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for the inside and outside strakes, comprising about thirty or forty holes, and the exact positions of several holes should be carefully calculated as a check on the templets. In works where this plan is adopted the necessary appliances for drilling in place are doubtless wanting, and then the holes in the end plate flanges and in the longitudinal seams will have to be drilled by hand, the outer machine-drilled holes acting as guides. The latter should never be drilled before bending the plates, as they are thereby seriously weakened at points where strength is of the utmost importance.
Great care should always be taken that these holes cover each other perfectly, for if blind, the sectional area of the rivet in the plane of shear is reduced, or if this is put right by chipping and rimering, the section of the plate round the hole is reduced.
Another practice is to punch or even to drill the holes of a smaller diameter than required, and to drill away the superfluous material when fitted together; but if the view be true that the injury to the material extends slowly for a considerable distance, then this practice ought to be condemned.
The marking off of back plates for boilers and combustion chambers can be done from the drawings, but often the combustion chamber plates after being drilled are laid on the boiler plates for marking off. In either case the pitch in the boiler plate should be slightly greater than the other, and also higher, so as to allow of the stays being
on the slant.
Bending Operations. -Since the failure of the steel shell plate of the steam yacht · Livadia' there exists a very justifiable dread of bending such plates while hot, but as long as this operation is not carried out at a blue heat the plates ought to suffer no permanent injury, and where the rolls are not sufficiently strong to bend cold plates they will have to be heated. It must not be forgotten that the
Livadia' case is not the only one in which the shell plates cracked, and that several instances are known where this happened with plates that had been bent cold.
A very strong objection against bending shell plates while hot is the necessity of being possessed of a very long heating furnace, the extra time required for warming and then for cooling the plates, and the difficulty of obtaining a uniform temperature, which leads to irregular curvatures. These, and not the supposed injury done to the shell, are probably the most potent reasons which have induced manufacturers to adopt the plan of bending plates cold.
The Bending Rolls have necessarily to be proportionately stronger, and the following few notes will be a guide in the matter.
The resistance to bending beyond the limit of elasticity is independent of the curvature, and is approximately equal to 4.t?. b for iron and 5. t. b for steel. Here t is the thickness and b the breadth of the plate, measured in inches. If the plate is heated to redness the coefficients 4 and 5 are reduced to about } (see p. 195).
As the strength of the bending rolls is proportional to the cube of their diameters, and inversely proportional to the square of their length, and as the bending moment exerted by the rolls on the plate is, in the case of three rolls, of which the two smaller ones nearly touch (see
fig. 215), proportional to the diameter, we find that D2 = c.L.t (see p. 174).
Here D'is the diameter and L the length of the rolls, and ca constant which is equal to about 4} for the lower or outer ones of a system of three rolls, and the upper or inner one should be 25% larger than these, as it supports double the load.
For red-hot plates the constants may be reduced to 1.5, which would allow of the rolls being reduced to about half the above diameters.
The smaller the diameters are made, the shorter will be the unbent end pieces of plate, and, as this is a very desirable object, various devices are in use for attaining it.
The two outer (or lower) rolls are sometimes supported by antifriction rollers, as shown in fig. 216. Or instead of three rolls four are used (fig. 217). The leverage x of the bending forces of the rolls can be very much reduced by this means, but on account of the greater pressure the diameters have to be proportionately increased.
Another plan is to have the four rolls arranged as shown in fig. 218, but the advantages are not apparent, and it would even seem that by removing the central roll C and bringing the two rolls B closer together
the bending could be done better. In some machines the upper roll can be moved horizontally, but this also demands that the diameters should be large.
The bending rolls may be placed either horizontally (fig. 219) or vertically (fig. 220). The latter plan is certainly the most convenient, and is being generally adopted. The upper framework is shown in plan, and is so arranged that the inner roll can be lifted out, in order that shell or furnace plates may be rolled in one piece. Of the horizontal rolls it is usually only the two lower ones which are driven, while with the vertical rolls all three turn together. They thereby acquire a better grip of the plate, but even in that case it is advantageous to cut a few grooves into the driving rolls, as they materially assist in dragging in the plate.
Sketches of various types of bending rolls will be found in the following volumes of · Engineering':-Horizontal plate bending rolls : vol. xxxiii. p. 134; vol. xlix. p. 529; vol. 1. pp. 327, 480, 688. Vertical rolls : vol. xxxii. p. 135; vol. xliv.
Plate straightening machines with five rolls : vol. xl. pp. 9, 321, 619; vol. xlv. p. 135; vol. 1. pp. 276, 606. Bending presses: vol. xliii. p. 491 ; vol. xlix. p. 245; vol. lviii. p. 477.
In some works the plates are passed several times through the bending rolls while these are being gradually screwed closer together. When possible, and par
FIG. 220 ticularly if the plates are hot, the curving should be carried out in one pass, for, independently of the disadvantages of punishing the material repeatedly, it will be found that less
wide or very
force is required for a single bending than for several, if that is necessary. This, however, is only possible if the machine is in good working order, and if full reliance may be placed on the marks to which the rolls are set. On account of the spring of the rolls some allowance has to be made, according as to whether very narrow plates are being bent, and for the same reason the influence
of the extra stiffness of thick plates has also to be taken into account. After bending, the plates uncurl slightly, but absolute accuracy need not be aimed at.
With vertical rolls the shop floor should be square to their axis, and instead of using round iron rolling rods to support the plates,
small carriages (fig. 221) Fig. 221
will be found to follow
the curvatures of the plate more smoothly, and not give rise to jerky motions. The speed at which the rolls are worked is about 18 ft. per minute, but it takes altogether about thirty minutes to bend one piece of shell plate. Before commencing the bending, a circular chalk line, to judge of the curvature, is drawn on the floor, passing through the roll space; but it is also necessary to have curved templets, with which the upper edge is gauged, as the wear on the roller bearing is not an equal one.
Hydraulic Bending Presses are sometimes used instead of rolls. They seem to be most efficient for the bending of long narrow plates, particularly if the curvatures are all equal, as was the case with the tubes of the Forth Bridge. When the radii of the press moulds and the shell differ materially, liners have to be interposed, as in fig. 222, producing a rounded polygonal, instead of a perfectly circular shape. This is the case even, though to a much less extent, when no liners are used, for the press is never powerful enough to force the plate into absolutely close contact with both moulds.
Templets have to be applied to the plates while being pressed step by step, otherwise irregularities are sure to occur.
amount of care can do away with the irregular distribution of the stresses in the plates, and this plan cannot therefore be looked upon as a good one for furnaces, because with them the stresses are compressive.
A few sketches of some hydraulic bending presses are shown in figs. 223, 224. In the first of these the two rams act directly on the press frames, while in the other motion is imparted by means of