英文资料.docx

1、英文资料 Sand Casting The first stage in the production of sand castings must be the design and manufacture of a suitable pattern. Casting patterns are generally made from hard word and the pattern has to be made larger than the finished casting size to allow for the shrinkage that takes place during so

2、lidification and cooling. The extent of this shrinkage varies with the type of metal or alloy to be cast. For all but the simplest shapes the pattern will be made in two or more pieces to facilitate moulding. If a hollow casting is to be made the pattern design will include extension pieces so that

3、spaces to accept the sand core are moulded into sand. These additional spaces in the mould are termed core prints. Sand moulds for the production of small and medium-sized castings are made in a moulding box. The mould is made in two or more parts in order that the pattern may be removed. The drag h

4、alf of the mould box is placed on a flat firm board and the drag half of he pattern placed in position. Facing sand is sprinkled over the pattern and then the mould box is filled with moulding sand. The sand is rammed firmly around the pattern. This process of filling and ramming may be done by hand

5、 but mould production is automated in a large foundry with the mould boxes moving along a conveyor, firstly to be filled with sand from hoppers and then to pass under mechanical hammers for ramming. When ramming of the sand is complete, excess sand is removed to leave a smooth surface flush with the

6、 edges of the moulding box. The completed drag is now turned over and the upper, or cope, portion of the moulding box positioned over it. The cope half of the pattern is placed in position, correct alignment being ensured by means of small dowel pins. Patterns for the necessary feeder, runner and ri

7、sers are also placed so as to give an even distribution of metal into the mould cavity. The risers should coincide with the highest readily escape from the mould. The sizes of risers should be such that the metal in them does not freeze too rapidly. An important function of a riser is to act as rese

8、rvoir of liquid metal to feed solidification within the mould. A thin coating of dry parting sand is sprinkled into mould at this stage. This is to prevent the cope and drag sticking together when the cope half is moulded. The cope is now filled with moulding sand and this is rammed firmly into shap

9、e in the same manner as in the making of the drag. After the ramming of sand in the cope is completed the two halves of the moulding box are carefully separated. At this stage venting of the moulding box are carefully separated. At this stage venting of the mould can be done, if necessary, to increa

10、se the permeability of the mould. After venting the patterns are carefully removed from both cope and drag, and a gate or gates are carefully cut to connect the runner channel with the main cavity. Gates should be sited to allow for entry into mould with a minimum of turbulence. Any loose sand is ge

11、ntly blown away and if a core is to be used it the cope upon the drag and it is then ready for use. Liquid metal is poured smoothly into the mould via the feeder. Pouring ceases when liquid metal appears at the top of the risers and the feeder channel is also full. When the metal that has been poure

12、d into a sand mould has fully solidified the mould is broken and casting is removed. The casting still has the runner and risers attached to it and there will be sand adhering to portions of the surface. Runners and risers are cut off and returned to the melting furnace. Sand cores are broken and ad

13、herent sand is cleaned from the surface by vibration or by sand blasting with dry sand. Any fins or metal flash formed at mould parting lines are removed by grinding and the castings are then ready for inspection. Engine An engine is a device that uses the energy in a fuel to do work. The energy in

14、the chemicals of the fuel is turned into heat energy. The heat is then used to move the metal parts of a machine. There are many kinds of fuel. Most engines use gasoline, oil, kerosene, coal, or coke. The heat that comes from burning the fuel makes a gas expand. This expand gas drives pistons or tur

15、bine blades. The pistons or turbines turn shafts. The turning shafts move gears and other wheels. We use these rotating wheels and shafts to move automobiles, airplanes, and other transport. We can also use them for pumping, drilling, digging, and other such activities. Early engines burned coal or

16、wood to heat water. The steam was used to drive steam engine. Until the middle of this century, most locomotives were powered by steam. At the beginning of the century, even some automobiles were run on steam. We still use steam engines, but most of them are being replaced by more efficient engines.

