In a normal lift operation, a crane has to roll much of its rope off the drum to drop the hook down to the ground. Then all of this rope tightens as the load rises and it spools back on the drum under tension.
Lowering jobs work differently. For example, consider a crane that is replacing all three blades of a wind turbine. First, the operator raises the boom to the height of the nacelle, hooks up a blade and lowers it safely. The lowering operation goes smoothly, because the rope was stored on the drum under tension. Then the crane raises the hook to attach the next blade. This is where the problems start, say experts. “The crane is winding the rope under no tension, except the weight of the rope and the block. Then it lifts the second blade on a full drum of slack rope. Little or no tension is a recipe for disaster. It destroys the rope, it crushes it,” says David Hewitt, crane technical manager at wire rope manufacturer Bridon.
Most crane ropes stretch a little bit under tension, which means that they get thinner. It is easier for a crane rope under tension to slice in to a lower layer of wound rope. And because the rope layer on the drum is slack, it yields to the force of the tense rope; it cannot push back. As the tensioned rope cuts in, the spooled rope’s outer layers are destroyed, and lifting suddenly stops.
The shock can destabilise a load, potentially enough to even knock the crane over. The ensuing snarl-up, at least, stops lifting work until a replacement drum of rope can be arranged. Ropes cutting in have even forced the flanges off the rope drum, says rope designer Roland Verreet, of Wire Rope Technology.
Lowering operations work against the fundamental design of the crane, says Hewitt. “The application dictates that the rope on the lower layers of drum installed have very little or no tension. The way the crane operates, the rope on the top of the drum actually starts to take high loads. This is the reverse to the way mobile cranes were envisaged to work,” he says. “In very old cranes, all you had was a single drum, which had a big diameter, and the wire rope was laid in a single layer, so it didn’t contact itself,” he says. “Over the last 30 years the applications to which we use cranes have changed. Those demands are for longer lengths of rope on smaller-diameter crane drums.” In other words, lowering loads with modern cranes-without raising them first-can be risky.
The offshore solution
Hewitt continues: “The crawler crane market is just experiencing this problem with wind turbines. But the offshore industry has this problem all the time: it wants to lower loads to the seabed.”
Offshore cranes may need to store 2.5km (1.7mi) of wire rope on a drum. “For deep water operations an enormous amount of rope needs to be stored on a normal winch, with many layers and many windings per layer. For such situations proper spooling becomes more and more difficult,” says Eric Romeijn, technical manager of the mechanical design department at Dutch offshore barge crane manufacturer Huisman-Itrec. “If spooling problems occur, the loss of operation time and even the risk of damaging the rope may be huge.”
Like other barge crane makers, Huisman-Itrec has installed a different kind of winch to deal with these problems. A construction cranes lowers a load by paying out rope from a single moving part, the drum. Offshore winches come in two pieces: a large storage drum and a friction winder (also called a double-capstan winch). In a lowering operation, the rope rolls off the drum and then snakes through a series of pulleys inside the friction winder. An individual motor powers each pulley wheel. As the rope winds through the reeving, it gets more and more tense, until it emerges on the load side taking full line pull. During a lowering operation that would normally have too low a rope tension, the friction winder’s wheels can work in opposition to each other and maintain the required minimum tension on the wire rope as it winds back on the drum.
Verreet of Wire Rope Technology and Hewitt at Bridon both argue that construction cranes would also benefit from these winches because they would help reduce the risk of drum crushing. Hewitt says that for him, moving to a double-capstan winch is the ‘only solution’ to the problem. Verreet says that friction winders pay out rope much more smoothly than drums, which can shudder when unwinding rope drops down on to the next layer on the drum.
On the other hand, friction winders do bend a wire rope many times, and all of that bending can wear out a rope. Robert Traxl, head of research and development at wire rope maker Teufelberger, says that in his experience, the fatigue caused by the double capstan winch does as much damage as rope crushing in drums. He said that Teufelberger has supplied rope for winders that pull snow-grooming equipment up mountain slopes. The winders come with either normal rope drums, or with friction winders. Teufelberger has supplied both, and has compared the lifetimes of both ropes. The result: no difference in rope life, he says.
Hewitt and Verreet both disagree with this conclusion. They say that even with this wear, ropes last longer in a friction winder. Verreet says: “Many people think that going on and off a multilayer drum is one bending cycle. The same rope will make 10 bends going through a capstan. But the one bending cycle in a multilayer drum will damage the rope a lot more than going through the double capstan with maximum line pull.”
In any case, even Traxl admits that double capstan winches may be the only option for construction cranes for some applications: “It might be that there are some applications, such as dismantling wind turbines, where it is impossible now to do it with drum winding. Then the friction winder has the benefit of making it possible.”
The onshore solution
Crawler crane manufacturers are looking at double-capstan designs, Verreet says. “Everybody is thinking about it. I know of crane makers who have employed students to work on capstan winch design.”
Manufacturers were not quite so willing to share their trade secrets with Cranes Today, however. Terex-Demag lattice-boom crane product manager Ruediger Zollondz said that the issue of rope tension and lowering is not a major priority for the manufacturer. He added that he could not comment about whether the company is developing a double-capstan winch system for its cranes. Manitowoc declined to comment entirely. Kobelco said it pre-tensions ropes at the factory, and helps users pre-tension ropes at job sites.
Liebherr seems to have moved away from friction winches, but has developed other ways of pre-tensioning the wire rope. Liebherr Ehingen design engineer Hans-Dieter Willim said that for all terrain cranes, the company is not looking into double-capstan winches, partly because of the amount of wear on the rope. He continues: “To reduce the wear of the rope, we would have to use a winch with two drums consisting of several, independently-driven sheaves, where each sheave has got different variable speed according to the elongation of the rope, which depends on the rope pull. This is very complex, costly, and requires a lot of space.”
Liebherr received two patents in 2004 and 2005 for devices that brake the wire rope as it spools on to the drum to create the necessary tension. The first system runs the rope through a pair of rollers that freewheel as the rope lowers, but brake the rope as it runs the other way. The braking force-created by hydraulic cylinders or springs-is adjustable by a set screw. This device is shown mounted near the tip of a crane’s telescopic boom. The second system does what appears to be the same thing by running the hoist rope around a couple of wheels. It is shown mounted near the tip of a lattice-jib extension of an all-terrain crane with telescopic boom.
However, Liebherr is not commercialising either of these ideas, because it is currently working on a third, which, Willim said, it does not want to publicise until it is tested and patented.
It may be that this issue is not yet urgent, and perhaps it will be 10 years before manufacturers come out with a double-capstan winch. But in the meantime, more and more wind turbines are being installed, and their generators continue to grow larger and heavier. This problem is unlikely to go away.
