The sudden and powerful outward movements of the piston under the pressure from the combustion are transmitted to a crank shaft, which must be of great strength in order to resist the heavy strains under which it operates. It is made of the best steel available for the purpose, and has as many cranks as the engine has cylinders. The cranks are generally made in one piece with the shaft for the sake of strength, and for stiffness there are as many bearings as possible. The number of bearings for the crank shaft of an engine with four or more cylinders depends on the arrangement of the cylinders. If the cylinders are evenly spaced, there will be room for a bearing between each pair of cranks, so that a four-cylinder engine will have five bearings, one at each end, the other three being between the cranks. If the cylinders are in pairs, there will not be room between the cranks of a pair for a bearing, the only space for it being between the pairs; a four-cylinder engine built in this way will thus have but three bearings, one at each end and one in the center. Crank shafts are described by their bearings as three, five, etc., point crank shafts.
Fig. 3.—Gasoline Engine in Section.
Fig. 4.—A, Two-throw crank shaft; B, four-throw crank shaft, 180°.
The relative positions of the cranks of a crank shaft are expressed in degrees of a circle; if, for instance, the cranks project from opposite sides of the shaft so that they are a half revolution apart, it is called a 180-degree crank shaft.
The outer ends of the crank arms, which correspond to the cranks of a bicycle, support the crank pin, which may be likened to the pedal, and to this the large end of the connecting rod is attached, the small end being connected to the piston. The connecting rod must be of great strength, tough but not brittle, and is made of steel or bronze.
Fig. 5.—One-Throw Crank Shaft.
