December 2, 2012

Building the Differential

I started out with a foot of 12 tooth, 20 pitch spur gear rod, two 20 tooth pinions, a pile of bushings and bearings, some 5/16 precision steel rod, a 2.5" diameter solid aluminum cylinder, and some miscellaneous other hardware.

I cut the spur gear rod roughly to length on the horizontal bandsaw, and faced the ends on the mill.  I bored out the centers, and pressed in pairs of bronze bushings.

The configuration of the gears inside the differential looks something like this, with the two 20 tooth pinions connected to the two output shafts:

I made the end caps from some 2.5" aluminum round found in the MITERS scrap pile.  I faced the end to make it easier to align properly in the lathe.

So I wouldn't have to constantly refer back to my computer, I printed out some engineering drawings with all the dimensions I needed.  Some are penned in, because I forgot to double check if I had all the dimensions before printing them.

This is the finished end cap that goes opposite the drive sprocket.  Things I learned from this:  Go very slowly when making press fits.  A tiny bit of material can make a huge difference.  I botched one of them, and made the bearing hole a hair too wide.  Fortunately, it was nothing some precision shim couldn't fix.

Six steel rods span the differential, serving as both axles for the planet spur gears to spin around, and as the structure that transfers torque from the sprocket side of the differential to the non-sprocket side.  I wanted these rods to all be the exact same length, and I didn't want to rely on the sketchy MITERS calipers to make them all the same.  To make sure they were identical, I decided to face their ends simultaneously.  To hold all six rods in the mill vice, I made a fixture out of some aluminum square rod.  I drilled 6 holes in the rod, and cut alternating slits through to the holes.  When the square rod is clamped form its ends, the slits compress, and clamp the rods together.

It also came in handy for tapping the ends of the rods without grunging their surfaces in the vice.

The rods are countersunk 1/2" into the end plate, and are held by stainless steel socket cap screws from the other side.

Here's the mostly assembled differential.  I still need to add keyways and set screws to the output pinions, and make spacers to keep the planet gears axially aligned 

And a rendering, for reference:

I found some 1/2" aluminum tubing lying around, so I used to temporarily assemble the drivetrain.

Wheels are on their way, so I should have a rolling frame relatively soon, assuming the last couple weeks of classes and finals don't eat all my time.


  1. Oh. My. God. That is awesome.

  2. May I ask how the motor reaches the differential to rotate it?

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