What is the difference between leaf chain wear and chain stretch?
Chain wear and chain stretch both increase the length of leaf chain but it is important to understand the difference and what you are actually measuring when carrying out inspections with a chain wear gauge.
Leaf chain wear is the removal or deformation of the pin diameter or link plate bore and is caused by the motion of the chain under load around the pulley. In general, this is mechanical erosion of the surfaces but could also be a result of corrosion. The wear rate of leaf chain is affected by factors such as the level of loading, speed of articulation, temperature and lubrication.
Wear on the component parts, together with other processes such as fatigue and stretch, causes the functional surfaces to degrade, eventually leading to chain failure. Regular measurement of chain elongation ensures replacement is scheduled before the chain is at risk of failing, thereby avoiding potentially catastrophic damage, injury and unplanned downtime.
All leaf chain components are generally heat treated to increase their resistance to wear and the pins will have a surface hardening. This is where the surface of the pins is made hard while allowing the steel underneath to remain soft and ductile. The thin layer of harder metal is often referred to as the case and will be around 0.3mm deep. When the wear goes through this hard layer, the rate of wear and elongation increases rapidly. This is one of the reasons why, when a chain reaches 2% elongation, service technicians are advised to start the replacement process, using their experience to determine the timescale for this to be completed.
As the leaf chain rotates around the pulley, approximately a third of the pin diameter will come under load and only the parts of the pin that are in contact with the articulating chain links. On BL644 leaf chain, for example, only four out of the eight links will be in contact with the pin. The image below shows grooves worn into the surface of the pin and the castellations in this example show severe wear.
When measuring chain as part of a leaf chain inspection, people sometimes refer to measuring chain ‘stretch’. We think about leaf chain stretch as the temporary or permanent elastic elongation of the leaf chain plates. Like all metal parts, leaf chain will deform elastically when under load and will return to its original length when the load applied has been removed. With any leaf chain, once the yield point is passed, the deformation will be permanent and irreversible.
Chain stretch is important to bear in mind as it can impact the measurement of chain wear. When measuring a leaf chain for elongation due to wear on the pin and plate joint, the leaf chain needs to be placed under a small load (1% of its tensile strength) to engage the clearances. If the chain is placed under too much load, however, you will be measuring chain stretch. As an example, BL644 leaf chain, which has a tensile strength of 12.7t, would have an elastic elongation of about 2mm per tonne of load so would require a measuring load of about 150kg.
If you measure a chain for wear when it is fully loaded, you could be over reporting the wear as there could also be some elongation due to chain stretch. This could result in you changing the chain sooner than you need to and, if done consistently over time, spending more money on chain due to the unnecessarily shorter replacement cycle. If you measure the chain without a load, however, the clearance between the chain components will not have been pulled together so you could be under reporting the wear. This may result in the chain being left in service for too long and there being a risk of chain failure.
For further advice on leaf chain measurement or any chain requirements, feel free to get in touch with our leaf chain experts.