CHAPTER XII. BELTS FOR TRANSMITTING POWER.

The traction¹ of belts upon pulleys, like that of locomotive wheels upon railways, being incapable² of demonstration³ except by actual experience, for a long time hindered the introduction of belts as a means of transmitting motion and power except in cases when gearing or shafts⁴ could not be employed. Motion is named separately, because with many kinds of machinery⁶ that are driven at high speed—such as wood machines—the transmission of rapid movement must be considered as well as power, and in ordinary practice it is only by means of belts that such high speeds may be communicated from one shaft⁵ to another.

The first principle to be pointed⁷ out in regard to belts, to distinguish them from shafts as a means of transmitting power, is that power is communicated by means of tensile instead of torsional strain, the power during transmission being represented in the difference of tension between the driving and the slack side of belts. In the case of shafts, their length, or the distance to which they may be extended in transmitting power, is limited by torsional resistance; and as belts are not liable to this condition, we may conclude that unless there are other difficulties to be contended with, belts are more suitable than shafts for transmitting power throughout long distances. Belts suffer resistance from the air and from friction⁸ in the bearings of supporting pulleys, which are necessary in long horizontal belts; with these exceptions they are capable of moving at a very high rate of speed, and transmitting power without appreciable⁹ loss.

Following this proposition into modern engineering examples, we find how practice has gradually conformed to what these properties in belts suggest. Wire and other ropes of small diameter, to avoid air friction, and allowed to droop¹⁰ in low curves to avoid too many supporting pulleys, are now in many cases employed for transmitting power through long distances, as at Schaffhausen, in Germany. This system has been very successfully applied¹¹ in some cases for distributing power in large manufacturing establishments. Belts, among which are included all flexible bands, do not afford the same facilities for [49]taking off power at different points as shafts, but have advantages in transmitting power to portable machinery, when power is to be taken off at movable points, as in the case of portable travelling cranes, machines, and so on.

An interesting example in the use of belts for communicating power to movable machinery is furnished by the travelling cranes of Mr Ramsbottom, in the shops of the L. & N. W. Railway, at Crewe, England, where powerful travelling cranes receive both the lifting and traversing power by means of a cotton rope not more than three-fourths of an inch in diameter, which moves at a high velocity¹², the motion being reduced by means of tangent wheels and gearing to attain¹³ the force required in lifting heavy loads. Observing the operation of this machinery, a person not familiar with the relations between force and motion will be astonished at the effect produced by the small rope which communicates power to the machinery.

Considered as means for transmitting power, the contrast as to advantages and disadvantages lies especially between belts and gearing instead of between belts and shafts. It is true in extreme cases, such as that cited at Crewe, or in conveying water-power from inaccessible¹⁴ places, through long distances, the comparison lies between belts and shafts; but in ordinary practice, especially for first movers, the problem as to mechanism¹⁵ for conveying power lies between belts and gear wheels. If experience in the use of belts was thorough, as it is in the case of gearing, and if the quality of belts did not form so important a part in the estimates, there would be but little difficulty in determining where belts should be employed and where gearing would be preferable. Belts are continually taking the place of gearing even in cases where, until quite recently, their use has been considered impracticable; one of the largest rolling mills in Pittsburg, Pennsylvania, except a single pair of spur wheels as the last movers at each train of rolls, is driven by belts throughout.

Leaving out the matter of a positive relative movement between shafts, which belts as a means of transmitting power cannot insure, there are the following conditions that must be considered in determining whether belts or other means should be employed in transmitting power from one machine to another or between the parts of machines.

1. The distance to which power is to be transmitted.

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2. The speed at which the transmitting machinery must move.

