Sprocket teeth are the pointed tips on the outer edge of a sprocket. The chain rides on the sprocket teeth, which provides power as the teeth pull against the links of the chain. The size of a sprocket can be found by counting the number of sprocket teeth. When choosing a final drive ratio for a chain-driven machine, the number of teeth on the large sprocket are divided by the number of teeth on the small sprocket to arrive at the correct final ratio of the two sprockets.
A roller chain is sized according to the size of the sprocket teeth that it fits properly onto, which is known as the pitch of a chain. The teeth are made with the width and the height being used to determine the size of the chain that will work properly on the sprocket. The size of the chain will dictate the number of teeth that must be on the sprocket. This is due to the chain links not being able to be bent as the chain rides along on the sprocket teeth. Therefore, a sprocket can be no smaller in diameter than the smallest number of chain links that will fit properly around the sprocket.
A sprocket should be replaced when the sprocket teeth are noticeably hook-shaped. As the drive sprocket pulls against the rollers of a chain, the drive face of the teeth begin to wear. As the wear becomes more noticeable, the teeth become worn and appear to have a C-shaped face on the teeth. This is caused from the chain wearing into the driving face of the sprocket. When this has occurred, the chain and sprockets should be changed.
Simply replacing the chain or the sprockets alone will cause the other to wear prematurely. It is also best to replace sprockets in pairs in order to prevent uneven chain wear. As the teeth on a driven sprocket become worn, such as the rear sprocket on a bicycle, they will continue to wear down until the teeth are mere bumps on the sprocket if unchanged. This will result in a loud grinding or clunking sound and will cause the chain to jump the teeth and slip over the sprocket teeth. As a roller chain begins to wear, the bushings in the links begin to become egg-shaped. This causes the links to stretch out and be longer than the sprocket teeth, effectively changing the pitch of the chain.