The compressor belt has snapped. You turn it over to copy the code, and the printing has long since worn away. The costly approach is to take it to a hardware store and match the length, because a V-belt code is never simply the measured length: classical inch sizes and metric sizes mean different things. This guide goes through four steps: understand which length the code refers to, measure the cross-section, measure the length, check the construction, and then set the tension.
1. First, which “length” does the code mean?
One V-belt has three different lengths: the outside length (around the outer edge), the inside length (around the inner edge) and the datum length (the length at the tensile cord layer inside the belt). The two main systems each use a different one as the code:
| System | Common sections | Code means | Unit | Example |
|---|---|---|---|---|
| Classical (inch) | A, B, C, D, E | Inside length | inch | A29 = inside length 29 in |
| Metric narrow V-belt | SPZ, SPA, SPB, SPC | Datum length | mm | SPA1157 = datum length 1157 mm |
ISO 4184, Belt drives — Classical and narrow V-belts — Lengths in datum system (current edition 2025), covers the datum length system.
A common mistake follows from this table: converting the measured length to millimetres and then looking for an SP code. It does not work. Inside length and datum length are measured on different circles, and a unit conversion cannot fix a difference in definition. Use the inch table for inch belts and the metric table for metric belts.
2. Step one: measure the cross-section to find the letter
Measure the top width and the thickness (height) with calipers or a ruler, and compare:
| Classical | Width × height | Metric narrow V-belt | Width × height |
|---|---|---|---|
| A | 13 × 8 mm | SPZ | 10 × 8 mm |
| B | 17 × 11 mm | SPA | 13 × 10 mm |
| C | 22 × 14 mm | SPB | 17 × 14 mm |
| D | 32 × 19 mm | SPC | 22 × 18 mm |
| E | 38 × 23 mm | — | — |
These are nominal sizes. Measured values may be slightly smaller; pick the closest set.
Watch for one trap: measuring the width alone can put you in the wrong system. A and SPA are both 13 mm wide; the difference is the thickness (8 mm vs 10 mm). B and SPB are both 17 mm wide; again the difference is the thickness (11 mm vs 14 mm). Same width, different thickness means they are not interchangeable, because the pulley groove depth and angle are designed for one section. Thickness matters as much as width.
3. Step two: measure the outside length and convert it to a code
Take the belt off the machine, lay it flat and do not stretch it. Wrap a soft tape measure once around the outer edge. If the belt is still on the machine, or you pull it while measuring, the reading will be too long.
For inch sections, subtract the fixed value for that section from the outside length to get the code:
| Section | Subtract from outside length | Example |
|---|---|---|
| A | 2 in | 44 in outside → 44 − 2 = A42 |
| B | 3 in | 59 in outside → 59 − 3 = B56 |
| C | 4 in | 94 in outside → 94 − 4 = C90 |
| D | 5 in | 125 in outside → 125 − 5 = D120 |
For section E, sources give different values (6 in and 7 in both appear) and the belts are less common, so check the supplier’s specification table instead of converting yourself.
Do not use this method for metric SP belts. The number after SP is a length in mm, but catalogs may give it as Ld (datum length) or Lw, which are defined differently. The Optibelt catalog we use writes codes such as SPA1157Ld, putting the length type into the code. In the Optibelt narrow V-belt system, for example, the SPA outside length is about the datum length plus 18 mm, and the difference is not the same for every section. So the outside length only narrows the range; you still need to check which length that manufacturer’s catalog uses and then confirm the code from its table.
If the belt has broken or cannot be removed, measure the diameters of the two pulleys and the center distance instead and calculate the belt length with the belt length formula. This is also why writing the original code in the maintenance record saves more effort than any measuring afterwards.
4. Step three: check the construction (one code, more than one belt)
Measuring B56 only settles the size, not the specification. One code commonly comes in four constructions, with large differences in price and life:
| Construction | Characteristics | Typical use |
|---|---|---|
| Standard | Wrapped, general purpose | General plant equipment, pump motors, fans |
| Heavy duty | Reinforced tensile layer, resists shock loads | Equipment with frequent starts and stops or changing loads |
| Cogged (XPA / XPB / AX, etc.) | Cross notches on the underside; lower bending resistance, better heat dissipation | Small pulleys, high speed, CNC spindles |
| Narrow, high-power (3V / 5V / 8V) | Narrow section, high power density | Limited space with high power to transmit |
We keep Optibelt SPA / SPB / SPC belts and Optibelt XPA / XPB / AX cogged belts in regular supply.
5. Step four: wrong tension undoes accurate measuring
Tension is the last step after fitting a new belt, and it is often set by feel. The common objective method is the deflection method:
- Measure the span (the length of belt between the two pulleys).
- Press down at the middle of the span, at right angles to the belt.
- The standard deflection is 1/64 in per inch of span, about 1.6 mm per 100 mm of span.
- Compare the force needed to reach this deflection with the force range recommended by the belt manufacturer.
Both directions cause problems. Too loose and the belt slips; slipping produces heat, then glazed sides, carbon in the grooves and lost efficiency. Too tight and the load moves to the bearings and shaft; the bearings fail before the belt does. Also, re-tension a new belt after running in. Initial stretch is normal, and a belt left alone after fitting will be loose a few days later.
6. Replace multiple belts as a matched set
On drives with two or more belts side by side, replacing only the broken one is common but expensive. A technical bulletin from Timken Belts puts it plainly: multi-belt drives need matched belts so that the load is shared evenly. Mixing new and old belts causes uneven load sharing, uneven pulley wear and more vibration, and the belts do not reach their expected life.
For the same reason, do not mix belts from different manufacturers. Even with the same code, each manufacturer’s actual dimensional tolerances differ, so mixing them creates length differences on purpose. Replacing the whole set usually costs far less than one unplanned stoppage.
FAQ
The code is unreadable. Can I just measure the inside length? Yes, but the error is larger. Inside length needs a flat pulley to measure accurately; a tape around the inner edge on site is affected by how the belt bends. Measure the outside length and convert it with the values in section 3.
Can SPA1157 replace A42? No. They belong to different standard systems, the cross-sections differ (A is 13 × 8 mm, SPA is 13 × 10 mm), the codes use different length definitions (inside length in inches vs datum length in mm), and the pulley grooves are matched to each section.
The measurement falls between two standard lengths. What now? First check that the cross-section is right. If it is, take the shorter length and confirm that the machine’s center distance or idler still has adjustment left. Tensioners have limited travel; stretching a belt to make up a length difference is the wrong approach.
Only one belt broke and the others look new. Can I replace just one? Not recommended; see section 6. Belts that have run have stretched, so next to a new belt the load is shared unevenly and the new belt carries more.
If you need a code checked against the section, length and machine conditions of your current belts, or a stock check of all drive belts in a plant, contact JIAHON. Our sales engineers can compare specifications and quote for batches.
Standards cited
Please refer to the current edition published by each organization.
- ISO 4184:2025, Belt drives — Classical and narrow V-belts — Lengths in datum system, Ed. 3. International Organization for Standardization (ISO).