Crank Pulleys

A crank pulley damper(also called harmonic balancer , crankshaft damper, torsional damper, or vibration damper) is a device connected to the crankshaft of an engine to reduce torsional vibration and serves as a pulley for drive belts.
A car runs by converting chemical energy (the combination of fuel and air) into mechanical energy. This energy is transferred to the moving parts of the engine through a system of car engine pulleys.
The engine usually has an idler and/or tensioning pulley as well as a few pulleys that use the power of a travelling serpentine belt to drive components such as the alternator or water pump. As this energy transfer is essential to the running of the engine, high quality car engine pulleys are recommended if you need to replace any or all of your car engine pulleys. 
Vibration
Every time the cylinders fire, torque is imparted to the crankshaft. The crankshaft deflects under this torque, which sets up vibrations when the torque is released. At certain engine speeds the torques imparted by the cylinders are in sync with the vibrations in the crankshaft, which results in a phenomenon called resonance. This resonance causes stress beyond what the crankshaft can withstand, resulting in crankshaft failure.