For decades, drive belts, V-belts, multi-vee-belts, and serpentine belts have been used to transmit power from the engine crankshaft Car Pulley Belt 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 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 well, or for very long, if at all, with incorrect tension. A loose drive belt won’t drive the accessory reliably, slipping 
and making noise. Conversely, an excessively limited belt may cause accessory or pulley bearing damage. Various kinds of tensioner pulley keep long-term engine and accessory quietness and reliability.
Tightening or Loosening
Sometimes, maintenance or repair will demand 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 new belt, as the new belt is smaller compared to the worn drive belt.
You’ll need to tighten a tensioner pulley, in most cases, after the installation of a new drive belt, or to modify for a stretched drive belt that hasn’t worn enough to warrant replacement. Extend 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 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 will be the simplest, most common, and least prone to failure. There is usually one caveat, nevertheless, as mechanical tensioner pulleys need manual adjustment. This makes them susceptible to user error, resulting in insufficient or extreme belt stress. Additionally, they have to be adjusted to compensate for belt stretch as time passes.