2024年1月12日发(作者：电脑怎么连接wifi)

Lesson 3 Diesel Engine Construction （１）

The Bedplate

The bedplate (Fig.7a) is the foundation on which the 2-stroke engine is built. It must be

rigid enough to support the weight of the rest of the engine, and maintain the crankshaft,

which sits in the bearing housings in the transverse girders, in alignment. At the same time it

must be flexible enough to hog and sag with the foundation plate to which it is attached and

which forms part of the ship’s structure.

If the bedplate was too rigid, then as the hull flexed, the holding down bolts, which secure

the engine into the ship would be likely to break, and there would be a danger of the bedplate

cracking.

Basically the bedplate consists of two longitudinal girders, which run the length of the

engine. Connecting these longitudinal girders are the transverse girders, which are positioned

between each crankshaft throw, and either side of the thrust collar. Built into the transverse

girders are the main bearing pockets for the crankshaft to run in.(1)

(a) (b)

Fig.7

The A-Frames

The A -Frames (Fig. 7b) carry the crosshead guides and support the engine entablature (the

cylinder block). On large engines, the A-frames are individually erected on the bedplate

directly above the transverse girders. When boxed in with plating they form the crankcase. The

trend nowadays is to build the frame box as a separate fabricated construction and then, after

stress relieving and machining the mating surfaces, to mount it on the bedplate. This has the

advantage of saving weight.(2)

When the frames are aligned on the bed-plate they are secured together by drilling and

reaming and using fitted bolts.

The tie rods

To hold the bed-plate, frames and entablature firmly together in compression, and to

transmit the firing forces back to the bedplate, long tie bolts ( Fig. 8a) are fitted through these

three components and then tightened hydraulically. To prevent excessive bending moments in

the transverse girders, the tie bolts are positioned as close to the center of the crankshaft as

possible. Because the tie bolts are so close to the crankshaft, some engines employ jack bolts

to hold the crankshaft main bearing cap in position instead of conventional studs and nuts.(3)

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(a) (b)

Fig. 8

The crankshaft

The crankshafts on the large modern 2-stroke crosshead engines can weigh over 300 tons.

They are too big to make as a single unit and so are constructed by joining together individual

forgings.(4) On older engines the so-called fully built method was used. This consisted of

forging separate webs, crankpins and main journals. The crankpins and journals were

machined and matching holes bored in the webs, which were slightly smaller in diameter. The

webs were heated up and the crankpins and journals fitted into the holes (which due to the

heat had expanded in size). As the webs cooled down, so the diameter of the bored holes

would try and shrink back to their original size. In doing so, the crankpins and journals would

be gripped tightly enough to stop them being able to slip when the engine was being operated

normally.

Today, crankshafts for large 2-stroke crosshead engines are of the semi built type. In this

method of construction the crankshaft "throws" consisting of two webs and the crankpin are

made from a single forging of a 0.4% carbon steel. The webs are bored to take the separately

forged and machined main journals which are fitted into the webs using the shrink fitting

method described above. The shrink fit allowance is between

1/570 and

1/660 of the diameter.

The advantages of this method of construction is that by making the two webs and crankpin

from a single forging the grain flow in the steel follows the web round into the crankpin and

back down the other web.(5)

The connecting rod

The Connecting Rod (Fig.8b) is fitted between the crosshead and the crankshaft. It

transmits the firing force, and together with the crankshaft converts the reciprocating motion

to a rotary motion.(6) Connecting Rods on the later engines are produced as a single drop

forging incorporating the top half of the crankpin bearing housing and the bottom half of the

solid crosshead pin bearing housing. On older engines the bearings were white metal thick

wall bearings, scraped to fit. Clearances were adjusted by inserting or removing shims

between the bearing halves. Modern bearings are of the "thin-wall" type, where a thin layer of

white metal or a tin aluminum alloy is bonded to a steel shell backing.(7)

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New Words and Expressions

foundation n. 基础

rigid adj. 坚固的

transverse adj. 横向的

girder n. 梁

flexible adj. 柔性的

hog v. 使拱曲，上拱

sag v. 下沉，下陷

flex v. 弯曲并伸展

holding down bolts 底脚螺栓，地脚螺栓

longitudinal adj. 纵向的

thrust collar 推力环

individually adv. 独立地

erect v. 建立，竖立

fabricated adj. 锻造的

ream v. 铰孔

jack bolts 撑杆螺栓

grip v. 紧握，抓紧

Notes to the text

1. Built into the transverse girders are the main bearing pockets for the crankshaft to run in.

制造在横梁上的是主轴承座，供曲轴转动。

该句是倒装句。正常的语序是：“The main bearing pockets for the crankshaft to run in

are built into the transverse girders.”。因主语较长，故倒置。

2. The trend nowadays is to build the frame box as a separate fabricated construction and

then, after stress relieving and machining the mating surfaces, to mount it on the bedplate.

This has the advantage of saving weight.

目前的趋势是按独立的锻造结构制造机架箱，在释放构件应力并对接触面光车后，安装在机座上。这种结构具有重量轻的优点。

3. Because the tie bolts are so close to the crankshaft, some engines employ jack bolts to

hold the crankshaft main bearing cap in position instead of conventional studs and nuts.

因为贯穿螺栓与曲轴靠得很近，一些发动机采用撑杆螺栓来代替传统的螺栓和螺母把曲轴的主轴承盖定位好。

4. They are too big to make as a single unit and so are constructed by joining together

individual forgings.

曲轴太大了，不能像单个部件一样来制造，因此通过把独立的锻件组装起来的方法

来建造。

5. The advantages of this method of construction is that by making the two webs and

crankpin from a single forging the grain flow in the steel follows the web round into the

crankpin and back down the other web.

采用这种结构的优点在于，通过把两个曲臂和曲柄销锻造成一体，钢结构内，力的流线能从曲臂圆滑地过度到曲柄销，然后向下回到另一曲臂。意即：半组合式的曲

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轴，由于在曲臂和曲柄销的连接处有较大的过渡圆角，能较好地消除曲轴内的应力集中。

6. It transmits the firing force, and together with the crankshaft converts the reciprocating

motion to a rotary motion.

连杆用来传递动力，并与曲轴一起，把往复运动转变成回转运动。

注意句中“together with”的用法，谓语动词应与前一个名词在人称和数上保持一致。再如：The main engine together with other driven machinery is running smoothly.

7. Modern bearings are of the "thin-wall" type, where a thin layer of white metal or a tin

aluminum alloy is bonded to a steel shell backing.

新型轴承是“薄壁”式的，一薄层白合金或锡铝合金附着在钢质轴瓦的内壁上。

句中第一个介词“of”作表语，表示属性。该句是由“which”引导的定语从句。

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