Text 10. Tower Cranes

1. A tower-mounted crane moves loads by executing three motions: the hook is raised and lowered by means of a winch and fall, carried in a circular path by the swing gear, and carried in a radial motion by either luffing the jib or rolling a trolley carriage. The simplest of tower cranes have only these motions, but more complicated cranes include mechanisms that allow the base to roll on a track, the crane to change elevation by climbing, or the jib to articulate on a hinge point.

2. Tower cranes are the lifting machines of choice worldwide for most mid- and high-rise building construction. They are used also on expansive sites where the broad hook sweep and the relative ease of coordinating multiple tower cranes is an advantage; cable stay and suspension bridges, offshore oil platforms, and power plants are some examples.

3. Freestanding hammerhead tower cranes range up to about 300 feet (91 m) in height; luffing tower cranes are less limited. Though most tower cranes remain at a fixed height, various self-climbing arrangements permit a tower crane to rise with a building under construction, so a tower crane can ascend to any building height.

4. Lift capability of tower cranes is rated in tonne-meters (AmE ton-meter). The tonne-meter rating is obtained by multiplying rated capacity in metric tons by the average working radius in meters. The smallest machines have ratings of about 20 meter-tons and the very largest in production exceed this by a factor of about a hundred. Most used for heavy construction are in the range of 150 to 650 meter-tons.

5. With the exception of a few diesel-powered machines with hydrostatic drives, tower cranes are powered by electricity, and motors driving all the machinery are almost universally electric, too. The hoist motors on older machines have high- and low-speed ranges with stepped increments in each range. More recent models have variable-frequency drive or other forms of continuously adjustable speed motors. Automatic acceleration for all motions is typical on many cranes. Remote controls are sometimes offered.

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in a radial motion

the relative ease

a building under construction

300 feet in height

far above limits

to exceed by a factor of

with the exception of

high- and low-speed ranges