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.
[Illustration: Fig. 17--A Four-cylinder Crank Shaft.]