Category: Challenge Questions

How do you make strong, 120 degree cranks for 12-legged walkers?

10/28/2016

Crank/axle system with left and right cranks 180 degrees out of phase, as is used by 8-legged walkers

Walkers typically stress axles and cranks more than wheeled vehicles and other applications, especially at large scales where heavy-duty parts may be required. LEGO-scale walkers also stress axles and cranks more than the plastic LEGO parts are designed to handle since:

their crank/axle systems can extend far from the robot's frame
in addition to transferring torque, their crank/axle systems have to bear the robot's weight

So, cranks should be built with sturdiness as the primary goal.

To handle the stress, we try to use LEGO's cam-shaped parts like in the image above, which have a greater resistance to bending than do linear parts. With 8-legged walkers, the inner and outer legs need to be 180 degrees out of phase, so we simply connect two such cranks together to make the double length cranks required to put the legs 180 degrees out of phase. Note: for large-scale walkers we don't recommend using double-length cranks between the inner and outer legs. Instead, single-length cranks should be used to return to the axle's center of rotation, and this axle section should be supported with diagonal beams to the support rods. Then, another single-length crank is used to put the outer legs 180 degrees out of phase from the inner legs. A simple example of this is shown in the animated GIF at the top of this post, with a yellow supporting beam between the left and right side's cranks.

12-legged walkers have 3 leg-pairs on each side of the frame, and each of these leg-pairs needs to be 120 degrees out of phase with its adjacent leg-pairs. So, adjacent cranks need to be 120 degrees out of phase, like in the simulation of TrotBot below.

One side of a 12-legged TrotBot, with each leg-pair 120 degrees out of phase of its adjacent leg-pair

The only LEGO part we found with an 120 degree angle was the Technic Rotor 3 Blade Liftarm shown below, which required additional parts to make into cranks of sufficient length, added width to the walker, and the resulting crank was weak.

Can you come up with a better solution?

(spoiler alert - one possible solution is presented further down)

Technic Rotor with Liftarms Separated by 120 Degrees (part #44374)

Below is one idea that utilizes triangles to make sturdy, 120 degree cranks with a width of 2 LEGO beams.

You can make an equilateral triangle with 3 Technic beams of the same length.

Add a 2nd equilateral triangle to it to create a 120 degree angle:

To transfer the rotation from one axle to another, LEGO parts with cross-shaped holes are needed. Below is one idea for making 120 degree cranks of length 4. The white and yellow 3-hole beams have a tight fit, but the resulting crank is sturdy. NOTE: The tight fit of the 3-hole beams increases the 120 degree angle slightly, which doesn't impact walking performance much but can be avoided by filing slightly the end of the 3 hole beams where they press against the gray beams.