For many years, 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, like 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 accessory reliably, slipping and producing noise. Conversely, an excessively limited belt may cause accessory or pulley bearing damage. Various kinds of tensioner pulley keep long-term engine and item quietness and reliability.
Tightening or Loosening
Sometimes, maintenance or repair will require tightening or loosening a tensioner pulley. Changing a drive belt or timing belt, for instance, would need you to loosen a tensioner pulley to create room for the brand new belt, as the new belt is smaller sized compared to the worn drive belt.
You will have to tighten a tensioner pulley, generally, after the installation of a fresh drive belt, or even 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 avoid 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 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 are the simplest, the majority of common, and least susceptible to failure. There is definitely one caveat, nevertheless, as mechanical tensioner pulleys require manual adjustment. This makes them prone to user error, leading 
to insufficient or extreme belt tension. Additionally, they need to be adjusted to pay for belt stretch as time passes.