For decades, drive belts, V-belts, multi-vee-belts, and serpentine belts have been used to transmit power from the engine crankshaft Car Pulley Belt pulley to add-ons, such as the power steering pump, air-con compressor, water pump, or cooling fans. Toothed timing belts and timing chains, too, are accustomed to transmit power from the crankshaft to the camshafts, plus some from the camshaft to camshaft, depending on engine design.
The drive belt, timing belt, or timing chain will not work very well, or for very long, 
if at all, with incorrect tension. A loose drive belt won’t drive the item reliably, slipping and producing sound. Conversely, an excessively restricted belt may cause item or pulley bearing damage. Various kinds of tensioner pulley preserve long-term engine and item quietness and reliability.
Tightening or Loosening
Sometimes, maintenance or repair will require tightening or loosening a tensioner pulley. Replacing a drive belt or timing belt, for instance, would require you to loosen a tensioner pulley to make room for the new belt, as the brand 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 even to adjust for a stretched drive belt that hasn’t worn enough to warrant replacement. Extend 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 classes: accessory-integrated (AI) and non-accessory-integrated (NAI). Think of AI tensioners as adjustable add-ons, such as an alternator, and NAI tensioners as adjustable idler pulleys. There are three types of tensioner pulleys and many methods to loosen them.
Mechanical tensioner pulleys will be the simplest, most common, and least susceptible to failure. There can be one caveat, nevertheless, as mechanical tensioner pulleys require manual adjustment. This makes them susceptible to user error, leading to insufficient or extreme belt pressure. Additionally, they have to be adjusted to compensate for belt stretch over time.