V-belts

For classical belts, A, B, C and D are the cross sections and the number is usually the length in inches, as in B-60. For narrow belts to ISO 4184, such as SPZ, SPA, SPB and SPC, the number is the datum length in mm. Makers may quote inside, outside or datum length for the same section, so confirm which length system applies.

Narrow belts such as SPZ, SPA and SPB carry more power for the same width but have a different groove angle and depth. Some pairs, like SPB and B, can run in each other's sheaves, but the rated power is limited to what the existing sheave allows. To get the full benefit of narrow belts, change to sheaves with the matching profile.

Cogged belts have transverse notches on the inner side, so they bend more easily around small pulleys and dissipate heat better. As a result they run on smaller diameters and carry more power than a wrapped belt of the same section. They still fit standard sheave grooves of the same profile.

Too little tension makes the belt slip, heat up and squeal, while too much tension cracks the belt back and overloads the bearings. Misaligned sheaves, polished worn grooves or oil on the belt also cause early failure. Never lever a belt over the sheave flange; reduce the centre distance to fit it.

The common method is to press the middle of the span with the force given in the maker's table and measure the deflection, typically about 16 mm per metre of centre distance. A slack belt slips, heats up and wears its sides, while an overtight belt overloads the motor and shaft bearings. A sonic tension meter gives more repeatable results.

New V-belts stretch and seat into the grooves fastest in their first hours, so check and retension after about 24 hours of running. After that, check on a routine schedule, for example monthly depending on load. When retensioning, also recheck sheave alignment because the motor may have shifted.

It is not advisable, because a new belt is shorter than the stretched old ones and will carry most of the load and fail early. Replace the whole set at once with belts of the same make and batch, or a matched set. If length variation remains a problem, a banded belt shares load evenly and prevents belt turnover.

No, prying a belt over the rim breaks the tensile cords inside even when the belt looks fine outside. The correct way is to slide the motor on its base until the belt drops into the grooves freely, then retension. Belt dressing should not be applied to stop slip either, as it degrades the rubber.