Another form of transmission may be said to combine the principal features of both the previous systems, and that is chain driving. Here, as before (see fig. 3), we have two wheels or pulleys connected by a flexible band, but the pulleys are not smooth as before. Their faces are cut into teeth suitably shaped to engage the links of a metallic chain which takes the place of the belt. As with belt driving the driven pulley or chain wheel, b, revolves in the same direction as the driving pulley or sprocket a. This method possesses the feature of positive driving belonging to the gear wheels, and whilst it has none of the slip of the belt there is some elasticity in the chain, which helps to take up driving
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Fig. 3
shocks and secures silent running. The extra friction on the bearings necessitated by belt driving is avoided, as the chain may be run fairly slack and the lower side hang loosely as shown in the diagram, the chain automatically tightening itself at the top as the different links are taken up by the teeth in the drive.
Where two chains are used there is frequently some rattle, owing to the two not being able to run exactly together, but a single chain is—like that of a bicycle, which it much resembles— practically silent. Wet, mud, and dust will also cause chains to 'grind' and become noisy, and therefore in order to obtain the best results they should be enclosed in a cover or gear-case, which will protect them from these enemies; and the
