шпаргалка по текстам 2 (english_shpora2)

Посмотреть архив целиком

Engineering materials, Metals, Non metals

There are two kinds of materials used in engineering – metals and non-metals. We can divide metals into ferrous and non-ferrous. The former contain iron and the latter don’t contain iron. Cast iron and steel are the two most important ferrous metals. Chromium can be added to iron to resist corrosion, and tungsten to increase hardness. Aluminium, copper, and alloys (bronze and brass) are common non-ferrous metals.

Plastics and ceramics are nonmetals; however, plastics may be machined like metals. Plastics are classified in two types - thermo-plastics and termosets. Thermo-plastics can be shaped and reshaped by heat and pressure, but termosets cannot be reshaped, because they undergo chemical changes as they harden. Ceramics are often employed by engineers when materials which can resist high temperatures are needed.


Engine

An engine produces power by burning air and fuel. The fuel and air are drawn into the engine cylinder by the piston. Then the fuel and air are compressed by the piston and ignited by the spark plug. They burn and expand very quickly and push the piston down. Thus the power is produced, air are expelled from the cylinder by the piston. There is an inlet valve allowing fresh fuel mixture into the cylinder and an exhaust valve which allows the burnt gases to escape. There are two basic engine operating cycles: a) the four-stroke cycle; b) the two-stroke cycle.

The complete four-stroke cycle comprises:

the induction stroke (the piston moves downwards);

the compression stroke (the piston moves upwards);

the power stroke (the piston moves downwards);

the exhaust stroke (the piston moves upwards).


Flexible production and robots

This country’s mashine-building industry is now facing the task of restructing on a large scale engineering production,and developing new methods of organization,new equipment and new technologies. Industrial robots play an important part in the process. Many institutes are currently engaged in developing them. The concept of designing robot modules is making successful headway with computer.

Also needed for the operation of flexible systems are robots which will transport works and parts between machine tools. All processes producing by robots are controlled a computer.

The combination of flexible systems with the general system of programmed production, the spreading of flexibility to the processes of preparatory productions — foundry, forging and welding — are also very complicated problems. The flexible system must embrace all the stages of machine building.


There five categories of cutting

There are five general categories of machine-tool. They are lathes, drilling, boring, milling and grinding machines. These machine-tools can perform turning, milling, grinding, boring and etc. operations. Finished parts possess symmetrical shapes, high dimensional accuracies and smooth surfaces. A lot of simple and complex shapes can be produced on screw machines. Screw machining is also used for finished operations.

The part of a computer system

Computer system may be disscussed in two parts.The first part is hardware – the physical,electronic, and electromechanical devices that are thought of and recoghnized as “computers”. The hardway consists of Central Processing Unit (CPU), input devices and output devices.The CPU is made up of a processor and a main memory,or main store.Input devices are used to provide data from CPU. The keyboard is a common data input device. By using a keyboard, a user can enter data directly into the computer system. Output devices receive data from CPU. The Visual Display Unit (VDU) and printer are common output devices. The VDU is similar to a television screen. The second part is software – the programs that control and coordinate the activities of the computer hardware and that direct the processing of data. Computer software can be divided into two very broad categories – systems software and application software.


Metal casting – a basic manufacturing process

One of the basic processes of the metal-working industry is the production of metal castings. A casting may be defined as “a metal object obtained by allowing molten metal to solidify in a mold”, the shape of the object being determined by the shape of the mold cavity. Numerous methods have been developed through the ages for producing metal castings but the oldest method is that of making sand castings in the foundry. Primarily, work consists of melting metal in a furnace and pouring it into suitable sand molds where it solidifies and assumes the shape of the mold. Most castings serve as details or components parts of complex machines and products. In most cases they are used only when they are mashined and finished to specified manufacturing tolerances providing easy and proper assembly of the product.


Forging process and equipment

Open die forging with modern hammers and presses is a technological extension of the pre-industrial blacksmith working with a hammer amd anvil. Open die forgings are produced on flat dies, round swaging dies and V-dies, either in pairs or in combination with a flat die. As workpiece is formed during open forging, it is moved via a manipulator in small increments untill hot working forces the metal into the desired dimensions. After forging the part is rough, then finish machines to net dimensions. Impression die forging compress the majority of commercial forging production. Close die forging, a variation of impression die forging, does not depend on the formation of flash to complete die filling. In true closed die forging, the metal is deformed in a cavity that allows little or no escape of excess metal.


Plastics are a large and varied group of materials consisting of combinations of carbon and oxygen, hydrogen, nitrogen, and other organic and inorganic elements. There are over 40 different families of plastics in commercial use today? And each may have dozens of subtypes and variations.

A successful design in plastics is always a compromise among highest performance, attractive appearance, efficient production, and lowest cost.

Most people have now outgrown the impression that plastics are low-cost substitute materials. Those that still view plastics as cheap and unreliable have not kept up with developments in polymer technology for the past ten years.

Today’s engineering resins and compounds serve in the most demanding environments. Their toughness, lightness, strength, and corrosion resistance have won many significant applications for these materials in transportation, industrial and consumer products. The engineering plastics are now challenging the domains traditionally held by metals: truly load-bearing, structural parts.

Plastics and ceramics are non-metals; however, plastics may be machined like metals. Plastics are classified into two types – thermoplastics and thermosets. Thermoplastics can be shaped and reshaped by heat and pressure but thermosets cannot be reshaped because they undergo chemical changes as they harden. Engineers often employ ceramics when materials, which can withstand high temperatures, are needed.

Fibers are probably the oldest engineering materials used by man. Jute, flax, and hemp have been used for “engineered” products such as rope, cordage, nets, water hose, and containers since antiquity. Other plant and animal fibers have been used for felts, brushes, and heavy structural cloth.

The fiber industry is clearly divided between natural fibers (from plant, animal, or mineral sources) and synthetic fibers. Many synthetic fibers have been developed specifically to replace natural fibers, because synthetics often behave more predictably and usually more uniform in size.

For engineering purposes, glass, metallic, and organically derived synthetic fibers are most significant.




Случайные файлы

Файл
43034.rtf
185464.rtf
161498.rtf
Shpora.doc
2777.rtf




Чтобы не видеть здесь видео-рекламу достаточно стать зарегистрированным пользователем.
Чтобы не видеть никакую рекламу на сайте, нужно стать VIP-пользователем.
Это можно сделать совершенно бесплатно. Читайте подробности тут.