When conducting a leaf chain inspection of any form of safety-critical industrial equipment, there can often be an over-reliance on the measuring of chain elongation.
While chain elongation is a significant factor, it should also be considered in the context of a wide range of other chain wear signs and symptoms including the condition of surfaces, pins and links or the effects of rust.
It is also important to be able to differentiate between leaf chain stretch and leaf chain wear. And in particular to be able to recognise the difference between temporary elongation (due to an increased load) and permanent elastic elongation that indicates the chain needs to be replaced.
In this blog post we explain some of the most common signs and symptoms of leaf chain wear, we take a closer look at the subject of chain elongation and we explore some of the other contributing factors that could well be impacting on the operational life of your chain.
Signs and symptoms of leaf chain wear
First, let's consider some of the most common types of leaf chain wear to look out for:
- Worn contours - These can occur as the result of normal wear on the sheave or abnormal wear due to rubbing on guides.
- Worn surfaces - Signs of wear on either the outer plates or pinheads can often occur due to misalignment or rubbing on side flanges.
- Tight joints - Tight joints are most commonly the result of rust, corrosion, bent pins, dirt or a foreign substance becoming caught in the leaf chain joints.
- Missing parts - Sometimes parts can be overlooked or missed at the point of assembly, meaning the entire chain will need to be replaced.
- Abnormal pins - Abnormal protrusion or turned pins are most commonly caused by excessive internal friction as a result of high loading or inadequate lubrication.
- Cracked plates - Often due to leaf chain being loaded beyond its dynamic capacity.
- Fractured plates - High overload increases the chance of material distortion which can fracture the leaf chain plates.
- Enlarged holes - Enlarged holes on the leaf chain plate can often occur due as a result of the equipment operating under excessive load.
- Corrosion - Any corrosive atmosphere (such as industrial plants or coastal environments) can have an adverse affect on leaf chain wear life.
- Worn leaf chain anchor bolt connecting pins - Normal wear and tear can result in worn leaf chain anchor bolt connecting pins, so these should always be replaced whenever you fit new leaf chains.
Temporary vs permanent elongation
Like all metal parts, leaf chain will deform elastically when placed under load, but will generally return to its original length once that applied load is removed.
It is only at the point that the yield point has been exceeded that any chain deformation would be considered permanent and non-reversible.
Temporary chain stretch can provide an inaccurate indication of chain wear. So it is advisable to make sure your chain is operating under a small load (i.e no more than 1% of its tensile strength) in order to gain an accurate result.
Measuring a chain for wear when it is fully loaded could mean you end up over-reporting the wear by as much as 0.25%.
BL644 leaf chain, for example, has a tensile strength of 12.7 tonnes and a maximum operating load of 2 tonnes - so will have an elastic elongation of approximately 2mm per ton of load. In order to achieve an accurate result, it would therefore require a measuring load of about 150kg.
Other contributing factors of leaf chain wear
Other factors that also have a significant impact on the speed, and the extent of, chain wear are:
- The chain's operating environment - heavy industrial locations or coastal/marine environments can often mean a greater risk of exposure to moisture or corrosives that can cause rusting, pitting or microscopic cracking
- The nature of the loads - if leaf chain is subject to dynamic impulse or shock loads, if you're "inching" loads beyond the capacity of the lifting mechanism or if loads are being carried over rough or uneven terrain, these factors can all have an adverse effect on chain wear life.
- The frequency of work - the number of duty cycles that the equipment is being asked to perform per day/per week is another significant factor - and especially so as machines are now often being asked to operate over double shifts or to provide 24-hr operation.
- The size of the chain - the chain used on forklifts is getting smaller. A 2 ton truck that once used to operate using a BL8 chain, for example, will now more often be working using a BL6.
For independent guidance, and recommendations on chain wear, our advice is to refer to the following standards: The international chain standard ASME B29.8 2010; The guidance notes from the British Industrial Truck Association GN15; The Safety Assessment Federation MLCC01.
The frequency of forklift truck inspections must also comply with the minimum legal requirements as laid out in LOLER 98 (regulations 9, 10 and 11) for the Thorough Examination and Inspection of Industrial Truck Leaf Chain.