CHAPTER X. MACHINERY FOR TRANSMITTING AND DISTRIBUTING POWER.

To construe¹ the term "transmission of power" in its full sense, it will, when applied² to machinery³, include nearly all that has motion; for with the exception of the last movers, or where power passes off and is expended⁴ upon work that is performed, all machinery of whatever kind may be called machinery of transmission. Custom has, however, confined the use of the term to such devices as are employed to convey power from one place to another, without including organised machines through which power is directly applied to the performance of work. Power is transmitted by means of shafts⁶, belts, friction⁷ wheels, gearing, and in some cases by water or air, as various conditions of the work to be performed may require. Sometimes such machinery is employed as the conditions do not require, because there is, perhaps, nothing of equal importance connected with mechanical engineering of which there exists a greater diversity of opinion, or in which there is a greater diversity of practice, than in devices for transmitting power.

I do not refer to questions of mechanical construction, although the remark might be true if applied in this sense, but to the kind of devices that may be best employed in certain cases.

It is not proposed at this time to treat of the construction of machinery for transmitting power, but to examine into the conditions that should determine which of the several plans of transmitting is best in certain cases—whether belts, gearing, or shafts should be employed, and to note the principles upon which they operate. Existing examples do not furnish data as to the advantages of the different plans for transmitting power, because a given duty may be successfully performed by belts, gearing, or shafts—even by water, air, or steam—and the comparative advantages of different means of transmission is not always an easy matter to determine.

Machinery of transmission being generally a part of the fixed⁸ plant of an establishment, experiments cannot be made to institute comparisons, as in the case of machines; besides, there are special or local considerations—such as noise, danger, freezing, and distance—to be taken into account, which prevent any rules of general application. Yet in every case it may be assumed that some particular plan of transmitting power is better than any other, and that plan can best be determined⁹ by studying, first, the principles of different kinds of mechanism¹⁰ and its adaptation to the special conditions that exist; and secondly¹¹, precedents¹² or examples.

A leading principle in machinery of transmission that more than any other furnishes data for strength and proper proportions is, that the stress upon the machinery, whatever it may be, is inverse¹³ as the speed at which it moves. For example, a belt two inches wide, moving one thousand feet a minute, will theoretically perform the same work that one ten inches wide will do, moving at a speed of two hundred feet a minute; or a shaft⁵ making two hundred revolutions a minute will transmit four times as much power as a shaft making but fifty revolutions in the same time, the torsional strain being the same in both cases.

This proposition argues the expediency¹⁴ of reducing the proportions of mill gearing and increasing its speed, a change which has gradually been going on for fifty years past; but there are opposing conditions which make a limit in this direction, such as the speed at which bearing surfaces may run, centrifugal strain, jar, and vibration¹⁵. The object is to fix upon a point between what high speed, light weight, cheapness of cost suggest, and what the conditions of practical use and endurance demand.

(1.) What does the term "machinery of transmission" include, as applied in common use?—(2.) Why cannot direct comparisons be made between shafts, belts, and gearing?—(3.) Define the relation between speed and strain in machinery of transmission.—(4.) What are the principal conditions which limit the speed of shafts?

1 construe   

v.翻译，解释

参考例句：

• He had tried to construe a passage from Homer.他曾尝试注释荷马著作的一段文字。
• You can construe what he said in a number of different ways.他的话可以有好几种解释。

2 applied   

adj.应用的；v.应用，适用

参考例句：

• She plans to take a course in applied linguistics.她打算学习应用语言学课程。
• This cream is best applied to the face at night.这种乳霜最好晚上擦脸用。

3 machinery   

n.(总称)机械，机器；机构

参考例句：

• Has the machinery been put up ready for the broadcast?广播器材安装完毕了吗？
• Machinery ought to be well maintained all the time.机器应该随时注意维护。

4 expended   

v.花费( expend的过去式和过去分词 )；使用（钱等）做某事；用光；耗尽

参考例句：

• She expended all her efforts on the care of home and children. 她把所有精力都花在料理家务和照顾孩子上。
• The enemy had expended all their ammunition. 敌人已耗尽所有的弹药。 来自《简明英汉词典》

5 shaft   

n.(工具的)柄，杆状物

参考例句：

• He was wounded by a shaft.他被箭击中受伤。
• This is the shaft of a steam engine.这是一个蒸汽机主轴。

6 shafts   

n.轴( shaft的名词复数 )；（箭、高尔夫球棒等的）杆；通风井；一阵（疼痛、害怕等）

参考例句：

• He deliberately jerked the shafts to rock him a bit. 他故意的上下颠动车把，摇这个老猴子几下。 来自汉英文学 - 骆驼祥子
• Shafts were sunk, with tunnels dug laterally. 竖井已经打下，并且挖有横向矿道。 来自辞典例句

7 friction   

n.摩擦，摩擦力

参考例句：

• When Joan returned to work,the friction between them increased.琼回来工作后，他们之间的摩擦加剧了。
• Friction acts on moving bodies and brings them to a stop.摩擦力作用于运动着的物体，并使其停止。

8 fixed   

adj.固定的，不变的，准备好的；(计算机)固定的

参考例句：

• Have you two fixed on a date for the wedding yet?你们俩选定婚期了吗？
• Once the aim is fixed,we should not change it arbitrarily.目标一旦确定，我们就不应该随意改变。

9 determined   

adj.坚定的；有决心的

参考例句：

• I have determined on going to Tibet after graduation.我已决定毕业后去西藏。
• He determined to view the rooms behind the office.他决定查看一下办公室后面的房间。

10 mechanism   

n.机械装置；机构，结构

参考例句：

• The bones and muscles are parts of the mechanism of the body.骨骼和肌肉是人体的组成部件。
• The mechanism of the machine is very complicated.这台机器的结构是非常复杂的。

11 secondly   

adv.第二，其次

参考例句：

• Secondly,use your own head and present your point of view.第二，动脑筋提出自己的见解。
• Secondly it is necessary to define the applied load.其次，需要确定所作用的载荷。

12 precedents   

引用单元； 范例( precedent的名词复数 )； 先前出现的事例； 前例； 先例

参考例句：

• There is no lack of precedents in this connection. 不乏先例。
• He copied after bad precedents. 他仿效恶例。

13 inverse   

adj.相反的，倒转的，反转的；n.相反之物；v.倒转

参考例句：

• Evil is the inverse of good.恶是善的反面。
• When the direct approach failed he tried the inverse.当直接方法失败时，他尝试相反的做法。

14 expediency   

n.适宜；方便；合算；利己

参考例句：

• The government is torn between principle and expediency. 政府在原则与权宜之间难于抉择。 来自《简明英汉词典》
• It was difficult to strike the right balance between justice and expediency. 在公正与私利之间很难两全。 来自辞典例句

15 vibration   

n.颤动，振动；摆动

参考例句：

• There is so much vibration on a ship that one cannot write.船上的震动大得使人无法书写。
• The vibration of the window woke me up.窗子的震动把我惊醒了。