# pulley ratios on pump

Discussion in 'Homeowner Assistance Forum' started by horselogger, Jul 6, 2005.

1. ### horseloggerLawnSite Memberfrom St. maries, IDMessages: 2

I am trying to hook up a pump for irrigation. The pump seems to be what I need but the gas motor is a Honda GX240 with a 1:6 reduction. It normally is supposed to function at a maximum of 3900 rpms but with the reduction it is a much lower rpm. I am trying to use it to run a centrifigul pump that requires 3200rpm. I am not sure how to figure out what size pulley I should put on the motor to bring the rpms back up to the optimal number. Can you help me? Thank you

2. ### betterlawnLawnSite Senior Memberfrom MIMessages: 426

Just use the ratio of the diameters. If it runs at 3900 with 1/6 reduction, than the output is running at 650 RPM. If you want to bump it back to 3200 (with the reducer in place), you need 3200/650 for the pulley ratio, i.e. ~5:1 speed increaser.

Or, just get rid of the slow side of the reducer and put a pulley on that is 3200/3900 the size of the motor pulley (about 82%), i.e. if you had an 8" pulley on the motor, put a 6.5" pulley on the pump. And of course you want to underspeed the pump, not overspeed it.

In short, the belts don't matter - its strictly the pulley diameter.

3. ### horseloggerLawnSite Memberfrom St. maries, IDMessages: 2

I guess I got some of the numbers wrong as I look at Honda's website and see that they calculate the reduction to equal 420 rpm for some reason. 3900 rpm was what they quote for the max without a load and 3600 is the regular rpm that is expected I guess).

Anyway, I see what you mean, I think. The pulley coming from the reducer is 2" so to raise the rpm by 5 or 6 I need to multiply that number by the 2" which is the 1 in the 1:5 ratio and that would give me a 10 " pulley--12" if I want to increase it by 6. Is that correct?

Thanks for your help!