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.
History:
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, and 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.