Sprockets are wheels with teeth that mesh with chains as the wheel turns. The chains typically connect two sprockets. Together the sprockets and roller chain form the chain drive. Chain drives are used in applications where slippage must not occur, such as bicycles and motorcycles, elevators and escalators, chain drive garage door opener systems, and certain engines. A worn or broken sprocket can stop the whole mechanism. Don't wait until your old sprockets break down to order replacements. Know these things about purchasing sprockets: match your sprocket to your roller chain pitch (usually a pitch, or “P” number), and be aware of the number of chain strands your sprocket is turning - if you have a double strand chain, you need a double chain sprocket (has 2 rows of teeth, one row for each chain strand). Zoro is a great source for everything having to do with chains, sprockets and parts.