The main parts of the crank are the main bearing journals (which allow it to spin within the cylinder block), the big end bearing journals or pin (where the conrods bolt on) and webs (which join the main and big end bearings but also protrude past the big end bearings to form counterweights). These webs help to balance the firing forces and the out-of-phase loadings form each cylinder.
Typically, for a four-cylinder engine, the crank will have four big end journals, and five main bearings, one at each end and one between each cylinder. This is to stop the crank from flexing. Some older engines used three (main) bearings, to get the overall engine length shorter. But this can make the crank bend or whip.
There are three main manufacturing methods for a crank:
Cast cranks nowadays are pretty good but there can be differences between them in terms of quality, due to variables in the casting process.
A forged crank tends to be stronger than a cast one, since the process means the grain structure within the material follows the overall shape of the crank, increasing strength and fatigue-resistance.
Billet cranks are reserved for high performance or race engines. Fabricating them is time-consuming, but they offer excellent strength and fatigue-resistance properties, handling much higher combustion and pressure loads. Additionally, they can handle higher engine speeds.
The design of the crank can have a direct effect on its strength. For example, the corners where the bearing surfaces meet the webs are radiussed, since sharp corners can give rise to stress concentrations and create failure points. The bearing journals themselves also get a surface treatment, to improve their surface hardness.
Two examples of this process are known as Nitriding or Tuftriding. Both increase hardness by altering the structure of the surface of the metal, either by heat or chemical action, or a combination of both.