This article describes the simple heat engines, made of rubber bands, that can be built by yourself.
In the 19 century the English physicist James Prescott Joule noted that if stretched rubber band is exposed to heat, it gets shorter, because it loses elasticity. The another scientist, Paul Archibald, has used this phenomenon in a heat engine of his own design. His device consisted of a set of rubber bands stretched between two disks. The angle between this two disks was about 8° (see Fig. 1). Each disk had its own shaft, this two shafts has a flexible joint between them that compensates a force of the rubber bands, and makes them to rotate synchronously.
While turning, the rubber bands can stretches or getting shorter. When the rubber bands passing warm water, then the rubber bands getting shorter. On the top position it stretches, but because of the longer distance between rims of the disks the rubber bands has almost the same tensile force as in the lower position, so the rotor, formed by two disks will rotates. But remember, that very hot water can decrease the rotating speed and even damage the rubber bands.
This heat engine uses a liquid (the water in our case) as means for heat transportation because the water has better thermal conductance than air.
Some decades earlier a similar heat engine was developed by chemist William B. Wiegand. His engine (Fig. 2) is a balanced pendulum swinging forth and back and powered by a rubber band. When the rubber band exposed to heat, it shrinks, so the pendulum moves to the shadow zone. In this zone the rubber band is obscured by the vane, so the rubber band doesn't get heat anymore and as a result it is stretches. Because of this the pendulum moves back and so on.
Another heat engine developed by William B. Wiegand (Fig.3) consists of a bicycle wheel, an iron frame, a rod, a crank and tiny rubber strips. The frame and the wheel are connected together and rotates on a vertical axle. The axle has the crank in its middle.
The heat is provided by a candle located under a metal plate. The candle heats the plate and it radiates the heat to the rubber strips. This protects the rubber from overheating. By the way, the air heated by this metal plate heats the rubber too. The rubber contracts at the heat source, so the heat engine works.
The heat engine designed by Roger Hayward is very simple (see Fig. 4). It is made of a cardboard, aluminium plates end rubber bands. The wheel can be made of aluminium or thick cardboard. A needle or a steel rod serves as a shaft on which the wheel rotates. When the rubber strips exposed to the heat of a lamp, the strips will be getting shorter on the one side, so the center of mass moves and the wheel rotates.
The advanced model of the heat engine has been build by L. Stong (Fig. 5). It uses some features of all previous designs. Instead of the frame it uses a 16mm film reel as a wheel. The crankshaft of the third model (Fig. 3) is used too. The rubber bands are connected between the edge of the reel and the crankshaft. Teflon discs used as bearings. This heat engine works in warm water, just like the first engine (see Fig. 1).
The starting torque moment for the heat engine shown in Figure 5 can be calculated by using Joule's law. If stretch a rubber band with force P=5 kg at the temperature T=300°K (27°C) and heat it till it reaches T=339°K (66°C) then the force increasing (ΔF) can be calculated as:
ΔF = P*(T2 / T1 - 1) ;
ΔF = 5*(339 / 300 - 1) = 0.65 kg.
If the value of this force and angle are known, then the torque moment can be calculated. A natural rubber can be stretched in six times, but it is better to stretch the rubber only in 3 times to avoid breaking the rubber.
Although the simple design of this rubber heat engines, they did not find practical application. Wiegand has planned to build an engine with power of 5 hp, and his imagination created the fantasy about gigantic heat engines working somewhere in tropics, converting the sun's energy into power.