Usually, shipboard weight- and cargohandling gear is subdivided as follows:
Boom-and-kingpost weight handling systems have wide application because of their versatility and reliability. The capacities of cargo booms range from less than 5 tons to more than 500 tons. When designing a boom-and-kingpost system, or when assessing the ability of existing gear to handle an oversized lift, load on individual components and foundations must be determined and matched against the strength of the structure.
The following loads are important:
Component loads in a cargo boom or other weight-handling system are determined by vector diagrams. In addition to loads caused by suspended weights, boom parts are subject to loads resulting from their own weight. The mast is subject to varying compression due to its weight and the weight of the boom and gear, which is at its maximum at the foot of the mast. The boom is subject to a bending moment caused by its weight.
Booms are subject to additional bending because the cargo whip or purchase block and the topping lift block are not attached at the same point. As boom angle increases, component loads increase for the same suspended weight. Since boom angle determines outreach, operating capacity for booms is a function of outreach or swing radius. In the example, the topping lift is attached directly opposite a single backstay, so the horizontal components of the topping lift and backstay tensions did not cause shear or bending in the mast. Only boom thrust caused shear and bending in the mast.
Two conditions cause additional stresses:
If there are two or more stays, the resultant of the tension in the stays will be in the same plane as the topping lift, so the opposing forces are directly opposite and will not cause shear and bending in the mast. Tension in the multiple stays is found by determining the resultant tension as for a single backstay then resolving it into components in line with the stays. Bending in a stayed mast can be avoided altogether if the boom is stepped at the foot of the mast. Masts for light cargo booms are sometimes unstayed and always subject to bending.
Stress at any section of the mast or boom is the sum of bending, axial (compressive), and torsional stresses. To find bending stress, the bending moment at the section in question is divided by the section modulus. Bending stress is compressive on the boom side of the mast and tensile on the opposite side. To find axial stress, the axial compression is divided by the section area. To find torsional shear, the torque (if any) on the section is divided by the torsional section modulus. The torsional section modulus of a circular section is one-half the bending section modulus.
The effective height of the mast-that is, the height above the boom step-influences the loads in the boom, topping lift, stays, and mast itself. Effective mast heights less than two-thirds of the boom length will cause very high component loads relative to the weight lifted.
Before a ship may be worked, the booms must be topped (raised), guyed, and properly spotted. Each man in the hatch section should understand the procedures for topping, spotting, and lowering the booms. Most of the newer cargo ships are equipped with separate topping-lift winches. Booms can be topped or lowered simply by operating the topping-lift winch.
Cargo booms are rigged in various configurations, depending on the amount and location of material handled and speed of handling. In a single swinging boom rig, the boom is swung by tending the vangs and is topped with the topping lift to spot the suspended load. Cargo transfer by a single swinging boom is relatively slow and tedious. Variations of the of the single swinging boom rig, such as the wing lead, backweight, or Liverpool rigs, or multi-boom rigs, such as the yard-and-stay or housefall rigs, can move cargo at a higher rate than a swinging boom or most cranes.
The rated load for a boom is its safe working load when rigged as a single swinging boom. Safe working load of yard-and-stay and housefall rigs is one-half of the rated load. Cargo booms on US Navy ships have 10-ton working loads unless otherwise specified. Many ships have heavy lift booms or derricks for making lifts greater than the 5-to 10-ton lifts associated with ordinary cargo loading. Navy LKA-116 Class ships have 78-ton Stuelken-type booms. Special-purpose heavy lift ships may have boom capacities of up to 500 tons. Rigging for modern heavy lift booms swings the boom by altering the length of two topping lifts rather than by using vangs. The topping lifts are slaved together so that the boom is controlled from a single point.
Cranes and Davits
Cranes offer greater speed and ease in handling loads than single swinging booms. Cranes are rotating or nonrotating. Rotating cranes are topping or nontopping. Raised runway and traveling support cranes are similar in concept and operation. Port facilities have numerous types of cranes in permanent, temporary, or mobile installations.
Navy ships have kingpost, pedestal, jib, and pillar cranes. Stores-handling davits (J-bar davits) are essentially simple pillar cranes. Most shipboard cargo cranes are kingpost or pedestal cranes. Some large special-purpose vessels, such as floating drydocks, have portal cranes. A portal crane consists of a large pedestal or kingpost crane with legs forming a portal for traffic. Large floating cranes are usually kingpost cranes.
Mobile cranes are usually pedestal or kingpost type cranes mounted on truck or crawler chassis. Large Navy vessels-aircraft carriers, amphibious warfare vessels, and repair ships-carry truck cranes. Mobile cranes are extremely useful in certain conditions:
Loads on crane components are determined by force and free-body diagrams - the same way as they are determined for boom systems. Because cranes are unstayed, the pivoting mechanism of a rotating crane is always subject to bending. Counterweights on many cranes reduce the moment on the crane base. Inertia loads caused by rolling or pitching increase the moment at the base.
