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Варианты заголовков:

A – The height of tower cranes

B – Means to rate tower cranes capabilities

C – Possible degrees of freedom for tower crane units

D –Conventional power drives used in tower cranes

E – The range of application of tower cranes

F – The range of operating height for tower cranes

G – Mechanisms of more complicated cranes

Ответьте на вопросы

1. What are the main moving parts of a tower crane?

2. What are the usual applications of tower cranes?

3. How can we use a tower crane to erect a super tall building, say 800 meters in height?

4. How is the geometry of a tower crane taken into account when we rate its lifting capability?

5. What’s the difference between the power drives of old tower cranes and the new models?

Дополнительный текст
Clearances

With each work cycle, a crane picks up a load, swings it and places it where needed. The path of both load and boom must clear obstructions through the full range of movement. The boom must be long enough to raise the load to the required height without danger of collision between the load and the boom, but not so long that it will swing into nearby structures. The tail end of the superstructure requires a clear swing path, too. And, after completing the work, the crane must be able to extricate itself and lower its boom for dismantling.

It is embarrassing, to say the least, to send a crane to a job only to find that it is incapable of placing the loads where needed. Embarrassment may be only the first problem in a series, as some accidents come about because field crews try to improvise and work around an unexpected limitation.

Clearance problems can come up in various ways.

· The lifted load is at risk of fouling the boom or jib. A wide load lifted close to the head machinery at a high boom angle could be at risk. This is sometimes a dilemma faced by riggers placing unwieldy vessels or machinery. Several styles of boom heads such as the hammerhead tip or offset tip are configured to lessen the possibility of fouling. The head sheaves of telescopic booms are often similarly offset, too.

· The suspended load or the boom tip fouls a nearby building or other obstruction. In tight quarters, the boom might not be capable of being raised high enough to swing past the obstruction. Urban and industrial sites can be places of heightened exposure to this peril.

· The boom head is obstructed from above by an overhanging structure or object.

· Reaching some distance past the leading edge or parapet of a building, the underside of the boom or jib cannot clear it. The authors refer to this as a swing clearance problem.

· The hook cannot reach sufficient height to place the load. This is known as a drift deficiency. It is sometimes caused by an underestimation of the length slings or height of the rigging supporting the load.

· The aft end of the crane superstructure cannot clear an obstruction. The counterweights, for instance, interfere with a tree or a live mast fouls a building.

· Any part of the crane or the suspended load encroaches within a restricted zone surrounding a power line.

Basic clearance checking can be done using the range diagram provided with the crane documentation, a copy of which is often also mounted in the crane cab. For most work, the rough data obtainable with a range diagram are sufficient to verify that the job can be done and that boom and jib lengths and jib offset will offer satisfactory clearance. These problems can be difficult to visualize with two-dimensional tools even for experienced planners. Until the arrival of the day when a worker will be able to don virtual reality goggles and walk through a comprehensive 3-D model of a crane set up on the site, it will be necessary to hone visualization skills and make the best possible use of some rudimentary analytical tools.