How to measure pulley diameter

You're on the right track. The pitch diameter of the pulleys is measured at the neutral axis of the belt. If you imagine the belt being made up of many thin layered loops, the loop that doesn't change length as the belt is flexed is the neutral axis. The reinforcing cords in a v-belt define the neutral axis, which is perhaps 1/3 of the belt thickness from the top surface of a typical v-belt.

I do see your problem. You allowed 1/2" for the center of the belt on the 2.5" pulley, and then neglected to do so for the 5" pulley. Why is that? By your logic, the 5" pulley would actually have an effective diameter of 4.5", wouldn't it?

And then you should probably allow for the exact spot the belt rides on the pulleys - it may be a bit over the top of the groove on one and slightly below on another. And, as Ned mentioned, you should make your calculations based on about 1/3 the thickness of the belt from the outside surface.

On the other hand, does it really matter if your saw blade turns at exactly 3600 RPM? Or at the 3450 RPM that your original motor most likely runs at? I don't think there will be any trouble if your blade turns fast or slow by 10 to 15% or so. I wouldn't worry about it, but I could be wrong.

John Martin

Won't the blade lose power with the pulley reduction? I know it will run faster than the motor but won't it lose torque? I know a small pulley driving a big one will gain power to move objects so it stands to reason that reversing that will give more RPM but less power. Am I wrong on that?

Jim Chandler

If the motor is the same HP, it will produce twice the torque at half the speed. The "gearing" will bring it back to the same speed and torque.

As for the pulleys, pretty close just measuring either the root or the crest of the pulley on both - and if they are the same width the speed ratio will be the same as the measurement ratio. (within a VERY small percentage of error)

According to Machinery's Handbook (13 ed. P.877-8) "These pitch diameters equal approximately the outside diameter of the sheave minus an amount equal approximately to the belt thickness plus from 1/16 to 1/8 inch, depending upon the belt size. For example, if the belt is size E or 1 1/2 by 1 inch, the pitch diameter would equal ( approximately at least) the outside sheave diameter minus 1 1/8 inch."

Approximately approximation of an approximate answer! This was the first thing I learned after I bought this book for five bucks ten years ago, something that I had pondered for many years. Gerry :-)} London, Canada

To measure the blade's pulley "pitch diameter": make a chalk mark on the pulley and belt just where the belt "exits" the pulley, rotate the pulley one revolution so the belt's chalk mark moves away from the pulley, and measure the distance on the belt from its chalk mark to the pulley's chalk mark. This is the pitch circumference, divide by pi for the pitch diameter.

Buy a new pulley whose pitch diameter is twice this. Pulleys *are* sold by pitch diameter. Maybe not the ones on the rack at Ace Hardware, but the ones at MSC, McM-C, etc.

HTH, Bob