In order for the engine to transfer rational energy to various components, like the alternator, there needs to be some way to connect the crankshaft pulley to the pullies of the other components. The serpentine belt is what makes this possible.
Serpentine belts have a variety of patterns which help keep it attached to the pulley system, but in almost every example, one side of the belt is completely flat, and this side usually faces towards the outside most of the time.
Some vehicles have multiple belts, so the crankshaft pulley can have two different sizes, which keeps the component pullies from having to be quite large. Sometimes custom sizes can be made so someone modifying their vehicle can bypass non-crucial components like the air conditioning pump or power steering.
Inspecting Belts
Belts can wear in multiple ways! Belts can most easily be looked at when removed from the engine, but can be given a basic inspection while still on the pulleys. Do not attempt to remove or touch a belt/pulley while the engine is running or could run. General wear occurs on the ribbed section faster than the smooth side, so checking the depth of the ribs is the most basic way to inspact a belt. The rubber material of the belt can age and dry out, causing small cracks to form that weaken the belt. Damage can also occur in individual spots on the belt, so the entire belt should be observed on both sides. Automotive chemicals like oil and coolant can weaken the integrity of a belt. A loose belt can slip on the crank pulley or a pulley that has a lot of momentum in its spin, and the pulley spinning faster than the belt will melt away.
Stretch Belts
Stretch belts are being used to eliminate the need for an automatic belt tensioner, which allows for smaller engine packages and slightly less tension on pulley bearings. The characteristics of a stretch belt also reduce vibration and noise.
Inspecting Stretch Belts
Inspecting a stretch belt is similar to traditional belts, but a tool needs to be used to inspect the grooves because the difference between a good belt and a worn belt is too small to feel or see. The material used to make these belts also tends to not crack with age, either.
Replacing Stretch Belts
Installing a stretch belt typically requires an installation tool. While it is possible to remove a stretch belt and reinstall it, it is not recommended. Typical removal involves simply cutting the old belt.