There are several methods of forging crank shafts. The one more commonly used is the commercial method, as described in detail below. When forgings were mostly made of wrought iron, the cranks were welded up of several pieces. One piece is used for each of the shafts, one piece for each cheek or side, and another piece for the crank pin. Cranks are sometimes bent up out of round stock, but this method is only used on small work. Common Method of Making. The common method now employed where machine steel is used, is to forge the crank from one solid piece of material. The stock is taken large enough to shape the largest part of the crank without any upsetting. If a crank is required similar to Fig. 91, the size of stock to be used should be 1 1/2 inches by 4 inches in section. The first thing to do of course is to calculate the amount of stock required. The long end would contain 10.13 cubic inches. As each inch of stock contains 6 cubic inches, it would require 1.7 inches of stock to form this end, provided there was no waste from scale. Waste does take place, however, and must be allowed for, so about 2 inches of stock should be taken. The short end contains 5.22 cubic inches and would require 0.87 inch of stock, without allowance for scale. About 1 1/8 inches should be taken. The total stock then required would be 7 1/8 inches.
Fig. 92. First Steps in Making Crank Shaft.
The first step is to make the cuts, and spread the ends as shown in Fig. 92. These ends may then be forged down with a sledge hammer, as illustrated, or may be worked out under the steam hammer, the finishing up against the shoulders being done as illustrated in Fig. 93. The shaft may be rounded down and finished between swages. Care must be taken to see that the cuts are properly spread before drawing out the ends. If the cuts are left without spreading, the metal acts somewhat after the manner shown in Fig. 94. The top part of the bar, as it is worked down, folds over and leaves a crack or cold-shut as illustrated. When the metal starts to act in this way the fault should be corrected by trimming off the overlapping corner along the dotted line shown in the upper sketch.
When the forging leaves the shop, it will be left in a shape similar to that shown by the solid lines in Fig. 95, the dimensions shown here allowing for the necessary finishing. The crank itself would be left in a solid block, the throat being afterwards cut out as indicated by the dotted lines. A line of holes is first drilled as shown, and the block of metal to be taken out is removed by making two slits with a cold saw and the block then knocked out with a sledge hammer. It is possible to form this throat by chopping out the surplus metal with a hot chisel in the forge shop, but on small cranks in particular, such as here shown, it is generally cheaper in a well equipped shop to use the first method.
Fig. 93. Finishing Up Shoulder.
Fig. 94. Results of Improper Spreading.
Fig. 95. Crank-Shaft Forging Ready for Machining.