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!

1 comment:

  1. I had no idea you could build a pull-back motor with Lego and elastics! Very educational post. Thanks for sharing your great ideas.