All About Engines

T56 All About Engines the whole of the mass acts as though it were concen- trated at a point called the centre of gravity, it will be seen that if the centre of gravity is not concen- trated at the geometrical centre of the shaft, very large forces indeed are called into play. If the force were i lb. at 100 revolutions per minute it would be 4 lb. at 200 revolutions, 9 lb. at 300 revolutions, 16 lb. at 400, 25 lb. at 500, 36 lb. at 600, 49 lb. at 700, 64 lb. at 800, 81 lb. at 900, and 100 at 1,000 revolutions per minute. Quite insig- nificant at low speeds, this force would become suf- ficient at 30,000 revolutions a minute to tear the machine to pieces. Not only, therefore, has the disc to be most accurately balanced, but it must be made of the finest quality of steel to resist the stresses set up in the material, and the blades must be securely fixed lest they be flung off by centrifugal force. But there is a more wonderful phenomenon arising from high speed than even the forces which are brought into play. When a shaft is supported be- tween bearings it sags a little, and when it is rotated there is a. tendency for it to whirl round a,s though it were a bow. This effect becomes very marked in the neighbourhood of a certain speed, called the critical speed, which depends upon the length and thickness of the shaft. In a long, thin shaft this speed is low, and in a short, thick shaft it is very high. As a matter of fact, there are several critical speeds for each shaft, but as a rule only one is important. When the critical speed is passed whirling becomes less, the shaft straightens, and the motion becomes steady.