All About Engines

Steam Turbines 157 Now the speed of a de Laval turbine is so high that it is practically impossible to avoid whirling. By making the shaft long and thin, however, the critical speed is low, and when the turbine is started the danger point is passed rapidly. Thus the dia- meter of the shaft in a 5-horse-power turbine is only J inch, and in the 300-horse-power turbine only z8 inches. But the bearings must be flexible —a condition which is secured by making the bushes* spherical in form, and there must be plenty of room in the casing. In the reaction turbine, which will be described later, this clearance space would be a dis- advantage ; but in the impulse turbine it is of but little consequence. The steam expands in the nozzles, so that pressure is converted into velocity, and it is this velocity, not pressure, which forces round the vanes. Each vane, as it passes a nozzle, receives a tap or impulse which forces it round. No machine—not even an electric generator__ needs to be driven round at from 10,000 to 30,000 revolutions a minute, and though they could be driven at this rate they would have to be specially built, and would be costly in consequence. So the de Laval turbine is generally connected with the machine it is to drive by toothed gearing, a small wheel on the turbine shaft, and a large wheel on the shaft of the electric generator, which reduces the number of revolutions by ten or more to one. The toothed wheels used for this purpose are very wide on the face, the teeth are small and are cut spirally * Bushes are the brass blocks or cylinders which form the bearing.