Lego Six-Speed Gearbox

Gearboxes are ubiquitous as a component of power transmission. I recall being fascinated by automobiles when I was youngI had many questions then, most of which concerned the hidden machinery, or perhaps magic, beneath the hood, purring ever so steadily as the car drove on. I had first built this gearbox to explore the workings of a transmission.

The vague objective, as I recall, was to build something compact with the parts I had, that can readily be used elsewhere, say, a model vehicle or some other sort of mechanical contraption. I had not bothered to stick to the usual H-pattern on manual transmissions.

Lego does provide specialized parts for dog clutches—they were released just months prior to the project. These are the red barrel-shaped pieces visible on the three top-layer shafts. 

 

Corresponding pairs of dark-grey gears couple with these clutches, and two more layers of gearing beneath transmit power to the output shaft. Input is connected to the top yellow connector, and output is tapped from the bottom counterpart.

The gear ratios of the gearbox are:

I'll discuss in brief the internals of the device, with some illustrations on the power flow through the gearbox. The generation of these diagrams took some meddling with software; if you are interested in the behind-the-scenes happenings, I have written a blog post on it here.

First Gear. Power is transmitted to the right-hand shaft through a 20:12 gearing at the rear of the unit, and then to the front middle idler gear through the dog clutch. A 1:1 gearing through the bottom two layers connects the idler gear to the output shaft.

Second Gear. Power is transmitted to the rear middle idler gear through the dog clutch. A 24:16 gearing through the bottom two layers connects the idler gear to the output shaft.

Third Gear. Power is transmitted to the left-hand shaft through a 16:20 gearing at the rear of unit. The left rear idler gear is engaged through the dog clutch, and power is relayed to the output shaft via a 24:16 gearing through the bottom two layers.

Fourth Gear. Power is transmitted to the output shaft through a straightforward 1:1 gearing.

Fifth Gear. Power is transmitted to the left hand shaft through a 12:20 gearing at the rear of the unit. A 1:1 gearing through the bottom two layers relays power to the output shaft.

Reverse Gear. Power is transmitted to the right hand shaft through a 20:12 gearing at the rear of the unit, and then down to the output shaft via a 24:16 gearing. An extension piece coupled to the dog clutch allows an even number of gears to be driven in this alternative gear-train, hence achieving direction reversal.

Digital models and building instructions are available for download below.

© 2023 by Exorus Koh