Ex. 4.2. Read the first sentence of each paragraph of the text and put the headlines in the correct order.

___ Dragline bucket system ___ Typical excavation cycle

___ Use of draglines ___ Limitations of draglines

___ Walking draglines ___ Throwing the bucket

Ex. 4.3. Look through paragraphs 2, 5, and 6, and then answer the questions in one sentence or phrase.

1. How is the bucket manoeuvred?

2. How do walking draglines accomplish their movement?

3. What is the speed of their movement?

4. What are the primary limitations of draglines?

Ex. 4.4. Read the text carefully to understand the construction and operations of draglines.

Draglines

¹A dragline excavator is a piece of heavy equipment used in civil engineering and surface mining¹. In civil engineering, draglines are used for road, port construction, and canal dredging.

²A dragline bucket system consists of a large bucket which is suspended from a boom with wire ropes. The bucket is manoeuvred by means of a number of ropes and chains. The hoist rope, powered by large diesel or electric motors, supports the bucket and the hoist coupler from the boom. The dragrope is used to draw the bucket horizontally. By skilful manoeuvre of the hoist and the drag ropes the bucket is controlled for various operations. A schematic of a large dragline bucket system is shown below.

³In a typical cycle of excavation, the bucket is positioned above the material to be excavated. The bucket is then lowered and the dragrope is then drawn so that the bucket is dragged along the surface of the material. The bucket is then lifted by using the hoist rope. A swing operation is then performed to move the bucket to the dumping place. The dragrope is then released causing the bucket to tilt and empty. This is called a dump operation.

⁴The bucket can also be thrown by winding up to the jib and then releasing a clutch on the drag rope. This would then swing the bucket like a pendulum². Once the bucket had passed the vertical, the hoist cable would be released thus throwing the bucket. On smaller draglines, a skilled operator could make the bucket land about one-half the length of the jib further away than if it had just been dropped. On larger draglines, this is not a common practice.

⁵All but³ the smallest of draglines are walking excavators. The movement is accomplished by using feet, as caterpillar tracks place too much pressure on the ground, and have great difficulty under the weight of the dragline. Maximum speed is only at most a few metres per minute since the feet must be repositioned for each step. If travelling medium distances, (about 30–100 km), a special dragline carrier can be brought in to transport the dragline.

⁶The primary limitations of draglines are their boom height and boom length, which limits where the dragline can dump the waste material. Another primary limitation is their dig depth, which is limited by the length of rope the dragline can utilize. Inherent with their construction⁴, a dragline is most efficient excavating material below the level of their base⁵. While a dragline can dig above itself, it does so inefficiently and is not suitable to load piled up material.

Notes to the text:

Date: 2016-03-03; view: 993