For decades, drive belts, V-belts, multi-vee-belts, and serpentine belts have already been used to transmit power from the engine crankshaft pulley to accessories, such as the power steering pump, air conditioning compressor, water pump, or cooling fans. Toothed timing belts and timing chains, too, are used to transmit power from the crankshaft to the camshafts, plus some from the camshaft to camshaft, based on engine Car Pulley Belt design.
The drive belt, timing belt, or timing chain won’t work very well, or for very long, if, with incorrect tension. A loose drive belt won’t drive the item reliably, slipping and making sound. Conversely, an excessively restricted belt may cause accessory or pulley bearing damage. Various kinds of tensioner pulley maintain long-term engine and item quietness and reliability.
Tightening or Loosening
Sometimes, maintenance or repair will demand tightening or loosening a tensioner pulley. Replacing a drive belt or timing belt, for example, would need you to loosen a tensioner pulley to create room for the brand new belt, 
as the new belt is smaller than the worn drive belt.
You will have to tighten a tensioner pulley, generally, after the installation of a fresh drive belt, or to adapt for a stretched drive belt that hasn’t worn enough to warrant replacement. Stretch belts don’t need tensioner pulleys but are “stretched” into place using a special tool-always use the special tool to prevent belt damage.
Tensioner pulleys generally fall into two categories: accessory-integrated (AI) and non-accessory-integrated (NAI). Think of AI tensioners as adjustable components, such as an alternator, and NAI tensioners as adjustable idler pulleys. There are three types of tensioner pulleys and several ways to loosen them.
Mechanical tensioner pulleys are the simplest, the majority of common, and least prone to failure. There can be one caveat, however, as mechanical tensioner pulleys need manual adjustment. This makes them susceptible to user error, leading to insufficient or excessive belt stress. Additionally, they have to be adjusted to compensate for belt stretch as time passes.