Drive Belt and Timing Belt

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What causes wear in ordinary belt or serpentine type?
Why do we have to replace them once in a while?

Friction, flexing, tension and heat.

Friction - Some rubbing naturally occurs as the belt lines up in and out of the pulley and grooves. This is analogous to tires. Rolling friction causes it to wear... doing burnouts or sudden acceleration and braking just speeds up the wear.

Flexing and tension - Most belts are bent more than 270 degrees as it goes around different pulleys... doing that while the tensioner keeps it tight enough to prohibit slipping. Rubber fatigue will eventually weaken the belt's construction.

Heat - Any material will change chemically when repeatedly exposed to heat. Think of it as the material's expiration date.