For decades, drive belts, Car Pulley Belt V-belts, multi-vee-belts, and serpentine belts have already been used to transmit power from the engine crankshaft pulley to accessories, such as the power steering pump, air-con compressor, water pump, or cooling fans. Toothed timing belts and timing chains, too, are used 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 very long, 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 damage. Various types of tensioner pulley maintain 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 make room for the new belt, as the new belt is smaller sized compared to the worn drive belt.
You’ll need to tighten a tensioner pulley, generally, after the installing a new drive belt, or to adapt for a stretched drive belt that hasn’t worn enough to warrant replacement. Stretch belts don’t require tensioner pulleys but are “stretched” into place utilizing 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 about AI tensioners as adjustable components, such as for example an alternator, and NAI tensioners as adjustable idler pulleys. There are three types of tensioner pulleys and several ways to loosen them.
Mechanical tensioner pulleys are the simplest, the majority of common, and least prone to failure. There is certainly one caveat, however, as mechanical tensioner pulleys require 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 over time.
