Visual inspections and internal breaks in wire rope
Thursday, 2 July 2020
Even the most rigorous inspection regime for wire ropes on cranes could leave you exposed to risk if visual inspections are the only methods used. During a visual rope examination, only the condition of the visible parts of the outer wires can be evaluated. Unfortunately, the metallic cross-section of the outer strands only represents about 40% of the metallic cross-section of the rope, and only about half of the length of these wires is visible as they twist from the inside to the outside.
This means that during a visual rope inspection, we can only examine the condition of 20% percent of the metallic cross-section of the rope, and we can only assume that the other 80% is in good condition. Quite often, the visible 20% looks good, while a great number of wire breaks are concealed in the invisible part of the rope. Wire ropes with internal wire breaks and no external signs of damage are extremely dangerous.
How can we deal with this situation?
One way is to frequently replace your ropes, but a wire break can occur even on the first lift. The ideal way is to introduce electromagnetic wire rope testing into your testing program, also known as non-destructive wire rope testing, or NDT. These tests involve running the rope through a specialised instrument that uses a sensor to induce a magnetic field across the rope as it passes through. Changes in the magnetic field are recorded using a computer and these are then interpreted by a specialist inspector who can pinpoint where single or multiple breaks exist inside the rope, or other issues such as internal abrasion and corrosion.
In Australia, these tests are undertaken by specialist testing contractors, consulting engineers, and wire rope suppliers, preferably who are in turn independently tested and accredited to undertake the tests by the national testing body, NATA. These tests can be undertaken offsite during repairs or routine lubrication or where this may not be practical, can be undertaken onsite. One of the key benefits of offsite testing is that the full length of the rope can be checked; identifying issues at the rope ends that may not be able to be discovered while the rope is installed and terminated.
Loose strands and birdcages
Sometimes, rope deformations such as loose strands or birdcages can be found on crane wire ropes. They are often attributed to shock loading (for more information on shock loading – read our article ‘The effects of shock loading’), but this may not always be the case. In most cases, loose strands and birdcages are generated by twisting a wire rope around its own axis. For example, a rope which is fixed at both ends, if we take this rope and twist it once, we will lengthen it on one side and shorten it on the other, resulting in the outer strands being too long on one side and the inner strands being too long on the other, thus causing the wires to ‘stand up’ or ‘birdcage’.
But what could cause a rope to twist on a crane around its own axis? Every sheave and drum. If a rope enters a sheave under a fleet angle, it will first touch the flange and then roll down into the bottom of the groove, twisting the wire rope slightly every time.
With increasing fleet angle, the amount of twist increases. If the rope enters the sheave at a fleet angle of 1° it will touch the flange in a very deep position and will only be twisted by 5°. If the rope enters the same sheave at a fleet angle of 5°, it will touch the flange at a very high position and will be twisted by up to 50°.
Of course, a wire will not always roll down the flanges – the downward movement will be a mixture of rolling and sliding. Therefore, the amount of twist induced into the rope depends on the amount of friction between the rope and the sheave.
So, how can we address this issue?
Keep ropes lubricated
The easiest way of reducing the friction between the wire rope and the sheave is to lubricate the rope. A well lubricated rope will twist much less than a dry or corroded one, as there will be more ‘slide’ and less ‘roll’ into the sheave grooves.
Avoid using plastic sheaves
Another solution is to use steel sheaves instead of plastic sheaves wherever possible. The coefficient of friction between steel and plastic is higher than between steel and steel. Under the same conditions, a wire rope will therefore be twisted much less by a steel sheave than a plastic one. Plastic sheaves should only be used in positions where the fleet angles are small.
Use wider grooves in sheaves
If the groove of a sheave is wide, the wire rope will enter much deeper in the groove before touching the flange and rotating. As such, sheaves with a groove angle of 45° are recommended, and angles of 52° are even better.
Use wire ropes with an internal plastic layer
Wire ropes with an internal plastic layer are much more resistant to birdcaging than any other rope design.
Avoid tight sheaves
Loose strands and birdcages can also be the result of tight sheaves. If the groove radius is too small, the rope will be compressed when travelling over the sheave. As a result, the outer strands will be too long for the reduced rope diameter, and the extra length will be pushed along to one point where it will stand up as a loose strand or birdcage.
The groove radius should measure 0.53 to 0.54 times the nominal rope diameter. If it is less, the sheave should either be re-grooved, or replaced. If this is not possible, a rope with a smaller diameter could be used which will then comply with the required conditions
Nobles have a team of highly qualified and professionally trained technicians who are experienced in wire rope inspections, lubrications and non-destructive testing. To have your equipment inspected by one our lifting and rigging specialists, please call us on 1300 711 559 or send them an email at firstname.lastname@example.org.
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