Why the same belt has two different numbers
Fractional horsepower (FHP) belts and classical belts are measured on different length bases, and that single difference is the source of almost every cross-reference mistake.
- 4L and 5L (FHP) are numbered by the outside circumference in tenths of an inch. A 4L460 is 46.0 inches measured around the outside of the belt.
- A and B (classical) are numbered by the inside length in inches. An A44 is 44 inches measured around the inside.
A 4L460 and an A44 are the same physical belt described two ways. Because the offset between the two measuring points is fixed for each cross-section, any size converts with simple arithmetic.
The conversion rule
Subtract the section offset from the outside circumference to get the classical number:
- A-section: classical number = (4L number ÷ 10) − 2
- B-section: classical number = (5L number ÷ 10) − 3
Worked examples. A 4L460 is 46.0 inches outside; 46.0 − 2 = A44. A 5L240 is 24.0 inches outside; 24.0 − 3 = B21. Going the other way, add the offset: an A50 needs a belt 52.0 inches outside, which is a 4L520.
4L to A and 5L to B conversion chart
| FHP belt | Outside length (in) | Classical belt | Inside length (in) |
|---|---|---|---|
| 4L220 | 22.0 | A20 | 20 |
| 4L250 | 25.0 | A23 | 23 |
| 4L280 | 28.0 | A26 | 26 |
| 4L310 | 31.0 | A29 | 29 |
| 4L340 | 34.0 | A32 | 32 |
| 4L370 | 37.0 | A35 | 35 |
| 4L400 | 40.0 | A38 | 38 |
| 4L430 | 43.0 | A41 | 41 |
| 4L460 | 46.0 | A44 | 44 |
| 4L490 | 49.0 | A47 | 47 |
| 4L520 | 52.0 | A50 | 50 |
| 4L550 | 55.0 | A53 | 53 |
| 5L240 | 24.0 | B21 | 21 |
| 5L280 | 28.0 | B25 | 25 |
| 5L320 | 32.0 | B29 | 29 |
| 5L360 | 36.0 | B33 | 33 |
| 5L400 | 40.0 | B37 | 37 |
| 5L440 | 44.0 | B41 | 41 |
| 5L500 | 50.0 | B47 | 47 |
| 5L560 | 56.0 | B53 | 53 |
Sizes not listed convert with the same rule. 3L belts have no direct classical equivalent and are omitted.
Cross-section dimensions
| Section | Top width | Depth | Length basis |
|---|---|---|---|
| 3L | 3/8 in | 7/32 in | Outside, in tenths of an inch |
| 4L | 1/2 in | 5/16 in | Outside, in tenths of an inch |
| 5L | 5/8 in | 13/32 in | Outside, in tenths of an inch |
| A / AX | 1/2 in | 5/16 in | Inside, in whole inches |
| B / BX | 21/32 in | 13/32 in | Inside, in whole inches |
4L and A share the same 1/2 inch top width and 5/16 inch depth, which is why they substitute cleanly. 5L and B share the same depth, but their top widths are close rather than identical (5/8 inch against 21/32 inch). Always check groove fit on the sheave before substituting a 5L for a B.
Can you interchange them?
Yes, in one direction only. A classical A, AX, B or BX belt can replace a 4L or 5L belt. The reverse is not true.
- FHP belts have lower horsepower ratings and roughly one third the design life of a classical belt of the same section.
- Using a 4L or 5L in an A or B application gives reduced performance and shortened belt life.
- Notched belts (AX and BX) run about 3 percent more efficiently than wrapped belts of the same section.
What the upgrade is worth in horsepower
On a 5.4 inch pitch diameter sheave, the difference between a 5L belt and a B belt of the same section family is substantial:
| Belt on a 5.4 in pitch diameter sheave | Horsepower transmitted | Versus the 5L |
|---|---|---|
| 5L (FHP, wrapped) | 2.39 | Baseline |
| B (classical, wrapped) | 7.21 | About 3 times the 5L |
| BX (classical, notched) | 9.19 | 285 percent gain, plus about 3 percent efficiency |
Both the 5L and the B belt fit the same sheave groove, and they still deliver dramatically different horsepower. These ratings are illustrative: actual capacity varies with drive ratio and center distance.
Check the sheave before you upgrade
- Use only FHP belts in die-cast sheaves.
- Pressed steel sheaves cannot transmit the horsepower available in notched belts.
- Notched belts are often problematic in moveable-side sheaves.
Substituting a higher-rated belt into a sheave that cannot carry it moves the failure from the belt to the drive.
Cross-referencing a belt you cannot read
Measure the outside circumference with a tape held flat against the belt, without pulling it tight, and divide by ten for the FHP number. Add the section offset to get the classical number. If the belt is unmarked and worn, measure the top width first: 1/2 inch is an A or 4L application, 5/8 to 21/32 inch is a B or 5L application.
Our sales team are trained engineers and will cross-reference the part for you. Send the part number or the measurements with a photo, tell us when you need it, and we will give you a firm delivery date the same day. Call 915-845-8188 or request a quote.