To follow out the application of power to the propulsion of vessels4 and trains, with the many abstruse5 problems that would of necessity be involved, would be to carry this work far beyond the limits within which it is most likely to be useful to the apprentice6 engineer; besides, it would be going beyond what can properly be termed manipulation.
Marine7 and railway engineering have engrossed8 the best talent in the world; investigation9 and research has been expended10 upon these subjects in a degree commensurate with their importance, and it would be hard to suggest a single want in the many able text-books that have been prepared upon the subjects. Marine and railway engineering are sciences that may, in a sense, be separated from the ordinary constructive11 arts, and studied at the end of a course in mechanical engineering, but are hardly [61] proper subjects for an apprentice to take up at the beginning.
In treating of machinery for transport, as a class, the subject, as far as noticed here, will be confined to moving and handling material as one of the processes of manufacturing, and especially in connection with machine construction. If the amount of time, expense, labour, and machinery devoted12 to handling material in machine shops is estimated, it becomes a matter of astonishment13 to as many as have not previously14 investigated the subject; as an item of expense the handling, often exceeds the fitting on large pieces, and in the heavier class of work demands the most careful attention to secure economical manipulation.
It will be well for an apprentice to begin at once, as soon as he commences a shop course, to note the manner of handling material, watching the operation of cranes, hoists15, trucks, tackle, rollers; in short, everything that has to do with moving and handling. The machinery and appliances in ordinary use are simple enough in a mechanical sense, but the principles of handling material are by no means as plain or easy to understand. The diversity of practice seen in various plans of handling and lifting weights fully16 attests17 the last proposition, and it is questionable18 whether there is any other branch of mechanical engineering that is treated less in a scientific way than machinery of this class. I do not allude19 to the mechanism20 of cranes and other devices, which are usually well proportioned and generally well arranged, but to the adaptation of such machinery with reference to special or local conditions. There are certain inherent difficulties that have to be encountered in the construction and operation of machinery, for lifting and handling, that are peculiar21 to it as a class; among these difficulties is the transmission of power to movable mechanism, the intermittent22 and irregular application of power, severe strains, also the liability to accidents and breakage from such machinery being controlled by the judgment23 of attendants.
Ordinary machinery, on the reverse, is stationary24, generally consumes a regular amount of power, is not subjected to such uncertain strains, and as a rule acts without its operation being controlled by the will of attendants.
The functions required in machinery for handling material in a machine shop correspond very nearly to those of the human hands. Nature in this, as in all other things, where a comparison is possible, has exceeded man in adaptation; in fact, we cannot conceive of anything more perfect than the human hands for handling material—a duty that forms a great share of all that we term labour.
Considered mechanically as a means of handling material, the human hands are capable of exerting force in any direction, vertically26, horizontally, or at any angle, moving at various rates of speed, as the conditions may require, and with varying force within the limits of human strength. These functions enable us to pick up or lay down a weight slowly and carefully, to transport it at a rapid rate to save time, to move it in any direction, and without the least waste of power, except in the case of carrying small loads, when the whole body has to be moved, as in ascending27 or descending28 stairs. The power travelling cranes, that are usually employed in machine-fitting establishments, are perhaps the nearest approach that has been made to the human frame in the way of handling mechanism; they, however, lack that very important feature of a movement, the speed of which is graduated at will. It is evident that in machinery of any kind for handling and lifting that moves at a uniform rate of speed, and this rate of speed adapted, as it must be, to the conditions of starting or depositing a load, much time must be lost in the transit29, especially when the load is moved for a considerable distance. This uniform speed is perhaps the greatest defect in the lifting machinery in common use, at least in such as is driven by power.
In handling a weight with the hands it is carefully raised, and laid down with care, but moved as rapidly as possible throughout the intervening distance; this lesson of nature has not been disregarded. We find that the attention of engineers has been directed to this principle of variable speed to be controlled at will. The hydraulic30 cranes of Sir William Armstrong, for example, employ this principle in the most effective manner, not only securing rapid transit of loads when lifted, but depositing or adjusting them with a care and precision unknown to mechanism positively31 geared or even operated by friction32 brakes.
The principles of all mechanism for handling loads should be such as to place the power, the rate of movement, and the direction of the force, within the control of an operator, which, as has been pointed33 out, is the same thing in effect as the action of the human hands.
