Metal idler sprockets maintain proper chain stress, and guideline the chain around obstacles and prevent excessive chain wear and vibration. You don’t need any particular tightener shafts for ball bearing idler sprockets. Composite sprocket idlers need no lubrication and are corrosion resistant and wear-resistant.
An idler sprocket is a device used to maintain the strain in a chain or chain drive system. Often consisting of nothing more than a sprocket mounted on a spring tensioned arm, the idler sprocket pulls against the chain in a continuous manner to keep the chain tight at all times. The size of the sprocket used in an idler sprocket assembly does not have any effect on the efficiency of the chain drive; however; a more substantial sprocket will often last longer because of the slower acceleration of the sprocket, which will save wear on the sprocket’s bearings. Maintenance for the idler assembly is commonly no more than an intermittent greasing of the sprocket’s bearings.
When traveling a machine simply by chain, the strain of the chain must be held at a constant in order to avoid the chain coming off of the drive sprockets. By setting up an idler sprocket in the drive program, the 
chain is kept taut while not being over-tightened. Working a chain in an over-limited condition can lead to premature bearing and chain failure while an idler sprocket placed in the program is often a way to greatly extend the life of the chain, sprockets and the bearings on the machine’s sprocket shafts.
The ideal installing the idler sprocket is on the contrary side of the chain between the drive sprocket and the driven sprocket. The application form should place the idler sprocket in a position which has the sprocket pushing or pulling the chain towards itself since it loops the two main sprockets in a shape similar to the letter B. This design will allow the pulleys to draw the chain hard without hindering the idler at all as the drive chain passes over the sprocket. If a condition presents itself which requires the drive to exert excess strain on the drive chain, the idler will flex against the chain, and can expand while staying in contact with the drive sprockets.
While the vast majority of idler sprockets are manufactured of steel, many materials are used to produce an idler sprocket. Many poly or composite sprockets have already been used in combination with great success and some wooden sprockets have also been used on some machinery without issue. Many machines, in an attempt to reduce the use on the drive chain, use an aluminum, cast iron or steel sprocket coated in a nylon materials. The metallic hub enables the idler sprocket to remain very strong while the nylon covering is soft on the chain links.