13.9 or 14 mm belts and pulleys

What is the most popular belt and pulley set up at the track? I have a mis matched set of pulleys and belts both 13.9 and 14mm. Would like to have a full matching set so if ever in need at the track could borrow or buy one. Also I heard you can use either pulley on the top as long as the bottom pulley and belt match,is this true. thanks for any help.

 

The 13.9 MM belt and pulley system was, as I remember it, a commie plot by Cragar to monopolize the blower drive market by preventing anyone from using industrial 14 MM belts. They evidently paid Gates (PLEASE correct me if I'm in error here) the substantial nonrecurring to have them fabricate a 13.9 MM belt mold with approximately 111 teeth. They also sold idlers, idler brackets, pulleys, and snouts.

Unfortunately, with only the one rather short belt available, many people had to buy another set of bottom pulleys when the center to center distance of their application changed.

The difference in the tooth spacing between a 13.9 and a 14 MM belt is .1 MM, or about .004 inch. Since this is cumulative when the belt engages the pulley, it will mesh until one half of the cumulative error exceeds the backlash, or the amount the tooth is cut oversize. The obvious downfall of this is that as the error accumulates, the load sharing among teeth diminishes. When the load is carried by a small number of teeth, the belt life is severely reduced.

I've used 24 and 25 tooth 13.9 MM pulleys in a pinch with a 14 MM belt, and the belt really didn't like it. However, I don't think I'd recommend going much farther than this.

As a sidelight, I've seen pulleys that were confirmed belt killers. Invariably, the problem was found to be that whoever had made the pulley hadn't bothered (or didn't know) to cut the teeth such that they exactly matched the tooth spacing of the belt. This is detectable by wrapping the belt as closely to all the way around the pulley as possible, and then looking to see if the tooth spacing on the pulley advanced, fell behind, or matched that of the belt. Same problem, different cause.

Norm Drazy

 

Norm,

Seems to me that your method for checking the tooth spacing on a pulley would also work for checking acceptable belt stretch, this would help determine the cycle life of a given belt.

Would you agree?
Thanks Jim Thurston

 

Jim, Today's belts are all Kevlar, which has an amazingly high modulus of elasticity. This number determines how much something will deflect when a given load is applied. Kevlar essentially doesn't stretch. Example: Note the belt tension before and after a warmup. Kevlar belts get A LOT tighter when stretched by the amount the block and manifold expand, which isn't very much. Norm