The safety, simplicity34, and reliable action of hydraulic machinery has already led to its extensive employment for moving and lifting weights, and it is fair to assume that the importance and success of this invention fully entitle it to be classed as one of the most important that has been made in mechanical engineering during fifty years past. The application of hydraulic force in operating the machinery used in the processes for steel Bessemer manufacture, is one of the best examples to illustrate35 the advantages and principles of the hydraulic system. Published drawings and descriptions of Bessemer steel plant explain this hydraulic machinery.
There is, however, a principle in hydraulic machinery that must be taken into account, in comparing it with positively geared mechanism, which often leads to loss of power that in many cases will overbalance any gain derived36 from the peculiar action of hydraulic apparatus37. I allude to the loss of power incident to dealing38 with an inelastic medium, where the amount of force expended is constant, regardless of the resistance offered. A hydraulic crane, for instance, consumes power in proportion to its movements, and not as the amount of duty performed; it takes the same quantity of water to fill the cylinders40 of such cranes, whether the water exert much or little force in moving the pistons41. The difference between employing elastic39 mediums like air and steam, and an inelastic medium like water, for transmitting force in performing irregular duty, has been already alluded42 to, and forms a very interesting study for a student in mechanics, leading, as it does, to the solution of many problems concerning the use and effect of power.
The steam cranes of Mr Morrison, which resemble hydraulic cranes, except that steam instead of water is employed as a medium for transmitting force, combine all the advantages of hydraulic apparatus, except positive movement, and evade43 the loss of power that occurs in the use of water. The elasticity44 of the steam is found in practice to offer no obstacle to steady and accurate movement of a load, provided the mechanism is well constructed, while the loss of heat by radiation is but trifling45.
To return to shop processes in manufacturing. Material operated upon has to be often, sometimes continually, moved from one place to another to receive successive operations, and this movement may be either vertically or horizontally as determined46, first, by the relative facility with which the material may be raised vertically, or moved horizontally, and secondly47, by [64] the value of the ground and the amount of room that may be available, and thirdly by local conditions of arrangement. In large cities, where a great share of manufacturing is carried on, the value of ground is so great that its cost becomes a valid48 reason for constructing high buildings of several storeys, and moving material vertically by hoists, thus gaining surface by floors, instead of spreading the work over the ground; nor is there any disadvantage in high buildings for most kinds of manufacture, including machine fitting even, a proposition that will hardly be accepted in Europe, where fitting operations, except for small pieces, are rarely performed on upper floors.
Vertical25 handling, although it consumes more power, as a rule costs less, is more convenient, and requires less room than horizontal handling, which is sure to interfere49 more or less with the constructive operations of a workshop. In machine fitting there is generally a wrong estimate placed upon the value of ground floors, which are no doubt indispensable for the heaviest class of work, and for the heaviest tools; but with an ordinary class of work, where the pieces do not exceed two tons in weight, upper floors if strong are quite as convenient, if there is proper machinery for handling material; in fact, the records of any establishment, where cost accounts are carefully made up, will show that the expense of fitting on upper floors is less than on ground floors. This is to be accounted for by better light, and a removal of the fitting from the influences and interference of other operations that must necessarily be carried on upon ground floors.
For loading and unloading carts and waggons50, the convenience of the old outside sling51 is well known; it is also a well-attested fact that accidents rarely happen with sling hoists, although they appear to be less safe than running platforms or lifts. As a general rule, the most dangerous machinery for handling or raising material is that which pretends to dispense52 with the care and vigilance of attendants, and the safest machinery that which enforces such attention. The condition which leads to danger in hoisting53 machinery is, that the power employed is opposed to the force of gravity, and as the force of gravity is acting54 continually, it is always ready to take advantage of the least cessation in the opposing force employed, and thus drag away the weight for which the two forces are contending; as a weight when under the influence of gravity is moved [65] at an accelerated velocity55, if gravity becomes the master, the result is generally a serious accident. Lifting may be considered a case wherein the contrivances of man are brought to bear in overcoming or opposing a natural force; the imperfect force of the machinery is liable to accident or interruption, but gravity never fails to act. Acting on every piece of matter in proportion to its weight must be some force opposing and equal to that of gravity; for example, a piece of iron lying on a bench is opposed by the bench and held in resistance to gravity, and to move this piece of iron we have to substitute some opposing force, like that of the hands or lifting mechanism, to overcome gravity.
