Ok,I think I got this right. Just figure the outside dia of pulleys and the ratios will still be close. Take no offense,but this isn't a nuclear power plant.
2" motor pulley to 12" jackshaft pulley = 6 :1 ratio.Motor RPM 1725 divided by 6 =287RPM jackshaft speed.
26" bicycle wheel divided by 2" pulley on jackshaft output end = 13: 1 ratio. 287 RPM divided by 13= 22.1 RPM.
If you put a 13" pulley on jackshaft input,you'll wind up w/ 20.4 RPM.

Hope this helps,
Mike

You need a reduction of about 86.25:1 if you need a motor turning at 1725RPM to turn a drum at 20RPM.

The circumference of the nominally 26" bicycle wheel is 81.7" This means you need a pulley with a circumference of .94", which is a pulley .3" in diameter.

This is probably infeasable in this manner, so you're right to devise a 3-pulley system.

What you can do is put a 2" pulley on the motor, and have that drive a 12" pulley on the middle shaft. This slows the middle shaft down to 1/6 the speed of the motor- a 6:1 reduction. Now.... all you need is a 14.5:1 reduction to the bicycle wheel. To do this, you need a 1.75" diameter pulley on that common middle shaft to drive the bicycle wheel. A 2" pulley here would spin you a skosh faster than 20RPM.

Remember all these sizea are relative on the pulleys. You want to wrap your belt around the pulley and measure the width of the circle formed by the belt going around the pulley. The deeper a belt rides in a pulley, the smaller the effective diameter of the pulley with that belt. So... when I say a 2" pulley, That's the OD of the belt going around the pulley.

Someone correct my calculations- I ran these through as a quick mental exercise, but I could be playing fast and loose with theory. Ratios are all based on circumferences.

Mike- your calculations are identical to mine, and I hadn't seen your post yet. Pretty funny!

That loud sound you all just heard was the slap of my hand against my forhead. How could I have not seen that! UUGGHH!!

I guess I was so involved with the construction of the stupid thing that the math just escaped me.

Lloyd

Wait a second. I'm an idiot. I forgot a basic piece of algebra- the circumference two circles is proportional to the diameters of the two circles by a ratio of pi. Duhh....

The calculations still stand, but the circumferences just needlessly complicated my math. :)

pt03: and to think that those months spent in algebra class seemed like a colossal waste of time back then.

Now, solve for X. :)

I spent the last 2 hours trying to do the frickin math for the pulleys and you guys pull it off in, what, 15 minutes? I ain't even going to try anymore. As long as I have the internet, I'm going to get you guys to do it for me! For some reason the trees were blocking my view of the forest.

Thanks to both you guys.

Lloyd

As said, pulley ratios are easy to figure just using the diameters and shaft speeds. As also stated, if the belt doesn't ride in the top of pulley you'll have to make small corrections for that if you want final speed to the nitty gritty.
Glad to help,
Mike

If you need to fine tune the speed and if motor shaft and jackshaft are the right dia.,get two variable pitch pulleys as are used on swamp cooler motors to change fan speed.
Mike

Is there any way to put the motor on some sort of variable speed circuit? Would you ever need to run this setup at, say, half-speed?

You might actually want to measure the diameter of the bicycle rims.
A nominal 26" rim has a TIRE OD of 26". The bead seating dia. of the rim is about 22".

Without reading all the posts, the actual calculations that I use is to divide the drive pulley by the driven pulley. This method provides the final drive RPM. You can keep going on down the line with this calculation if intermediate pulley's are required.
If variable speed is needed, then either use a VFD for AC motors or DC drive for DC motors. This will allow you to fine tune any mistakes or issues with final pulley size to come up with the exact RPM your wanting. DC drives are very inexpensive these days ($100). VFD (variable Frequency Drive) are a little more expensive, but not to bad for fractional HP motors. DC has more low end torque, if that's an issue. VFD's low end torque has really improved in the last several years.

I have an old self propelled Bluebird leaf collector .It has a 6 to 1 gear box mounted on a B&S 3 HP engine.At a goverened idle,power out shaft turns about the RPM needed.

Thanks for all your help guys.

Lloyd

Here is a link that might be useful: Pulley reduction

Glad to help, Lloyd. Looked thru your pics,that is a professional looking job!!! Did the speed wind up about where you wanted it? I know of an old drag elevator you could use to feed that.
Mike

That does indeed look pretty good- I'd call it factory I'd even go so far is to say.

Glad to have helped!

Hi Mike

We got somewhere around 16-17 rpm, which is the ballpark we were shooting for. I now have to wait for spring to run some compost through it to check if that speed is adequate. We can easily adjust by changing the small pulley on the motor as we have several a few sizes bigger just laying around.

We are thinking of feeding it with an old grain combine hopper that will be freestanding at the inlet end. Of course one can just shovel/fork the material in as well. One guy wants to run pails of used coffee grounds through it to seperate the paper filters from the grounds. I think it will work fine for that as well.

Once again, thanks guys,

Lloyd

Just a follow up for the guys that helped me out. The sifter worked very well and the rotation speed is perfect. It just purrs along.

Lloyd

Here is a link that might be useful: Sifter in use.