Active Vocabulary:

consideration - рассмотрение, соображение

account - счет, расчет

remain - оставаться

stable - стабильный, устойчивый

tip - наклонять(-ся), опрокидывать(-ся)

create - создавать

beam - балка

point - точка

opposite - противоположный

equal - равный

strut - стойка, подпорка

support - поддерживать

fix - укреплять, устанавливать

wind - наматывать, поднимать при помощи лебедки

conservation - сохранение

exceed - превышать

remote control - дистанционный контроль

value - значение, величина

cause - быть причиной, вызывать

Exercise 1.Answer the following questions:

1. What considerations are taken into account in the design of cranes?

2. What kinds of simple machines does the use of cranes illustrate?

3. What does the principle of the lever allow to do?

4. How does the pulley system increase the applied force?

5. What does the principle of energy conservation mean?

6. How can cranes be controlled from the ground?

7. How is the crane stability provided?

Exercise 2.Translate the following sentences, paying attention to Modal Verbs:

1. The crane must be able to lift a load of specified weight and must remain stable.

2. The hydraulic cylinder can be used directly to lift the load or indirectly to move

the jib.

3. The energy delivered to the load cannot exceed the energy put into the machine.

4. Cranes can mount several utensils to be able to pick up several objects.

5. In order for a crane to be stable, the sum of all moments about any point must be

equal to zero.

6. The front assembly of a loader may be a removable attachment or permanently

mounted.

7. Backhoe loaders are to be used in urban engineering and small construction

projects.

8. Agricultural implements may be towed behind or mounted on the tractor, and

the tractor may also provide a source of power.

9. The outriggers have to be retracted when the vehicle needs to change its

position.

10. For larger projects, a tracked excavator should be used.

Exercise 3.Translate the following sentences, paying attention to the underlined words:

1. As an option, bulldozers can be equipped with ripper claws.

2. Bulldozers grew more complex as time passed.

3. Bulldozers are very powerful tractors and have excellent ground hold, as their

main tasks are to push or drag things.

4. Superdumps can be rated as high as 80,000 pounds.

5. Because of their large volume buckets loader tractors are very useful.

6. A bulldozer can push soil because it has a blade.

7. Backhoe loaders can be used for a wide variety of tasks.

8. For many years loaders have been further modified.

9. Hydraulic arms (instead of cable operated) are preferred for they enable more

precise manipulation.

10. Steam cranes have been used since the 18-th century.

11. Since the effectiveness of a roller depends on its weight self-propelled vehicles

replaced horse-drawn rollers.

12. Widespread use of the bull-graders does not seem to appear before the mid-

1930s.

13. The cranes had been constructed from wood before cast iron and steel replaced

it.

Exercise 4.Translate the text without a dictionary:

Text B

ROMAN CRANES

The Romans adopted the Greek crane and developed it further. We are relatively well informed about their lifting techniques thanks to rather lengthy accounts by the engineers Vitruvius and Heron of Alexandria . There are also two surviving beliefs of Roman treadwheel cranes offering pictorial evidence, with the Haterii tombstone from the late first century AD being particularly detailed.

The simplest Roman crane, the Trispastos, consisted of a single-beam jib, a winch, a rope, and a block containing three pulleys. Having thus a mechanical advantage of 3:1, it has been calculated that a single man working the winch could raise 150 kg (3 pulleys x 50 kg = 150), assuming that 50 kg represent the maximum effort a man can exert over a longer time period. Heavier crane types featured five pulleys (Pentaspastos) or, in case of the largest one, a set of three by five pulleys (Polyspastos) and came with two, three or four masts, depending on the maximum load. The Polyspastos, when worked by four men at both sides of the winch, could already lift 3000 kg (3 ropes x 5 pulleys x 4 men x 50 kg = 3000 kg). In case the winch was replaced by a treadwheel, the maximum load even doubled to 6000 kg at only half the crew, since the treadwheel possesses a much bigger mechanical advantage due to its larger diameter. This meant that, in comparison to the construction of the Egyptian Pyramids, where about 50 men were needed to move a 2.5 ton stone block up the ramp (50 kg per person), the lifting capability of the Roman Polyspastos proved to be 60 times higher (3000 kg per person).

