An important piece of work is the making of crank axles for steam engines either for marine or stationary service. In order to forge a small two-arm crank-shaft like that shown in Fig. 83, a soft, tenacious piece of metal should be selected, one that can be easily upset. It is well to use iron whose diameter is equal to that of the finished shaft. If the shaft is to be very long it will be well to forge the crank separately and then weld it in position. First determine the position of the crank-pin by adding the length of one arm to one end of the shaft. This gives the commencenient of the crank-pin. From this point to a short distance beyond the other end of the crank-pin the metal should first be upset. This upsetting should be done at or near a welding heat and should make the bar about one-quarter larger than the original diameter. The work must next be upset at the base of each arm. These points are located on each side of the crank-pin at distances equal to the length of the arm. The upsetting at these points should 1m such as to produce about the same increase of diameter as before. This will leave the piece in the condition shown by Fig. 84. After the upsetting the first bending should be done at one end of the intended crank-pin; the next should be at the other end. If, however, the pin is to be very short, a single bend may be used, leaving the piece in the form of a longlegged II. During the whole of this work the diameter of the crank-pin is of far less importance than the length. The latter should be so adjusted that the metal in the arms at each end will just clean up in the lathe, and leave the pin of the proper length. The only precaution necessary in regard to the diameter is that it shall be large enough. If, when the bends have been made it is found that the pin is too short, it may be stretched with the fullers. If it is too long, it should be reheated and upset.



Crank Axles 100547

Fig. 83.

Crank Axles 100548

Fig. 84.

After the length of the pin has been properly formed the throw is made by bending back the two ends. This should be done when the pin is cool enough to retain its shape while the work is being done on the adjoining parts. It is well to cool the pin in water before doing the work. When this bending it of iron at 60° and at 900° is .0054 of its length when cold. Suppose then, we have the side rods of a locomotive to weld. If they are to be 9 feet long when finished, they must be 9 feet 0.58 inch long when at a red heat. This means that in the rough slightly more than inch must be allowed for contraction. If only a portion of the rod is heated the allowance is to be proportionately less.

The tempering of steel tools has already been discussed. In such work only a small portion of the metal needs to be tempered. Springs require a slightly different treatment. They must be tempered throughout their whole extent. The ordinary method of tempering a steel spring is to heat it to a cherry red and then cool it by plunging into a bath of oil. This treatment hardens the metal but does not give it the same degree as plungiug into water. The reason is that the oil does not absorb the heat of the metal as rapidly as the water, with the result that the cooling takes place more slowly. such treatment leaves the metal harder than in its annealed condition, and yet not as hard as when plunged into water. This intermediate condition is suitable for service as a spring. The l>est results can probably be obtained if whale oil is used for the bath.

Crank Axles 100549