Fig. 137.—Crank-Shaft of Thomas-Morse Eight-Cylinder Vee Engine.
Crank-shaft form will vary with a number of cylinders and it is possible to use a number of different arrangements of crank-pins and bearings for the same number of cylinders. The simplest form of crank-shaft is that used on simple radial cylinder motors as it would consist of but one crank-pin, two webs, and the crank-shaft. As the number of cylinders increase in Vee motors as a general rule more crank-pins are used. The crank-shaft that would be used on a two-cylinder opposed motor is shown at [Fig. 136]. This has two throws and the crank-pins are spaced 180 degrees apart. The bearings are exceptionally long. Four-cylinder crank-shafts may have two, three or five main bearings and three or four crank-pins. In some forms of two-bearing crank-shafts, such as used when four-cylinders are cast in a block, or unit casting, two of the pistons are attached to one common crank-pin, so that in reality the crank-shaft has but three crank-pins. A typical three bearing, four-cylinder crank-shaft is shown at [Fig. 134], A. The same type can be used for an eight-cylinder Vee engine, except for the greater length of crank-pins to permit of side by side rods as shown at [Fig. 137]. Six cylinder vertical tandem and twelve-cylinder Vee engine crank-shafts usually have four or seven main bearings depending upon the disposition of the crank-pins and arrangement of cylinders. At [Fig. 138], A, the bottom view of a twelve-cylinder engine with bottom half of crank case removed is given. This illustrates clearly the arrangement of main bearings when the crank-shaft is supported on four journals. The crank-shaft shown at [Fig. 138], B, is a twelve-cylinder seven-bearing type.
Fig. 138.—Crank-Case and Crank-Shaft Construction for Twelve-Cylinder Motors. A—Duesenberg. B—Curtiss.
Fig. 139.—Counterbalanced Crank-Shafts Reduce Engine Vibration and Permit of Higher Rotative Speeds.
In some automobile engines, extremely good results have been secured in obtaining steady running with minimum vibration by counterbalancing the crank-shafts as outlined at [Fig. 139]. The shaft at A is a type suitable for a high speed four-cylinder vertical or an eight-cylinder Vee type. That at B is for a six-cylinder vertical or a twelve-cylinder V with scissors joint rods. If counterbalancing crank-shafts helps in an automobile engine, it should have advantages of some moment in airplane engines, even though the crank-shaft weight is greater.