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Design of the Crane
The crane is based on a design patented by Sir William Fairbairn in November 1850, derived from his experience of building iron bridges in the late 1840s. Its core concept is a hollow box girder, constructed of wrought-iron panels on each of its four sides, rivetted together at their flanges. The structure of the box, with four panels reinforcing one another, makes it stronger and less susceptible to buckling than a solid beam of the same dimensions and, being hollow, it weighs much less. Its defining feature is a curved tapering jib. This replicates the strength and structural rigidity of the straight box girders used in bridge-building, with opposing iron panels reinforcing each other, and transfers the concept from the static weight-bearing of a bridge to the variable weightlifting of a crane. The smaller panels on the inside of the curve “push” against compression, the larger panels on the outside (or top) of the curve “hold” the tension from above.
The intrinsic strength of the jib removed any need for the diagonal strut that was an essential part of the design of previous cranes with their straight booms, and the absence of this obstruction enabled large items to be lifted higher, fitting into the curve of the crane. Moreover, the fact that the jib was hollow enabled the lifting chains to be contained inside the shaft, removing the risk of their becoming entangled around the item being lifted, as well as protecting them from the weather.
Plan for the crane, showing the jib and the underground pit.
Fairbairn’s design of crane was, in short, very well suited to lifting heavy and bulky loads into and out of ships on windy and inhospitable quaysides. It was also relatively inexpensive. In the second half of the 19th Century, many dozens were bought by the Royal Navy for its various dockyards and by commercial ports in Britain and overseas. While the curved jib is the most conspicuous and celebrated element of the design, many more parts of the crane are out-of-sight. The chains are concealed within the structure of the jib, passing from the lifting hook, over three internal rollers (one at the head of the jib and two more, positioned one third and two thirds of the way down the shaft) to the base of the jib where they are wound around a grooved barrel, fixed horizontally and controlled by the large external gear wheel.
The innovative design of the crane was an entirely different from previous generations of dockside crane, like those on the adjacent Ordnance Quay (known as “Gun Wharf”)
Moreover, almost half of the crane is underground. The base of the jib sits on top of an inverted iron cone, which itself fits within a cast-iron cylinder. At the bottom of this cylinder, the tip of the cone is fixed with a pintle bearing to the centre of a steel base plate; and, just below ground level, a ring of rollers around its circumference enables the cone to revolve smoothly. The turning of the cone rotates the jib of the crane out over the dockside and the ships tied up alongside. All parts of the mechanism are worked by hand. Gear wheels magnify the pressure applied at the manual winding wheel, to turn the winch barrel and lift loads of up to 30 tons; and a pinion enables the same winding wheel to be used to rotate the crane. Lever brakes lock all wheels still when needed.
The crane could rotate through 360°, supported by a ring of rollers within its baseplate.
