HP and pulley size

[Wasatchmtnbike]

Ok, I'm new to this forum and I have a problem I need help with. Please keep in mind that I'm not very experienced in Physics, so I thought I would ask the experts.

 

A while back I bought a used chipper-shredder with a Tecumseh 8HP gas engine that was bad, it broke a piston and had a hole in the side of the engine. Anyhow, on the original 8hp engine it had a side shaft that turned a 4"diameter pulley. I have replaced the 8hp engine with a 6.5hp engine. I'm hoping to try to put a pulley wheel on the new 6.5 hp engine that will get me close to the same power that the 8hp engine had. Do I go with a larger diameter pulley wheel or a smaller diameter pulley wheel? My brother says a smaller diameter pulley wheel, he said it would turn a little slower but should be close to the same power. I think that I would agree with my brother but I'm not sure how much smaller that pulley needs to be.

 

 

I hope I have given all the info you need to help me with this. If I am on the wrong form please tell me. Anyhow, I thought I would give it a try.

 

 

Thanks guys,

 

Gary

 

[TonyMcC]

First things first - Whatever you do the maximum power you will get from an 8 hp engine is 8 hp and the maximum power you will get from a 6.5 hp engine is 6.5 hp. In other words the smaller engine will be less powerful.

I imagine a belt from the pulley you describe runs round another pulley that rotates the shredder blades. In order to to maintain rotation when shredding you need to gear down the link between motor and shredder blades. This can be achieved by either reducing the size of the motor's pulley or increasing the size of the pulley that rotates the blades. This will reduce the speed of blade rotation but allow the blades to apply sufficient force. I haven't done any calculation, but its probably not that critical - I would try a 3.5 inch pulley in place of the 4 inch pulley.

[Wasatchmtnbike]

Thanks! I think I'll try a 3 1/2" pulley.