Improvised Pull-Back Motor

During a recent afternoon, I found myself bored. Then, I experienced a spontaneous urge to build a sort of homemade pull-back motor. (My junior high shop teacher would comment about what happens when I get bored. Refer to The Gear Train of Doom). I didn't want to use the type with an elastic band wrapped around an axle, because I've seen that done before. Instead I decided on a model with a rack sliding along a track, meshing with gears driving the wheels. The rack is securely locked within the casing, able only to slide back and forth.Three rubber bands stretch from the rack to fixed points on the frame of the vehicle, which pull the rack along the gears. In this respect, it is more like an automatic rip-cord than a tension coil, but it still works.

The final product

As usual, my first attempt did not work. I tried to use all studless pieces, but the frame was not strong enough to withstand the elastic tension. The frame warped just a bit, allowing the rack to slip by without turning the gears.

Though slight, the distorted angle of the rack is visible.

This picture shows the basic mechanism, which did not have
enough support to function correctly.

For my second attempt, I used a combination of studded and studless pieces. While most technic builders have drifted away from studded beams, they have their merits. Their dimensions are more rigidly set, so a track using bricks and tiles virtually eliminated the vertical breathing room seen in the first model. However, bricks can pull apart vertically, which is why I reinforced it with studless beams crossing the casing vertically. When reinforcing in this way, it is important to note that there must be exactly two plates (two thirds of a brick) between two technic bricks in order for a beam to cross them vertically. This formula follows for greater distances, though it is sometimes easier to fiddle with it until it works than to calculate it. The new, sturdier casing has no vertical give, thus forcing the rack to mesh with the gears at all times. The casing is long enough that the rack can pull all the way off the gears when fully contracted, thus allowing the vehicle to coast.

The rubber bands take up the space where the cockpit should
be. Note that the diagonal beams keep the front end from
bending upward under high tension.

This picture shows how the studless beams (in gray) reinforce
the studded beams (green) of the main casing.

This picture shows what it looks like when wound up to maximum.

This picture shows how the gears interact when wound up.

This one shows how the rack (black) does not mesh with
the gray eight tooth gears when at rest. This allows the car
to coast after all the tension has been released.

Though outperformed by the one-piece Lego pull-back motors, I still managed to make it go about fifteen feet. The one-piece motors can go twice as far, but they are easy to find, and I tend not to do things the easy way. I enjoy knowing that I created a working mechanism, though not perfect. I encourage everyone to try to build something different, just to see if you can. Happy building!

The Gear Train of Doom

Sometimes it's fun to create something completely and utterly pointless. Then, after you create something pointless, you want to share it with the entire world, right? Maybe not. In any case, I want to properly display one of my great feats of uselessness: The Gear Train of Doom. The principle is simple. It consists of lots of sets of gears stacked against each other, such that the gear reduction reaches ridiculous levels.

This gear train uses seven 40-tooth-to-8-tooth pairings, and nine 36-tooth-to
12-tooth pairings, with a few 24-tooth gears in there for some reason.

Here are some stats:

• The gear ratio is one billion, five hundred thirty seven million, seven hundred thirty four thousand, three hundred seventy five to one, or 1,537,734,375 to 1.
• When driven by a Lego RC motor, which can operate at 3000 RPM, it would take two years and four months nonstop to finish one rotation of the output.
• When driven by a Lego RC motor, the output has 20 million, 755 thousand, 396.46 foot pounds of torque, which approximately equals 27,131 Chevy Silverados. With that kind of power, one could easily snap the Empire State Building in half. (It would take decades, though).
• When driven by a Lego NXT motor, it takes an abysmal twenty five years to complete a rotation, but has nine times as much torque as the RC motor.
• Under no circumstances would you need to use this. EVER.

I built the Gear Train of Doom when I had spare time in shop class, out of the forgotten remnants of a few Mindstorms NXT's. I pretty much raided every gear in the box to build it. My gear train became quite popular among my classmates, and my teacher even mentioned it at a meeting once. (My shop teacher is awesome, by the way. You can find his YouTube channel here). The conversation went something like this:

Other Teacher: "I wrote this great new assignment to make students calculate gear ratios!"

My Teacher: "I have a student who does that for fun, except REALLY OVERKILL."

I think he almost passed out when I showed him all the numbers I calculated. In spirit of this, I encourage everyone to build something completely useless, as time well wasted is not wasted at all.