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Chain jumping its sprockets?

Chain jumping its sprockets?
Have you ever had problems with transmission chain jumping or climbing its sprockets? David Turner of Renold Chain looks at the causes and the solutions.

A common cause of chain climbing or even jumping its sprockets is excessively worn sprockets, or worn chain, or both. Worn sprockets will also cause chain to wear rapidly, so it's important to replace it and check the condition of the sprockets before fitting new chain.

Look at the faces of the sprocket's teeth. Any wear will be seen as a polished worn strip, about the pitch circle diameter (PCD), on each of the teeth as shown in the diagram below. 

If the depth of the wear, X, has reached a value equal to 10% of the dimension Y, then the sprocket has reached the end of its life and needs to be replaced. It's worth pointing out that low cost, poor quality sprockets on most industrial applications are a false economy. A good quality sprocket should last through the life of several chains before anything like the amount of wear shown here will have occurred.
If you're fitting new chain don't forget to correctly lubricate it. The lubrication that's on the chain when it comes out of the box is only sufficient to prevent corrosion during transit and to aid initial bedding in.

If neither the chain nor the sprockets have worn then check the chain isn't excessively slack. If it is then adjust the centre distances to increase tension. This adjustment should result in the slack strand of chain having a calculated degree of movement based on the horizontal centre distance of the drive.

This distance, A, in horizontal drives, is calculated by dividing the horizontal centre distance, C, by factor K, which has a value of 25 for smooth drives and 50 for shock drives. For vertical drives A should be approximately equal to half the chain pitch P. Care should be taken to avoid over tensioning.

Other ways of removing slack from the chain include using a jockey sprocket or, if it's permissible to do so, the chain itself could be shortened.
 
If there is no sign of wear or excessive slack in the chain then it's possible that there is insufficient wrap around the drive sprocket. Particularly for large ratio drives it's possible that the driver sprocket may  not have enough teeth to absorb the working tension. If the drive cannot be altered the solution is to fit a jockey sprocket to increase wrap.

A final check is to look for a build up of foreign material in the sprocket tooth gap. If any is present then clean the sprockets teeth so that the chain can once again engage correctly.
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