Bulk-handling crane

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.

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 (e.g. is it wet or dry) and, in some cases, degree of settlement (e.g. a ship's cargo has compacted due to vibration, rolling, pitching and yawing over a long voyage).

Other applications (e.g. rape seed) may require to be self-sealing and have offset lower jaws with serrated, angle cut side cutters to restrain easy-flow properties.

The weight of the grab is supported by the holding rope(s) and pinned to the upper girder allowing some angular freedom.

A linked chain requires a much small diameter pulley than the equivalent rope which makes a smaller, more compact, design of girder.

The clamshell bucket on the right hand side of the photograph on this page at Cardiff is a chain ring discharge grab.

The clamshell bucket on the left hand side of the photograph on this page at Cardiff is a self-dumping chain grab.

Some grabs are electro-hydraulic, having an electric motor in the top girder which drives a hydraulic pump and all control gear.

An additional feature sometimes fitted to the crane, used to restrict the tendency of the grab to rotate on the axis of the holding ropes, is a tagline (sometimes referred to as a grabline) mechanism.

The design can vary but is typically a coiled spring contained within a horizontal tube with a rope drum on the end.

Dumping the grab contents into the hopper now only requires the quicker luffing movement, without needing to slew for each load.