TWO HOOK TUSSLE

One of the more often debated issues is whether or not two hoists should be used at the same time for turning a suspended object.

Opponents of this technique argue that safety is jeopardised when using out of vertical lines. Others say that it is quite safe, and is a cost effective way of using a crane. But what is actually the truth? And what do crane manufacturers say about what you can do with their cranes? In Germany it is legal to use two hoists simultaneously, although the general perception among the crane rental companies seems to be the opposite, and the operators coming north of the German border are surprised to see how frequently it is done in Scandinavia. This is interesting, as most cranes operating in Scandinavia are made in Germany. When ordering a crane, a Danish customer will specify two hoists as standard, and a tip extension or jib capable to lift anything from about 5t to 15t, for handling concrete panels. These panels can easily weigh 25t or more and, needless to say, it requires a very strong jib or tip extension to do the job.

The length of the panels – or height when turned to vertical – can be as much as 16m or 17m, so a crane of at least 140t (telescopic boom) or 100t (lattice boom) is needed to lift the elements from the truck. For a 16m panel, a radius of at least 15m is required by the boom tip in order to reach the middle of the panel. At a weight of 25t, this is a delicate task, and yet it is done hundreds of times every day.

The German manufacturer is asked to deliver the crane to meet these requirements, and has the opportunity to say yes or no.

In the UK, it is not strictly legal, but then again it is not actually illegal either. It seems that nobody can say for certain. The practice has certainly been used (in construction work in London Docklands, for example), but it is not popular with the legislators.

Peter Oram, a regular Cranes Today contributor, crane safety consultant and former government safety inspector, is a fierce opponent of using two hoists simultaneously. He warns that lifting out of vertical line with the crane will damage the boom, lines and sheaves, and thereby lead to accidents. This view is supported by Dennis Eckstine, senior director of product safety at Grove Cranes. Eckstine believes that when carefully examined, the method could perhaps be performed as an engineered lift, but he is concerned about the possible damage to the crane. Also, he adds, there is the possibility that the operator makes an error, by placing more than half the load on the tip extension, and thereby overloading or overturning the crane.

The trend is popular in Canada but Grove is not keen, as twin hoisting has led to accidents, Eckstine says. He is surprised to learn, however, that Grove and other US-built cranes are being used to perform this kind of lift even though they are not designed to.

Another concern is that the LMI system may not give the right reading, thus leading the operator to believe that he is in a safe working area, when he is actually overloading the crane.

Talking to Ron Kohner of Landmark Engineering reveals that in the USA it is actually legal to use two hoists to turn an object in mid air, but the trend is not widespread, he believes. He foresees two main problems, one being that the crane has to have either twice the normal amount of horsepower, or one hoist – preferably the main – in a higher amount of falls in order to keep up sufficient pump pressure to actually lift the load. This falls well inside the example shown in the pictures, although the amount of falls on the main hoist is exaggerated. Three falls would be sufficient. But it gives rise to some questions: Do US manufacturers not anticipate the possibility of using two hoists? Maybe this is one reason why American makes are in the minority in Europe, since the German manufacturers design their cranes for this specific feature.

Or is it the mere possibility of a fatal operator error that gives the manufacturers second thoughts, fearing product liability litigation? In Germany, Liebherr takes a bold approach and simply designs its cranes to meet the forces imposed on the boom, although Hans-Jörg Kohn of Liebherr backs Eckstine’s warning that the LMI system could give false readings.

This is a minor problem though, as the load lifted shall and must always be within the rated capacity limit. This remains the sole responsibility of the crane operator, who alone determines when the lift is safe and when not. Therefore, says Kohn, almost all accidents, regardless of nature, are due to some kind of operator error. Liebherr actually gives guidance to two hoist lifting in its operators manuals.

Kohn also states that all Liebherr cranes going to the Asia Pacific region have two hoists fitted as standard. Two hoist lifting is so commonplace there that it is hard to sell a crane with just one hoist. Thus such Asian cranes as Kato’s come with two hoists as standard. Kato’s European distributor AB Kranlyft in Sweden enjoys good sales, in part, because the customer has no need to fit a second hoist on his new crane.

What is the final outcome of this debate? Should it be illegal to use two hoists. Is it in fact dangerous, putting both the crane and crew at peril? Or is it a very elegant and timesaving operation, that also saves the contractor a substantial amount of money? Certainly the latter point of view will be argued by the building industry.

And the fact that at least some of the manufacturers take into consideration that the crane should be able to perform this kind of work, encourages the view that the safety of two hoist lifting is simply a question of design.