Lifting empty hooks or failing to use lower layers can play havoc with your rope drum. Cris Seidenather, managing director of Lebus International Engineers GmbH, offers some remedies
The Strong Man at the circus withstands punches to his stomach by tensing his muscles. With this tension, the ability to absorb pressure is significantly increased. This is well known to civil engineers, who use concrete that is prestressed and post-tensioned for extra structural strength.
Tensioning is just as important in wire rope that is spooled around a winch drum in multiple layers.
It is one of the basic requirements for multi-layer spooling that the wire rope is initially installed on the drum under tension. Without tension in the rope, the lower layers are unable to withstand the impact of the pressure exerted by the upper layers as the drum is loaded with rope.
Even if the rope has been installed correctly there are certain applications where this is not enough. Usually cranes are used for lifting heavy objects. The rope unspools to lower the hook to ground level, a heavy load is attached to the hook and, as the load is raised, the rope spools onto the drum under the tension of the downward force exerted by the load.
Sometimes, however, the crane may be used to lower a heavy load - perhaps placing tunnelling machinery down a shaft or to lift off the blades of a wind turbine for repair. In such applications, force is applied as the rope is unspooled but, when re-spooling back onto the drum, there is only the weight of the hook block and tackle acting on the drum. This does not normally generate sufficient tension in the rope as it goes back onto the drum. This slack spooling is the crane operator's enemy. It leads to the rope snagging, cutting in and becoming damaged.
Slack spooling prevention
There are several options to prevent slack spooling in applications that lift empty hooks, none of which is perfect, but all should be considered.
- Get a heavier hook
A heavier hook obviously uses up more of the system's lifting capacity but, if you have some to spare, it can help to prevent slack spooling. Alternatively, consider adding weighted plates to the hook block to put extra tension in the rope.
- Get a bigger drum
Although this is unlikely to be a realistic option in most applications, a bigger rope drum - either longer or fatter - means that the rope can be spooled in fewer layers. A theoretical ideal is to have the drum big enough to accommodate your entire rope in a single layer. The fewer the layers, the less scope there is for damage on lower layers. A fatter drum increases the bending radius, which reduces the amount of pretension required. Still, 0.75-1% of the nominal breaking load at a safety factor of 5:1 is necessary to avoid damaging the rope.
- Use the ultimate non-crushable rope
Sadly, this rope has not yet been invented. Wire rope manufacturers are continually making advances in the crush-resistance properties of their special ropes, which help significantly in preventing the damage caused by slack spooling. But even the best of these is not absolutely non-crushable, especially if there is insufficient tension in it.
- Always spool under tension
Some crane owners, in certain applications, have found ingenious solutions to avoid lifting an empty hook. One of these, for example, is to put water bags on the hook at ground level and then empty the bag when the hook is at full height and the rope fully spooled onto the drum.
- Minimise dead turns
For safety, it is always necessary to have dead turns that never spool off the drums. These should be kept to a minimum, however, and spooled with maximum possible tension at the outset.
A related problem to spooling with an empty hook is only using the upper layers of the rope. For example, a tower crane on a high-rise building will conduct many lifts using just a fraction of its wire rope before being jumped or climbed to the next height as the building progresses. Only when the crane reaches its full height will the full length of the rope come into play.
A customer came to us recently with a similar problem, although this involves just a winch rather than a crane. The application here was the construction of a 6 kilometre tunnel. In such projects, as the tunnel boring machine (TBM) progresses underground, a winch at the opening of the tunnel pulls the trucks that carry excavated spoil out of the tunnel for disposal.
The 2,600 mm wide winch drum holds 3.8 km of 43 mm diameter wire rope in eight layers. In the early stages of construction, the winch needs to let out only short lengths of rope. As the TBM progresses, the distance travelled by the spoil trucks increases and more rope comes into play.
Just like the climbing tower cranes, this is another example of heavy loads being lifted using only the upper layers of the rope until later stages.
A good solution, in both examples, is to use three separate ropes for different stages of the project: a short rope; a mid length rope; and a long rope, so that throughout operations, a much larger proportion of the rope is being unspooled form the drum and spooled back onto it under tension.
In the tunnelling example above, this is exactly the solution adopted after it was proposed by Lebus to the winch supplier Paul Reber AG of Switzerland and the TBM manufacturer Aker Wirth.
While this may seem more expensive than relying on a single rope, the extra cost of the additional ropes is far outweighed by the cost of mis-spooling bringing a project to a standstill, and then still having to buy another full length rope.