Fig. 21.—Four-throw
Crankshaft
The Crankshaft, as its name implies, is a shaft with one or more cranks or right-angled bends in it. Its function is to convert the sliding motion of the piston into the rotary motion of the flywheel and revolving shaft. A crankshaft with a single throw (or single crank) is shown in Fig. [20]; a four-throw crankshaft is shown in Fig. [21]; and Fig. [22] shows how an equivalent motion can be obtained by a single pin fixed into the face of a flywheel. This device (Fig. [22]) is frequently used for motor-cycle engines. Crankshafts are always made of steel; sometimes mild steel is used, but more usually special alloys of steel containing chrome, nickel, vanadium, etc., are used. The general practice at the present time is to machine the crankshaft direct out of a solid bar of steel; this requires special jigs for holding the work and special tools for cutting the metal, but is the quickest, cheapest, and most satisfactory method to adopt. A few firms specialize in this class of work with high-grade steel and can supply crankshafts from stock.
Fig. 22.—Motor-Cycle Crankpin
fixed into the Flywheel.
It is easily seen by examining Fig. [18] that the crankshaft is being twisted in overcoming the engine resistance, while Fig. [20] shows that the crankshaft is being bent under the push from the connecting rod, so that we say the material of a crankshaft is subjected to combined bending and twisting, and as such a combination is not easy to resist we see now why special steel alloys are required for safety, combined with economy in material and reduction of weight. In Fig. 20 the crankpin is shown at A, the crank cheeks or webs at B1, B2 and the crankshaft proper at C. The portions of the crankshaft C which work in the bearings D1, D2 are termed journals. A crankshaft must be very stiff and not bend or twist sensibly, otherwise the shaft will vibrate when the engine runs up to speed—which would be very undesirable. It must be perfectly true with all the bearings absolutely in line and the journals well bedded down in their respective brasses (or bearings), otherwise mechanical troubles will arise. Each crank with its crankpin and webs forms a lop-sided or unbalanced mass, so that either (1) each crank must have its own balance weight as in Fig. [23], or (2) special balancing masses must be fitted at each end of the crankshaft. A convenient method of balancing the crankshaft is to have a fan pulley at one end and the flywheel at the opposite end, so that by drilling holes in the faces of these discs an amount of metal may be removed from them sufficient to balance the excess weight of the respective crankpins and webs. In Fig. [24] the shaded area indicates that portion of the crank which constitutes an unbalanced mass. Crankshafts for high-speed engines have always to be very carefully balanced, otherwise the engines will never run satisfactorily, the want of balance being greatly aggravated as the speed of rotation increases. Fig. [25] shows how the crankshaft of a two-cylinder engine may be balanced by drilling holes in the flywheel and fan pulley respectively, but the same effect may be produced by attaching balancing masses—this latter method would, however, be more inconvenient and expensive. The crankpins and journals are ground truly circular after being turned in the lathe as true as possible.
Fig. 23.—Sketch of a Balanced Crank.