CHAIN A SHORT NOTE

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The sketch shows the assembly of a single chain link and as can be seen it consists of:

•              outer link plates (Cr. Mo.)

•              hardened steel pin which is an interference fit into the outer link plate.

•              inner link plates (Cr. Mo.)

•              bushes pressed into inner link plates and free on pins.

•              rollers which are a clearance fit on the bushes.

Link of a Roller Chain

Advantages of chain drive

•              Allows total flexibility in the positioning of driven equipment.

•              Easy adjustment for timing of camshaft to crankshaft.

•              Cheap with minimum spares requirement.

•              Can cope with some misalignment between chain wheels caused by axial movement of shafts.

•              High efficiency (98.4 -98.9%).

Due to arduous load conditions with greatly varying cyclic stresses chains normally have a factor of safety in the region of 40 to 50. Because of this there is little risk of the chain actually stretching. Elongation of the chain is due to wear between the link components.

Power requirements

For engine powers in the following ranges the chain drive absorbs the given percentage of engine output.

                        200 -  500 kw            6%

                        500 - 7000 kw            5%

                        7000 plus  kw            3%

For badly worn or adjusted chains the following will result :

Elongation

•              This will alter the camshaft position relative to the crankshaft so affecting fuel and valve timing.

Slack chain

•              Will cause variation in timing due to backlash, especially during sudden load changes and manoeuvring.

Tight chain

•              Will result in excess wear and fatigue failure due to overloading of the rollers.

•              Will also result in overloading of chain wheel bearings.

Adjustment of chain

Facility provided to take up initial wear by adjustable jockey. This is normally done when play in the slack span of chain is greater than 1 pitch. After this is at its limit it is usually allowed to remove a number of links and to adjust with jockey up to the limit again.

Maximum allowable elongation varies but is usually 2% it may be advisable to renew when 1.5% reached. (A 1% maximum tolerance is given by some manufacturers/engine builders)

Elongation checked by turning engine until chain is taut. Then measure length of a number of links, say 10, from pin centre to pin centre.

Elongation is then difference between measured length and as new length of number of links.

Using a profile gauge to check wear on chain wheel teeth.