Making the right choice20 October 2011
Rope discard does not just apply to all cranes. “Most people apply this standard to any scenario where a rope comes off a drum and passes over or around a pulley/sheave,” says Adrian Kirkham, a training manager for Certex. Cristina Brooks reports.
That’s why the latest revisions to the ISO (International Organization for Standardization) 4309 are critical. “Anybody who is responsible for, uses, inspects, maintains or examines crane ropes needs to be aware of the changes that have been made within this standard.”
The revisions to the ISO have changed the criteria for deciding when rope discard happens, recognising that ropes stretch slightly.
The standard has always listed tables for different types of crane. Kirkham continues, “Depending on the type of machine the rope is installed on, it will show how that rope deteriorates over time. This is also defined in the standard in that the allowable number of wire breaks is given in a number of tables.”
The ISO gives the largest number of wire breaks or amount of wear, corrosion and deformation that inspectors can find on a rope before it must be thrown away.
A committee writes the ISO, known to some as the ‘rope inspection bible’. It is made up of representative rope experts from each country, elected from national standard organisations as well as crane, rope and safety industries.
Regarding his selection for thecommittee, David Hewitt says, “It’s a structured process through BSI (British Standards Institution).
“You have to be a member of the crane committee for BSI, that is the British Standard Crane Committee.” He is the chosen representative for Britain and was a speaker at the Cranes Asia conference in Singapore in September 2011.
Hewitt says new diameter calculation criteria in the latest standard revision does not substantially change practice. “The way we inspect wire ropes has been the same for a number of years.”
He continues, “To clarify, with the adjustments we made in this document, we’re talking about numbers which are so, so small, it’s not going to make any significant difference. But we technically need to be correct.”
Though this ISO has been around for many years, the ISO committee still makes improvements, such as the latest revision, ISO 4309:2010, to refine the values and calculation method for the minimum rope diameter.
The slimming of rope that the revision targets is caused by external and internal wear and abrasion. The most critical area on a rope to examine for decreased diameter is the area on the rope that runs over steel sheaves and/or spools on single layer drums.
“This is about uniform decrease in rope diameter due to external and internal wear and abrasion.
“The wires are rubbing on each other and the rope is rubbing on the steel sheave,” says Hewitt.
The ‘uniform decrease in rope diameter’ is the name for this slimming of diameter.
While the old standard that would have measured this slimming as a percentage decrease in rope size from the nominal diameter, which is the diameter of rope in the manufacturer’s catalog, the new standard requires the customer to use a ‘reference diameter’ when calculating it.
The ISO committee has created this new term, ‘reference diameter’ because ropes vary slightly during the manufacturing process.
Hewitt says, “The reference diameter is always larger than the catalog value due to the manufacturing tolerances. ‘Manufacturing tolerance’ means that we say the rope is 22mm diameter, but when that rope is made, it can be anything from 22mm to 22mm+5%.
“So it could be 22mm +1% or 22mm +3%, but it should never be smaller than 22mm.”
The ‘reference diameter’ is a measure of the diameter of the new rope taken by the customer immediately after being broken in.
If the rope is already in service, the reference diameter may also be taken from a section of rope that does not pass over sheaves or spool on the drum, normally a section of rope towards the outboard anchor.
An indication of the rope’s actual diameter when manufactured can be seen on some suppliers’ test certificates.
Kirkham explains this stretching. “It is recognised that after installation, and once a loading cycle has taken place, the rope diameter will normally change from the nominal stated on the test certification.
“Therefore the allowable reduction in diameter is based upon a reference diameter that should be recorded after the rope has been installed.”
Most inspectors didn’t realise they had to take this second measurement.
Hewitt says: “People weren’t taking into account the difference in the rope when it was being manufactured. It was a pretty common mistake.”
Another change reflects not tiny differences in manufacturing, but the behavior of materials within different designs of rope.
The original standard referred to all kinds of rope when it pegged the unacceptable level of diameter reduction for an abraded rope at 7% for the nominal rope diameter.
