Finding Pulley Sizes


| February/March 1994



PULLEY A DIAMETER X RPM

3952 Norman Road Clarkston, Georgia 30021

This is some information which I thought might be interesting and helpful to people who like to 'belt' their engines to various equipment. A friend of mine recently belted two of his engines to some hammermills he had acquired. When I asked him how fast he was turning the mills, he said, 'I don't know, I just found some pulleys that made them sound right.' That is one way and he has two nice hammermills. However, I proceeded to explain how to determine pulley sizes and RPM. With my shaft tachometer I checked the engine RPM (no center hole in mill shaft) and computed the mill speed at 1821 RPM using the pulley sizes. This procedure is explained on the attached sheet.

My interest comes from my experiences around my dad's sawmill and shop. There we had numerous occasions to use such equipment as the shaft tachometer. One application was checking such things as saw mandril RPM. As you may know, the circle saw has to be operated at the specified RPM for which it was 'hammered' and is rather critical. I remember on one occasion he had a power unit which ran 1800 RPM with a 14' drive pulley. It only took him a few minutes with his 4th grade education and a pencil to figure he needed a 48' pulley on the mandril to run the saw 525 RPM. He was well aware of dangers involved with flywheels, belts, pulleys, and etc., and always kept people clear of such equipment. The main drive belt was one of his major concerns besides the circle saw. This belt was 10' or 12' wide, probably 50' or 60' overall length, and I would guess weighed in excess of 100 lb. This belt could create a lot of havoc when it started fraying and coming apart. He probably never knew its actual travel speed so I thought it might be interesting to compute.

Pulley circumference = 14' x 3.1416 = 43.98'

Rim speed (per minute) = 43.98' x 1800 RPM = 79164'

Feet per minute = 79164'/12' = 6597' (App 110' per second)