Background
A crane is a machine that is capable of raising and lowering heavy objects and moving them horizontally. Cranes are distinguished from hoists, which can lift objects but that cannot move them sideways. Cranes are also distinguished from conveyors, that lift and move bulk materials, such as grain and coal, in a continuous process. The word crane is taken from the fact that these machines have a shape similar to that of the tall, long-necked bird of the same name.
Human beings have used a wide variety of devices to lift heavy objects since ancient times. One of the earliest versions of the crane to be developed was the shaduf, first used to move water in Egypt about four thousand years ago. The shaduf consists of a long, pivoting beam balanced on a vertical support. A heavy weight is attached to one end of the beam and a bucket to the other. The user pulls the bucket down to the water supply, fills it, then allows the weight to pull the bucket up. The beam is then rotated to the desired position and the bucket is emptied. The shaduf is still used in rural areas of Egypt and India.
As early as the first century, cranes were built that were powered by human beings or animals operating a treadmill or large wheel. These early cranes consisted of a long wooden beam, known as a boom, connected to a rotating base. The wheel or treadmill powered a drum, around which a rope was wound. The rope was connected to a pulley at the top of the boom and to a hook that lifted the weight.
An important development in crane design occurred during the Middle Ages, when a horizontal arm known as a jib was added to the boom. The jib was attached to the boom in a way which allowed it to pivot, allowing for an increased range of motion. By the sixteenth century, cranes were built with two treadmills, one on each side of a rotating housing containing the boom.
Cranes continued to rely on human or animal power until the middle of the nineteenth century, when steam engines were developed. By the end of the nineteenth century, internal combustion engines and electric motors were used to power cranes. By this time, steel rather than wood was used to build most cranes.
During the first half of the twentieth century, European and American cranes developed in different ways. In Europe, where most cranes were used in cities with narrow streets, cranes tended to be built in the form of tall, slender towers, with the boom and the operator on top of the tower. Because quiet operation was important in crowded cities, these tower cranes were usually powered by electric motors when they became widely available.
In the United States, cranes were often used in locations far away from
residential areas. Cranes tended to be built with the boom connected to a
trolley, which could be moved easily from place to place. These mobile
cranes tended to be powered by internal combustion engines. During the
1950s, the availability of stronger steels, combined with an increased
demand for taller buildings, led to the development of cranes with very
long booms attached to small trucks, or to crawlers with caterpillar
treads. Mobile cranes and tower cranes of
many different kinds are used extensively in construction sites around
the world.
Raw Materials
The most important substance used to manufacture cranes is steel. Steel is an alloy of iron and a small amount of carbon. For structures that do not require very high strength, a common form of steel known as carbon steel is used. By definition, carbon steel contains less than 2% of elements other than iron and carbon. Carbon steel exists in a wide variety of forms. The most important factor in determining the properties of carbon steel is the amount of carbon present, which ranges from less than 0.015% to more than 0.5%.For structures that require great strength, particularly in cranes designed to lift very heavy objects, a variety of substances known as high-strength low-alloy (HSLA) steels are used. HSLA steels contain relatively low levels of carbon, typically about 0.05%. They also contain a small amount of one or more other elements that add strength. These elements include chromium, nickel, molybdenum, vanadium, titanium, and niobium. Besides being strong, HSLA steels are resistant to atmospheric corrosion and are better suited to welding than carbon steels.
Depending on the exact design of the crane, a wide variety of other materials may be used in manufacturing. Natural or synthetic rubber is used to make tires for mobile cranes. Certain structural components may be manufactured from various metals such as bronze and aluminum. Electrical components may include copper for wires and semiconducting elements such as silicon or germanium for electronic circuits. Other materials that may be used include ceramics and strong plastics.
Design
Very few machines exist in as wide a variety of designs as cranes. Before the crane is constructed, the manufacturer must consider the site where it will be used and the weight it will need to lift. In addition, cranes are often modified to suit the needs of the user. For these reasons, it is not much of an exaggeration to say that no two cranes are exactly alike.Cranes used for industrial purposes are generally designed to remain permanently in one location. These cranes often perform repetitive tasks that can be automated. An important type of industrial crane is the bridge crane. Traveling on tracks attached to two horizontal beams, known as a bridge, a trolley enables the movement of the bridge crane. Usually, the bridge itself can be moved along a pair of parallel rails, allowing the crane to reach a large, rectangular area. A bridge crane may also be designed so that one end of the bridge is supported by a central pivot while the other end moves on a circular rail, allowing a large, round area to be reached.
An overhead traveling crane is a kind of bridge crane in which the rails are located high above the ground. Usually supported from the ceiling of a building, an overhead traveling crane has the advantage of causing no obstruction in the work area.
