what causes a crankshaft to go bad

To elaborate malinga, then crankshaft is what all the piston rods are connected to. By what's called a connecting rod. From the piston, when in the combustion chamber, the top of the piston gets forced down--by an explosion--inside of the combustion chamber--of the fuel/air mix when the spark plug fires. The piston has a 'rod' from the bottom,. on the top it's called a 'wrist pin' that forces then piston down, therefore creating the energy to push on the crankshaft in a circular motion to create torque.