chain elongation

Students sometimes have difficulty in visualising the effects of the elongation of a timing chain on the timing of the camshaft. This model may help in understanding what happens.

It should be made clear from the start that timing chains do NOT stretch, The factor of safety is too high. However, they wear on the bushings and rollers, and this causes elongation and thus a change in the pitch of the chain.

For the model, I used bicycle parts, two large wheels of 42 teeth and a smaller 21 tooth wheel for the jockey on the tensioner. Two chains were used; a brand new chain with a pitch of 0.5 inch (12.7mm) and an old worn chain. The percentage elongation between old and new chains is about 2.5% which is way above the max allowable on an actual engine of 1%, but because my model is not to scale, will show the effects more clearly.

It will be noticed that a series of 29 links have been painted red and white on both chains. This is to highlight the difference in length and also to highlight why the camshaft timing alters as the chain gets longer. The 29 links spans the distance between the chain leaving the camshaft drive wheel and engaging with the crankshaft drive wheel. These points on the wheel teeth are also highlighted.

For our model the crankshaft and camshaft are turning clockwise when running in the ahead direction as indicated by the arrows. TDC of the crankshaft and camshaft for No 1 unit are indicated by the marks on the crankshaft and camshaft wheels. It can be seen that with the new chain the timing marks are correctly aligned.

As the chain gets longer so the pitch of the chain increases. BUT there is still the same number of links between the camshaft wheel and the crankshaft wheel. Because of this the camshaft chain wheel is effectively retarded when turning the crankshaft wheel in an ahead direction. This can be clearly seen from the timing marks. Because of this, chain elongation results in alteration of valve, fuel pump and air start distributor timing.