17、 Today we have powerful gasoline and diesel engines to work for us. The steam engine is an external combustion engine. This means that fuel is combustion, or burned, outside the cylinder that produces power. Since the invention of the steam engine by James Watt in 1769s, steam engines had a great ef

18、fect on the industrial revolution in the eighteenth century. Until the middle of the twentieth century, most locomotives were powered by steam. At the end of the century, even some mobiles were run on steam. Today most of the functions of steam engine have been taken over by internal combustion engi

19、nes fueled by gasoline and diesel oil. However, there are many steam engines in use at sea. Steam is also used to generate electricity. The kind of steam engine that is used today does not have cylinders. It is generally a steam turbine. The steam turbine was invented by Sir Charles Parsons. Steam t

20、urbines can handle steam at higher pressures than piston engines can, and they are more compact than piston engines of similar power. The gasoline engine The engine of most automobiles and small vehicle use gasoline as fuel. The gasoline engine is a combustion engine. The fuel is burned in combustio

21、n (burning) chambers inside the engine. The combustion chambers are placed at one end of the cylinders. Pistons move up and down in the cylinders. They are pushed by the hot gases from the burning fuel. When the fuel is mixed with air it burns so quickly that it explodes. The combustion chambers and

22、 cylinder apart like a bomb, the explosion simply kicks hand against the head of the piston. It pushes it as far as it can. Each movement of a piston up or down in its cylinder is called a stroke. Most gasoline engines work on a four-stroke cycle. This means that each piston goes up and down twice f

23、or each explosion. That makes four movement or strokes. This cycle of events is repeated over and over again. On the first down stroke, the piston moves to the lowest part of the cylinder. A mixture of gasoline droplets and air is drawn into the cylinder above it. Now the piston moves up again. This

24、 is its second stroke. It squeezes the mixture into a small space. An electric lights the mixture, and it explodes. The piston is force down again for its third stroke. This is called the power stroke. For the fourth stroke, the piston moves to the top again. This time it pushes the burnt gases out

25、of the cylinder. The gases leave the engine as exhaust fumes. The first engine that used the four-stroke cycle was made in about 1876. It was designed by a German engineer, August Otto. He used coal gas, not gasoline. The first engines to burn gasoline were developed by Karl Benz and Gottlieb. These

26、 two men were famous as automobile pioneers. A piston simply going up and down cannot push an automobile along. Its movement must be changed to a turning movement. To do this, a crankshaft is used. Each piston of the engine is linked to part of the crankshaft. Each push it gives makes the shaft turn

27、. The spinning shaft passes the power on to the automobiles transmission system. It usually does this through a heavy flywheel. The transmission system transmits power to the clutch and to the propeller shaft, through a gearbox. The propeller shaft drives the road wheels by means of axles. To keep a

28、n automobile engine going, there need to be several systems. There must be a fuel system. This has to supply gasoline to the engine cylinders in the right amounts. It has also to mix it with the right amount of air, so that it will explode properly. There must be an ignition system. This has to prov

29、ide sparks to ignite the explosive mixture fat exactly the right time. There has to be a cooling system, otherwise the engine would overheat. The lubrication system must keep all the moving parts oiled and moving freely. Too much friction causes wear of the metal and makes the engine overheat. The e

30、ngine unit Gasoline engine has two basic parts. They are called the cylinder head and the cylinder block. The cylinder block is machined from solid metal. The metals usually cast side the cylinder blocks are the cylinders. The walls of the cylinders have to be very accurately made, and are highly po

31、lished. The pistons that move up and down in the cylinders must be accurately made, too. They have springy bands of metal around them to press tight against the cylinder walls and stop gases leaking. The bands are called piston rings. They are often made of aluminum alloy for strength and lightness.

32、 An engine may have any number of cylinders. They may be arranged in a line, or in opposite pairs. They are often arranged in a V shape. In many airplanes with piton engines, the cylinders are arranged in a ring around the crankshaft. The lower part of the cylinder block is called the crankcase. Thi

33、s is where the crankshaft lies. The crank shaft is linked to each piton by a connecting rod. The crankshaft is made in a single piece. It must be tough and accurately machined. It may spin as many as 6000 times a minute. It changes up-and-down motion. It does this by means of cranks, one for each piston. The cranks are set at different ang