3. The course or direction of transmission, whether in straight lines or at angles.

4. The cost of construction and durability¹⁶.

5. The loss of power during transmission.

6. Danger, noise, vibration¹⁷, and jar.

In every case where there can be a question as to whether gearing shafts or belts will be the best means of transmitting power, the several conditions named will furnish a solution if they are properly investigated and understood. Speed, noise, or angles may become determinative conditions, and are such in a large number of cases; first cost and loss of power are generally secondary conditions. Applying these tests to cases where belts, shafts, or wheels may be employed, a learner will soon find himself in possession of knowledge to guide him in his own schemes, and enable him to judge of the correctness of examples that come under his notice.

It is never enough to know that any piece of work is commonly constructed in some particular manner, or that a proposition is generally accepted as being correct; a reason should be sought for. Nothing is learned, in the true sense, until the reasons for it are understood, and it is by no means sufficient to know from observation alone that belts are best for high speeds, that gearing is the best means of forming angles in transmitting power, or that gearing consumes more power, and that belts produce less jar and noise; the principles which lie at the bottom must be reached before it can be assumed that the matter is fairly understood.

(1.) Why have belts been found better than shafts for transmitting power through long distances?—(2.) What are the conditions which limit the speed of belts?—(3.) Why cannot belts be employed to communicate positive movement?—(4.) Would a common belt transmit motion positively¹⁸, if there were no slip on the pulleys?—(5.) Name some of the circumstances to be considered in comparing belts with gearing or shafts as a means of transmitting power.

1 traction   

n.牵引；附着摩擦力

参考例句：

• I'll show you how the traction is applied.我会让你看如何做这种牵引。
• She's injured her back and is in traction for a month.她背部受伤，正在作一个月的牵引治疗。

2 incapable   

adj.无能力的，不能做某事的

参考例句：

• He would be incapable of committing such a cruel deed.他不会做出这么残忍的事。
• Computers are incapable of creative thought.计算机不会创造性地思维。

3 demonstration   

n.表明，示范，论证，示威

参考例句：

• His new book is a demonstration of his patriotism.他写的新书是他的爱国精神的证明。
• He gave a demonstration of the new technique then and there.他当场表演了这种新的操作方法。

4 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

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 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。

7 pointed   

adj.尖的，直截了当的

参考例句：

• He gave me a very sharp pointed pencil.他给我一支削得非常尖的铅笔。
• She wished to show Mrs.John Dashwood by this pointed invitation to her brother.她想通过对达茨伍德夫人提出直截了当的邀请向她的哥哥表示出来。

8 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。

9 appreciable   

adj.明显的，可见的，可估量的，可觉察的

参考例句：

• There is no appreciable distinction between the twins.在这对孪生子之间看不出有什么明显的差别。
• We bought an appreciable piece of property.我们买下的资产有增值的潜力。

10 droop   

v.低垂，下垂；凋萎，萎靡

参考例句：

• The heavy snow made the branches droop.大雪使树枝垂下来。
• Don't let your spirits droop.不要萎靡不振。

11 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。

12 velocity   

n.速度，速率

参考例句：

• Einstein's theory links energy with mass and velocity of light.爱因斯坦的理论把能量同质量和光速联系起来。
• The velocity of light is about 300000 kilometres per second.光速约为每秒300000公里。

13 attain   

vt.达到，获得，完成

参考例句：

• I used the scientific method to attain this end. 我用科学的方法来达到这一目的。
• His painstaking to attain his goal in life is praiseworthy. 他为实现人生目标所下的苦功是值得称赞的。

14 inaccessible   

adj.达不到的，难接近的

参考例句：

• This novel seems to me among the most inaccessible.这本书对我来说是最难懂的小说之一。
• The top of Mount Everest is the most inaccessible place in the world.珠穆朗玛峰是世界上最难到达的地方。

15 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。

16 durability   

n.经久性，耐用性

参考例句：

• Nylons have the virtue of durability.尼龙丝袜有耐穿的优点。

17 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。

18 positively   

adv.明确地，断然，坚决地；实在，确实

参考例句：

• She was positively glowing with happiness.她满脸幸福。
• The weather was positively poisonous.这天气着实讨厌。