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, 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, 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 very long, if, with incorrect tension. A loose drive belt won’t drive the accessory reliably, slipping and producing noise. Conversely, an excessively limited belt may cause item or pulley bearing harm. Various types 
of tensioner pulley preserve long-term engine and item quietness and reliability.
Tightening or Loosening
Sometimes, maintenance or restoration will require tightening or loosening a tensioner pulley. Changing a drive belt or timing belt, for example, would need you to loosen a tensioner pulley to create room for the brand new belt, as the brand new belt is smaller compared to 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 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 utilize the special tool to avoid belt damage.
Tensioner pulleys generally fall into two categories: accessory-integrated (AI) and Car Pulley Belt 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 methods to loosen them.
Mechanical tensioner pulleys will be the simplest, most common, and least susceptible to failure. There can be one caveat, however, as mechanical tensioner pulleys need manual adjustment. This makes them susceptible to user error, resulting in insufficient or extreme belt tension. Additionally, they have to be adjusted to pay for belt stretch over time.