Thread: pulley sizing -air compressor

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I bought a new pump for my air compressor but here is the deal- I need to slow it down. Pump manual calls for 1200 RPM and has a 10 3/8" flywheel, My motor is 3150 Rpm . SO what Dia. pulley do I need for my motors shaft? I have a 4" pulley on there now and it is WAY too fast.

Heres a calculator for you.
http://www.csgnetwork.com/pulleybeltcalc.html

Can't make it work. I have the-
Driven RPM (1200) ,
Driven pulley size (10")
Motor RPM (3150),
Center to center distance(12.5")but center to center depends on pulley sizes as does belt length.
Which is what I am trying to figure out is what I need for a drive pully(motor side)

Works for me ?
4 inch powered drive pulley @ 3150 rpm. ( pulley on motor)
10.425 driven pulley = 1200.15 rpm @ driven pulley (on compressor)

Switch it to a 3 inch power pulley and it would slow the driven pulley to 900.9 rpm.

For some reason I could not get my brain wrapped around that one. I did find another that came up w the same answer. I need a 4" pulley or rather 3.81". but close enough to 4" to use a 4"
http://www.pumpcalcs.com/calculators/view/98/

That's pretty close I come up with 1250 rpm 4x 3.141 = 12.56 in per rpm and 10.375 x3.141 =32.50 in. per rpm. that means 2.6 revolutions to turn the pump 1 rev. the math comes out 1250 rpm maybe I'm missing something I don't know. but that's how I got it figured......ted

Hell i screwed up, shoulda used 10.375 instead of 10.425 for my driven pulley size.......DUHHHHHHH ,but it still comes out @ 1212.75 rpm.