时间：<2020-06-03 11:01:40 作者：XY我的大哥大H5E 浏览量：9777
《关于福利彩票在线直播开奖最新相关内容》:CHAPTER XVI. MACHINERY OF APPLICATION.4. The speed at which shafts should run is governed by their size, the nature of the machinery to be driven, and the kind of bearings in which they are supported.
【福利彩票在线直播开奖】Trip-hammers employed in forging bear a close analogy to, and were no doubt first suggested by, hand-hammers. Being the oldest of power-forging machines, and extensively employed, it will be proper to notice trip-hammers before steam-hammers.
Next in order are strains. As the cutting action is the source of strains, and as the resistance offered by the cutting tools is as the length or width of the edges, it will be found in the present case that while other conditions thus far have pointed to small proportions, there is now a new one which calls for large proportions. In displacing the metal between teeth of three-quarters of an inch pitch, the cutting edge or the amount of surface acted upon is equal to a width of one inch and a half. It is true, the displacement may be small at each cut, but the strain is rather to be based upon the breadth of the acting edge than the actual displacement of metal, and we find here strains equal to the average duty of a large planing machine. This strain radiates from the cutting point as from a centre, falling on the supports of the work with a tendency to force it from the framing. Between the rack and the crank-shaft bearing, through the medium of the tool, cutter bar, connection, and crank pin, and in various directions and degrees, this strain may be followed by means of a simple diagram. Besides this cutting strain, there are none of importance; the tension of the belt, the side thrust in bearings, the strain from the angular thrust of the crank, and the end thrust of the tool, although not to be lost sight of, need not have much to do with problems of strength, proportion, and arrangement.The term application has been selected as a proper one to distinguish machines that expend and apply power, from those that are employed in generating or transmitting power. Machines of application employed in manufacturing, and which expend their action on material, are directed to certain operations which are usually spoken of as processes, such as cutting, compressing, grinding, separating, and disintegrating.
The loss of effect by the inertia of the pieces acted upon increases with the weight of the work; not only the loss of power, but also the expense of heating increases with the size of the pieces. There is, however, such a difference in the mechanical conditions between light and heavy forging that for any but a heavy class of work there would be more lost than gained in attempting to operate on both sides of pieces at the same time.
Physical strength, bone and muscle, must be elements in successful engineering experience; and if these things are not acquired at the same time with a mechanical education, it will be found, when ready to enter upon a course of practice, that an important element, the "propelling power," has been omitted. This makes a statement in brief of what comprehends a knowledge of pattern-making, and what must be understood not only by pattern-makers, but also by mechanical engineers who undertake to design machinery or manage its construction successfully.The second includes what is called threading in America and screwing in England. Machines for this purpose consist essentially of mechanism to rotate either the blank to be cut or the dies, and devices for holding and presenting the blanks.
【福利彩票在线直播开奖】London, 1875.A great share of the alleged improvements in machinery, when investigated will be found to consist in nothing more than the combination of several functions in one machine, the novelty of their arrangement leading to an impression of utility and increased effect.
It has been already remarked that the boldness of young engineers is very apt to be inversely as their experience, not to say their want of knowledge, and it is only by a strong and determined effort towards conservatism, that a true balance is maintained in judging of new schemes.
【福利彩票在线直播开奖】Vertical 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 interfere 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.The invention is now complete, and as the principles are all within the scope of practical mechanism, there is nothing left to do but to devise such mechanical expedients as will carry out the principles laid down. This mechanical scheming is a second, and in some sense an independent part of machine improvement, and should always be subservient to principles; in fact, to separate mechanical scheming from principles, generally constitutes what has been called chance invention.
To direct the attention of learners to certain rules that will guide them in forming opinions in this matter of machine combination, I will present the following propositions, and afterwards consider them more in detail:】【
【福利彩票在线直播开奖】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 inverse 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 shaft 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.