A canal barge lift which has superseded ten locks at Foxton, Leicestershire. Two tanks, balancing one another, run on separate tracks up and down an incline. At the bottom and top of the incline the tank is submerged so that a barge may float in or out.
A SHIP-RAISING LIFT
The writer has treated one form of lift for raising ships out of the water—the floating dry dock—elsewhere,[21] so his remarks in this place will be confined to mechanism which, having its foundations on Mother Earth, heaves mighty vessels out of their proper element by the force of hydraulic pressure. Looking round for a good example of an hydraulic ship-lift, we select that of the Union Ironworks, San Francisco.
Some years ago the works were moved from the heart of the city to the edge of Mission Bay, with the object of carrying on a large business in marine engineering and shipbuilding. For such a purpose a dry dock, which in a short time will lift a vessel clear of the water for cleaning or repairs, is of great importance to both owners and workmen. By the courtesy of the proprietors of Cassier's Magazine we are allowed to append the following account of this interesting lift.
The site available for a dock at the Union Ironworks was a mud-flat. The depth of soft mud being from 80 to 90 feet, would render the working of a graving dock (i.e. one dug out of the ground and pumped dry when the entrance doors have been closed) very disagreeable; as such docks, where much mud is carried in with the water, require a long time to be cleaned and to dry out. Plans were therefore prepared by Mr. George W. Dickie for an hydraulic dock, including an automatic control, which the designer felt confident would meet all the requirements of the situation, and which, after careful consideration, the Union Ironworks decided to build. Work was begun in January, 1886, and the dock was opened for business on June 15th, 1887—a very fine record.
This dock consists of a platform built of cross and longitudinal steel girders, 62 feet wide and 440 feet long, having keel blocks and sliding bilge blocks upon which the ship to be lifted rests. The lifting power is generated by a set of four steam-driven, single-acting horizontal plunger pumps, the diameter of the plungers being 3 1 / 2 inches and the stroke 36 inches. Forty strokes per minute is the regular speed.
There is a weighted accumulator, or regulator, connected with the pumps, the throttle valve of the engines being controlled by the accumulator.[22] The load on the accumulator consists of a number of flat discs of metal, the first one about 14 inches thick and the others about 4 inches thick, the diameter being about 4 feet. The first disc gives a pressure of 300 lbs. per square inch. This is sufficient to lift the dock platform without a ship, and is always kept on.
In lifting a ship, as she comes out of the water and gets heavier on the platform, additional discs are taken on by the accumulator ram as required. The discs are suspended by pins on the side catching into links of a chain. The engineer, to take on another disc, unhooks the throttle from the accumulator rod, runs the engine a little above the normal speed, the accumulator rises and takes the weight of the disc to be added; the link carrying that disc is thus relieved and is withdrawn. The engineer again hooks the accumulator rod to the engine throttle, and the whole is self-acting again until another weight is required. When all the discs are on the ram the full pressure of 1,100 lbs. per square inch is reached, which enables a ship of 4,000 tons weight to be raised.
There are eighteen hydraulic rams on each side of the dock. These rams are each 30 inches in diameter and have a stroke of 16 feet; and as the platform rises 2 feet for 1 foot movement of the rams, the total vertical movement of the platform is 32 feet. When lowered to the lowest limit there are 22 feet of water over the keel blocks at high tide.
The foundations consist of seventy-two cylinders of iron, which extend from the top girders to several feet below the mud line. These cylinders are driven full of piles, no pile being shorter than 90 feet. The cylinders are to protect the piles from the teredo (the timber-boring worm), which is very destructive in San Francisco Harbour. A heavy cast-iron cap completes each of the foundation piers, and two heavy steel girders extend the full length of the dock on each side, resting on the foundation piers and uniting them all longitudinally. The hydraulic cylinders are carried by large castings resting on the girders, each having a central opening to receive a cylinder, which passes down between the piers. There are thirty-six foundation piers, and eighteen hydraulic cylinders on each side of the dock.