时间：<2020-07-06 06:33:31 作者：wP全经联D6R 浏览量：9777
4. Any force resulting from water rebounding from the vanes parallel to their face, or at any angle not reverse to the motion of the wheel, is lost.
Another peculiar feature in trip-hammers is the rapidity with which crystallisation takes place in the attachments for holding the die blocks to the helves, where no elastic medium can be interposed to break the concussion of the dies. Bolts to pass through the helve, although made from the most fibrous Swedish iron, will on some kinds of work not last for more than ten days' use, and often break in a single day. The safest mode of attaching die blocks, and the one most common, is to forge them solid, with an eye or a band to surround the end of the helve.It is exceedingly difficult for an apprentice engineer, without experimental knowledge, to choose plans for his own education, or to determine the best way of pursuing such plans when they have been chosen; and there is nothing that consumes so much time, or is more useless than attempting to make original designs, if there is not some systematic method followed.Seventh.—Draught, the bevel or inclination on the sides of patterns to allow them to be withdrawn from the moulds without dragging or breaking the sand.
It is generally useless and injudicious to either expect or to search after radical changes or sweeping improvements in machine manufacture or machine application, but it is important in learning how to construct and apply machinery, that the means of foreseeing what is to come in future should at the same time be considered. The attention of a learner can, for example, be directed to the division of labour, improvements in shop system, how and where commercial interests are influenced by machinery, what countries are likely to develop manufactures, the influence of steam-hammers on forging, the more extended use of steel when cheapened by improved processes for producing it, the division of mechanical industry into special branches, what kind of machinery may become staple, such as shafts, pulleys, wheels, and so on. These things are mentioned at random, to indicate what is meant by looking into the future as well as at the present.Steam-hammers are divided into two classes—one having the valves moved by hand, and the other class with automatic valve movement.
Lastly, proportions; having estimated the cutting force required at one ton, although less than the actual strain in a machine of this kind, we proceed upon this to fix proportions,  beginning with the tool shank, and following back through the adjusting saddle, the cutting bar, connections, crank pins, shafts, and gear wheels to the belt. Starting again at the tool, or point of cutting, following through the supports of the rack, the jaws that clamp it, the saddle for the graduating adjustment, the connections with the main frame, and so on to the crank-shaft bearing a second time, dimensions may be fixed for each piece to withstand the strains without deflection or danger of breaking. Such proportions cannot, I am aware, be brought within the rules of ordinary practice by relying upon calculation alone to fix them, and no such course is suggested; calculation may aid, but cannot determine proportions in such cases; besides, symmetry, which cannot be altogether disregarded, modifies the form and sometimes the dimensions of various parts.One thing more must be noticed, a matter of some intricacy, but without which, all that has been explained would fail to give a proper idea of steam-hammer-action. The valve motions are alluded to.
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.First. Gravity wheels, acting directly from the weight of the water which is loaded upon a wheel revolving in a vertical plane, the weight resting upon the descending side until the water has reached the lowest point, where it is discharged.CHAPTER XLI. WORKSHOP EXPERIENCE.
1. The value of pneumatic apparatus in reaching places where steam furnaces cannot be employed.By examining the history of great achievements in the mechanic arts, it will be seen that success has been mainly dependent upon predicting future wants, as well as upon an ability to supply such wants, and that the commercial value of mechanical improvements is often measured by conditions that the improvements themselves anticipate. The invention of machine-made drills, for example, was but a small matter; but the demand that has grown up since, and because of their existence, has rendered this improvement one of great value. Moulded bearings for shafts were also a trifling improvement when first made, but it has since influenced machine construction in America in a way that has given great importance to the invention.It may be mentioned in regard to rack gearing for communicating movement to the carriages of planing machines or other purposes of a similar nature: the rack can be drawn to the wheel, and a lifting action avoided, by shortening the pitch of the rack, so that it will vary a little from the driving wheel. The rising or entering teeth in this case do not come in contact with those on the rack until they have attained a position normal to the line of the carriage movement.
A true movement of carriages is dependent upon the amount or wearing power of their bearing surface, how this surface is disposed in reference to the strain to be resisted, and the conditions under which the sliding surfaces move; that is, how kept in contact. The cutting strain which is to be mainly considered, falls usually at an angle of thirty to forty degrees downward toward the front, from the centre of the lathe. To resist such strain a flat top shear presents no surface at right angles to the strain; the bearings are all oblique, and not only this, but all horizontal strain falls on one side of the shear only; for this reason, flat top shears have to be made much heavier than would be required if the sum of their cross section could be employed to resist transverse strain. This difficulty can, however, be mainly obviated by numerous cross girts, which will be found in most lathe frames having flat tops.INTRODUCTION.(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 positively, 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.
Fourth. Pressure wheels, acting in every respect upon the principle of a rotary steam-engine, except in the differences that arise from operating with an elastic and a non-elastic fluid; the pressure of the water resting continually against the vanes and "abutment," without means of escape except by the rotation of the wheel.But now all is changed: natural phenomena have been explained as being but the operation of regular laws; so has mechanical manipulation been explained as consisting in the application of general principles, not yet fully understood, but far enough, so that the apprentice may with a substantial education, good reasoning powers, and determined effort, force his way where once it had to be begged. The amount of special knowledge in mechanical manipulation, that which is irregular and modified by special conditions, is continually growing less as generalisation and improvement go on.