Hey guys, Steve here . I'm wanting to possibly build up a compressor for my shop . I've got a 60 gal. single stage and even tually want to move up to a 2 stage pump. My question is : Does anyone know how to figure out the pully sizes for a rated rpm. ? Here is an example of the compressor : HP- 5 Flywheel -13.8" RPM - 1450 How would the formula work to choose a motor / engine / pullies ? Sorry this may be basic to some but I never passed Pre-Alg . Variables: Engine RPM. when charging ? Engine Pully size - ? Known : Compressor Flywheel - 13.8" / Rated Compressor Engine RPM is 1450 MAX. Hope someone here likes to crunch numbers . Thanks , Steve
Basically the important thing is the ratio between the two sizes. If your motor is spinning 1450 RPM with a 13.8" flywheel, and your compressor has a 27.6" flywheel (a ratio of 1:2), your compressor will turn a 1/2 of 1450, or 725 RPM. If the size difference goes the other way (where the driven pulley is smaller than the driving pulley), the speed of the compressor will be higher. If the compressor pulley has a diameter of 6.9", it will go twice as fast as the motor, or 2900 RPM. The same formula applies to alternators- if the alt. pulley is 1/2 the diameter of the crank pulley, it will turn at 2X the crank speed. If it is 1/3 the diameter, it will turn at 3X the crank speed, etc. -Bob Cunningam