As molecular56 adhesion keeps the particles of matter together so as to form solids, so the force of gravity keeps objects in their place; and to attain57 a proper conception of forces, especially in handling and moving material, it is necessary to familiarise the mind with this thought.
The force of gravity acts only in one direction—vertically, so that the main force of hoisting and handling machinery which opposes gravity must also act vertically, while the horizontal movement of objects may be accomplished58 by simply overcoming the friction between them and the surfaces on which they move. This is seen in practice. A force of a hundred pounds may move a loaded truck, which it would require tons to lift; hence the horizontal movements of material may be easily accomplished by hand with the aid of trucks and rollers, so long as it is moved on level planes; but if a weight has to be raised even a single inch by reason of irregularity in floors, the difference between overcoming frictional contact and opposing gravity is at once apparent.
One of the problems connected with the handling of material is to determine where hand-power should stop and motive59-power begin—what conditions will justify60 the erection of cranes, hoists, or tramways, and what conditions will not. Frequent mistakes are made in the application of power when it is not required, especially for handling material; the too common tendency of the present day being to apply power to every purpose where it is possible, without estimating the actual saving that, may be effected. A common impression is that motive power, wherever applied to supplant61 hand labour in handling material, produces a gain; but in many cases the [66] fallacy of this will be apparent, when all the conditions are taken into account.
Considered upon grounds of commercial expediency62 as a question of cost alone, it is generally cheaper to move material by hand when it can be easily lifted or moved by workmen, when the movement is mainly in a horizontal direction, and when the labour can be constantly employed; or, to assume a general rule which in practice amounts to much the same thing, vertical lifting should be done by motive power, and horizontal movement for short distances performed by hand. There is nothing more unnatural63 than for men to carry loads up stairs or ladders; the effort expended in such cases is one-half or more devoted to raising the weight of the body, which is not utilised in the descent, and it is always better to employ winding64 or other mechanism for raising weights, even when it is to be operated by manual labour. Speaking of this matter of carrying loads upward, I am reminded of the fact that builders in England and America, especially in the latter country, often have material carried up ladders, while in some of the older European countries, where there is but little pretension65 to scientific manipulation, bricks are usually tossed from one man to another standing66 on ladders at a distance of ten to fifteen feet apart.
To conclude. The reader will understand that the difficulties and diversity of practice, in any branch of engineering, create similar or equal difficulties in explaining or reasoning about the operations; and the most that can be done in the limited space allotted67 here to the subject of moving material, is to point out some of the principles that should govern the construction and adaptation of handling machinery, from which the reader can take up the subject upon his own account, and follow it through the various examples that may come under notice.
To sum up—We have the following propositions in regard to moving and handling material:
1. The most economical and effectual mechanism for handling is that which places the amount of force and rate of movement continually under the control of an operator.
2. The necessity for, and consequent saving effected by, power-machinery for handling is mainly in vertical lifting, horizontal movement being easily performed by hand.
3. The vertical movement of material, although it consumes more power, is more economical than horizontal handling, because less floor room and ground surface is required.
4. The value of handling machinery, or the saving it effects, is as the constancy with which it operates; such machinery may shorten the time of handling without cheapening the expense.
5. Hydraulic machinery comes nearest to filling the required conditions in handling material, and should be employed in cases where the work is tolerably uniform, and the amount of handling will justify the outlay68 required.
6. Handling material in machine construction is one of the principal expenses to be dealt with; each time a piece is moved its cost is enhanced, and usually in a much greater degree than is supposed.
(1.) Why has the lifting of weights been made a standard for the measure of power?—(2.) Name some of the difficulties to contend with in the operation of machinery for lifting or handling material.—(3.) What analogy exists between manual handling and the operation of hydraulic cranes?—(4.) Explain how the employment of overhead cranes saves room in a fitting shop.—(5.) Under what circumstances is it expedient69 to move material vertically?—(6.) To what circumstances is the danger of handling mainly attributable?
