Junctions Of Iron Work. Several forms of joints used in putting iron frame work together will be found illustrated in a succeeding paragraph, when describing Iron Roofs and Bridges. The present paragraph will concern itself with the elements of the subject. In iron framing, the following, as illustrated in figs. 394, 398, and 399, are used. In fig. 394, a is a section of a rolled beam, sometimes called an "I"-beam, b is " channel "-iron, c "T"-iron, d " angle "-irons, f "flat bar"-iron, e "L"-iron, g "round bar" or "rod"-iron. These are joined together either by "rivets," as in fig. 398, by "pins and split keys," as in fig. 399, or by "bolts and nuts," as in fig. 8, Plate LIX. We shall now notice these more in detail.
49. Rivets are made of wrought-iron, and have two parts; the "head," as a, fig. 398, and the "tail" or "shank" b; this is made of so much greater a length, as at c, than the thickness of the two parts d e to be joined together, that when the extra length, as shown by the dotted lines c, are hammered down, a second head is formed, thus clasping, as it were, the two parts d and e together. The rivet is put red hot into the holes of the two pieces d and e, and while one man presses against the "head" a with his tool, the other man hammers at the end c of the " shank " or " tail," till it closely presses against the surface of the piece d, and is made to assume the form of a cone as at c, or the rounded form as at f. If to be left rounded, as at f, so as to correspond with the head g, the end f is finished off with a tool called a " snap," the end of which is provided with a cup-shaped hollow corresponding with the form of f. In place of riveting by hand, the operation is now often done by machine; when this is used the rivets are finished, as f and g, alike on both sides of the pieces to be joined. When the rivet is not to project beyond the surface of the piece, as d, the upper end of the hole in the piece d is sloped off as at h and i by means of a drill; and the end, as c, is made by hammering to fill up this conical cavity; the rivet is then technically said to be "countersunk." This is sometimes done at both ends, as at j and k. When two plates are joined together the ends are generally sloped or bevelled off, as at I and m, and if in the case of a boiler the joint is required to be water-tight, the points of junction of the two plates, as n and o, are "caulked" or driven tight with a caulking tool. Plates of iron, as the parts of roof and bridge framing, are put together and retained in position by means other than rivets, as by screwed "bolts and nuts," by "keys, gibs, and cottars," already illustrated, and by pins and split keys, as in fig. 399; in this a is the "pin," b the side view of "split key," so called from its edge being split or divided into two by a cut, as shown at edge view at c. The pin is passed through the holes in the two pieces as d e, the "split key" is next passed through the slot g in the pin, and the two halves of c forced open as at f; the key is thus kept in place. The strength of a joint, as that shown in fig. 398, does not depend altogether upon the strength of the rivets, or the way in which the rivets are put in, that is, upon good and bad workmanship; but, as shown by the great authority on iron work, Fairbairn, also upon the way in which the rivets are placed in relation to one another. Thus there is a great difference in the strength of a riveted joint when the rivets are placed as in fig. 400 and in fig. 401. In the method illustrated in fig. 400, which is called " single riveting, if the strength was represented by 56, that of the method known as "double riveting," fig. 401, would be 70. The advantages of " double riveting" would not be obtained by simply making two rows, or another row parallel to that in fig. 400, or as at a b c d in fig. 402; but the rivets must be placed so as to form a series of triangles, as in fig. 401. These triangles are not equilateral, as at ab c, but rather isosceles, as at d e f. Fig. 402 illustrates the method of joining plates together by what is called "chain riveting," in which rows, as 1, 2, 3, 4, are placed parallel to one another; this kind of work is used chiefly in bridge and large frame work of iron, as illustrated in Plate LX. But not merely upon the good workmanship given to the "hammering," etc., of the rivets, and the placing of them in proper position in relation to each other does the strength of riveted plates depend. Another important point is the accuracy with which the holes are bored which receive the rivets, so that the two holes shall exactly coincide with each other.
