All About Engines

The Modern Reciprocating Engine 111 are 400 reversals of motion in that time. The move- ment is stopped partly by the crank, which exerts a push or a pull on the connecting-rod, and partly by the cylinder covers through the cushion of steam left in each end of the cylinder at the end of the stroke. The crank communicates the force to the engine bed through the bearings, and the cylinder covers do likewise through the cylinder. The bed, therefore, is subject to stresses, now in one direction, now in the other, and vibration is only prevented by rigid connection with a heavy founda- tion. Anyone who has seen a small engine bolted down to a couple of planks and shaking as though it would pull itself to pieces, will realise what this means. But that is not the point it is proposed to discuss in this section. Let the reader fix his attention, not on the reciprocating but on the rotating parts of a single-cylinder engine. Here we have the crank with the heavy connecting rod end being whirled round the shaft, now above, now below, now on this side, now on that. If a stone be tied to the end of a piece of string and whirled round in this way the string will become tighter the faster the stone is whirled, and if it be not strong it will break. If the string breaks or is cut the stone will fly on in a straight line in the direction it was going at the moment of release. All whirling bodies, in fact, exert a force outwards from the centre about which they are revolving, and this force is known as centrifugal force. In order to get an idea of the importance of centrifugal force in an