# How to Build a Rubber Band Heat Engine

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Hello Everybody!

Have you ever wanted to impress your friends with a cool physics project? Well you're in luck!!

My instructable will teach you how to build the simplest kind of heat engine. This instructable is a cheap (roughly \$20 cost) and a simple idea to build for a basic physics demonstration. The total time to build takes only an hour from start to finish. The importance of this project stresses the simplicity of demonstrating a simple heat engine through heat energy converted into mechanical motion.
Building this rubber band heat engine is one of the simplest and coolest ways to demonstrate how work is done through the use of heat. You can also impress your friends by telling them you're building an "engine".

What is a heat engine?
A heat engine is a machine that converts heat energy into mechanical motion. The Rubberband engine will convert heat engery, using a heat lamp, into a mechanical motion by spinning the wheel.

How does this work?
Rubber bands react opposite of metals when exposed to hot and cold temperatures. When exposed to a (hot) heat lamp, the rubber bands will contract and become smaller. As they contract, the center axle will shift towards the outer rim of the wheel causing the wheel to become unbalanced. When the wheel is unbalanced, it will cause the weight to shift and begin to spin! Henceforth, heat energy has now been converted into mechanical motion.

Follow the next 6 steps carefully and you will soon have a fully functional rubber band heat engine!

NOTE: Building this heat engine is a bit challenging for children but could easily be accompanied by someone of the age 12+.

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## Step 1: Gather the Materials

Essentials for your rubber band heat engine:

- Fully functional bike rim (no tire or rubber tube)

The bike rim should be in good shape. Don't use a bent up old rim.

- Small bolt cutters

Pliers will not cut through the aluminum spokes. You will need a sharp and sturdy pair of small bolt cutters.

- Heat lamp bulb

You may use a regular heat lamp from any hardware store or you can purchase just the bulb and use a standard desk lamp as the source to put the bulb.

- 3inch Rubberbands.

Rubber bands react the same to heat no matter what size, shape, or width. However, for the best result, I recommend 3 inch rubber bands that are a little thicker and stronger to hold the wheel and axle together.

After you have these materials gathered, you are now ready to begin building your rubber band heat engine.

## Step 2: Marking the Spokes

1. Measure the length of one spoke.

My bike wheel is 26 inches in diameter. Therefore each spoke has a length of 11.5 inches.

2. Mark 1.75 inches off the tip of the spoke near the axle.

3. Mark 1.75 inches off the tip of the small aluminum stud.

4. Repeat sub-steps 2 and 3 so that every spoke is marked the same around the wheel.

Tip: If you don't have a 26 inch diameter bike wheel, no worries. You can still mark 1.75 inches off each end of the spoke because it gives you the best room for the next step, cutting and bending the hooks.

## Step 3: Cutting & Bending Hooks

WARNING!

Using bolt cutters can be a pinch point and will need to be used with caution when cutting.

Aluminum is sharp when cut. Use caution and to not poke yourself when bending hooks.

1. Cut only ONE spoke at the marks you made in step 2.

2. Remove the small aluminum piece near the axle

3. Bend a hook on the small piece of axle you have just removed.

CAUTION! Bending the aluminum piece too much can cause it to break.

Tip: Hold the piece tightly in the pliers and press against a hard surface to make it a little easier to bend.

4. Replace the bent aluminum piece back into the axle.

5. Bend a hook into the small aluminum piece of the rim with pliers or bolt cutters to have a hook at the "cut" end.

## Step 4: Replacing and Repeating

Warning!

Stretching rubber band to far could cause it to snap and leave a welt on your hand. Stretch with caution.

1. Stretch a rubber band over the two hooks you have made in step 3.

You are now replacing the spoke with a rubber band.

2. Repeat step 3-4.

Tip: Cut only one spoke at a time replacing it with the rubber band. This helps so that you don't lose track when remembering the configuration of the spokes of a bike rim. It also keeps the wheel balanced

## Step 5: Setting Up Rubber Band Heat Engine

1. Place the wheel on a level foundation with the axle rod resting in a balanced position.

Outcome: The wheel should spin freely without rolling of the platform. You may use the bolt already in the axle of the wheel, or since mine doesn't have one, I used a wooden rod allow the wheel to spin.

WARNING!

Heat lamps become very hot when in use. Use caution not to burn your skin on the bulb.

2. Stand the heat lamp 6 inches away from the rubber bands, level with the axle.

Tip: Position the lamp closer to the rim to get a better spin on the wheel.

