John King Chains

19.8 Chain extension / elongation

John Kings are often asked to assess when the useful life of a mechanical handling chain has ended.

Assuming that chain and sprocket wheel alignment are correct, chain life is directly dependant on the amount of wear / elongation which can be accommodated by the depth of case hardening / heat treatment on the chain bearing pin body and chain bush / barrel bore.

It could be argued that the greater the depth of heat treatment to these chain component interfaces, the greater the potential chain operating life.

Two points however govern this, one being that it is essential to maintain a certain degree of ductility in the core of the components to prevent brittle fracture and the second being the ability of the chain sprocket tooth form to accommodate a limited amount of chain length extension.

For most commonly encountered chain in general industrial applications the heat treatment depths for chain components will be of the order of 0.040” / 1.00mm. Chain wear will only take place during articulation of the chain bearing pin within the chain bush / barrel bore as the chain engages and dis-engages with the sprocket tooth on the loaded / driving strand.

Adjustment to cater for wear / elongation is normally provided and is based on the following formula:

Total adjustment on chain centre distance = Number of chain links / Pitches in one complete chain circuit x factor “Y” (see below).

Factor “Y” = Case depth of component = 0.040” / 1.00mm

Applying the above formula is simple and based for example on a chain length of 392 pitches in a full circuit, the following will result:

Total adjustment in Inches = 392 x 0.080” = 31.36 inches

Total adjustment in millimeters – 392 x 2.00 = 784 mm

When the chain has reached this adjustment level replacement should be considered.

Should any further confirmation of wear be needed, a visual examination of the chain bearing pin body and chain bush bore will clearly indicate that wear has taken place on both these components. Rapid chain length extension takes place when heat treatment has been worn away.
Normally heat treatment hardness values are in the region of Rockwell 54/62 “C” scale.

Whilst the wear on the chain bearing pin and bush are considered to be the main criteria, consideration should also be given to roller bore wear. This wear has no effect on chain length extension but it can give rise to tooth flank wear on sprocket chain wheels. This can cause mal-gearing and the chain may start to climb out of the sprocket teeth, which then exerts additional loads within the chain circuit.

Assessment of roller bore wear can usually be made by viewing the chain as it engages with the sprocket chainwheel teeth. If there is a significant rise of the chain roller as it engages the tooth, then a further check must be made by manually examining the clearances when the chain has stopped.

Where this is appropriate to reduce chain wear to an absolute minimum, adequate lubrication is essential and this must be directed at the component faces which dictate chain extension. This as above is the bearing pin and bush / barrel Interface.

Penetration of lubricant to these areas is vital and whilst appreciating that oils and greases are difficult and expensive to apply correctly and in the right quantities, lubrication of the chain Side link plates should be avoided wherever possible. If an excessive build up of oil / grease takes place on the chain linkplates, this can prevent the ingress of lubricant to the internal load carrying faces of the chain. Equally important is the use of non-carbonising oils which do not produce a build up of debris in / on the chain.

Specialist advice should be taken on all aspects of lubrication from the appropriate sources.