However, numerous Roman buildings which feature much heavier stone blocks than those handled by the Polyspastos indicate that the overall lifting capability of the Romans went far beyond that of any single crane.

Roman engineers accomplished lifting extraordinary weights by two measures: First, a lifting tower was set up, whose four masts were arranged in the shape of a quadrangle with parallel sides with the column in the middle of the structure. Second, a multitude of capstans were placed on the ground around the tower, for, although having a lower leverage ratio than treadwheels, capstans could be set up in higher numbers and run by more men (and, moreover, by draught animals).

Notes:

adopt - заимствовать

assume - предполагать, допускать

effort - усилия

exert - напрягать(силы), осуществлять

crew - бригада

ramp - скат, наклонная плоскость

capstan - кабестан, ворот

Unit 7

Grammar: Infinitive;

Polysemantic Words

Text A

TYPES OF CRANES ( PART I)

Different types of cranes are used for maintenance work, recovery operations and weight loading.

Mobile Crane

The most basic type of a mobile crane consists of a steel truss or telescopic boom mounted on a mobile platform, which may be a rail, wheeled (including "truck" carriers) or caterpillar tracks. The boom is hinged at the bottom, and can be raised and lowered by cables or by hydraulic cylinders. A hook is suspended from the top of the boom by wire rope and sheaves. The wire ropes are operated by whatever prime movers the designers have available, operating through a variety of transmissions. Steam engines, electric motors and internal combustion engines (IC) have all been used. Older cranes' transmissions tended to be clutches. The operational advantages of this arrangement can now be achieved by electronic control of hydrostatic drives, which for size and other considerations is becoming standard. Some examples of this type of crane can be converted to a demolition crane by adding a demolition ball, or to an earthmover by adding a clamshell bucket or a dragline and scoop, although design details can limit their effectiveness.

To increase the horizontal reach of the hoist, the boom may be extended by adding a jib to the top. The jib can be fixed or, in more complex cranes, luffing (that is, able to be raised and lowered).

Telescopic Crane

A telescopic crane has a boom that consists of a number of tubes fitted one inside the other. A hydraulic or other powered mechanism extends or retracts the tubes to increase or decrease the total length of the boom. These types of booms are often used for short term construction projects. The relative compactness of telescopic booms makes them adaptable for many mobile applications.

Tower Crane

The tower crane is a modern form of a balance crane. Fixed to the ground (or "jacked up" and supported by the structure as the structure is being built), tower cranes often give the best combination of height and lifting capacity and are used in the construction of tall buildings. To save space and to provide stability the vertical part of the crane is often mounted on large beams, braced onto the completed structure, being lifted from one floor to the next as the structure grows. The jib (colloquially, the 'boom') and counter-jib are mounted to the turntable, where the slewing bearing and slewing machinery are located. The counter-jib carries a counterweight of concrete blocks, and the jib suspends the load from the trolley. The hoist motor and transmissions are located on the mechanical deck on the counter-jib, while the trolley motor is located on the jib. The crane operator either sits in a cabin at the top of the tower or (rarely seen) controls the crane by radio remote control from the ground. In the first case the operator's cabin is most usually located at the top of the tower attached to the turntable, but can be mounted on the jib, or partway down the tower. The lifting hook is operated by using electric motors to manipulate wire rope cables through a system of sheaves.

In order to hook and unhook the loads, the operator works in conjunction with a signaller (known as a 'rigger'). They are most often in radio contact, and always use hand signals. The rigger directs the schedule of lifts for the crane, and is responsible for the safety of the rigging and loads.

A tower crane is usually assembled by a telescopic jib crane of greater reach. In the case of tower cranes that have risen while constructing very tall skyscrapers, a smaller crane (or derrick) will be lifted to the roof of the completed tower to dismantle the tower crane afterwards. A self-assembling tower crane lifts itself off the ground using jacks, allowing the next section of the tower to be inserted at ground level. It is often claimed that a large fraction of the tower cranes in the world are in use in Dubai.