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1
machinery
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| n.(总称)机械，机器；机构 | |
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applied
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| adj.应用的；v.应用，适用 | |
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civilisation
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| n.文明，文化，开化，教化 | |
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vessels
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| n.血管( vessel的名词复数 )；船；容器；（具有特殊品质或接受特殊品质的）人 | |
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abstruse
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| adj.深奥的，难解的 | |
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apprentice
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| n.学徒，徒弟 | |
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marine
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| adj.海的；海生的；航海的；海事的；n.水兵 | |
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engrossed
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| adj.全神贯注的 | |
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investigation
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| n.调查，调查研究 | |
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expended
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| v.花费( expend的过去式和过去分词 )；使用（钱等）做某事；用光；耗尽 | |
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constructive
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| adj.建设的，建设性的 | |
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devoted
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| adj.忠诚的，忠实的，热心的，献身于...的 | |
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astonishment
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| n.惊奇，惊异 | |
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previously
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| adv.以前，先前（地） | |
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hoists
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| 把…吊起，升起( hoist的第三人称单数 ) | |
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fully
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| adv.完全地，全部地，彻底地；充分地 | |
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attests
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| v.证明( attest的第三人称单数 )；证实；声称…属实；使宣誓 | |
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questionable
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| adj.可疑的，有问题的 | |
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allude
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| v.提及，暗指 | |
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mechanism
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| n.机械装置；机构，结构 | |
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peculiar
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| adj.古怪的，异常的；特殊的，特有的 | |
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intermittent
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| adj.间歇的，断断续续的 | |
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judgment
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| n.审判；判断力，识别力，看法，意见 | |
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stationary
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| adj.固定的，静止不动的 | |
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vertical
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| adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置 | |
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vertically
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| adv.垂直地 | |
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ascending
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| adj.上升的，向上的 | |
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descending
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| n. 下行 adj. 下降的 | |
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transit
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| n.经过，运输；vt.穿越，旋转；vi.越过 | |
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hydraulic
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| adj.水力的；水压的，液压的；水力学的 | |
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positively
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| adv.明确地，断然，坚决地；实在，确实 | |
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friction
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| n.摩擦，摩擦力 | |
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pointed
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| adj.尖的，直截了当的 | |
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simplicity
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| n.简单，简易；朴素；直率，单纯 | |
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illustrate
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| v.举例说明，阐明；图解，加插图 | |
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derived
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| vi.起源；由来；衍生；导出v.得到( derive的过去式和过去分词 )；（从…中）得到获得；源于；（从…中）提取 | |
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apparatus
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| n.装置，器械；器具，设备 | |
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dealing
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| n.经商方法，待人态度 | |
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elastic
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| n.橡皮圈，松紧带；adj.有弹性的；灵活的 | |
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cylinders
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| n.圆筒( cylinder的名词复数 )；圆柱；汽缸；（尤指用作容器的）圆筒状物 | |
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pistons
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| 活塞( piston的名词复数 ) | |
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alluded
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| 提及，暗指( allude的过去式和过去分词 ) | |
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evade
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| vt.逃避，回避；避开，躲避 | |
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elasticity
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| n.弹性，伸缩力 | |
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trifling
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| adj.微不足道的；没什么价值的 | |
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determined
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| adj.坚定的；有决心的 | |
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secondly
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| adv.第二，其次 | |
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valid
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| adj.有确实根据的；有效的；正当的，合法的 | |
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interfere
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| v.(in)干涉，干预；(with)妨碍，打扰 | |
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waggons
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| 四轮的运货马车( waggon的名词复数 )； 铁路货车； 小手推车 | |
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51
sling
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| vt.扔；悬挂；n.挂带；吊索，吊兜；弹弓 | |
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52
dispense
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| vt.分配，分发；配(药)，发(药)；实施 | |
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hoisting
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| 起重，提升 | |
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acting
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| n.演戏，行为，假装；adj.代理的，临时的，演出用的 | |
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velocity
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| n.速度，速率 | |
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molecular
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| adj.分子的；克分子的 | |
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attain
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| vt.达到，获得，完成 | |
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accomplished
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| adj.有才艺的；有造诣的；达到了的 | |
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motive
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| n.动机，目的；adv.发动的，运动的 | |
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justify
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| vt.证明…正当(或有理)，为…辩护 | |
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supplant
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| vt.排挤；取代 | |
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expediency
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| n.适宜；方便；合算；利己 | |
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unnatural
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| adj.不自然的；反常的 | |
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winding
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| n.绕，缠，绕组，线圈 | |
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pretension
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| n.要求；自命，自称；自负 | |
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standing
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| n.持续，地位；adj.永久的，不动的，直立的，不流动的 | |
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allotted
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| 分配，拨给，摊派( allot的过去式和过去分词 ) | |
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outlay
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| n.费用，经费，支出；v.花费 | |
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expedient
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| adj.有用的，有利的；n.紧急的办法，权宜之计 | |
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