I'm trying to reduce the rpm of a combustion motor. It operates at 2000 rpm with 3HP, and I need it to work at around 12 rpm (no matter the power). I've made the math with sprocket and chains, but the biggest sprocket is of 472 mm of diameter, which is too big. Is there any cheap and easier way to achieve this? Thanks!

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    $\begingroup$ use more pairs of smaller sprockets... $\endgroup$ – Solar Mike Oct 10 '19 at 19:14
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    $\begingroup$ It's going to cost more than the motor, that's for sure. Hydraulics or a decent multistage gearbox with chain final will set you back a couple grand US. But you need to provide design details of what you are doing. Three hp @ 12 rpm is a lot of torque. What are you building - a Ferris wheel? $\endgroup$ – Phil Sweet Oct 11 '19 at 9:49
  • $\begingroup$ The big sprocket will be about 2.5 feet in diameter using a fairly efficient and compact design. You need a 60 series chain on the final, or things get even more expensive with multistrand chain such as four strand 40 series. The final sprocket will cost 700 - 1000 dollars. That's using an 18 to 112 tooth 60 series final chain drive. You want a linear chain speed of about 50 ft per minute for this sort of final drive. That means big diameter. Running slower than that generally adds to cost. $\endgroup$ – Phil Sweet Oct 11 '19 at 10:38

2000/12 is about 167, so you need to gear it down a lot.

To a sensible 1st-order approximation, you don't want to gear down by more than 10:1 in any one stage (I'm not a mechanical engineer; if one of them speaks up feel free to believe them).

If I were throwing this together from surplus parts, or even building a one-off machine or a short production run, I think I'd run belt drive to a gearbox with my desired total reduction. So maybe 4:1 belt reduction to a 40:1 box, or 8:1 belt reduction to a 20:1 box.

One stage of reduction on a belt drive to one more stage of reduction with a gear drive wouldn't be horribly impractical. Because of the large torque multiplication, you'd need an enormous belt drive on the final stage which is why I'm suggesting gears or chains.

If you're building 10,000 a year, then you can afford to get an integrated motor/gearbox -- I suspect that's another story.

Or I'd go with 200:1 reduction because it's a nice round number, and run the motor at 1200 RPM.

Keep in mind that if the motor can do 3HP then the potential torque output at 12 RPM is around 1300 foot-pounds*, which is ginormous. Size your shafts accordingly or put in torque-limiting clutches, and don't get in its way.

* For those who noted my earlier math error -- I did the torque calculation from the RPM and power, without regard to gear ratio (and I used a calculator). So it should be correct.

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