Donkey Mill Project

I've had the idea for a donkey-powered mill for awhile, and recently I came across a good automotive differential to be the primary hub and gearcase for the project.  With typical abandon I went to work and had the basic machine running about 10 days later.

Sebastian was the test pilot for this project.  I hope to have three sweeps eventually and use the whole team.  A threshing machine will require all the power I can get.  This sweep is a carbon-fiber windsurfer mast, much stronger than it looks in the picture.  I might go for something a bit longer eventually.  The skid is just spiked into the ground right now.  The sheller is just sitting on the ground, too.

Here is how the differential looked the day I brought it to the farm.  The seller told me it was from a rear-wheel drive Mazda, but I have no way of confirming that.  It took some doing to get the old driveshaft and rear axle off.

I liked this differential because all three shafts terminated in nice 4-bolt flanges.  Easy for fabricating.

One of the axles needs to be prevented from turning.  This differential offered an easy solution--I added a piece of angle iron to serve as both mounting bracket and the stop-turn mechanism.  It got a little trickier, however, when I found that any pressure on the axle flange caused the gears inside to bind up.  I later learned that the seal around the axle is not very good, and oil leaks out.  If I get the unit inside again, I think I will clean it up and put some good silicone around the lower axle to keep the oil in better.

So I fabricated another mount to hold the unit up around the driveshaft flange.  Since nearly every differential has a modest gear reduction from the driveshaft to the axle, it makes sense to drive the donkey mill from one of the axles.  In theory there would be no problem attaching the sweep arms to  the driveshaft input.  You would just lose some rpm on the output shaft.  By driving one of the axles I'm getting about a 1.5:1 speed increase.  My donkeys pulling on 9' sweeps are doing about 3 rpm I figure.

Here I am using some bicycle sprockets to get an additional 43:13 gear ratio.  I used 1/2" shaft for my interim driveshaft, but now I wish I had used at least 3/4".  It looks like a potential area of weakness.

The long shaft coming out of the center is 1/2" water pipe.  Again, maybe I should have gone bigger.  I fabricated some simple flex couplings from pipe flanges, little pieces of rubber cow mats, and 1/4-20 bolts.  I put these flex joints at both ends of the 1/2" pipe driveshaft.

Here is the unit to receive the power coming off of the sweep.  Again, very quick and dirty.  I'd like to improve this with some kind of shut-off clutch and some additional shafts to make additional gear ratios easy to achieve.  I'd like to run a fanning mill and a small threshing machine eventually, in addition to the corn sheller.  The corn sheller made gearing pretty easy for the first time out, since it's made to run with a hand crank at around 20-30 rpm I think.  The final output speed tested was 30-32 rpm, with Cassie on the sweep.  Other items will require higher rpm and more gearing.  I felt good about using a v-belt for the final drive, since the slippage would provide some safety as I built up the system and learned how it worked.

After running about 100 lbs of corn through the mill, the 1/2" pipe shaft seems a little undersized.  It would buck under loads, usually when the donkey would slow down and the sheller get stuck with an ear of corn half way through.  I'd like to rebuild it so the shaft has a higher rpm, and therefore lower torque on it.  I think a second chain on the center before transferring power to the main shaft is a good idea.  Also, I'm wondering if I can find some sort of slip clutch to act a safety overload.