Cranes are always designed for a specific ultimate load range. The lifting of heavy ultimate loads is then potentially not possible with such a crane. For lesser loads, such a crane is not economical, because the purchase and operation costs are higher than the costs of a smaller crane. Nevertheless, in order to lift increased loads without having to purchase a larger crane, it is already known to lift a load with multiple cranes simultaneously. However, the necessary safety is ensured only under specifically defined conditions with such so-called tandem or multiple-lifts, as a rule, due to the problematic load distribution or the fact that the crane is equipped with individual controllers. Consequently, the load to be lifted with such tandem or multiple-lifts is often permitted to be only so high that maximally 60 to 80 percent of the load carrying capacity is used.
It is already known from GB 1,162,405A to connect two cranes by a portal, which is affixed to the free ends of each boom, in order to lift heavy loads. The problem with this arrangement is that the position of the two cranes is not fixed relative to each other and thus it results in constraining forces, whereby the load must be reduced.
A double portal crane for bulky loads, in particular a container crane, is known from AT 28 28 76. This double portal crane comprises two separate, movable crane portals. Each crane portal carries a crane trolley, preferably designed as an angled trolley, with its lifting unit. The two crane portals are latchable with each other at specified intervals. The trolleys are independent from each other in a known manner and are also synchronously movable with each other by an operator's stand. According to one exemplary embodiment, one drive train can also comprise only one of the crane portals.
The ring lift crane shown in U.S. Pat. No. 6,516,961 B1 comprises, in one exemplary embodiment, a parallel connection of two lattice booms on a common lower chassis. The booms mutually stabilize each other, whereby the achievable ultimate load amounts to more than twice that of a single boom. In other respects, the crane includes a single basic unit like a usual crane, which unit is designed for the ultimate load of the double boom. It cannot be operated economically with only one boom. Moreover, a ring lift crane is, as a rule, not portable and is consequently severely limited in its work area, because this is possible only with a relatively high technical effort.
A twin slewing crane is described in DE 27 45 059 A1, which finds application especially as a deck crane. The twin slewing crane described therein comprises two cranes disposed the same distance from a common vertical pivot axis. The two cranes each comprise a boom and are individually operable. Both cranes rest on a common foundation that is, e.g., mounted in a ship hull. A sprocket is located on this foundation, which sprocket is part of the slewing gear of the one crane as well as a part of the slewing gear of the other crane.
Lastly, a ring lift crane is disclosed in DE 30 26 850 A1, which crane is also alleged to be portable when loaded.
For the purpose of a comprehensive discussion, a lifting gear, in particular according to the crane type of DE 29 02 767 A1, also should be pointed out. A rotatably-supported main ring is affixed to a self-propelled vehicle or a stationary plane. A horizontal platform is rigidly and fixedly provided on the main ring, which platform takes part in the rotation with the main ring and which projects in a cantilever manner on one side with respect to the main ring. Furthermore, two turret-like, rotatably-borne supports are provided, which supports are borne on the horizontal platform on the par provided with respect to the main ring. Two extendable booms, which are angularly adjustable in arbitrary positions, are provided on the turret-like supports, which booms are operable independently from each other. Similar to the ring lift crane of U.S. Pat. No. 6,516,961 B1, a common chassis is again provided, on which chassis the two independently-operable booms are disposed.
A deck crane device with multiple derrick crane booms, which are rotatable independently of each other, is known from DE 1 531 631 OS. Two of the derrick booms are coupleable with each other and are commonly rotatable about the central axis in the coupled position and are commonly pivotable about horizontal axes. The horizontal pivot axes of the derrick booms are disposed on separate, concentric rim bearings, which are formed as ring-shaped and box-like. At least two derrick booms are operably coupleable with each other in one position near to each other in the vertical and the horizontal planes.