机械英语第一课

LESSON 1 MACHINE ELEMENTSHowever simple, any machine is a combination of individual components generally known as machine elements or parts. Thus if a machine is completely dismantle, a collection of simple parts remains, such as nuts, bolts, springs, gears, cams, and shafts- the building blocks of an machinery.A machine element is, therefore, a single unit designed to perform a specific function and capable of combining with other elements. Sometimes certain elements are associated in pairs, such as nuts and bolts or keys and shafts. In other instances, a group of elements is combined to form a assembly, such as bearing, coupling, and clutches.The most common example of a machine element is a gear,which, fundamentally, is a combination of the wheel and the lever to form a tooth wheel. The rotation shafts gear on a hub or shaft drives other gears which may rotate faster or slower, depending upon the number of teeth on the basic wheels. The material from which the gear is made establishes its strength, and the hardness of its surface determines its resistance to wear. Knowledge of the forces on the gear makes possible the determination of its size. Changes in its shape allow modifications in its use. These applications, as in most machine elements have developed into many standard forms, such as spur bevel, helical, and worm gears. Each of these forms has requires the development of a special technology for its production and use.Other fundamental machine elements have evolved from wheels and levers. A wheel must have a shaft on which it may rotate. The wheel is fastened to the shaft with a key, and the shaft is joined to other shafts with couplings. The shaft must rest in bearings, such as journal bearings, ball bearings, or roller bearings. The shaft may be started by a clutch or stopped with a brake. It may be turned by a pulley with a flat belt, a V-belt, or a chain connecting it to a pulley in a second shaft. The supporting structure may be assembled with bolts or rivets or by welding. Proper application of these machine elements depend upon a knowledge of the forces on the structure and the strength of the materials employed. In the design, calculations must accommodate the forces to the materials in the simplest construction.Other machine elements have been evolved whose applications are more specific in construction. Machine parts which are more commonly used have been developed into standardized designs. Manufacturing specialists have concentrated upon the development of standard elements and have mass-produced these parts with a high degree of perfection at reduced cost.The course machine elements, as a branch of engineering, outlines the methods, rules, and standards for designing elements on the basis of the conditions of their operation in a machine, with a view to giving them the most advantageous forms and sizes, choosing the required materials,the degree of accuracy and surface finish and providing for