When a crank shaft has more than one crank or crank pin, it is spoken of as a multiple-throw crank; a double-throw crank is a crank shaft with two cranks; a three-throw or triple-throw, one with three cranks; etc. As a general rule, multiple-throw cranks are forged flat, i.e., the cranks are all forged in line with each other. The shafts and pins are then rough turned and the cranks are heated and twisted into shape. The forging for the double-throw crank shown finished in Fig. 96 would first be made in the general shape shown in Fig. 97. The parts shown by the dotted lines would then be cut out with a drill and saw as described above, and the shafts and pins rough turned, i.e., turned round, but left as large as possible. The forging is then returned to the forge shop where it is heated and the cranks twisted to the desired angle. When twisting, the crank would be gripped just to the right of the point marked A. This may be done with a vise, or a wrench if the crank is small, or it may be held under the steam hammer. The twisting may be done with a wrench similar to that shown in Fig. 98, which may be easily made by bending up a U of flat stock and welding on a handle.
Fig. 96. Finished Double-Throw Crank Shaft.
A three-throw crank without any intermediate bearings is shown in Fig. 99. The rough forging for this is shown in Fig. 100. The extra metal is removed, as indicated by the dotted lines, and the twisting is done as described before.
Fig. 97. First Step in Making Double-Throw Crank Shaft.
Fig. 98. Wrench for Crank Twisting.
Fig. 99. Diagram of Finished Three-Throw Crank Shaft.
Fig. 100. Three-Throw Crank Forging Ready for Machining.