In this Instructable, we will be combining the powers of 3D printing and rubber bands to create an unusual engine used to demonstrate important concepts of thermodynamics.

An engine is a machine used to transform one kind of energy into another to produce work. In this project, we will be using thermodynamics to convert thermal energy into mechanical energy. This conversion will cause our wheel to begin a continuous rotation and, therefore, achieve the properties of an engine.

Normally when a material gets hotter, its molecules cause the material to expand. Rubber is different due to the structure of its polymers. When rubber is exposed to heat, it ends up shrinking in size. This interesting fact will help us harness the mechanical properties of rubber in order to create a working engine.

Step 1: How Does the Engine Work

We know that heat causes our rubber bands to contract while cold temperatures result in expansion. If a localized heat source is applied to a section of bands on our wheel, this change in temperature will result in the contraction of the rubber bands. The contracting will subsequently cause the ring to become lopsided. The lopsided ring will become unbalanced and the heavy side, which is opposite to the heat source, will be pulled down to the bottom. After the rotation has occurred, the coldest (and longest) rubber bands should find themselves in close proximity to the heat source. This process is continuously repeated causing a slow rotation.

Step 2: Materials

Four quarter sections for the outer ring (3D print)
One centerpiece (3D print)
One base piece (3D print)
Two arms (3D print)
20 identical rubber bands
30 1/2" hooks
One center shaft with a 2.8mm diameter
Two pieces of 0.38" lamp pipe cut to roughly 7.5"
A localized heat source: I used a heat gun but a heat lamp or other similar source would also work well.

Tools:

A saw to cut the lamp pipe to size
An old soldering iron to 'weld' the quarter sections together

Step 3: Building the Base

First, we will assemble the base. Cut the pipe into two pieces of identical length. The exact length isn't important as long as it is longer than the radius of the outer circle (I chose a length of 7.7"). Lamp pipe is convenient because it comes pre-threaded allowing me to directly screw the pipe into the baseplate. As an alternative, a regular rod of similar diameter can be used with some glue to achieve a similar effect. Once I gently screwed the pipe into the base to prevent stripping, I then attached the arm pieces. The arm pieces will hold the wheel in a localized area while limiting friction. Finally, although it is completely optional, I adhered a piece of felt to the bottom. Once the base is assembled, I adjusted the arms by slightly unscrewing the rods to ensure they were perfectly level.

Step 4: Making the Outer Ring

The outer ring is printed in four identical pieces. Although there are many ways to fuse 3D printed parts together I prefer to 'weld'. Using an old soldering iron and some extra filament I melted the pieces together creating a solid bond. During the assembly process, it is important to ensure that a perfect circle is being formed. To assist in the process, I printed a graphic to scale that I could use to check for accuracy. Just like metal welding, I found that it was helpful to first tack weld my pieces together before making a final completion pass.

Once again, any sturdy attachment method would work for this step as long as the weight around the ring is equally distributed and it can withstand the force applied by the rubber bands.

Step 5: Attach Hooks

Once you have an inner and outer ring, the hooks can be attached to the prefabricated holes. During this process, be gentle to avoid stripping the plastic. If the holes do become stripped, superglue should be enough to hold the hook in place. As long as the weight is evenly distributed, the direction of the hooks is not important.

Some people might prefer installing the hooks before fusing the outer ring together. In doing so, any stripped pieces can be reprinted.

Step 6: Attaching the Rubber Bands

Attach the rubber bands onto the hooks one at a time in a star pattern (like lug nuts on a tire). Following the image above, every inner hook should receive two rubber bands from individual hooks on the outer rim. Once all the bands are installed, check to be sure the inside ring is close to the center. If it appears to be off-center by a large amount, check to ensure the rubber bands are uniform.

Step 7: Final Assembly

Install the shaft and balance the contraption on the arms of the base. Although I have found rubberbands have different levels of elasticity, check that the wheel isn't too off-balance.

Step 8: What to Do

Being careful not to melt the rubber or plastic, apply a localized heat source close to the base of the wheel. It might take some time for bands to begin contracting or expanding. Once the thermodynamic process occurs, the wheel will start turning. It is important to note that a very slow rotation will be achieved.

Before building notes (Buyer beware)

If you are interested in building one, I would recommend it, but a lot of patience is required for a good result.
This engine cannot be used to efficiently harness energy.
This process will result in a very slow cyclic rotation similar to a Ferris wheel.
This is a neat science trick but needs perfect conditions to perform well.
Nevertheless, it is an interesting concept to be able to visualize.