Pulley Size and RPM Calculator

A bigger motor pulley does not give you more air

It gives you more pump speed. Whether that turns into more air depends on whether the pump was rated to run there, and most weren't. The pump's rating plate or manual lists a maximum rpm. Run above it and you get heat, oil carryover and a short life, not extra CFM. So the first number to find is the pump's rated speed, and the pulleys are chosen to hit it with the motor you have.

The math is a ratio. Pump rpm equals motor rpm times the motor pulley diameter divided by the compressor pulley diameter. A 1725 rpm motor with a 4 inch pulley driving a 12 inch flywheel turns the pump at 575. Swap the motor pulley to 4.5 inches and the pump runs at 647. That's what the three modes above do, in whichever direction you need.

Belt speed, the number nobody checks

The belt speed figure is the surface speed of the belt in feet per minute. Cast iron pulleys are generally rated to about 6,500 ft/min. A 3450 rpm motor on a 7 inch pulley is already there. Go past it and the pulley rim is the thing at risk, not the belt. If the calculator flags it, either drop to a smaller motor pulley and accept a slower pump, or move to a 1725 rpm motor with a larger pulley to get the same pump speed at half the belt speed.

Bore and bushings

Fixed bore pulleys come in 1/2, 5/8, 3/4 and 1 inch and fit the shaft directly with a key and set screw. Most of what we stock is bushing type: the pulley has a tapered hub and you buy the H, SH, SK, SD or SDS bushing separately in your shaft size. More parts, but one pulley casting fits a dozen shafts and the taper grips better than a set screw. If the bore filter above shows nothing in a fixed bore, switch to bushing type and look at the pulley collection for the matching bushing.

Pulley OD is used throughout. Belt manufacturers rate drives on pitch diameter, which sits a little inside the OD, so a drive built off these numbers will run within a percent or two of the figure shown. For a compressor that is well inside normal. Motors are in the motor collection if the one you have won't get there.