Obtain proper chain tension when neither driving nor driven shaft is adjustable, and to take up slack chain developed through normal chain wear
Guide chain around an obstruction
Prevent whipping action in theslackspan of chain transmitting an uneven load
Bring about greater chain wrap around a small sprocket, particularly if it is the lower sprocket in a vertical drive
Provide for reversed direction of rotation of a sprocket in contact with the outside of the chain.
Idler sprockets should not rotate at greater speeds than are allowable for drive sprockets of the same size. They should be mounted in contact with the slack span of chain. Mount them on the outside ...