All About Engines

ii4 All About Engines will have observed that the sides of the crank are extended backwards in a fan-shaped block as shown in Fig. 64. This block is sometimes forged in the same piece with the crank, and sometimes of cast- iron, bolted in place. In size it is such that its weight multiplied by the distance of its centre of gravity from the crankshaft centre is as nearly as possible equal to the product of W and r in the case of the necessary unbalanced rotating parts of the engine; and as it is placed on the opposite side of the crank and connecting rod end, the centrifugal forces pro- duced by these crank balance-weights act in the opposite direction to those which it is desired to oppose. Owing to the small effect of the connecting rod acting as a rotating body at the crank pin, the balance can only be obtained approximately, but it is near enough to be effective. If the crank-pin is fixed on a disc it will be noticed that this disc is of a “ lid ” form, with the solid counterweight inside and opposite to the pin. Again, in a locomotive the weight is fixed between the spokes of the driving wheels ; but this form of engine will be considered in detail later, and it will be better to consider now the general question of balancing in a two-cylinder engine. At first sight it would seem that there was no need for any special device in an engine with two cylinders, when the cranks are set in opposite directions. The weight on one side of the shaft will certainly always be equal to the weight on the other; but the centrifugal forces will not be