Movable Eyes for Mechanical Puppets

Our STEM club’s mechanical puppet project was filled with interesting challenges. I addressed one in a previous blog: building a working jaw using a lever structure. In this article I will discuss the lessons learned while building movable eyes.

The design challenge for the Movable Eyes lesson was to simulate opening and closing eyes. The idea was that as the eyeball rotated down it would appear to close, particularly if the top half of the ball was decorated to look like an eyelid. Downward rotation was to be produced by pulling a string and reversed by the opposing force of a piece of elastic.

In order to get the students noodling on the problem, we used the first few minutes to explain the concept of a wheel and axle. Then we demonstrated a very specialized type of wheel and axle, the crankshaft. A crankshaft is a classic mechanical design that converts a push or pull motion to rotational motion. We demonstrated two sample crankshafts, made by bending stiff 18-gauge wire as shown below. The teams quickly observed that they could rotate the ping pong balls by pulling on the loops of the crankshaft. But, how could these structures be controlled by string and elastic?

By this stage of the project, the puppet heads were partially built, either with a cardboard shell or a wire frame. This made it difficult to work out the design of the eye mechanism inside the head. Instead, we had the teams experiment with creating eyeball assemblies on the Gizmo Rig platform. Since this was a prototyping activity, teams used cardboard discs mounted on a dowel rod. Some teams supported the dowel rod in the bar brackets of the Gizmo Rig while others fabricated cardboard supports. Our requirement was that the string that produced downward rotation be routed over a pulley somewhere behind the eyes. The elastic was to be mounted in such a way as to return the eyes to the front position when tension on the string was released.

Teams fashioned their crank arms from bent paper clips taped to the dowel rod. This let them experiment with both single arm and double arm crank designs. Both options have their pros and cons as we soon discovered. 

  • A single arm crank was easier to build, but the elastic and string needed to pull from opposite directions in order to get reversing rotation.
  • The double arm crank made it possible for the elastic and string to both pull from the same direction since the top arm and bottom arms rotated the axle in opposite directions.

During this activity, one team came up with a design that was altogether unique. They called it the wheel crank. The wheel was made from a bottle cap with cardboard clued on both sides to create a raised edge. The string pulled on a paperclip taped to the side of a wheel. As the string pulled, the elastic wrapped around the wheel and provided the tension to return the eyes when string tension was released.

This activity served as a very helpful step in designing the movable eyes. Armed with their prototypes, the teams moved on to recreating the design inside the puppet head. The steps for building the eyeball apparatus include:

  1. Build a bracket on each side of the head to hold the eye assembly firmly in place. The brackets should be strong and stable while providing free rotation of the wire axle. 
  2. Add the eyeballs to the wire axle – one on either side of the crank. Secure the axle so it can rotate in the brackets.
  3. Fasten the string to a crank arm so that pulling the string rotates the eyes down. Route the string over a pulley in the top of the body tube and down to the control area.
  4. Fasten elastic to a crank arm and anchor it so the elastic returns the eye to open position when tension on the string is relaxed.
  5. Decorate the eyes so the viewer can tell when the eyes are open and shut.

The eyeball challenge certainly provided teams an opportunity to stretch their imagination and utilize every ounce of their understanding of how wheels, axles, and pulleys work. While this was more challenging than it seemed like it should have been, everyone learned a lot about these simple machines. . .  even the Kool Kat team!

Bring your simple machine lessons to life with the newest Kool Kat Science STEM unit: Explore Simple Machines with Mechanical Puppets

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