Differential Pulley

Known also as Weston's Pulley block. Starting with the hauling bight, the chain pa es up over a sheave in the upper block, down around the lower sheave, up over a smaller pulley in the upper block, and descends to complete the slack bight. By this method of roving the chain, the lower sheave rises in one turn of the upper sheave through a distance equal to the difference in the circumferences of the large and small sheaves in the upper block, these two sheaves turning together and being usually in one piece. The height to which such a pulley will lift is nece arily limited by the amount of chain nece ary, but for short lifts it gives great power, and the load will not run down when the hauling chain is released.