What's the difference between leaf chain wear and chain stretch?

What is the difference between leaf chain wear and chain stretch?

Understanding the difference between chain wear and chain stretch in leaf chains

Leaf chain wear occurs under load, causing pin and link plate deformation. In general, this is mechanical erosion of the surfaces but could also be a result of corrosion.

Chain stretch is a specific consequence of chain wear. It refers to the elongation of the entire chain due to the wear on individual links.

Factors affecting leaf chain wear

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.

Importance of regular chain elongation measurement

Prevent Catastrophic Failure: Prioritize Chain Elongation Measurement.

Regularly monitoring chain elongation is not optional. It’s the definitive way to prevent catastrophic equipment failure, potential injuries, and crippling unplanned downtime. By staying ahead of wear, you ensure the safety and smooth operation of your machinery.

Surface hardening and wear rate

Leaf Chain Durability Relies on Precise Engineering:

  • Heat Treatment: All components undergo a heat treatment process, maximizing wear resistance. This creates a crucial hardened surface on the pins, while the inner core remains ductile for optimal functionality.
  • Precise Case Depth: This hardened surface, often called the “case,” is approximately 0.3mm deep and plays a critical role. Once wear breaches this layer, the rate of wear and elongation accelerates significantly.
  • 2% Elongation = Replacement Trigger: When chain elongation reaches 2%, it’s a clear warning sign. Service technicians should immediately initiate the replacement process, leveraging their expertise to determine the appropriate timeframe for completion.

Load distribution on leaf chain

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.

Elastic elongation

Measuring Chain Stretch and Elastic Deformation in Leaf Chain Inspection

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. Leaf Chains, Like All Metal Components, Exhibit Predictable Behaviour Under Load:

  • Elastic Deformation: Up to a specific point (the yield point), leaf chains will temporarily deform under load but return to their original length upon release. This is a normal characteristic of metals.
  • Permanent Deformation: Exceeding the yield point results in permanent and irreversible chain lengthening. This highlights the criticality of staying within safe load limits.

Consideration for measuring Chain stretch and Wear in the Leaf Chain Inspection

Chain stretch is important to bear in mind as it can impact the measurement of chain wear. To properly assess leaf chain elongation due to pin and plate joint wear, a specific load is crucial. A minimal load of 1% tensile strength is essential to engage clearances and guarantee accurate measurement. Exceeding this load will result in measuring chain stretch, not true wear. 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.

Considerations for Accurate Measurement of Chain Wear and Stretch

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.

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