Greg Pierce, age 13, of Johnson City, Tenn., for his question:
WHAT IS THE DIFFERENCE BETWEEN A CRANE AND A DERRICK?
Cranes and derricks are hoisting machines used to lift and move heavy loads in such places as factories, shipyards and at construction sites. Most derricks are stationary while most cranes can move from place to place under their own power.
Engineers, however, sometimes use either name to describe the same machine.
Simplest form of derrick is called the gin pole. It has a mast, or pole, supported by four guys, or cables, staked to the ground. A pulley at the top of the mast supports ropes to lift the weight.
The sheers derrick has two crossed masts and two guys. Other derricks have a long boom, or pole. The boom slants out from the base of the mast and supports the hoisting cable that carries the load. A cable attached to the top of the mast supports the boom.
Oil derricks are tall steel structures that raise and lower the equipment used to drill an oil well.
The hand operated jib crane is the simplest type of crane. It has a long jib, or arm, that extends several feet from a heavy base. The base keeps the crane from tipping over.
The end of the jib has a pulley. A rope or a cable, with a lifting hook in the end, runs from this pulley to a winch or crank in the foundation. The operator turns the winch to lift or lower the hook.
The arm of the crane can be swung in a circle before the weight is lowered. Thus, a jib crane can move a weight to any point around the circumference of the circle its jib makes.
The pillar jib crane has a pillar rising from its base. A cable attached to the top of the pillar raises and lowers the end of the jib so the jib can be moved up and down as well as from side to side. When the jib moves up, it carries the load toward the base. When lowered, it moves the load away from the base.
Factories and foundries often use bridge cranes, sometimes called overhead traveling cranes. This type of crane moves back and forth on a bridge extending across the width of a factory room. The bridge travels the length of the roof on overhead rails.
Locomotive cranes are mounted on railroad cars and have long, power operated booms.
Crawler cranes are mounted on tractors while truck cranes, frequently used by building contractors, are mounted on trucks.
Tower cranes are used to construct high rise buildings. Most tower
cranes, called climbing cranes, have built in jacks that raise the cranes through openings in the floor as the building goes up. Workers take the cranes apart to lower them after the building has been completed.