For many years, drive belts, V-belts, multi-vee-belts, and serpentine belts have been used to transmit power from the engine crankshaft pulley to components, such as the power steering pump, air conditioning 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, based on engine design.
The drive belt, timing belt, or timing chain will not work well, or for very long, if at all, with incorrect tension. A loose drive belt won’t drive the accessory reliably, slipping and producing sound. Conversely, an excessively restricted belt may cause item or pulley bearing harm. Various kinds of tensioner pulley maintain long-term engine and accessory 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 brand new belt, as the new belt is smaller sized than the worn drive belt.
You will have to tighten a tensioner pulley, in most cases, after the installing a fresh drive belt, or to adjust for a stretched drive belt that hasn’t worn enough to Car Pulley Belt warrant replacement. Extend belts don’t need tensioner pulleys but are “stretched” into place using a special tool-always use the special tool to avoid belt damage.
Tensioner pulleys generally fall into two types: accessory-integrated (AI) and non-accessory-integrated (NAI). Think of AI tensioners as adjustable accessories, such as for example 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 prone to failure. There is one caveat, nevertheless, as mechanical tensioner pulleys need manual adjustment. This makes them susceptible to user error, leading to insufficient or extreme belt tension. Additionally, they need to be adjusted to compensate for belt stretch as time passes.