Water may be considered as a rigid5 medium for transmitting power, corresponding to shafts6 and gear wheels; air as a flexible or yielding one, corresponding to belts. There is at this time but a limited use of pneumatic apparatus7 for transmitting power, but its application is rapidly extending, especially in transporting material by means of air currents, and in conveying power to mining machinery.
The successful application of the pneumatic system at the Mont Cenis Tunnel in Italy, and at the Hoosac Tunnel in America, has demonstrated the value of the system where the air not only served to transmit power to operate the machinery but to ventilate the mines at the same time. Air brakes for railway trains are another example illustrating8 the advantages of pneumatic transmission; the force being multiplied at the points where it is applied9, so that the connecting pipes are reduced to a small size, the velocity10 of the air making up for a great force that formerly11 had to be communicated through rods, chains, or shafts. The principal object attained12 by the use of air to operate railway brakes is, however, to maintain a connection throughout a train by means of flexible pipes that accommodate themselves to the varying distance between the carriages. Presuming that the flow of air in pipes is not materially impeded13 by friction14 or angles, and that there will be no difficulty in maintaining lubrication for pistons15 or other inaccessible16 parts of machinery when driven by air, there seems to be many reasons in favour of its use as a means of distributing power in manufacturing districts. The diminished cost of motive17 power when [56] it is generated on a large scale, and the expense and danger of maintaining an independent steam power for each separate establishment where power is employed, especially in cities, are strong reasons in favour of generating and distributing power by compressed air, through pipes, as gas and water are now supplied.
Air seems to be the most natural and available medium for transmitting and distributing power upon any general system like water or gas, and there is every probability of such a system existing at some future time. The power given out by the expansion of air is not equal to the power consumed in compressing it, but the loss is but insignificant18 compared with the advantages that may be gained in other ways. There is no subject more interesting, and perhaps few more important for an engineering student to study at this time, than the transmission of power and the transport of material by pneumatic apparatus.
In considering pneumatic machinery there are the following points to which attention is directed:—
1. The value of pneumatic apparatus in reaching places where steam furnaces cannot be employed.
2. The use that may be made of air after it has been applied as a motive agent.
3. The saving from condensation19, to which steam is exposed, avoidance of heat, and the consequent contraction20 and expansion of long conducting pipes.
4. The loss of power by friction and angles in conducting air through pipes.
5. The lubrication of surfaces working under air pressure, such as the pistons and valves of engines.
6. The diminished cost of generating power on a large scale, compared with a number of separate steam engines distributed over manufacturing districts.
7. The effect of pneumatic machinery in reducing insurance rates and danger of fire.
8. The expense of the appliances of distribution and their maintenance.
In passing thus rapidly over so important a subject, and one that admits of so extended a consideration as machinery of transmission, the reader can see that the purpose has been to touch only upon such points as will lead to thought and investigation21, and especially to meet such queries22 as are most likely to [57] arise in the mind of a learner. In arranging and erecting23 machinery of transmission, obviously the first problem must be, what kind of machinery should be employed, and what are the conditions which should determine the selection and arrangement? What has been written has, so far as possible, been directed to suggesting proper means of solving these questions.
(1.) In what respect are air and water like belts and gearing, as means to transmit power?—(2.) What are some of the principal advantages gained by employing air to operate railway brakes?—(3.) Name some of the advantages of centralising motive power.—(4.) Are the conditions of working an engine the same whether air or steam is employed?
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1
machinery
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| n.(总称)机械，机器；机构 | |
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2
elasticity
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| n.弹性，伸缩力 | |
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analogous
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| adj.相似的；类似的 | |
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4
hydraulic
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| adj.水力的；水压的，液压的；水力学的 | |
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rigid
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| adj.严格的，死板的；刚硬的，僵硬的 | |
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6
shafts
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| 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） | |
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apparatus
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| n.装置，器械；器具，设备 | |
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8
illustrating
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| 给…加插图( illustrate的现在分词 )； 说明； 表明； （用示例、图画等）说明 | |
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9
applied
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| adj.应用的；v.应用，适用 | |
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10
velocity
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| n.速度，速率 | |
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11
formerly
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| adv.从前，以前 | |
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attained
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| （通常经过努力）实现( attain的过去式和过去分词 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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impeded
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| 阻碍，妨碍，阻止( impede的过去式和过去分词 ) | |
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14
friction
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| n.摩擦，摩擦力 | |
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pistons
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| 活塞( piston的名词复数 ) | |
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16
inaccessible
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| adj.达不到的，难接近的 | |
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17
motive
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| n.动机，目的；adv.发动的，运动的 | |
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18
insignificant
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| adj.无关紧要的，可忽略的，无意义的 | |
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19
condensation
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| n.压缩，浓缩；凝结的水珠 | |
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contraction
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| n.缩略词，缩写式，害病 | |
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21
investigation
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| n.调查，调查研究 | |
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22
queries
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| n.问题( query的名词复数 )；疑问；询问；问号v.质疑，对…表示疑问( query的第三人称单数 )；询问 | |
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erecting
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| v.使直立，竖起( erect的现在分词 )；建立 | |
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