I wrote in the January issue, you may recall, about the jargon we use to describe our machines and expressed mild disapproval of the term pedestrian operated tower crane, or POT crane.
I am prompted to return to the theme by the prevalence of cranes – and I think it is primarily an issue for telescopic mobiles – that are unable to lift (at least with the required margins of safety) their official rated capacity. I have no doubt you know exactly what I mean, the so-called 200 tonne crane whose actual physical lifting capacity, according to DIN/ISO principles, is only 150 tonnes. The manufacturers get away with calling it a 200 tonner because with a special attachment added, it can just about lift 200 tonnes over the rear.
It may seem a bit of a con trick at first sight but I don’t entirely blame the manufacturers. The categorising of cranes by capacity needs to be simple, and therefore is by necessity rough and ready. The headline figure is generally an accurate representation of the capability of the crane throughout its entire chart, and if a new model does not actually lift 200 tonnes but throughout its chart is clearly a far stronger crane that an alternative 180 tonner, say, then it seems entirely reasonable to say that, for all practical circumstances, the new machine competes in the 200 tonne capacity class. After all, how often is a crane needed to lift something at just 3m radius to the height of only the retracted main boom? The whole point of telescopic cranes is to reach up and out, and it is at distance that a crane’s capacity should really be judged.
It is for this reason that there is a body of opinion that believes it would be more useful to categorise mobile cranes by their maximum load moment (tonne/metre), as tower cranes and knuckle boom truck loader cranes are. For all its faults, I prefer the simplicity of the current system. Besides, the manufacturers all play the same game – their customers understand it, and so the playing field is level.
Where I do draw the line, however, is for the crane hire companies to add another 15 or 20% to the given headline rated capacity to extract rental revenue from their customers. This seems particularly prevalent at the top end.
Innovations in crawlers
I have been doing some research into how easy it is to assemble large crawlers. A couple of years ago, at an open house at its factory, a small group of Demag technicians unloaded and assembled a 300t-class CC 1800 in just 87 minutes. It was very impressive demonstration. I have seen similar feats achieved at other factories. While such displays demonstrate the great strides all the manufacturers have made in making crawler cranes easier to deploy, they bear little resemblance to what happens on site.
Even in the USA, where there is supposed to be more room on sites, there is almost never space to lay all the boom out. I spoke to two of the biggest crane owners there who both told me that, while some of the self-assembling features are very helpful, they always had to send an assist crane along as well.
More significant than self-assembly, therefore, is the ability to minimise the amount of setting up and stripping down on site. There are some really interesting innovations here.
Visitors to Bauma ’04 were arrested by the sight of the 400t-class Liebherr LR 1400/2-W, which has two slew rings and narrow tracks to negotiate tight corners around wind farms and so drive from turbine to turbine.
In the same class and for the same market, the new Manitowoc 16000 has an optional upper boom point that just pins on. It does away with the need for de-reeving and reeving between each lift. Very cunning.
Most recent of all is the twin boom version of the Demag CC 8800, doubling its capacity to a thumping 3,200t. It offers an alternative to ring mounted cranes that take three to four weeks to erect. Not only can it be erected much quicker (six shifts with a crew of 20, the manufacturer estimates) but, as a fully mobile crawler, it can be set up off site where there might be more space and then driven to the point of use fully equipped.