A crankshaft pulley is a pulley which attaches directly to the crankshaft, outside the engine block, where it drives the serpentine belt. It is the component which transfers rotational energy to components such as the alternator and water pump. Its size, in relation to the size of all the other pulleys, determines the amount of the engine's power gets transferred to the various components. A large crankshaft pulley with small component pulleys will give the components a lot of power, and less of the engine's power is sent to the drive wheels.
However, if the crankshaft pulley is too small or a component's pulley too large, not enough power will be sent to the components. Because of this, there needs to be a balance between power sent to the components and power sent to the wheels. When production cars are designed, there is usually a bit of bias towards the components, because if there is just enough for the components, there won't be enough as engine power is lost with age, or if something is modified later. Because of this, a popular inexpensive performance modification for many cars is a performance crankshaft pulley. This doesn't actually add any power to the engine, it just frees up otherwise wasted power. One made from lightweight materials also helps the engine change RPM more quickly.
Built into almost all crankshaft pullies are harmonic balancers, which dampen rotational vibrations that can negatively impact the engine internals and, to a smaller degree, belt accessories. The damping part of the pulley is usually a rubber or similar material ring within the pulley. Usually this softer material is what wears and gives way over time.
Is a Harmonic Balancer The Same as a Crankshaft Pulley?
In most cases, yes, but a crankshaft pulley can be just simply the engine's drive pulley. A harmonic balancer is practically always a crankshaft pulley with the necessary dampening components built in. While it is possible to put a balancing element elsewhere, it is very impractical to. A crankshaft pulley without a harmonic balancer is typically used only in vehicles that have to make maximum horsepower and have no concern for internal engine wear or vibration, so many race cars have harmonic balancers, too.
Is a Harmonic Balancer Necessary?
Some performance engine builders incorrectly assume that if the internal engine components are finely balanced, that a harmonic balancer is not required. Even the most well-balanced engine will have natural imbalances that need to be dampened. A harmonic balancer can be made more effective for a well-balanced engine, but there should definitely always be one, except in special cases.