While speaking of crank-shafts, the three names, shaft, spindle, and axle, are synonymous, of which three "axle" is correct. The arm of a crank is a lever, and if a crank has two arms, it is a two-lever or double-lever crank. That part of the lever at winch the power is applied is a handle, and in a crank is named a crank-pin. The two arms, together with the crank-pin, are termed the crank, and also the throw.

Fig. 43 represents a two-arm crank, and Fig. 44 indicates a one-arm crank. In both these Figures the letters L signify lever, and the letters P signify crank-pin. S denotes the portion named the axle or spindle, and B shows the bearing surfaces.

Small crank-axles are sometimes forged with cranks of circular section, which are named round-throw cranks. These are used for small machinery, such as foot-lathes, small pumping-engines, steam-cranes, and similar work.

Small round crank-axles are made by two principal methods. The one consists in bending a straight rod of iron or steel on an anvil, or anvil-beak, until the cranks are produced; and the other method is by forcing the work into cast-iron shapers or dies, which arc of suitable dimensions to produce the throws to the necessary length, width, and shape.

To forge a small two-arm crank-shaft of round iron or steel, and without dies, it is usual to proceed by selecting a soft, tenacious metal which may be upset without opening or splitting. The diameter of the iron selected is that which is required when the crank is forged. If the entire crank-axle is to be of great length, the crank part is forged separately, and afterwards welded to the other part of the spindle ; but if only a short axle is wanted, the entire length of metal necessary is ascertained, when the length is cut from a bar or rod, and the crank-axle made of one piece. The first requisite is to determine the position of the intended crank-pin. This is done by adding the length of the intended arm to the length of one end of the axle. These two lengths added together indicate the commencement of the crank-pin, and from this spot to a short distance beyond the other end of the crank-pin is the part of the work which is to be first upset. This portion is heated to a welding heat and upset till the rod's diameter is about a fourth greater than its previous diameter. The next step is to upset two other portions of the work, and the situations of these two portions are equidistant from the portion that was upset for the crank-pin. A dot is put into the work at an equal distance from either end of the crank-pin to indicate the length of the lever, because both levers must be of the same length. The work is then heated and upset at the place indicated by one of the dots, and the diameter increased to about the same as that of the crank-pin. Another upsetting is next performed at the place marked by the dot showing the length of the other lever. (See Fig. 135.)

After the three portions are upset, the first bending is effected at one end of the intended crank-pin. The next bend will be at the other end of the pin, but if the crank-pin is to be very short, instead of two bendings, one is sufficient.

When this portion of the crank is made, the two ends of the intended shaft are parallel to each other, and the distance between the two centres should be the same as the length of the crank-pin, if measured from the centres of the lever ends. If the length of the heated part previous to bending were too short or too near the centre of the crank-pin, the bent part must be re-heated and adjusted, or stretched and lengthened by fullers of proper thickness; and if the heated part were too long, the crank-pin is also too long; in such cases it is shortened by re-heating and cooling to the right place and closing the two ends of the work together.

During forging, the diameter of the crank-pin is not so important a consideration as the length of it. If the pin is upset beyond the finished diameter, and the pin too long to admit of being stretched, it is afterwards reduced to the right diameter by the lathe process.

When the crank-pin is formed, the throw is then produced by bending back the two ends of the shaft while the pin is cool enough to prevent any alteration of it during the bending which produces the levers. The cooling is effected by placing the bent portion already made into the water until a proper amount of the intended two arms is cooled, while the remainder is still hot enough for bending. It is then placed between two studs of a suitable length and width on a heavy block, and bent by forcing the end back with levers or tongs which are fixed at the end of the work.

The length of the heated portion at the time of bending should be the length of the curve desired ; consequently, the iron is cooled until the punch-mark is exactly midway between the two extremities of the curve intended. If the curve is not made in the right place, the throw will be either too long or too short; the work is then re-heated and cooled to lengthen or shorten the throw to its proper length. During this lengthening or shortening, top and bottom tools and fullers are also needed to produce the necessary curved outline of the work.

Adjusting the crank and shaft is next performed by making the two levers parallel to each other, and in the same plane with the crank-pin and crank-axle. Another sort of adjustment also is performed by the aid of a long straight-edge. This is applied to the axle of the work to indicate whether the longitudinal axis of one end is nearly in line with the longitudinal axis of the other end. If the work is not properly adjusted at the first forging, it must be adjusted at some other time previous to being turned, and also at a needless expense.

Cutting the extremities to a suitable length is next performed. The precise length will depend upon whether or not the axle-ends are. to be steeled. Steel ends may or may not be necessary, according to the intended use of the axle. Such ends are often used for lathe crank-axles when each end is to be supported by a screw whose end fits the end of the axle.