloth tape measure or a length of string. If the belt is still installed, wrap the tape snugly along the belt’s outer path over the pulleys. Using string? Mark the overlap point, then measure the string’s full length — that’s your outside circumference. If the belt is off the machine, lay it flat and wrap the tape or string tightly around its back side (not edge to edge). Always apply slight tension while measuring, as belts can stretch. Avoid folding the belt or using rigid tools like steel rulers, which can distort the measurement.
V belt lengths can be confusing as not everyone means the same thing when they say “belt length.” Here’s a breakdown of the most commonly used length definitions, reinterpreted for clarity:
Outside Length - This is the measurement taken around the outside of the belt using a tape with no tension. It’s only an estimate, as belt tops may vary in shape (some are arched, some flat), which affects accuracy.
Inside Length - Measured using flat pulleys (not V-pulleys), this dimension also varies between manufacturers and isn’t generally relied upon. If needed, it can be calculated using the pulley diameters and the centre distance between them.
Datum (Pitch) Length - This is the industry standard for classical V-belts. It represents the length at the belt’s pitch diameter — roughly near the cord line or neutral axis — and is calculated based on the pulley’s pitch circumference and centre distance. “Datum” is the updated term, previously referred to as “pitch.”
Effective Length - Refers to the distance measured around the effective outer diameter of the pulley groove, as defined by industry specs (like RMA, ASAE, or SAE). Like datum length, it’s calculated using pulley circumference and center spacing under standard tension.
Nominal Length - A label used for easy identification, like “A38” for an A-section belt with a nominal 38" length. It’s a catalogue term, not a physical measurement.
Confused now after taking the belts outside circumference as to what will be the correct V-Belt part number? Note most belts are measured in inches so you may have to do some conversions if you only have a metric measuring tape. 1” = 25.4mm. Use the below table to understand how the measurement relates to the part number.
|
Section |
Outside Length |
Subtract |
Inside Length |
V-belt designation |
|
Z |
21.5" |
1.5" |
20" |
Z20 |
|
A |
24" |
2" |
22" |
A22 |
|
B |
26" |
3" |
23" |
B23 |
|
C |
30" |
4" |
26" |
C26 |
|
D |
35" |
5" |
30" |
D30 |
|
E |
40" |
6" |
34" |
E34 |
|
SPZ |
625 mm |
13 mm |
612 mm |
SPZ612 |
|
SPA |
750 mm |
18 mm |
732 mm |
SPA732 |
|
SPB |
1422 mm |
22 mm |
1400 mm |
SPB1400 |
|
SPC |
2030 mm |
30 mm |
2000 mm |
SPC2000 |
|
AA |
30" |
2" |
28" |
AA28 |
|
BB |
33" |
3" |
30" |
BB30 |
|
CC |
40" |
4" |
36" |
CC36 |
|
3V |
50" |
- |
- |
3V500 |
|
5V |
60" |
- |
- |
5V600 |
|
8V |
75" |
- |
- |
8V750 |
|
2L |
20" |
- |
- |
2L200 |
|
3L |
30" |
- |
- |
3L300 |
|
4L |
40" |
- |
- |
4L400 |
|
5L |
50" |
- |
- |
5L500 |
Step 4: Future Proof
To make future belt replacements faster and hassle-free, it’s good practice to clearly label the drive with the correct V-belt part number. Use a durable sticker, tag, or sign mounted near the drive so the information is easy to spot at a glance. This small step can save time and guesswork down the line. For even better reliability, record the installation date as well — this helps track wear over time and plan proactive replacements before a failure disrupts operations.
Final Checklist for V belt identification
ü Cross section (width and depth)
ü Belt construction (wrapped, cogged, banded)
ü Outer circumference
ü Section constant (if applicable)
ü Final part number
ü Machine labelling for future reference
Above all please remember to stay safe.
- Lock it out before you look: Always isolate power and apply a lockout/tagout (LOTO) device before any maintenance. Never assume a machine is safe just because it’s quiet.
- Know your belt: Cogged belts aren’t the same as toothed timing belts — don’t confuse the two. If you’re working in a flammable or dusty environment, consider FRAS-rated belts or those with heat and oil resistance.
- Dress for the job: Avoid loose clothing, jewellery, or untied long hair. Stick to fitted workwear and tie back anything that could get caught in moving parts.
- Keep guards in place: Never run a belt system without its safety guards. They’re not optional — they’re your first line of defence.
- Inspect regularly: Check belts, pulleys, and tensioners for wear, misalignment, or cracking. Use proper tools like tension testers, alignment lasers, and pulley gauges to get it right.
- Keep it clean: Debris, oil, and clutter around belt drives can cause slips, fires, or entanglement. A tidy workspace is a safer one.
- Schedule preventive maintenance: Critical drives should be visually checked weekly. Full inspections — including shutdowns — should happen every 3–6 months depending on usage and environment.
- Use the right tools: Don’t eyeball tension or alignment. Use proper gauges, straightedges, and tension meters to ensure safe, efficient operation.
Cheers,
Mike