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 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 won’t work very well, or for very long, if at all, with incorrect tension. A loose drive belt won’t drive the item reliably, slipping and producing noise. Conversely, an excessively limited belt may cause item or pulley bearing harm. Various kinds of tensioner pulley preserve 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 instance, would need you to loosen a tensioner pulley to create room for the 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 installation of a new drive belt, or even to change 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 utilize the special tool to prevent belt damage.
Tensioner pulleys generally fall into two types: accessory-integrated (AI) and non-accessory-integrated (NAI). Think of AI tensioners as adjustable components, such as 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 susceptible to failure. There is one caveat, however, as mechanical tensioner pulleys require manual adjustment. This makes them susceptible to user error, resulting in insufficient or excessive belt tension. Additionally, they have to be adjusted to compensate for belt stretch over time.