Swaged ropes on multi-layer drums28 July 2011
Roland Verreet, founder of Wire Rope Technology Aachen in Germany, presented research at the recent 16th North Sea Offshore Cranes and Lifting Conference recommending the use of compacted strand, swaged Lang’s lay ropes on multi-layer drums to increase rope life.
Verreet identifies an increasing trend to use multi-layer drums for longer rope lengths which causes damage both to rope and to drums. “The world of drums can be divided in to single layer and multi-layer drums, and I would say into drums with small problems and drums with big problems,” he says. This is because of the amount of abrasion caused by rope on rope contact.
Verreet finds that when flattened, swaged ropes are used the result is 50% more surface area touching the sheave, which means a corresponding decrease in bearing pressure. Where the contact increases, bearing pressure decreases, leading to longer rope life.
He gives the example of a steel ball that lies on a steel plate. Even for a large ball the area of contact is still very small, which multiplies the bearing pressure with rope size and leads to greater abrasion.
“In conditions where round objects touch something else, the increase in bearing pressure is far higher than the linear increase in size and that is the problem with multi-layer spooling,” says Verreet. “A rope arrives, comes down scratching against the neighbour and finally has been damaged on all sides during only one bending cycle.”
Particularly for large diameter ropes, this can be a peril. Verreet cites an example of a rope on a ship crane which was damaged by an overload test, so much it needed to be replaced. The new rope, however, had to be subjected to the same overload test, leading again to the discard of the rope.
Swaged ropes are used to decrease bearing pressure. “If you have round elements it doesn’t help you because the contact areas will always remain small. You must flatten the surface of the rope, you have to increase the contact area.
“I made a few tests where I put rope around the sheave with carbon and white paper in between and I loaded the rope to get the imprint,” says Verreet. “I used my digital microscope to calculate the relationship between the white and black areas, and then I knew all the statistics. This was done in 10 minutes. And then I knew exactly what the footprint of the wire rope is in the sheave.”
He compared different ropes with compacted strands. With an increasing number of strands the area of contact increases. “17 or 100 strands won’t make a difference anymore, but increasing the number from six to eight strands gives a tremendous improvement. If you use a swaged rope then you get rid of the round surfaces. The same is true for rope touching other rope on a multi layer drum,” says Verreet.
Another way to ensure lower abrasion on the drum is to keep the load continuous rather than variable, Verreet says, by lifting from and to varying altitudes, ensuring that every time different areas of the rope are sliding against the lower layers or the drum body rather than the same ones over and over. Grating the rope against the drum body, always at the same location, can even cause the rope to cut through the drum barrel.