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A bulk-handling crane is one that, instead of a simple hook that can handle a range of slung loads, has an integral grab for lifting bulk cargoes such as coal, mineral ore etc.
Where the grab is a two-piece hinged bucket, it is known as a shell grab or shell bucket. Working the grab requires extra cables from the crane jib, so requires a specialised design of crane throughout, not merely an attachment. Some grabs use 2 cables for lift and control, others use 4.
In 1927, Stothert & Pitt of Bath, Somerset produced the first specialised bulk-handling crane. This was to unload coal at Barking power station in London.
The grab has to be selected so that the combined weight of grab and contents does not exceed the grabbing safe working load of the crane. The mass of the contents depends upon the properties of the product being handled; its density, flow characteristics, angle of repose, lump/grain size, purity and, in some cases, degree of settlement. Most bulk-handling cranes are dedicated to a single product, but others will handle various materials and need a selection of grabs. Some applications may also require the use of teeth fitted to the grab jaws to enable penetration. Other applications may require to be self-sealing and have offset lower jaws with serrated, angle cut side cutters to restrain easy-flow properties.When off-loading a ship, the grab may be more efficient if designed with a low-profile extended body which enables reaching under the ship's hatches, negating any need to centre the product with human and machinery access.The grab mechanism may be four rope, double rope, single rope ring discharge, single rope self-dumping, double chain, single chain self-dumping, single chain ring discharge, hydraulic or electro-hydraulic.Rope grabs have rope pulleys in both the upper and lower girders and close by drawing the closing rope to shorten the gap between the two girders. The minimum diameter of the pulley is restricted by the ratio of the pulley diameter to the rope diameter, the strands of the wire rope being subjected to fatigue bending stresses if the pulley is too small, giving premature failure. The physical size of the pulleys then determines the size of the girders, then determining a break point between a four rope and a two rope design. The closing force at the shell jaws, and, inversely, the speed of closure, are determined by the number of pulleys on each closing rope in the two girders. The closing rope is fed into the grab mechanism through either a bellmouth or an arrangement of small rollers. The weight of the grab is supported by the holding rope and pinned to the upper girder allowing some angular freedom. Holding and closing ropes require separate winding drums in the crane.The mechanism on chain grabs operates in the same way as on rope grabs. The chains are connected to ropes on the crane by couplings which allow rotation. A linked chain requires a much small diameter pulley than the equivalent rope which makes a smaller, more compact, design of girder. Chains used on chain pulleys are less easily inspected for wear and fatigue than ropes which show outer strand deformation well before failure. Failure of a rope or chain is a very high severity hazard.