Have you ever wondered how an automatic transmission works? I did, so I looked it up and then designed this desktop model. It has six forward speeds and one reverse. Real automatic transmissions have a hydraulic or electric system that engages different clutches and brakes to shift gears depending on the driving situation. With this model you control those simplified brakes and clutches yourself.
The clutch is actuated by sliding the drive shaft through to different positions (which each have two gear markings), whereas three distinct wheels each also have two gear markings. You select a gear by engaging the brake and clutch position associated with your desired gear. See demonstration video: http://www.youtube.com/watch?v=-FyC3dn3HJY
I tried to design the gear ratios to be fairly close to what some actual automobiles use, and this is the result, where the input is the crank and the output is the annulus:
1st gear: 1 : 4.29
2nd gear: 1 : 2.5, 71% growth
3rd gear: 1 : 1.67, 50% growth
4th gear: 1 : 1.3, 28% growth
5th gear: 1 : 1, 30% growth
6th gear: 1 : 0.8, 25% growth
Reverse: 1 : -3.93
The OpenSCAD document is included and is highly parametric in case you'd like to play with different gear ratios. If you select a different number of teeth, it will print out the resulting gear ratios at the beginning of the output. Additionally, I used Matlab to explore more thoroughly how the gear sizes affected the various ratios. I used transmission.m as a help in optimizing the ratios to be somewhat evenly dispersed.
This was all printed in PLA in 120 mm/s on a Replicator 1 with Sailfish firmware, default height (0.27mm). Everything came out perfectly on the first print. I swear, complex models don't have to be difficult, and who desires glue when you can print snap-fits?
All the STLs are there if you want to print the pieces individually, but using the plates is faster. Print one each of stand.stl, gears.stl, carrier.stl, shaft.stl, crank.stl, handle.stl, and three each of planets.stl and pins.stl. To make the tags more visible, you can pause and switch colors a few layers into stand.stl.
Each of the stand pieces has a brake, which is another element that is permanently captured by the larger part. Make sure these can rotate freely (they could require some force to free if your printer renders any strings). Then assemble the stand as shown in the pictures (they are designed to press-fit, so no adhesive ought to be required).
The herringbone gears are tricky to build because they can't slide past each other, but they run smoothly and don't require thrust bearings such as the helical gears in actual automatic transmissions. These steps are shown in the pictures above. Begin with the planet carrier right-side-up and insert two medium-length pins into two inner holes and snap idlers (smallest world) onto them. Then mesh an upside-down sun gear to both of them. Finally, mesh the last idler with the sun and push down its pin through from the top to secure it.
Next, mesh a medium world with an idler and push a long pin through from the back of the carrier to secure it. Attach a large planet to the other side of the pin (on the back of the carrier). Next, set the meeting with the idlers down in the middle of the annulus. Mesh the other two medium planets with the idlers and annulus and be sure they line up with their holes. Set a large planet on top of one of these holes and clock it till the flat sides of the holes line up between the big and medium planet. Then push a long pin all the way through to lock them together. Mesh an upside-down sun gear with the two big planets, then attach the last big world the same way (again, being careful with the clocking so the pin goes through).
Now that meeting will hold itself together. Place it in the stand so the annulus faces the side that says "1 3", then insert the drive shaft from that same side. You might have to rotate pieces to find the hexagonal part to slide through. Use the short pins to attach the crank and handle to the drive shaft, and you are done.
Mine started out pretty tight, but after greasing the pins and spinning it for awhile it loosened up. Mostly I think it had to grind off the very small lumps and strings that form in the gear teeth.
UPDATE: I've added front_loose. stl and back_loose. Stl for anyone who is having trouble getting the brakes to free up. They use the same tolerance (0.25mm), but I've opened up the remainder of the space around them to make them easier to break free. Notice the stand plate does not use them, so you may want to make your own printing plate.