## Step 6: Complete Rubber Band Heat Engine

1. Turn on heat lamp.

2. Wait 5 minutes for the lamp to become hot and for the rubber bands to begin contracting

3. Watch carefully as the wheel spins!

You now have successfully built your first rubber band heat engine!!!

Click HERE to watch the video of the wheel spinning.

Outcome:
The wheel will spin as the rubber bands continue to stretch, due to the heat, and shift the center axle.
You will notice that the wheel spins quickly in one direction, and then reverses in the other. This is because the axle is shifting towards one side of the rim, causing it to become unbalanced. The wheel will continue to rotate back and forth until a new set of rubber bands are heated up to continue rotating the wheel.

Note: This heat engine cannot be used to perform much work. However, the design is very unique and cool to watch as the bands stretch and the wheel continues to spin.

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## 33 Discussions

I've tried this and it isn't working... Could it be becouse the rubber bands aren't as tight as they should be or it isn't hot enough(on one side the heat of the rubber bands was around 70°C)

Ahh, just read the comments, I concur with macrumpton.

Very cool! I bet itd actually have some torque if you used nitinol or some other memory wire...

very cool i want to build one! just as a point of reference the spokes are actually steel, aluminum spokes would be very soft

Excellent! Great idea! You really deserve more followers :)

can u use metal springs instead of rubber bands. i know you will need more heat, but will it work, and will it have more power?

2 replies

An elastic band is more suitable than a heavier thing, with a spring sure you would need more heat to heat it but this is working by when the rubber band heats up it gets bigger, and as the air hits it or it leaves the heat source, it shrinks the spring would take a lot more time for it to cool down and shrink, less elasticity than the elastic surprising!

The amount of power you can get out of this type of engine is limited by the size and weight of the wheel (because the off center wheel is providing all the leverage), and the speed and amount of expansion of your bands/springs.

A better approach for using metal for a heat engine might be nitinol wire, which strongly contracts when heated.

Wear safety glasses in this step! Spokes are under a fair amount of tension to hold the rim true. Nothing ruins a day quite like a chunk of metal in the eye.

Yes, but I think it would cause it to not turn. The imbalance in heat is what makes it turn at all.
I think the reason it turns and then turns back is because the rubber bands take a while to heat up enough to overcome static friction. The dynamic friction is much less than static, so the wheel is able to turn until the warmed bands are at the top (and the system is back in equilibrium.) While turning, the bands move past the heat too quickly to expand them enough to continue the motion.
More heat or less sticky bearings should make it turn continuously, but too much heat will melt/burn/weaken the bands, and less sticky bearings may be tough to find.

In a heat engine using a gas, the gas expands as it drives a piston. If an engine uses rubber instead of gas, does the rubber really shrink? If it does then a rubber heat engine could gain heat at the cold side and deliver some heat to the hot side, and so the engine could run without waste heat, in theory.

4 replies

It does not "shrink", it contracts in length while getting thicker, i.e., it is converting the heat into the exertion of extra energy to resume it's natural shape.

But does the elastic's volume increase or decrease? I know that oil increases in volume greatly when heated. If oil is compressed to several thousand psi, its temperature rises several degrees.

I cannot be so precise in answering, because I've never explored the physics to that level.

As an experiment, stretch a large thick rubber band and then touch it to the sensitive part of your upper lip. You will definitely notice heat.

Then let the band relax and retouch it. It will be much cooler as it reabsorbs the heat.

In fairness to the author, this is only a demonstration of concept. If he had the funds to replace all the rubber bands with memory metal (muscle wire), he would have achieved something far more functional with a much lower heat differential, ,i.e., forget the heat gun and use a candle.

Engineering often makes use of chemistry. From principles of chemistry we should realize that systems oppose change, which means that application of heat to a stretched rubber band should result in the conversion of some heat into (potential) mechanical energy, thus increasing band tension, but most of the heat simply heats the elastic molecules.
A more interesting engine might use oil, instead of a solid (rubber).. Oil can be pumped easily. Oil expands a lot when it is heated. Using oil in a heat pump may be more efficient than using a refrigerant in a heat pump. Research needs to be done on using an oil in a refrigeration cycle. Oil generates an incredible amount of heat when it is compressed to several thousand PSI. I have experimented with compressing oil to incredible pressures, to measure its heat production.

If you think about the light as if it were a commutator of an electric motor, maybe multiple lights of varying heat levels would make the wheel spin continuously and efficiently. As the wheel rotates and reaches the point where it wants to reverse direction, it is heated by the next light, causing it to continue in the same direction. As the wheel speeds up, the heat level is increased, further increasing the speed. Neat project!