A harmonic balancer pulley is an engine component that is attached to the crankshaft of the engine. The purpose of this pulley is to help prevent or dampen vibration which can lead to damage in other components of the engine. It is usually mounted at the front of the engine, and the serpentine belt is usually routed around the harmonic balancer pulley in addition to other pulleys in the system. This pulley is made of a durable metal that is resistant to the high temperatures common in an engine compartment, and it may or may not be shaped to help keep the belt in place.
The main function of a harmonic balancer pulley is to prevent damage to the engine's crankshaft. When the engine cylinders fire during combustion, they cause torque to be imparted on the crankshaft. The crankshaft will then try to resist this torque, resulting in vibration. This process in itself is usually not damaging to the crankshaft, but at certain engine speeds, the force caused by the combustion in the cylinders and the deflection of the force in the crankshaft can lead to excessive forces that the crankshaft simply cannot handle. The crankshaft may then fail.
To prevent the failure of the crankshaft, a harmonic balancer pulley is affixed to the crankshaft. This pulley helps balance some of the forces being placed on the crankshaft during combustion, thereby preventing that force from leading to crankshaft failure. The pulley wheel itself will help slow the progression of vibration in the engine system, and the belt system will help absorb some of the residual vibration. The balancer often also features a rubber ring for added damping. The harmonic balancer pulley can also act as a guide for the drive belt, ensuring it stays tensioned and in place during its normal functioning.
If the engine's harmonic balancer pulley begins to wear out or malfunction, the serpentine or drive belt is likely to get thrown, or derailed off the pulley system. This can lead to catastrophic failure of the belt, thereby necessitating a belt replacement. If the worn harmonic balancer is not addressed before replacing the belt, the new belt may also get thrown and consequently damaged. The harmonic balancer will need to be replaced in this case to prevent further damage to the belt and any excess vibrational force being transferred to the crankshaft, which can risk even more damage to the vehicle.