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 accessories, like 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, depending on engine design.
The drive belt, timing belt, or timing chain won’t work very well, or for lengthy, if, 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 types of tensioner pulley keep long-term engine and accessory quietness and reliability.
Tightening or Loosening
Sometimes, maintenance or restoration will demand tightening or loosening a tensioner pulley. Replacing a drive belt or timing belt, for example, would require you to loosen a tensioner pulley to create room for the new belt, as the brand new belt is smaller sized than the worn drive belt.
You’ll need to tighten a tensioner pulley, generally, after the installation of a fresh drive belt, or even to modify for a stretched drive belt that hasn’t worn enough to warrant replacement. Stretch out 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 groups: accessory-integrated (AI) and non-accessory-integrated (NAI). Think about 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 will be the simplest, most 
common, and least susceptible to failure. There is certainly one caveat, however, as mechanical tensioner pulleys need manual adjustment. This makes them prone to user error, leading to insufficient or extreme belt stress. Additionally, they have to be adjusted to compensate for belt stretch as time passes.