Solar Thermal Motor

This instructable describes the construction of a simple solar thermal motor, made from commonly available, low cost materials.
This device operates by harnessing the ability of certain polymers (in the case black plastic bags) to shrink when exposed to heat, and relax back to their original length when cooled.
Normally, this shrinkage occurs in all directions within the material. However, the material can be stretched, causing its polymer strands to line-up, and directionalizing the shrinkage.

The solar thermal motor operates by using bands of stretched black plastic bag to continually pull a flywheel off-center as it rotates on an axle. The strips are heated by sunlight on one side of a drum/flywheel assembly, pulling the flywheel toward the sun-side. As the strips rotate to the back, they cool in the shadow of the drum and relax. This makes the flywheel continuously off-center on the sun side, causing it to rotate.

The following is a list of the materials you will need:

1x Black plastic trash bag

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2x Styrofoam cups
1x 1/8" wooden dowel 12" long
1x Styrofoam freezer tray
2x Sewing pins (Forgot these in picture)
1x Plastic yogurt (or similar) lid approx. 4" dia.
1x Scotch tape
2x Tin cans

Tools used:

-Single edged razor or xacto knife
-Drawing compass
-Felt tip marker
-Ruler (forgot this in picture)
-Sizers
-Glue gun

Unfold a bag and lay it out on your work surface. Cut the bag into 2-3 inch wide strips, the full length of the bag. You don't have to be perfect about making the cuts perfectly straight, just try not to make any jagged edges. If you have sharp sizers, you can start a cut, then slide them up the length of the bag.
You will probably need about 10-15 bag length strips, but it depends on how careful you are at stretching the material in the next step.
Cut each bag length strip into 8-12 inch sections.

Grip one of the strips between your fingers and the base of your thumb, ans shown in the picture.
Slowly, stretch the strip. It will begin to "neck out" at one location on the strip - the "neck" will then propagate up the length of the strip as you pull on it. See before and after picture.
Repeat the process with all your strips.
This might sound all very easy, but it takes practice. The strips frequently break before your done pulling them, so if it doesn't work at first, be patient and keep trying.
Cut the stretched strips into sections slightly longer than twice the length of the Styrofoam cups, leaving some extra length for error.

NOTE: Don't expose the stretched strips to direct sunlight, as it will per-shrink them. However, if the sun is out while you are working on this step, try laying laying one of the strips in the sun - watch it shrink!

Measure the diameter of one of the Styrofoam cups, and use your compass to draw an equal-sized circle on the freezer tray. You can trace the circle directly off the cup, but it will make it difficult to locate the center of the circle, which you will need to do.
Cut out the circle, and make a hole the size of your dowel at it's center.
I used my glue gun to melt the hole.

Make a hole in the center of one of the Styrofoam cups, and glue the Styrofoam disk to its bottom. place your dowel through both holes and glue it in place. You can also make a small (1") foam disk, and place it at the bottom of the cup, but this is not really necessary.
Make a smooth blob of glue around the dowel at the back of the cup. this will serve as a pivot for the second cup, which must be left free to wobble on its axis. You can also cut several 1/2" diameter Styrofoam disks and place them over the dowel instead.

Make a hole in the bottom of the second Styrofoam cup.
Apply glue to the inside lip of the cup and place the end of one of the stretched strips on top. You may need to let the hot glue cool for a few seconds to prevent the strip from melting, esp. if you are using a high temp. gun.
Repeat the process, gluing strips around the lip of the cup. Leave a 1/8" to 1/4" gap between strips.

Slide the cup with the glued strips onto the dowel. It should be able to wobble freely on the pivot. Use scotch tape to center the open end of the cup to the dowel.
Begin gluing the strips to the Styrofoam disk on the other cup. Make the strips taunt when you glue them, but don't pull to hard or you will cause the wobble cup to be off center.
Continue with all the strips.

Cut a 1-2 inch hole in the center of the plastic lid.
Remove the tape that you put on the wobble cup to center it and glue the lid to its rim, centering it on the dowel. This will act as a flywheel.
Stick a sewing pin into each end of the dowel, being careful to make them go in straight.

Make a notch in the rim of the two tin cans (note: In the picture I used foam cups instead of cans, because I didn't have any cans, but this does not work as well.)
Rest the axle pins in the notches of the cans.

Although your solar motor might be ready to use now, it is a good idea to balance test it first. Give it a spin. If it looks like it if heave on one side, add weight (pins, small brads, etc) to the opposite side until it is balanced.
Note: add the weight to the Styrofoam Back disc, Not to flywheel.

You are done!
Put the whole assembly in the sun - if everything is built right, if will begin the rotate. The more balanced it is, the faster it will spin.