Over on YouTube I found AviatorBJPs channel, in which he has built several automatic transmissions out of LEGOs. There is both a first generation design and a second generation design.
The central piece is an expanding flywheel that sits on the input shaft. As the input shaft speeds up the flywheel expands, creating linear motion. The linear motion slides a selector gear back and forth on the input shaft. At low RPM the selector gear engages the “downshift” gear to trigger a downshift, increasing the rpm, expanding the flywheel, and disengaging the selector gear from the downshift gear. Similarly at high RPMs the flywheel expands fully, pushing the selector gear forward to engage the upshift gear. The upshift lowers RPM, collapsing the flywheel, and disengaging the selector from the upshift gear. In between these two points, for most of the RPM range, the selector gear does not engage either the upshift or the downshift gears.
The upshift and downshift gears are then coupled (via several other gears and shafts) to a single gear that rotates one direction with the upshift gear engaged and the other direction with the downshift gear engaged, and is still when neither is engaged. This single gear generates the linear motion that ends up shifting gears. Cleverly, this gear is also friction fitted to its shaft, so that you can override the shifting by manually moving the linkage.
It’s probably a lot easier to see in the series of how-to videos, from phase 1 to phase 8, which describe this shifting mechanism. The linear 3 speed gearbox is described in yet another video.
Take a look,
Design News Gadgeteer
Send in your gadget! Design News is always looking for new gadets to feature in the print edition of the magazine. Mechanical, electrical, pneumatic, whatever. Believe me, if they’ll run my hot rodded Billy Bass they’ll run anything. If you’ve creating something clever and would like to see it in print, submit it to the Design News editors:
You’ll get the thrill of seeing your product show up in a print magazine right in your mailbox, and you’ll get a check for $500 as well.