Model Steam Turbine

FUN IN SCIENCE Make a model steam turbine from three needle valves and a couple of tin cans.

By H. McDougall

HE steam turbine finds its principal application T in ships. Its main advantage, besides its efficiency, is that it converts steam pressure to rotary

NEEDLE VALVE in lid of can directs jet of steam against vanes to drive turbine wheel. 118

motion without use of reciprocating parts and this makes it comparatively free from vibration. This little model demonstrates the principle of the steam turbine. It costs less than a dollar to build, involves no soldering and can be whirling merrily within a couple of hours after you start work. You'll have to buy three needle valves of the kind used to inflate footballs and four suitable nuts. The valves can be obtained at any sporting goods store but you may have to visit a bicycle repair shop for the nuts. The boiler is simply a can with a press-fit lid. We used a custard can but a paint can or anything similar would do. The press-fit lid is important because it acts as a safety valve; if the pressure gets too high, it will lift and allow the steam to escape. Thus there is no danger of the can blowing up. Mechanix Illustrated

The turbine wheel is made from another tin can hammered flat. Scrap aluminum is even better because it is lighter and easier to work. Mark out the wheel as shown, center and drill the hole and then cut and bend the blades. Next outline the bracket and drill the holes for the shaft and the attachment bolts before cutting and bending it to shape. Use the bracket to mark the positions of the bolt holes in the lid of the can and drill them too. Last, mark and drill the holes for the jet which should be about 7/8-inch from the centerline of the turbine wheel. The shaft for the turbine wheel is built up from two needle valves and two nuts. Thread a nut onto one valve and then add the turbine wheel. The thread must project only enough to go halfway through the nut on the other side of the May,

1960

wheel. To cut off the excess, estimate the number of threads that must be removed, take off the turbine wheel and the nut and grip the valve in a metalworking vise by the part that is to be discarded. Then use a fine-tooth saw to remove the unwanted material and clean up the end threads afterward with a fine file. Put the wheel back on the valve and then add the nut on the other side of the wheel. If you've cut the valve correctly, there should be just enough threads remaining inside this nut to accept the other valve. The second valve, in turn, must be cut short so that it has only enough t h r e a d s to penetrate halfway through the nut and butt up against the end of the first valve. Having completed the turbine wheel assembly, all that remains is to slip it into place and trim the projecting ends of the shaft with diagonal cutters. Since the shaft is tubular, this will also squash the ends and keep them from pulling back through the holes. The remaining valve is used for the jet. Cut it to a 5/8-inch length with a finetooth saw (not the cutters) and file the end flat, taking care not to burr the end. Check to see that it is clear inside and remove any fine chips with a needle. Make a simple stand as shown, the dimensions depending upon the size of the can. Use Sterno or a hot plate for the "furnace." Half fill the can with hot water and press the lid into place. Flick the wheel to make sure it has clearance and then turn on the heat. As soon as the water boils, the jet of steam will drive the wheel at surprising speed—a graphic demonstration of the principle of the steam turbine. • 119