THE CRANK SHAFT
Most of you are familiar with a crank as applied to a grindstone. A crank in a motor is practically the same shape except that it is supported on two bearings instead of one and is therefore made in the form shown in Fig. 17. The crank shafts for two and four-cylinder motors are only a combination of two or four of these single cranks. Crank shafts are made up of steel, carefully forged, and then turned and ground down to proper size to fit the bearings for which they are intended. They are hardened and every precaution taken to keep them from wearing. They form one of the most important parts of the motor because they change the back and forth motion of the piston into the rotary motion of the fly wheel. The fly wheel in our former illustration was represented by the grindstone itself. In the real motor the fly wheel is made of cast iron, and after being carefully balanced so that it turns evenly, it is securely bolted to the crank shaft, so that they practically form one piece.