Increase a gas engine's RPM to drive a water pump

[RTE]

Hello all,

This is my first post, but I'm no stranger to skimming the boards. Always can find helpful stuff, but I'm stumped on this one. The situation is we have a Kohler 17 hp electric start gas engine horizontal shaft that max's out at 3600rpm. We are attaching a self-priming water transfer centrifugal pump to it that can handle the same HP, but it starts at 3600rpms and max's at 6000rpms... I want to get the most out of my pump so I have to increase the ratio on the pully system it will be attached to.. I am looking for a 1:3 minimum, but 1:4 would be solid... I'll stick with the 1:3 for now... My question is if I would attach let's say a 3.5" pulley to the pump and around a 10" on the engine then would the engine have a tough time starting up on it's electric start system with that big of a pulley attached directly to it? Engine shaft is 1 1/4" and pump shaft is 5/8".. Everything double belt or triple? The entire system runs on a 12v battery which is charged by the command pro Kohler engine... The system turns on and off automatically using murphy switches and a control panel... Any input would be most appreciated... Thanks

 

[rbelli1]

Would something like this be an option?
https://en.wikipedia.org/wiki/Torque_converter

BoB

 

[russ_watters]

I think your odds are good. The nice thing about a pump is they have small, low inertia impellers and start with fairly low torque as long as they aren't speeding up too fast.

 

[Rx7man]

Can you have an idler between the two pulleys to tension the belts once it has started? Old technology, it's simple, and it should do the job for you.. a torque convertor would definitely work great, it's just more expensive.

If you wanted to automate the tensioning system you could connect a line from the engine oil pressure to a small cylinder that tightens the belts... as the engine starts it will slowly build pressure and engage the pump... might squeak a little as it engages.

 

[Nidum]

What rpm has to be achieved before engine will start ?

 

[Rx7man]

I'd say 'idle speed' on a 17hp industrial engine would be around 1200-1500 RPM, below that it probably doesn't make enough power to turn itself over well...

 

[Nidum]

What I am trying to get at is the engine catching speed and how that relates to the coming on load speed for the pump .

Say 3:1 speed up to a pump which starts to pick up significant load at - rough guess - 2400 rpm .

2400/3 = 800 rpm at the engine .

 

[Rx7man]

Perhaps if you could have a bypass on the pump so it can't build any pressure would help keep load off while cranking.
I'm kinda thinking out loud

 

[Nidum]

Not enough data to do proper sums but I think that limiting the speed up ratio to around 2 would be a good idea .

Otherwise you may need an unloading device as suggested .

 

[cjl]

Why would you want a 3 or 4 to 1 ratio? It seems to me that you'd want a ratio such that the pump and engine both hit maximum speed at the same time, which implies a 5:3 ratio. A 2:1 ratio would probably also work fine, so long as the gas engine is producing near full power at 3000RPM, but if you run a 4:1 ratio, you'll be expecting the engine to run at 1500RPM while driving the pump, and the engine likely doesn't make very much power at 1500RPM.