Metal idler sprockets maintain proper chain pressure, and guideline the chain around obstacles and stop excessive chain put on and vibration. You don't need any special tightener shafts for ball bearing idler sprockets. Composite sprocket idlers need no lubrication and so are corrosion resistant and wear-resistant.
An idler sprocket is a device used to maintain the tension in a chain or chain drive system. Often consisting of only a sprocket mounted on a springtime tensioned arm, the idler sprocket pulls against the chain in a constant manner to keep the chain tight at all times. The size of the sprocket used in an idler sprocket assembly has no effect on the efficiency of the chain drive; however; a larger sprocket will often last longer due to the slower acceleration of the sprocket, which will save use on the sprocket's bearings. Maintenance for the idler assembly is commonly no more than an intermittent greasing of the sprocket's bearings.
When driving a machine simply by chain, the tension of the chain must be held at a constant in order to avoid the chain coming from the drive sprockets. By installing an idler sprocket in the drive system, the chain is kept taut without being over-tightened. Operating a chain in an over-tight condition can result in premature bearing and chain failure while an idler sprocket positioned in the system is usually a way to significantly extend the life span of the chain, sprockets and the bearings on the machine's sprocket shafts.
The ideal installing the idler sprocket is on the opposite side of the chain between the drive sprocket and the driven sprocket. The application should place the idler sprocket in a position that has the sprocket pushing or pulling the chain towards itself since it loops the two major sprockets in a form like the letter B. This design will allow the pulleys to pull the chain hard without hindering the idler at all as the drive chain 
passes over the sprocket. If a condition occurs which needs the drive to exert extra 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 produced of steel, many materials are used to chain sprocket manufacture an idler sprocket. Many poly or composite sprockets have already been used with great success and some wooden sprockets are also used on some machinery without concern. Many machines, in an attempt to reduce the put on on the drive chain, use an light weight aluminum, cast iron or steel sprocket covered in a nylon materials. The metallic hub enables the idler sprocket to stay very strong while the nylon covering can be gentle on the chain links.