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《关于手机怎么购买中国福利彩票最新相关内容》:Machinery of transportation.
【手机怎么购买中国福利彩票】(1.) Name some of the differences between planing and slotting machines.—(2.) Why should the feed motion of a slotting machine act abruptly?—(3.) To what class of work are slotting machines especially adapted?
Besides this want or difference of facilities which establishments may afford, there is the farther distinction to be made between an engineering establishment and one that is directed to the manufacture of staple articles. This distinction between engineering-works and manufacturing is quite plain to engineers themselves, but in many cases is not so to those who are to enter as apprentices, nor to their friends who advise them. In every case where engagements are made there should be the fullest possible investigation as to the character of the works, not only to protect the learner, but to guard regular engineering establishments in the advantages to be gained by apprentice labour. A machinist or a manufacturer who employs only the muscular strength and the ordinary faculties of workmen in his operations, can afford to pay an apprentice from the beginning a fair share of his earnings; but an engineering-work that projects original plans, generates designs, and assumes risks based upon skill and special knowledge, is very different from a manufactory. To manufacture is to carry on regular processes for converting material; such processes being constantly the same, or approximately so, and such as do not demand much mechanical knowledge on the part of workmen.
CHAPTER XI. SHAFTS FOR TRANSMITTING POWER.
【手机怎么购买中国福利彩票】Strains caused by cutting action, in planing or other machines, fall within and are resisted by the framing; even when the tools are supported by one frame and the material by another, such frames have to be connected by means of foundations which become a constituent part of the framing in such cases.Different kinds of gearing can be seen in almost every engineering establishment, and in view of the amount of scientific information available, it will only be necessary to point out some of the conditions that govern the use and operation of the different kinds of wheels. The durability of gearing, aside from breaking, is dependent upon pressure and the amount of rubbing action that takes place between the teeth when in contact. Spur wheels, or bevel wheels, when the pitch is accurate and the teeth of the proper form, if kept clean and lubricated, wear but little, because the contact between the teeth is that of rolling instead of sliding. In many cases, one wheel of a pair is filled with wooden cogs; in this arrangement there are four objects, to avoid noise, to attain a degree of elasticity in the teeth, to retain lubricants by absorption in the wood, and to secure by wear a better configuration of the teeth than is usually attained in casting, or even in cutting teeth.Movements and strains may be called two of the leading conditions upon which designs for machines are based: movements determine general dimensions, and strains determine the proportions and sizes of particular parts. Movement and strain together determine the nature and area of bearings or bearing surfaces.
A cubic inch of water, by taking up a given amount of heat, is expanded to more than five hundred cubic inches of steam, at a pressure of forty-five pounds to the square inch. This extraordinary expansion, if performed in a close vessel, would exert a power five hundred times as great as would be required to force the same quantity of water into the vessel against this expansive pressure; in other words, the volume of the water when put into the vessel would be but one five-hundredth part of its volume when it is allowed to escape, and this expansion, when confined in a steam-boiler, exerts the force that is called steam-power. This force or power is, through the means of the engine and its details, communicated and applied to different kinds of work where force and movement are required. The water  employed to generate steam, like the engine and the boiler, is merely an agent through which the energy of heat is applied.Modern turbine wheels have been the subject of the most careful investigation by able engineers, and there is no lack of mathematical data to be referred to and studied after the general principles are understood. The subject, as said, is one of great complicity if followed to detail, and perhaps less useful to a mechanical engineer who does not intend to confine his practice to water-wheels, than other subjects that may be studied with greater advantage. The subject of water-wheels may, indeed, be called an exhausted one that can promise but little return for labour spent upon it—with a view to improvements, at least. The efforts of the ablest hydraulic engineers have not added much to the percentage of useful effect realised by turbine wheels during many years past.In no other branch of machinery has so much research and experiment been made during eighty years past as in steam machinery, and, strange to say, the greater part of this research has been directed to the details of engines; yet there has been no improvement made during the time which has effected any considerable saving of heat or expense. The steam-engines of fifty years ago, considered as steam-using machines, utilised nearly the same proportion of the energy or power developed by the boiler as the most improved engines of modern construction—a fact that in itself indicates that an engine is not the vital part of steam machinery. There is not the least doubt that if the efforts to improve steam-engines had been mainly directed to economising heat and increasing the evaporative power of boilers, much more would have been accomplished with the same amount of research. This remark, however, does not apply to the present day, when the principles of steam-power are so well understood, and when heat is recognised as the proper element to deal with in attempts to diminish the expense of power. There is, of course, various degrees of economy in steam-using as well as in steam-generating machinery; but so long as the best steam machinery does not utilise but one-tenth or one-fifteenth part of the heat represented in the fuel burned, there need be no question as to the point where improvements in such machinery should be mainly directed.
【手机怎么购买中国福利彩票】CHAPTER X. MACHINERY FOR TRANSMITTING AND DISTRIBUTING POWER.
【手机怎么购买中国福利彩票】To follow the same chain of reasoning still farther, and to show what may be gained by method and system in learning mechanics, it may be assumed that machine functions consist in the application of power, and therefore power should be first studied; of this there can be but one opinion. The learner who sets out to master even the elementary principles of mechanics without first having formed a true conception of power as an element, is in a measure wasting his time and squandering his efforts.In the workshop, mechanical knowledge of some kind is continually and often insensibly acquired by a learner, who observes the operations that are going on around him; he is continually availing himself of the experience of those more advanced, and learns by association the rules and customs of the shop, of the business, and of discipline and management. He gathers the technical terms of the fitting-shop, the forge and foundry; notes the operations of planing, turning, drilling, and boring, with the names and application of the machines directed to these operations. He sees the various plans of lifting and moving material, the arrangement and relation of the several departments to facilitate the course of the work in process; he also learns where the product of the works is sold, discusses the merits and adaptation of what is constructed, which leads to considering the wants that create a demand for this product, and the extent and nature of the market in which it is sold.