22 May 2008
In Lewis Caroll's novel Through the Looking-Glass, the Red Queen remarks that in her country “it takes all the running you can do to keep in the same place.” Well, maybe she was talking about the crane industry.
Rope makers had just begun to understand the fatigue mechanisms in reeving systems and come up with fatigue-resistant ropes when the crane industry invented a new rope killer: drums with multi-layer spooling.
In the good old days, when ropes were spooling on single layer drums, they were guided by a smooth groove and supported on about 130° of their circumference.
Today, almost every tower crane and every mobile crane has one, or several, multi-layer drums. In the second layer of a multi-layer drum, the rope is no longer guided by any grooves. It must find its position by scratching along a neighbouring wrap, getting the first surface damages in the process (Figure 1).
Then it lays down on a rough surface created by the first layer underneath it. The high bearing pressures at the small number of contact points creates additional surface damage (Figure 2). After that the next wrap will come in, damaging the rope section even further (Figure 3). Finally, a third layer will damage the rope on both sides at the top (Figure 4).
The scratching along the neighbouring wrap creates very high local temperatures, which will form martensite at the wire surface (Figure 5). Martensite is very hard and brittle, and martensite formation will lead to fatigue cracks and premature wire breaks in the rope.
At the crossover zones, conditions are even worse. The rope will no longer be supported on two sides, it will be forced out of its bed in order to cross an underlying wrap. This mechanism leads to very severe local rope damage (Figure 6).
Multi-layer spooling also created other dangers unheard of in the good old days of single layer spooling: If the rope was spooled onto the drum with no or only low tension and then a load was picked up at a high elevation, the rope would be pulled into the lower layers and get trapped. When later the rope was spooling off the drum, the trapped section would not come off, suddenly reversing the direction of load travel (Figure 7). More than one rope has broken under such circumstances, and more than one crane has tipped over because of the impact. And more than one drum has broken off its flanges under the wedging effect of the rope pulling in.
The working environment of the ropes had changed, so the ropes had to adapt to the new conditions.
In the good old days of the single layer drum, every crane rope was made in regular lay. On multi-layer drums, however, the outer wires of regular lay ropes would indent with the neighbouring wraps, severely damaging the rope surface when the rope was spooling on and off the drum (Figure 8).
In Lang's lay ropes, the outer wires cannot form indentations (Figure 9). Therefore, today, a vast majority of crane ropes spooling on multi-layer drums is Lang's lay.
This improved the situation, but drum crushing continued to be the main reason for discarding ropes operating in multi layer spooling. A plastic layer between the steel core and the outer strands helped stabilize the rope geometry (Figures 10-13), but the ropes still suffered from their fatigue resistant design in an environment that asked for robustness.
To improve the ropes' resistance to drum crushing, rope manufacturers finally developed ropes with compacted outer strands and with bigger and more robust outer wires (Figure 10).
Another approach was to swage the entire rope to smooth its surface (Figures 11 to 13). A smooth cylindrical rope surface avoids any indentation between neighbouring wraps and seems to be the best remedy against drum crushing.
The swaged rope designs have a very high fill factor, leading to extremely high breaking strengths. Crane makers love rope breaking strength: they cannot have enough of it. The high radial stiffness of the swaged ropes ensures the stability of the multi-layer package on the drum, reducing the danger of pulling the rope into the lower layers. And the smooth rope surface reduces wear and avoids negative imprints in the sheaves of the reeving system.
Crane technology develops fast and so does rope technology. Swaged ropes are certainly not the end of the story. Hybrid ropes with high strength fibres in the centre and steel strands outside are already waiting to be tested on multi-layer drums. We will keep you informed.