Cranes used in construction often perform a variety of tasks and must be controlled by highly skilled operators. Construction cranes are divided into mobile cranes and tower cranes. Mobile cranes are mounted on trucks or crawlers in order to travel from place to place. An articulating crane is a mobile crane in which there is a joint between two sections of the boom, allowing it to move in a way similar to a knuckle in a human finger. Articulating cranes are generally used to lift objects located a relatively short distance away, but with a wide range of motion. A telescoping crane is a mobile crane in which two or more sections of the boom can extend and retract, changing the length of the boom. Telescoping cranes are less versatile than articulating cranes, but are usually able to lift heavier objects located a greater distance away.
Tower cranes are used in the construction of tall buildings. They are installed when construction begins and dismantled when the building is completed. An external tower crane is installed outside the building. As the building increases in height, the crane is raised by lifting the upper part of the crane and adding a new section of tower beneath it. An internal tower crane is installed within the
A mobile crane.
The Manufacturing
Process
Making steel components
- 1 Molten steel is made by melting iron ore and coke (a carbon-rich substance that results when coal is heated in the absence of air) in a furnace, then removing most of the carbon by blasting oxygen into the liquid. The molten steel is then poured into large, thick-walled iron molds, where it cools into ingots.
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2 In order to form flat products such as plates and sheets, or long
products such as bars and rods, ingots are shaped between large rollers
under enormous pressure. Hollow tubes, such as those used to form the
latticed booms of large cranes, may be made by bending sheets of steel
and welding the long sides together. They may also be made by piercing
steel rods with a rotating steel cone.
An external tower crane. - 3 The cables used to lift weights are made from steel wires. To make wire, steel is first rolled into a long rod. The rod is then drawn through a series of dies which reduce its diameter to the desired size. Several wires are then twisted together to form cable.
- 4 Steel arrives at the crane manufacturer and is inspected. It is stored in a warehouse until it is needed. The many different components that will later be assembled into cranes are made using a variety of metalworking equipment. Lathes, drills, and other precision machines are used to shape the steel as required.
Assembling the crane
- 5 A crane is put together from the necessary components. As the crane moves along the assembly line, the steel components are welded or bolted into place. The exact procedures followed during this process vary depending on the type of crane being assembled. For a mobile crane, the components are then assembled to a standardized truck or crawler of the appropriate type.
- 6 The assembled crane is tested and shipped. Depending on the size and type of crane, it may be broken down into subsections to be assembled on site. It may also be shipped whole on special large trucks.
Quality Control
Safety is the most important factor to be considered during crane manufacturing. The steel used to make the crane is inspected to
An internal tower crane.
The United States government sets specific regulations through the Occupational Safety and Health Administration that limit the weight that a specific crane is allowed to lift. The Crane Manufacturers Association of America sets its own safety standards which exceed those required by the government. Special devices within the crane prevent the user from attempting to lift a weight heavier than that allowed.
A completed crane is first tested without a weight to ensure that all of its components operate properly. It is then tested with a weight to ensure that the crane is able to lift heavy objects without losing stability.
Safety ultimately depends on proper use of the crane. Crane operators must be specially trained, must pass specific tests, and must be examined for any visual or physical problems. The crane should be inspected each working shift, with a more thorough inspection of the motor and lifting apparatus on a monthly basis. Crane operators must be aware of changes in the environment in order to avoid accidents. For example, cranes should not be used during very windy conditions.
The Future
Manufacturers of cranes are constantly seeking new ways to incorporate new technology into their products. Future cranes will have improved safety and versatility with computers and video screens that will allow operators to move heavy objects with increased accuracy.Signs of the future can be seen in an unusual crane recently developed by James S. Albus, of the National Institute of Standards and Technology in Gaithersburg, Maryland. The Stewart Platform Independent Drive Environmental Robot (SPIDER) looks nothing like an ordinary crane. Instead, the SPIDER is shaped like an octahedron (a diamond-shaped solid consisting of eight triangles joined together in the form of two four-sided pyramids). Six pulleys support six cables from the top level of the SPIDER. The cables manipulate the lower level of the SPIDER, which is attached to tools or grip devices. The six cables can be operated together or independently, allowing the lower level to be moved in all directions. The SPIDER can lift heavy objects to within 0.04 in (1 mm) of the desired location, and hold them within one-half of a degree of the desired angle. The SPIDER can lift up to six times its own weight.
Where to Learn More
Books
Jennings, Terry. Cranes, Dump Trucks, Bulldozers, and Other Building Machines. Kingfisher Books, 1993.Shapiro, Howard I. Cranes and Derricks. McGraw-Hill, 1980.
Periodicals
"The Crane: A Versatile Truck-Mounted Tool." Public Works (November 1988): 62-63.Shapiro, Lawrence K. and Howard I. "Construction Cranes." Scientific American (March 1988): 72-79.
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