1. Screw conveyors can be considered as the world’s oldest materials handling aid. 2. They still remain one of the most versatile. 3. About 2,500 machines are manufactured in UK every year. 4. This number is growing at 5-6 per cent annually.
5. The main reasons for the screw’s popularity are its relatively low costs, compact shape, ability to handle a wide range of materials and to convey in a vertical and horizontal plane as well as its general simplicity and reliability. 6. Size and capacity can vary greatly from a diameter of 50 mm up to 2.5 m or more and from 20 kg an hour up to 500 tons an hour.
7. Lengths tend to be short; though screw conveyors have been manufactured with lengths of over 30 m, most of them are between 3 m and 10 m long.
8. The basic construction of a screw conveyor is a centre shaft with a helicoids flight round it. 9. The size and pitch of the flight depend on its function. 10. For a normal conveyor a pitch is equal to the diameter. 11. The conveying capacity is dependent on the pitch. 12. Capacity is also proportional to the speed of rotation.
13. The screw itself operates inside a casing which can be tubular or U-shaped. 14. Trough conveyors are more expensive and can only be used horizontally or inclined at less than about 35 degrees. 15. Above this angle a tubular casing has to be used. 16. Elevators are used at relatively high speed of rotation, upwards of 50 rpm. 17. Much lower speeds of rotation, less than 1 rpm, can be used with horizontal conveyors.
18. The most common material of construction is mild steel. 19. Finally, the mechanical features of the screw conveyor may be defined as follows: 1) generally compact; 2) simple in construction, installation and maintenance; 3) can be used at all angles; 4) can be made mobile; 5) remote and automatic controls can be fitted simply; 6) heavy construction is possible at little extra cost; 7) they are available is steel or other materials.
20. Recently there have been a number of developments in screw conveying. 21. Flexibility, great capacities and lengths are the features of the new equipment.
Remember the words:
screw, consider, handle, versatile, reason, low, shape, wide, range, reliability, capacity, vary, length, shaft, size, depend, equal, speed, rotation, inside, trough, incline, elevator, common, feature, installation, maintenance, remote, development, flexibility, equipment.
Note in the text:
1. Superlative degree, possessive case. 4. Predicate (tense). 5. Infinitive (function). 7. Subjects and predicates. 14. Comparative degree, ed-forms. 19. “be” (forms and functions). 20. Predicate (tense).
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