How to read belt markings: SPB, B, 8M and length

A belt carries two things in its marking: a profile and a length. The letters are the profile: A, B, C on classical V-belts, SPZ, SPA, SPB, SPC on narrow V-belts, PJ, PK, PL on ribbed belts, 5M, 8M, 14M on timing belts. The number is the length in millimeters or inches. The main trap is which length it is: inside, pitch or outside. They differ by tens of millimeters. So in a request write not only "B 1500" but also which length is meant.

V-belts: classical and narrow

V-belts come in classical and narrow types. A narrow belt with the same top width is taller and carries more power. The pulleys are different.

The sizes are from the Optibelt catalog (DIN 2215 and DIN 7753 standards). In its manual Gates gives nominal sizes rounded, and for B and C with a smaller height. This is not a different belt, only a different way of writing it. It is better to check against the pulley.

How to tell A from SPA by eye. The width is almost the same, but the height differs: SPA is taller. If the belt is worn, measure the pulley groove and look at the print on the old belt.

Which length is printed: Li, Ld, La

The international standard ISO 4184 (latest edition 2025) sets the lengths of classical and narrow V-belts in the datum length system Ld. Narrow belts are usually marked with the pitch length: SPB 2000 means Ld = 2000 mm. Classical belts come with both notations, inside and pitch length. On a good belt Li or Ld is printed next to the number.

The difference between the lengths depends on the profile. From the Optibelt table:

Example. If the old belt was marked "B 1500 Li" and you bought "B 1500 Ld", the new belt is 40 mm shorter. The tensioner may not take up such a difference.

American A, B, C belts are marked with the inside length in inches: A100 has an inside length of 100 inches.

Ribbed belts

A ribbed belt is flat, with longitudinal ribs. The profile sets the rib pitch.

The PJ, PL and PM pitches are from the Gates manual. PH and PK profiles also exist. Check their pitch in the manufacturer's catalog: a pulley of another profile will not work with the belt.

How to read a marking like 6PK1200: 6 is the number of ribs, PK is the profile, 1200 is the length in millimeters. For ribbed belts Gates states the effective length. Which length your manufacturer prints, check in its catalog.

Timing belts

For a timing belt three numbers matter: length, pitch and width.

An example from Gates: "HTD 1040 8M 30" is a pitch-line length of 1040 mm, a pitch of 8 mm, a width of 30 mm. Other manufacturers may put the numbers in a different order, but the set is the same.

Can one profile replace another. Gates writes that an HTD belt must run on pulleys of the same pitch. GT 8M and 14M belts from Gates run on HTD pulleys of the same pitch. GT belts of 2, 3 and 5 mm go only on their own pulleys. A T10 belt will not fit an 8M pulley: both the pitch and the tooth shape are different.

What to put in the request

1. Type: V-belt, ribbed belt, timing belt.
2. Profile: B, SPB, PK, 8M and so on.
3. Length and which one it is: Li, Ld, La; for a timing belt, along the pitch line.
4. For a ribbed belt, the number of ribs; for a timing belt, the width.
5. How many belts are on the drive. On a multi-groove drive belts are changed as a set: a new belt next to stretched old ones is loaded harder.
6. A photo of the print on the belt and of the pulley with its grooves.

How we do it

• From the print and a photo of the belt we look for the belt at the manufacturing plant; if the print is worn off, we ask for the pulley groove dimensions and the length.
• We send what we find before the order: profile, length with Li or Ld stated, number of ribs or width.
• At the plant we check the marking of the belts being shipped against the agreed item.

There are no ready-made belts in the catalog at the moment: they are selected on request.