The Technic Tracked Crane (9391) is a great, inexpensive Technic set, but it doesn’t have any power. I wanted to do a simple mod to make the tracks spin.
Getting power to it is fairly easy if you’re willing to include the trailer. This method replaces the axle that rotates the upper portion with an axle that drives the tracks.
The first step is modifying the Tracked Crane so that the tracks can be driven and a trailer can attach to it.
First just replace the axle on the very bottom that runs parallel to the tracks with a longer one. You’ll probably also want to get rid of the black gear that spins the top portion of the crane (since this gear will constantly turn).
Next attach a worm screw to the axle.
Now take another axle and place it across the tracks to connect the back gear on each track to each other. On this axle, attach an 8 tooth gear which will be driven by the worm screw. You may need to firm up the axles with some other pieces.
Finally, make a little trailer with a motor on it that can connect to the first axle. The only important thing here is that you make the connection the same height as the axle you’re connecting to on the crane. You may use a different motor than the one I used.
Connect the trailer to the crane and you’re ready to roll (literally)!