Cranes, unless specifically designed for heavy weather operation, cannot be used in as high a sea state as booms or sheer legs. Because they have no topping lifts, the booms of pedestal, pillar, and jib cranes are cantilever beams and are subject to shear forces and bending moments caused by both the suspended load and their own weight. Cranes are designed to carry loads in the same vertical plane as the boom axis. Side-loading should be avoided because the crane structures are not designed to resist athwartships thrust. Side-loading occurs when the crane rotates or travels with a swaying load or when it drags loads.
Ships are equipped with portable J-bar davits for handling stores, ammunition, and miscellaneous weights. Stores davits on Navy ships normally have 1,000-pound working loads (500-pound on destroyers and smaller ships). Ammunition davits have working loads equal to the weight of the heaviest piece of ammunition in the magazine served. U.S. Navy shipboard cranes have a 5-long-ton safe working load unless otherwise specified.
Floating Cranes Floating cranes perform a variety of lifts in salvage work. Light- to medium-capacity cranes can be moored alongside casualties without operating cargo gear to offload cargo, remove other weights, or position salvage gear. Large cranes with capacities as high as several thousand tons may be used to:
Gin Pole / Standing Derrick
A gin pole or standing derrick is a boom without a mast or topping lift. It has a single spar with its butt resting securely in a shoe so that it can pivot and rotate. The head of the spar is held in place by guys or stays. A guy is a stay that includes a tackle or is rigged to a winch to permit adjustment of its length. Two guys, spaced 60 to 90 degrees apart, are nominally sufficient to support the gin pole, because the weight of the load compresses the gin pole and holds its head against the guys. At least one additional guy or stay should be rigged to prevent the pole from falling over backwards if the load jerks or is released suddenly. Gin poles are rigged with three to six guys. Sheer legs, gin poles, stiff leg derricks, etc., can be improvised on salvage operations.
The compression in the gin pole is determined by the force diagram. Like the back guy, load in the pole approaches infinity as the pole approaches the horizontal. Increasing the distance between the after guy anchor and the foot of the gin pole reduces tension in the guy, but increases compression in the pole. When lifting weights of more than about 1,000 pounds, a large shoe or doubler plate is fitted under the foot of the gin pole and the deck shored to spread the load. Attachments for the stays or guys are also reinforced.
Gin poles erected on firm foundations may have lengths of 250 feet or more, with lift capacities to about 300 tons. Heavy gin poles do not swing under load but lift loads vertically at fixed radius. High-capacity gin poles lean only 5 to 10 degrees from the vertical. Because of the small angle with the vertical, the gin pole must be significantly taller than the height of the lift when bulky loads are lifted. The guys should be heavily preloaded to minimize the increase in pole angle when the guys stretch as heavy loads are lifted. The guy preloading must not be so great that the compressive load on the pole resulting from the combination of guy tension and the suspended load exceeds the capacity of the pole. Lighter poles can lean to greater angles and are sometimes rigged with adjustable guys so the load can be moved horizontally by topping the pole.
Gin poles can be erected in pairs to increase lifting capacity and clearance between the poles and the load. Two poles assembled with a horizontal beam across the pole heads form a gallows frame. If both poles are rigged to the same angle and guy arrangement, and the hoist tackle connected to the center of the beam, the loads in both poles will be equal. Gallows frames have higher capacities than twin gin poles. Shorebased gallows frames typically have lift heights of up to 200 feet, and capacities to 600 tons; shorter frames can lift up to 1,200 tons.
With an improvised swinging derrick, or cargo boom, the foot of the boom is either stepped to the foot of the mast by heavy lashings or set in a shoe against the mast. The boom can also be set in a shoe at some distance from the foot of the mast. The shoe carries the horizontal and vertical boom thrust. Shoes can be built up against hatch coamings or machinery foundations. It is best to use a mast, kingpost, or other structure that is in place as the derrick mast. A jury mast can be set up by inserting a steel or wooden spar through a hole cut in the deck and resting the foot of the spar on the deck below. The mast is secured by welding, angle clips, or wedges at its foot and the deck penetration. Alternatively, the jury mast is inserted through a hatch and braced firmly against one side or corner. There should be two or more stays.
A type of swinging derrick, called a Chicago boom, can be installed on an existing mast, kingpost or deckhouse. The lower end of the boom is attached to a vertical or horizontal surface by a combination hinge and swivel pin, or goose neck. Topping lifts and hoist tackles are rigged. Swing guys, running from each side of the boom head to convenient anchorages, swing the boom and load. Typically, a double drum winch powers the topping and hoisting lifts. If the boom is used only for lifting and swinging, the topping lift is replaced by a fixed stay. Chicago boom lengths typically range from 10 to 25 feet, with lift capacities from 500 pounds to about 35 tons.
Sheer legs are built from steel or wooden spars. The butts of the legs are separated by a distance equal to one-third of their length or less. Cross-members or stays prevent the sheer legs from spreading under load. A hoisting tackle rigged to the apex of the legs lifts the loads. Guys attached to the sheer head allow moving the load horizontally by increasing or decreasing the angle of sheer legs off the vertical. Slacking the load sharply increases loading on the legs and guys. Guy tension and leg compression are determined by viewing the rig edge-on and determining backstay tension and leg compression as if the rig were a gin pole with a single stay. Resultant guy tension and leg compression are resolved into their components by solving force polygons.
|Join the GlobalSecurity.org mailing list|