This accuracy, as we have elsewhere remarked, is a point of still greater importance to be aimed at when there are several plates placed together, as in the top or bottom flanges of box or plate girders; for it is obvious that if the holes do not coincide in all the plates, undue strain will be thrown upon the plates. The difficulty found in practice of getting the rivet holes so accurately adjusted, that the rivets will go fair in, is such, that " drifting," as it is called, is resorted to in a great many instances, the operation having for its aim the bringing in of the rivet holes into the straight as near as possible. "Drifting" being what may be called a "brute force" method of overcoming the difficulty brought about in nine cases out of ten by careless or indifferent marking out, and punching or drilling the holes at first, should not be allowed; but such rivet holes as do not exactly correspond, or are out of line, should be brought right by the use of the "rymer," by which the irregularities of metal in the line of the rivet holes will be pared off. To lessen the difficulty of obtaining accurate adjustment of rivet holes, the thickness of flanges, or top and bottom plates of " built beams," should not exceed 4 inches if the required sectional area to obtain the necessary strength in the beam is not given by this thickness, the breadth of the flanges should be increased in the desired proportion. The rivet holes should be spaced out by "templates" so as to insure accuracy in the meeting or coincidence of the holes in the various plates when put together in situ, and the edges of the plates butting against each other should be placed so as to insure fair joints. In no case, where good work is required, should the edges and ends be left rough and uneven where they are to butt against each other. It is scarcely necessary to say that, in order to reduce the number of riveted joints, the plates which make up built beams should be as long as possible.
A great deal has been said as to the relative merits of punching and drilling; and of this we may only remark, that drilling, beyond doubt, gives the best results; and, in addition to greater accuracy in the adjustment of the corresponding holes in different plates, it avoids the injury done to the fibre of the plates by punching. Any one has only to thoroughly examine drilled and punched plates, to be convinced of the mechanical accuracy of the drilled plates. It is, however, a question of paying and not paying, drilling being more expensive than punching. For small work, where thin plates are used, punching will be found to be, and is found practically to be, the best. If, however, where punching is decided upon, the rivet holes are accurately spaced out, and the adjustment of the plates be carefully looked to, the punching and the riveting honestly done, good work may be fairly expected. The standard of good work is accurately formed rivet holes, all the holes in the different plates coinciding with each other; the rivets filling up closely the interior of the holes, and the sectional area of the rivets equal to the sectional area of the plate, measured in a direction at right angles to the tensile strain exercised upon the plate, in other words, at right angles to the line of rivet holes; and finally, well made rivets of the best iron, put in red hot, and hammered tight in.
In figs. 398, 399, 400, and 402 the joint is called a "lap-joint," from one plate, as a, fig. 400, lapping or overlapping the other plate as b. In fig. 403 we illustrate the " butt joint," in which no lap is used; but the two plates, as a b, " butt" up against each other, as at c, the joint being made good by a plate termed a " fishing " plate d, this being riveted as shown, or two fishing plates may be used, a second as e being placed above. When several plates are joined together, the "butt joint" must not be common to all, as shown by the dotted line a b, fig. 404; but the several joints must be disposed so that they shall lie against the solid part of the plate below, as at c, or of the solid parts of two plates, as at d; e and f show the other two joints; one fishing plate as g g is here used. In fig. 404a, we illustrate the junction of two pieces of angle-iron a b, by a fishing plate of angle-iron c, as in section. " T "-iron, as at d' d, may be jointed, when in two pieces, by a flat plate e, or by angle-irons, as f, at each side, as at g g, and for additional security a flat plate h h may be riveted at top. The junction of two pieces of channel-iron is shown at i i, smaller fishing plates as j j of channel-iron being used. The arawings to the left are sections, those to the right elevations. When rafters of iron roofs are required to be strong, in cases where the length is great and the weight to be supported heavy, two " T "-irons are bolted together, back to back, as in fig.
405. Angle-irons are used for " purlins;" figs. 406 and 407 show different methods of joining them; in fig. 406, a a is the rafter, b b the purlin, secured to the rafter by the angle-iron c; in fig. 407 the purlin b b is secured to the under side of the rafter a a by bolts and nuts. In fig. 408, the angle-iron b forming the purlin, is riveted to the upper flange of rafter a a. This figure also illustrates the method of securing a tim-ber purlin, part of which is shown at c c, by a short piece of angle-iron, as c in fig. 406; a bolt at d passing through the angle-iron and the wood purlin. Various other methods of forming joints in iron work will be noticed further on.