JOCKEY
Jockey is the idle sprocket used for guiding the chainwheel or give tension in drive.
Basically it is an arrangement of bearing on sprocket to move freely. Its fitting is in between the driving sprockets. when alignment is straight, it can tilt the chain so chain will move in tension and drive the driven parts smoothly otherwise slippage or chain breaks can happen.
To avoid all these things jockey is used. Here is the arrangement bellow to give an clarity and exact idea about uses of jockey.
