Instructables
Picture of How to Build a Rubber Band Heat Engine
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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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?

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.

http://www.youtube.com/watch?v=oKmYqUSDch8

flamekiller2 years ago
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.
mechanically speaking , concept is good but of no use
reangerer2 years ago
if you used a light on both sides wouldn't that even out the heating?
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.
R.Blakely2 years ago
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.
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.
hhnewman2 years ago
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!
moon1612 years ago
Bolt cutters are overkill for cutting spokes. Any large diagonal cutters or small lineman's pliers will do easy. If it doesn't cut straight away, just bend the spoke where it's been scored by the pliers.
I'm wondering whether adding more heat to a wider range of rubberbands would make it spin completely. As it stands now, you have approximately one quadrant being heated, leaving the remaining three(approximately...) to rest at ambient temperatures, perhaps having half and half would balance it out long enough to affect significant temp. changes?
Wazzupdoc2 years ago
It moves. That is pretty much it. You could be burned at the stake for that in the 15th century if you said the Sun was at the center of the axis and the Earth was on the rim. It does not do much. Get back to work in the shop and keep at it. You are in the right direction. Ratchets! Gears! and Work!
aapurim2 years ago
To this visiting reader, it would seem apparent that ...
1. not explained is that each metal spoke is cut in two places, to remove the middle 2/3 of its length, and the remaining stubs are bent back to form hooks where the rubber band is mounted, replacing each removed spoke section. It is best to do the cutting, bending, and rubberband installation one spoke at a time.
2. the wheel would perform most efficiently when its axis is 90° to the direction of gravity, i.e., in the same position it is when in use on a bicycle.
3. in order to hold the wheel, a bicycle fork would be the easiest ready-made device for mounting the axle, although anything that holds the axle will do.
4. the heat-souce light, of course, must not be shining on the center of the wheel, but on the lower half, to best unbalance the wheel.
. . . The energy generated by the heat-engine wheel will never equal or exceed the energy used to heat it, but if the heat source is natural (a half-a-wheel-sized spotlight of sunlight, or directed flow of hot air or liquid passing over the lower wheel's rubberbands), the power output will be useful and sustained as long as the heat continues until the rubberbands deteriorate in a few days. The axis could be extended to a simple electric generator.
. . . The speed of the wheel is limited by friction, drag, and the ability of the rubber bands to cool before they rotate back into the heat, therefore a means of quickly chilling the bands as they rotate to the upper part of the wheel's rotation would increase its speed.
. . . For a giant-scale practical application of heat-exchange power generation, look up "OTEC" (Ocean Thermal Energy Conversion) developed by Lockheed Martin, described on the page http://Braun2012.US (scroll down, just short of half way down the long page of descriptions of renewable energy system descriptions). See more at http://www.lockheedmartin.com/ms2/features/otec.html
scraptopower... Spokes come in steel, stainless steel, carbon fibre and various alloys but not straight aluminium as this is to soft to withstand the forces imposed on a spoke - my friend is a specialist wheel builder, he gave me this information, and the wheel and hub to try to build my own heat engine. Thanks rcgroves, well written - even I can follow this one :)
Jim Cooper2 years ago
Maybe add a one way clutch to keep it going the same way?
Quester552 years ago
At this stage you may want to Letter or Number each set of wires. This will help keep the proper pattern in case of rubber band rot, takes out more than 1 set at a time. It will also help with storage when not in use.
raviolikid2 years ago
Very interesting - thanks for sharing! Does it ever go all the way around?
Wazzupdoc2 years ago
I didn't see much spinning. Would a focused beam and a black background help? You could then solarize it and make it do work. That is, I think, what engines do. You got some movement though, so a real good start! Keep at it
Great project! Really clear write up, thanks.

One minor thing, aren't most spokes steel or stainless steel ? The ones I've taken apart have always been steel and really tough to cut!
boysie2 years ago
Really clear description, and an awesome-looking project. Nice work man!
bertus52x112 years ago
Very nice!
WolfKodi2 years ago
Well done, Riley! I like how you explained what a heat engine is and how it functions in your introduction. Nicely put together.