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 add-ons, 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, and 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 lengthy, if, with incorrect tension. A loose drive belt won’t drive the accessory reliably, slipping and producing noise. Conversely, an excessively restricted belt may 
cause accessory or pulley bearing harm. Various kinds of tensioner pulley keep 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 instance, would need you to loosen a tensioner pulley to make room for the new belt, as the new belt is smaller than the worn drive belt.
You will have to tighten a tensioner pulley, generally, after the installing a new drive belt, or to change 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 utilize the special tool to avoid belt damage.
Tensioner pulleys generally fall into two classes: accessory-integrated (AI) and non-accessory-integrated (NAI). Think about 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 are the simplest, many common, and least prone to failure. There is definitely one caveat, nevertheless, as mechanical tensioner pulleys need manual adjustment. This makes them susceptible to user error, resulting in insufficient or extreme belt pressure. Additionally, they need to be adjusted to compensate for belt stretch as time passes.