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wedges, W. In both cases it is necessary to let the two plungers move together; this is easily done by working the two force pumps from one shaft, and by having an accumulator with two rams instead of one.
Shell Plate Ends. From previous remarks it will have been gathered that one of the chief difficulties to contend with is the bending of the ends of the plates in such a manner that the general curvature is uniform, no matter whether the joint is to be butt-strapped, welded, or lap-jointed as in fig. 225.
There are various ways of producing these forms. In some works the end of the plate is heated before bending; it is then laid on a slanting anvil block (fig. 226), with or without curvature, and hammered
with mallets. It is then heated over its entire length, and put through the rolls, or it is bent cold.
Another plan is to give the top or inner roll an extra screw-down when the end of the plate has been reached (fig. 227). In order to get the correct shape on the other end of a lap-jointed plate it has to be taken out of the rolls and reversed, as in fig. 228.
This plan is unsatisfactory unless carried out on hot plates, and even then it is clear that the joining surfaces are not flat but irregularly curved.
A better shape is obtained by the following plan, but it is attended with great danger, because the bending may accidentally be carried out at a blue heat, producing either fracture or brittleness. The plate is fixed as shown (fig. 229), and a heater placed at H, and when locally warmed the bending is done by hammering. The ends of plates to be butted are sometimes left flat, and are drawn together by the butt straps (fig. 230). Barbarous though this method is, the finished shape is a fairly true one, because the strength of the two butt straps is about equal to that of the shell plate, and the resultant curvature is the same as for the rest of the boiler. With thick plates this plan gives too much trouble while the riveting is going on, and under
circumstances it is not a satisfactory one. In some works the plates are ordered extra long and the ends cut off after bending and used as butt straps.
The most satisfactory results are undoubtedly obtained by bending the ends of the plates with suitable moulds in a strong riveting machine, either before or after bolting the plates together. The moulds (fig. 231)
Fig. 231 should be about 9 or 12 ins. long, and gently rounded at their ends. The bending or pressing is done cold. The same moulds can also be used for curving the butt straps.
Ends of Riveted Seams.-In lap-jointed shell plates the corners have to be tapered off previous to bending (figs. 232, 233). This is
usually done by heating them and drawing them out under a steam hammer or by hand. On account of the heaviness of the plates the latter plan is most convenient. Fin-shaped tools should be used, at least for the heavy work, and it is always well to re-heat the surroundings of such corners when finished to prevent cracking. Very satisfactory results are obtained by chipping or planing the corners. (See figs. 234, 235, and p. 247.)
In the case of butt-strapped joints the arrangements are various. They are sometimes left square and butted at X A (figs. 236, 237)
against the flanged end plates or the adjoining strakes, in which case it is very necessary to be careful that the lengths of the butt straps are correct, and also that they are correctly fitted. Another plan, but one in which the advantages obtained are doubtful, is to bevel the
butt straps (fig. 238). They are more difficult to make, to fit, and to caulk than the previous ones.
In both these cases, but especially in the first, it is customary to fit another cover plate, as shown in fig. 239.
Another plan-probably the most efficient—is to draw out or plane the end of the outer butt strap and chip away part of the shell, as shown in fig. 240. Instead of chipping these parts they may with advantage
be planed before bending, and the butt straps might also be planed to shape. Fig. 241 shows another, but not a satisfactory, arrangement. Sometimes the butt ends are welded, or the end rivet in a butt seam may with advantage be replaced by a screwed stud.
The Fitting together of the Shell Plates is comparatively simple, they being easily held together by temporary bolts and straps. With butt-strapped joints it is an advantage to be able to draw the butts tight together, and it is best done as shown in fig. 242, by bolting brackets to the circumferential seams and drawing these together by means of strong bolts, but as these butts can never be water tight fits it is not necessary to go to much trouble about them. The end plates, either front or
back, are made in two or three pieces, unless they are small enough to be made in one. The lower part of the end plate can now easily be secured in its correct position by bolts, starting at the bottom; this draws the shell up, and the upper part of the end plate can now be inserted and bolted to the top of the shell. Where the end plates are made of three pieces, the centre one is generally fitted last, but if it is desired to be prepared for having to renew the furnaces, it is the lower plate which should be fitted last.
Riveting.—The older types of power riveters were actuated by steam, while all newer ones are worked by hydraulic pressure. Formerly, too, it was thought necessary that the pressure should be applied suddenly, in imitation of the blows of a hammer ; but it has been found that better results are obtained if the pressure is steady, provided, of course, that it is sufficiently intense. The smallness of the cylinders required for hydraulic presses has made it possible to introduce contrivances which will press the plates firmly together before and while the pressure is applied to the rivet. Machines have also been constructed which will form heads at both ends, so that, instead of
rivets, pieces of round bars might be used. The results do not appear to have been very satisfactory, except that the heads thus formed required little or no caulking. Practically the same result can be obtained with an ordinary machine by using pan-headed rivets and a spherical die. The deformation which the head experiences assists in closing it up to the plate. Under any circumstances the rivets should be inserted from the inside of the boiler.
Riveting Machines.--Good illustrations of several types of riveters will be found in the following numbers of Engineering':-Vol. xxxiii.