【9号彩票开户平台】欢迎访问: 。 Flat or plane joints nearly always admit of adjustment between the fitted surfaces; that is, the material scraped or ground away in fitting can be compensated by bringing the pieces nearer together; but parallel cylindrical joints cannot even be tried together until finished, consequently, there can be nothing cut away in trying them together. Tapering, or conical joints, can of course be trial-fitted, and even parallel fits are sometimes made by trial, but it is evident that the only material that can  be cut away in such cases, is what makes the difference between a fit too close, and one which will answer in practice. 主播带你云游博物馆
9号彩票开户平台Another object gained by equal action on both sides of large pieces is the quality of the forgings produced, which is generally improved by the rapidity of the shaping processes, and injured by too frequent heating.
(1.) Why are shaping machines better adapted than planing machines for planing slots, key-ways, and so on?—(2.) What objects are gained by a quick return motion of the cutter bar of shaping machines?5. The lubrication of surfaces working under air pressure, such as the pistons and valves of engines.
It is evident that when the parts of a machine are joined together, each piece which has contact on more than one side must have specific dimensions; it is farther evident that as many of the joints in a machine as are to accommodate the exigencies of construction must be without space, that is, they represent continued sections of what should be solid material, if it were possible to construct the parts in that manner. This also demands specific dimensions.
The functions and adaptations of machinery constitute, as already explained, the science of mechanical engineering. The functions of a machine are a foundation on which its plans are based; hence machine functions and machine effect are matters to which the attention of an apprentice should first  be directed.The gearing of these machines is alluded to here mainly for the purpose of calling attention to what constitutes a new and singular mechanical movement, one that will furnish a most interesting study, and deserves a more extended application in producing slow reciprocating motion.
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 droop 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 applied 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 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.
Although a system but recently developed, the employment of hydraulic machinery for transmitting and applying power has reached an extended application to a variety of purposes, and gives promise of a still more extensive use in future. Considered as a means of transmitting regularly a constant amount  of power, water apparatus is more expensive and inferior in many respects to belts or shafts, and its use must be traced to some special principle involved which adapts hydraulic apparatus to the performance of certain duties. This principle will be found to consist in storing up power in such a manner that it may be used with great force at intervals; and secondly, in the facilities afforded for multiplying force by such simple mechanism as pumps. An engine of ten-horse-power, connected with machinery by hydraulic apparatus, may provide for a force equal to one hundred horse-power for one-tenth part of the time, the power being stored up by accumulators in the interval; or in other words, the motive power acting continuously can be accumulated and applied at intervals as it may be required for raising weights, operating punches, compressive forging, or other work of an intermittent character. Hydraulic machinery employed for such purposes is more simple and inexpensive than gearing and shafts, especially in the application of a great force acting for a considerable distance, and where a cylinder and piston represent a degree of strength which could not be attained with twice the amount of detail, if gearing, screws, levers, or other devices were employed instead.
Finally, it must constantly be borne in mind that what will be learned is no less a question of faculties than effort, and that the means of succeeding are closed to none who at the beginning form proper plans, and follow them persistently.
Connected with screw dies, there are various problems, such as clearance behind the cutting edge; whether an odd or even number of edges are best; how many threads require to be bevelled at the starting point; and many other matters about which there are no determined rules. The diversity of opinion that will be met with on these points, and in reference to taps, the form of screw-threads, and so on, will convince a learner of the intricacies in this apparently simple matter of cutting screw-threads.。
Beginning at the tool there is, first, a clamped joint between the tool and the swing block; second, a movable pivoted joint between the block and shoe piece; third, a clamped joint between the shoe piece and the front saddle; fourth, a moving joint  where the front saddle is gibed to the swing or quadrant plate; fifth, a clamp joint between the quadrant plate and the main saddle; sixth, a moving joint between the main saddle and the cross head; seventh, a clamp joint between the cross head and standards; and eighth, bolted joints between the standards and the main frame; making in all eight distinct joints between the tool and the frame proper, three moving, four clamped, and one bolted joint.。
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, hoists, 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 fully attests the last proposition, and it is questionable 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 allude to the mechanism 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 peculiar to it as a class; among these difficulties is the transmission of power to movable mechanism, the intermittent and irregular application of power, severe strains, also the liability to accidents and breakage from such machinery being controlled by the judgment of attendants.。