A Treatise On The Principles And Practice Of Dock Engineering

536 DOCK ENGINEERING. Jiggers—Closely akin to cranes are jiggers (figs. 549 and 550) actuated commonly by hydraulic power. The apparatus is so light as to not require a rail track. There is no jib, and goods are simply hoisted out ofa ship’s hold by means of a chain, or rope, passing over a sheave suspended to the rigging. It may be used as a useful auxiliary to quay cranes, and it has certain advantages in rapidly lifting light articles out of the holds. It has a close competitor in this respect in the winches with which steamships are usually furnished. Hydraulic Crane at Malta.*— This crane, the elevation of which is shown in fig. 551, has a maximum working load of 160 tons. This weight can be lifted through a height of 50 feet at a radius of 70 feet. Loads up to 35 tons can be lifted through a height of 90 feet at a radius of 75 feet. The larger loads are raised by means of a direct-acting hydraulic cylinder suspended in gymbals from the jib; the smaller loads by a chain purchase, worked by a rotary hydraulic engine. The structure, which was constructed and erected by Messrs. Sir W. G. Armstrong & Co., of Newcastle, is carried by and revolves upon 96 bevelled live rollers, 15 inches mean diameter and 16 inches wide, working on a lower roller path of cast iron, planed on the top and bottom, the top being bevelled to suit the rollers. The rollers are connected on the outside by links, 5 inches by f-inch, passing over the ends of the axles, each link taking two rollers ; and on the inside, the end of the axle is clipped in a wrought-iron circular frame, connected by bracing (figs. 552 and 553) to a collar, working on rollers round the centre column of masonry. The centre pivot is fixed to the masonry by four bolts, 15 feet long, extending into the centre chamber, and is made hollow to admit the hydraulic pipes. These pipes are concentric through the pivot, the internal one being for pressure and the other for exhaust. The foundation consists of a solid mass of Portland cement concrete, composed of 6 parts of hard limestone and 3 of sand to 1 of cement, faced above ground with limestone masonry. The main lifting cylinder is of cast iron, in three lengths, connected by fourteen Sj-inch bolts. Its internal diameter is 29 inches, and the thick- ness of the metal 3} inches. The piston is of cast iron and arranged for hemp packing. The piston-rod is of wrought iron, 8 inches in diameter, and fitted at the lower end with swivel eye and shackle. A platform is suspended from the cylinder, from which the inlet and outlet valves are controlled ; it is reached by a light iron bridge hinged to it, and resting upon the framework of the jib. The cylinder is carried in a wrought-iron trunnion ring, suspended from the jib by four forged iron links, so that it can be swung in towards the jib when the 30-ton purchase is in use. The rotary hydraulic engine for working the 30-ton purchase, the slewing machinery, and the swinging-in gear, has three oscillating cylinders, * C. and C. II. Colson on “ The 160-Ton Hydraulic Crane at Malta Dockyard Extension Works,” Min. Proc. Inst. C.E., vol. cxiv.