A tensioner, in mechanical engineering and automotive applications, refers to a device or mechanism used to maintain or adjust the tension of a belt, chain, or similar flexible element. Its primary function is to ensure that the belt or chain remains taut and properly aligned, which is crucial for the efficient operation and longevity of various systems. Idler pulleys are primarily used to route and maintain proper tension in the engine's accessory drive belts. By guiding the belts and maintaining tension, idler pulleys contribute to smooth operation of the engine's accessory drive system, thereby reducing noise and vibrations.