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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 solidification 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 spaces to accept the sand core are moulded into sand. The 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 half of the mould box is placed on a flat firm board and the drag half of the 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 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 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 rirs are also placed so as to give an even distribution of metal into the mould cavity. The rirs should coincide with the highest readily escape from the mould. The sizes of rirs should be such that the metal in them does not freeze too rapidly. An important function of a rir is to act as rervoir 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 shape 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
parated. At this stage venting of the moulding box are carefully parated. At this stage venting of the mould can be done, if necessary, to increa 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 loo sand is gently blown away and if a core is to be ud it the cope upon the drag and it is then ready for u. Liquid metal is poured smoothly into the mould via the feeder. Pouring ceas when liquid metal appears at the top of the rirs and the feeder channel is also full.
When the metal that has been poured into a sand mould has fully solidified the mould is broken and casting is removed. The casting still has the runner and rirs attached to it and there will be sand adhering to portions of the surface. Runners and rirs are cut off and returned to the melting furnace. Sand cores are broken and adherent 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幺九牌型图片
An engine is a device that us the energy in a fuel to do work. The energy in the chemicals of the fuel is turned into heat energy. The heat is then ud to move the metal parts of a machine. There are many kinds of fuel. Most engines u gasoline, oil, kerone, coal, or coke. The heat that comes from burning the fuel makes a gas expand. This expand gas drives pistons or turbine blades. The pistons or turbines turn shafts. The turning shafts move gears and other wheels. We u the rotating wheels and shafts to move automobiles, airplanes, and other transport. We can also u them for pumping, drilling, digging, and other such activities.什么运动瘦脸
Early engines burned coal or wood to heat water. The steam was ud 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 u steam engines, but most of them are being replaced by more efficient engines. Today we have powerful gasoline and diel 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 effect 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 cent
ury, even some automobiles were run on steam. Today most of the functions of steam engine have been taken over by internal combustion engines fueled by gasoline and diel oil. However, there are many steam engines in u at a. Steam is also ud to generate electricity. The kind of steam engine that is ud today does not have cylinders. It is generally a steam turbine. The steam turbine was invented by Sir Charles Parsons. Steam
turbines 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 u gasoline as fuel. The gasoline engine is a combustion engine. The fuel is burned in combustion (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 gas from the burning fuel. When the fuel is mixed with air it burns so quickly that it explodes. The combustion chambers and 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 for 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 is its cond 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 gas out of the cylinder. The gas leave the engine as exhaust fumes.
The first engine that ud the four-stroke cycle was made in about 1876. It was designed by a German engineer, August Otto. He ud coal gas, not gasoline. The first engines to burn gasoline were developed by Karl Benz and Gottlieb. The two men were famous as automobile pioneers.
国企实习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 ud. Each piston of the engine is linked to part of the crankshaft. Each push it gives makes the shaft turn. The spinning shaft pass the power on to the automobile’s 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 an automobile engine going, there need to be veral 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 provide sparks to ignite the explosive mixture fat exactly the right time. There has to be a cooling system, otherwi the engine would overheat. The lubrication system must keep all the moving parts oiled and moving freely. Too much friction caus wear of the metal and makes the engine overheat.
The engine 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 polished. 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 gas leaking. The bands are called piston rings. They are often made of aluminum alloy for strength and lightness.
An engine may have any number of cylinders. They may be arranged in a line, or in opposite pairs. T
hey are often arranged in a “V” shape. In many airplanes with piton engines, the cylinders are arranged in a ring around the crankshaft.
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The lower part of the cylinder block is called the crankca. This 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 t at different angles round the shaft. Each piston gives a push to its crank during its power stroke. During the other three strokes, the crank pushes the piston up ,down and up again. The heavy flywheel is bolted to one end of the crankshaft. It keeps the shaft turning smoothly between the power strokes. Strong main bearings support the crankshaft in its ca.
The cylinder head is bolted to the cylinder block. Inside it are the combustion chambers. Each combustion chamber is a space above a piston inside a cylinder. This is where the explosion of the mixture of fuel and air takes place. A spark plug is t into the top of the chamber. Each chamber has a pair of valves. There is an inlet valve to allow the fuel and air mixture into the chamber. The other valve is an outlet or exhaust valve. Through this pass the burnt gas after the explosion. The valves are opened and clod by push rods and springs. The push rods are moved up and down by l
inks with the crankshaft. The linkage is through a turning shaft called the camshaft.
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To carry water to cool the engine, there are passages in the metal of the cylinder head and cylinder block. Oil pass through other passages. Between the cylinder head and block is a gasket.
烂的成语This is a thin plate of metal that acts as a al. It is put in when the parts of the engine are bolted together. It is often made of copper.
The fuel system Fuel system of the engine supplies the gasoline to be burnt. The gasoline is stored in a large tank. In a powerful car, the tank holds many gallons of fuel. The tank is placed well away from the engine, to reduce the risk of fire .The gasoline is pumped through a fuel line. The pump may be driven by links with the engine camshaft. Sometimes it has its own electric motor. Before the gasoline reaches the cylinders, it must be mixed with air. The fuel line
开心豆leads to the carburetor. In the carburetor the gasoline is forced through a fine nozzle, or jet. It forms a spray of small droplets. The droplets vaporize as they mix with the air. Now the mixture is ready for ignition.
The speed of the engine is controlled by a valve. Opening and closing the throttle valve regulates the
amount of mixture leaving the carburetor. From the carburetor the mixture pass to the inlet valves of the combustion chambers. The mixture is delivered through a t of tubes called the inlet manifold. A similar t of tubes takes away exhaust gas form the exhaust valves of the combustion chambers. This is called the exhaust manifold. It leads to the outside air through the exhaust pipe.
In some automobiles, the fuel is delivered by a different system. Instead of passing through a carburetor, the fuel is sprayed into the air stream just before the inlet valves. It is sprayed in small amounts, which are carefully metered. The system is called fuel injection.
The ignition system Mixture inside each cylinder must be made to explode. A spark is ud to do this. The spark must jump across the gap in the spark plug at exactly the right times, each of the autombile’s cylinders must fire in turn. The ignition system depends upon very accurate timing. If the sparks are a fraction of a cond too early or too late, the engine will not run properly.
The electricity to make the sparks comes from the automobile’s electric storage battery. This battery is kept charged by a dynamo, or generator, run by the engine. The battery supplies electricity at only about 12 volts. To make a spark, volts are required. The voltage from the battery is boosted up to about 30 000 volts by means of a transformer. The transformer is called the ignition coil. The ignition
coil supplies high voltage to the distributor. This is a device that distributes in surges, or puls, to each of the spark plugs in turn. When a surge of electricity at high voltage reaches a plug, a spark leaps across a small gap. The spark is so hot that it makes the mixture of gasoline and air ignite and explode.
泡椒猪蹄The cooling system Heat produced by burning the gasoline in the engine is very great. The temperature inside each combustion chamber may reach more than 1 000℃ [over1 800].The engine must be constantly cooled. The cooling system supplies cool water through channels called water jack. The water jackets surround the cylinders. They carry away the excess heat as the water pass through them. The hot water is led away to be cooled in radiator. The radiator is a system of many tubes, linked together, with spaces between them. The hot water los its heat to the air. The cooled water is then pumped to the engine again.
Some automobiles do not have a water-cooled system. They u air cooling. Air from the front
Of the automobile is blown over the cylinder block and the cylinder head. The engine is filled