However, the new standard gives a table containing different discard criteria diameters for different types of rope, making adjustments to reflect the use of fibre core ropes, which are able to maintain their durability while stretching thinner.
For example, the uniform diameters of single layer ropes with fibre cores are allowed to decrease up to 10%, single layer ropes with steel cores or parallel closed ropes are allowed to decrease up to 7.5% and rotational resistant ropes should only be allowed to decrease up to 5% of their nominal diameter from the reference diameter.
Kirkham continues: “The new standard recognises the fact that different constructions of wire rope are expected to reduce in diameter by different amounts, and there is now different criteria for ropes with a fibre core, ropes with a steel core and multi-layer low rotational ropes.”
However, don’t throw away your multi-layer drum ropes using this criteria.
“Any piece of rope working on single layer drums is measured and discarded in this manner, but it’s not applicable to that piece of rope that goes on multi-layer spooling drums,” says Hewitt.
Along with fine-tuning this diameter measurement, the ISO committee gives a new Severity Rating percentage within a table, with columns for percentage diameter reduction and the rope type.
The Severity Rating table can be used to give an indication of how close the rope is to a point where the examiner must discard it.
For all types of ropes, a Severity Rating of 20% is ‘slight’, 80% ‘very high’ and 100% means that it is time to ‘discard’.
Rope Category Numbers and Cumulative Effects
Distributors of ropes and training are now changing their catalogues to include a Rope Category Number (RCN) for each rope, a figure based on rope type provided by the manufacturer.
The RCN can be used in other calculations.
Supplying rope and rope inspection in addition to crane services, Certex now gives the RCN to buyers of its rope products.
“Suppliers of crane ropes should be informing their customers of the RCN, so that the correct discard criteria can be used,” says Kirkham.
He adds, “This defines ropes by their construction and number of load bearing wires in the outer layer of strands. Some of the discard criteria is then based around this RCN.”
Kirkham says, “One of the other significant changes is the formalisation of a method of assessing rope deterioration known as the cumulative effect.
“This method may be used due to the fact that a rope deteriorates in a number of different ways and this can sometimes happen in the same portion of rope.
“The standard now recognizes this and gives advice as to the way we can assess the ropes percentage towards discard.”
Carl Stahl Evita, which has for years provided a two-and-a-half-day Wire Rope Inspection Course in Rotherham, UK, has recently updated its courses to take into account of all of the changes which have been introduced in the new standard, and offer a shorter course as a referesher course for returning graduates on the new method of measuring rope diameter.
Roy Fulthorpe, a manager at Carl Stahl Evita, says: “By making this calculation and referring to relevant tables within ISO 4309:2010 the examiner can determine whether the rope has reduced in diameter sufficiently to trigger discard.
“In the event that the diameter has reduced, but not to a level which would justify full discard, the examiner can allocate a percentage ‘severity rating’.”
Fulthorpe says this training allows examiners to prove dicardability to clients. “This is important as it allows the wire rope examiner to support his judgment with quantitative information based on actual measurements.”
Certex, a global training provider with offices in the US, UK and Europe is also offering courses on the update to the standard: a day-long rope examiners’ course, or a four-hour session designed for graduates.
Attendees of Certex’s training may have the good fortune to be in a class taught by Ray Allen, who is also the current chairman of the ISO 4309 committee.
To help their customers attain rope examiners’ certification, Bridon also provide a two-day rope examiners course at their manufacturing base in Doncaster, while offering tailor-made courses worldwide.
With the latest revision, the rope inspection industry faces a large-scale training challenge that will lead to improvements in safety in the years to come.
Hewitt notes that the use of large ropes will continue to shape inspection practice.
“I think it is getting more precise and I think it’s becoming more critical with the large and longer lengths of ropes now being deployed on cranes,” he says, adding, “Crane manufactures are simply building bigger cranes.”
Although this applies to all crane industries, it is particularly important to the offshore industry because of its tendency to cycle load.
Hewitt says, “Wire rope inspection is particularly important, and the knowledge of wire rope inspection is particularly important when ropes are being cycle loaded for long periods.”