Crank shaft pulley or harmonic balancer.
The crankshaft pulley also known as harmonic balancer drives all the vehicle accessories such as alternator, air condition compressor, power steering pump etc.
The harmonic balancer dampens the vibrations caused by the internal combustion of an engine. The crankshaft pulley has an inner and outer ring with a rubber padding in between. The outer or inner ring eventually separates due to heat and fatigue from the rubber padding. The result is the snapping or slipping of the accessory drive belt which may fall off and damage other components besides the bad crankshaft pulley damaging the engine.