![]() Put it in the stand so that the annulus faces the side that says "1 3", then insert the drive shaft from that same side. Now that assembly will hold itself together. ![]() Mesh an upside-down sun gear with the two large planets, then attach the last large planet the same way (again, being careful with the clocking so that the pin will go through). Then push a long pin all the way through to lock them together. Set a large planet on top of one of those holes and clock it until the flat sides of the holes line up between the large and medium planet. Mesh the other two medium planets with the idlers and annulus and make sure they line up with their holes. Next, set the assembly with the idlers down in the middle of the annulus. Attach a large planet to the other side of this pin (on the back of the carrier). Next, mesh a medium planet with an idler and push a long pin through from the back of the carrier to secure it. Finally, mesh the last idler with the sun and push its pin down through from the top to secure it. Then mesh an upside-down sun gear to both of them. Start with the planet carrier right-side-up and insert two medium-length pins into two inner holes and snap idlers (smallest planet) onto them. All these steps are shown in the photos above. The herringbone gears are tricky to assemble because they cannot slide past each other, but they run smoothly and don't require thrust bearings like the helical gears in real automatic transmissions. Then assemble the stand as shown in the pictures (they're designed to press-fit, so no glue should be required). Make sure these can rotate freely (they can require some force to free if your printer leaves any strings). To make the labels more visible, you can pause and switch colors a couple layers into stand.stl.Įach of the stand pieces has a brake, which is a separate element that is permanently captured by the larger piece. 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. I swear, complex models don't have to be difficult, and who needs glue when you can print snap-fits?Īll the STLs are there if you want to print the pieces individually, but using the plates is quicker. Everything came out perfectly on the first print. This was all printed in PLA at 120 mm/s on a Replicator 1 with Sailfish firmware, default layer height (0.27mm). I used transmission.m as an aid in optimizing the ratios to be somewhat evenly spaced. I also used Matlab to investigate more thoroughly how the gear sizes affected the various ratios. If you select a different number of teeth, it will print out the resulting gear ratios at the beginning of the output. The OpenSCAD file is included and is highly parametric in case you'd like to play with different gear ratios. I tried to design the gear ratios to be fairly close to what some real cars use, and this is the result, where the input is the crank and the output is the annulus: You select a gear by engaging the brake and clutch position associated with your desired gear. ![]() The clutch is actuated by sliding the drive shaft through to different positions (which each have two gear markings), while three separate brakes each also have two gear markings. With this model you control those simplified brakes and clutches yourself. ![]() Real automatic transmissions have a hydraulic or electrical system that engages different clutches and brakes to shift gears depending on the driving situation. It has six forward speeds and one reverse. Have you ever wondered how an automatic transmission works? I did, so I looked it up and then designed this desktop model.
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