船舶轮机英语lesson 13

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The gudgeon pin is carried in bosses in the piston skirt. Generally the gudgeon pin is fully floating in both connecting rod small end and the piston bosses.
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circlip
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Bearings The main bearing shells are made of steel with a lining of bearing metal, which may be white metal, copperlead or aluminium-tin(锡) alloy; they often have a thin flash of lead or indium(铟元素) to provide a layer giving protection against corrosion.
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Accordingly it is essential that the large end bolts are tightened correctly to the designed pull as under- or over-tightening will result in the housing bore departing from its true circular shape.
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This is required much more often in the case of the exhaust valves because of operation at higher temperature and because the gases flowing through may contain particles of carbonaceous matter which occasionally get trapped under the valve seat causing pitting.
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Being smaller also assists with keeping them cool which is important as exhaust valves often give rise to thermal problems.
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Inlet and Exhaust Valves The exhaust valves open against the pressure within the cylinder at the end of the working stroke.
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In another design the chamber is open and constructed so that the oil splashes about violently.
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This
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It is located endwise, either by pads(端盖) fixed to the piston or by means of circlips( 卡 簧 ) entered into small end grooves at each end of the piston boss bore.
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The simplest means is by splash(飞溅) or spray(喷射) on
the underside of the piston crown.
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A more complex design involves a chamber specially constructed for the oil to circulate through.
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Bowl/Toroidal shape Cut-off portion
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The cooling of the piston is carried out by circulating lubricating oil across the underside of the crown and inside the ring belt or through specially shaped passages.
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The two halves are held together by bridges( 轴 承 盖 ). The shells depend on the accuracy(精确性) of the bore to hold them in a truly circular shape.
the ‘cocktailshaker( 震 荡 式 )’; the motion of the oil providing extremely good heat transfer.
design
is
termed
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The pistons of high output(power) engines are often made in two pieces bolted together to form precisely shaped cooling chambers.
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Because of these considerations it is not unusual to find that the exhaust valves are designed to be of smaller diameter than the inlet valves.
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The combustion chamber is enclosed between the cylinder head and the piston and the most of it is contained in the top of the piston.
This pressure is considerably higher than that against which the inlet valves have to open. Furthermore, the pressure of the exhaust gases assists, once the valve is open, in expelling(排出) the gases through the open valve.
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Pistons
be made in aluminium alloy in order to keep the weight down for balancing purposes at high speed. In other designs, where the necessity for light weight is not so important, they are made wholly of cast iron.
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may
The high output two-piece design is usually used in conjunction with a heat resistant steel crown bolted to a skirt portion which may be of light alloy or of cast iron.
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The difference is termed ‘nip(压 隙)’. When the assembly is bolted up, the result is the equivalent of
interference fit( 过 盈 配 合 )
between the shells and the housing.
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In some designs this chamber takes the form of a coil cast into the material of the piston and conducting the heat away from the piston ring region.
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The correct tightening is obtained by the use of stretch gauges(拉伸 仪) to measure the extension given to the bolts, or by using a torque wrench(扭力搬手), or by means of special hydraulic tightening equipment.
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For this purpose the top surface of the piston crown may be bowl shaped or toroidal( 环 形 的 ) shaped.
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The rim of the piston may contain cut-out portions( 避 让 坑 ) to accommodate the valves when they open.
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They(bearing shells) are held in position and shaped by the bore of the bedplate or frame. The external circumference of a pair of main bearing shells is slightly larger than that of the bore of the housing which receives them.
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The design of the connecting rod large end bearing is similar to that of the main bearing.
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The bearing shell generally has tags( 标 记 ) for location and oil grooves round the center of its axial length which correspond with grooves in the bearing housing through which oil is transmitted up the rod to lubricate the small end and to cool the piston.
The inlet and exhaust valve seats may become damaged during operation and from time to time they have to ቤተ መጻሕፍቲ ባይዱe reconditioned(修理) by grinding in the valves.
LESSON 13
MARINE MEDIUM SPEED(300—1000r/min) DIESEL ENGINE Ⅱ
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Pistons The piston is a pot shaped(筒状) component. The crown is fairly thick with the cylindrical parts tapering to a thinner section.