A magnetic attraction to safety
Magnetic Rope Testing (MRT) is a great example of how technology can play a vital role in improving lifting safety, says Keith Tonge, Technical Officer at LEEA.
Regular inspection of a crane’s structure, its sheaves, the hook and block mechanisms are just some of the areas of focus for maintenance programmes. Wire ropes should also be a key part, yet too often this element is overlooked. Neglecting proper maintenance and inspection of the crane rope means the risk of missing internal degradation may lead to rope failure with potentially catastrophic results.
Many operations still rely only on visual inspection of wire ropes. However, plant engineers should consider the crucial role Magnetic Rope Testing (MRT) can play in maintenance – particularly on safety critical crane installations that are being used intensively throughout their working life.
MRT gives a view into the heart of a wire rope used on cranes in order to detect any deterioration that might have occurred in service. It gives plant engineers the ability to predict the life expectancy of the rope, allowing them to plan replacement in a just-in-time manner, without the need for bulky wire rope hogging storeroom space.
The technology involves passing the rope through a permanent magnet. This sets up an electromotive force, which is picked up with electronic sensors that can detect any breaks in the rope or any corrosion that occurs throughout the section of the rope, which is known as Loss of Metallic Area (LMA). There are different MRT equipment manufacturers but the method is exactly the same with each one, giving clear visibility of any broken wires, voids and corrosion.
To examine the core of the rope without MRT, is a time consuming process. On multi-strand crane ropes, you can never see the core because the multiple layers can’t be opened up due to the underlying layers being laid in the opposite direction to the outer strands. A rope may outwardly look perfectly OK, but if its core is utterly degraded through fatigue failure, it can drop a load. This failure can, however, be detected quickly using MRT, preventing the risk of accident.
Early MRT machines were cumbersome with the recorder and oscilloscope almost filling an entire room, which didn’t make using MRT easy for maintenance of wire ropes installed on cranes. However, advances in technology have made the technology eminently portable – it can even become a fixture of the crane itself in some cases, allowing the rope to be constantly monitored as it is used. An MRT unit today can send a signal directly to a manager’s computer screen to give a visual representation of how the wire rope is deteriorating over time.
The technology has been incorporated into ISO 4309 Cranes – Wire ropes – Care and maintenance, inspection and discard as a method of examination of in-service cranes ropes, but only as a supplement to visual examination.
MRT is, however, a vital tool for safety and maintenance systems and has become crucial for routine monitoring of ropes particularly in applications where minimising costly downtime is particularly critical – such as offshore.
Many safety critical cranes will use MRT to produce a ‘baseline’ as a ‘birth certificate’ for a length of wire rope. So when it enters service, the engineer knows precisely what that rope is like at the outset and can then accurately monitor any deterioration throughout its service life. When the rope gets to the point of rejection criteria, managers can plan in the change without fear of rope failure, maximising uptime in busy operations.
When it comes to the provision of lifting equipment maintenance and repair services it is always prudent to seek out the ‘gold standard’. In addition to offering expertise on MRT and condition monitoring of wire ropes, LEEA members are structured to provide specialised repair and maintenance as well as the inspection function, as separate parts of a one stop shop, in compliance with best practice.
