Page:Forth Bridge (1890).djvu/51

Rh bearing all round; and after these were found to be satisfactory, the rectangular portion of the key-plate was raised up and rivetted to the upper bed-plate. This was then lowered down into its place to see how it fitted, and whether all parts of the key were in proper contact. After thus lowering and raising it several times (a weight of about 57 tons), the surfaces were carefully cleaned and thick brown oil of a special character was poured into the key recess and on the whole area of the lower bedplate, and the upper bedplate lowered down upon it for the last time. Nothing else but oil was placed as lubricating medium between the two surfaces.

The movements described in connection with the keyplates require that a certain amount of play be given in the holes of the upper bedplates through which the holding-down bolts pass. The amount of play varied, of course, in the same manner as with the key-plates.

The lower bedplates being fixed and held down firmly by the bolts, it was necessary that the nuts should be screwed down upon them. These nuts, shown in Fig. 95, were therefore arranged with a long neck, circular in form, and with an enlarged head of hexagon form. The holes in the upper bedplate had to be therefore made of such size and shape that with the nuts screwed down tightly upon the lower bedplates, with a considerable stress upon the holding-down bolts, the upper bedplate could yet move in any desired direction without hindrance.

This was done by means of a washer of oblong form placed upon the carefully-levelled sides of the cells in the bedplates, which had bolts passing through them. The nuts were first put on and screwed down by means of a hydraulic spanner—that is, a heavy box spanner with a short lever on the top of it, to the end of which a hydraulic ram was applied with a given pressure. This assured an even stress to all the bolts. The distance between the side of the cell and the under side of the hutul of the nut was then carefully callibered for each nut, and the washers made to that measurement, one-sixteenth of an inch being allowed for play. The sixes and shapes of the holes in the upper bedplates are shown by the diagram here given, the holes for Queensferry bedplates being, of course, the same as those for Fife, but in reversed position. (See Fig. 96.)

The building of the skewbacks could now be proceeded with, and this was done in such manner that the work as erected was at once rivetted up by machine. The most difficult places to get to were the lower edges of the inner and outer webplates, with their double angles already rivetted to the bedplates. All the work in these cells which could not be done previous to the put ting in of these plates had to bo done by small hydraulic rivetters. These were simply cylindrical rams of 4 in. to 6 in. diameter, and in length from 6 in. to 10 in., and these were worked by a pressure of 3 tons per square inch. The cylinders were placed to either side of the rivet to be struck, facing each other, and backed by hardwood packings against the sides of the cells, and the pressure water was supplied to them through small copper pipes about $3/16$ in. in bore. The rivets were heated on the outside, and passed through holes in the webplates; one cylinder was set against the rivet-head, and the other then closed upon the free end, and formed a fiat head, the hole on that side being somewhat countersunk. To produce the 3 tons per square inch pressure, a multiplier was used, consisting of an 11-in. ram, which was acted upon by an ordinary accumulator pressure of 1000 lb. per square inch, and which had at its end another ram attached, 4 in. in diameter, which was forced into a cylinder already charged with 1000 lb. pressure water. The 4-in. ram forced the water to a small accumulator, loaded to produce a pressure of 3 tons per square inch. It needs no saying that this was a tedious kind of work, and took a long time to accomplish. On many days not more than half a dozen rivets were done—at no time more than about a hundred in the twelve hours, and the average would not probably amount to more than twenty-five per day.

Meanwhile staging had been erected between the piers on trestles of certain height, upon which were put together the horizontal tubes connecting the skewbacks and the cross-girders and diagonal girders. All these were now connected to the skewbacks, and preparations made to rivet them up. All the erection of the lower portions of the central towers was done by 3-ton or 5-ton steam derrick cranes placed on platforms at some height from the deck and in convenient position to pick the material off the bogies and at once place it in position. For the rivetting of the latticed girders the ordinary fixed or jointed machines were used, which are extremely simple in their construction and their action. (See Figs. 97, 98, 99, 100, the three first representing fixed machines, the last a jointed machine.) It depended entirely upon the places where rivets had to be struck whether one kind of machine or the other was used; but the direct-acting machine was surer in its action, because the double lever arrangement was apt to twist, and thereby bend the unclosed portion of the rivet before the latter was properly staved. Of course the longer the arms the greater becomes this tendency, and as some of the jointed machines had to be used with the arms fully 4 ft. 6 in. in length, great care had to be exercised to get good work done.

The horizontal tubes, built up of heavy plates $1 1/4$ in. thick, required a special machine for rivetting, and as this machine is the same, or at all events typical of others, which were used for the rivetting of the bottom members, the vertical columns and the diagonal struts in the central towers, a brief description will not be amiss.

In the main it consisted of two circular girders, the inside diameter being about 1 ft. larger than the outside diameter of the tube. They were placed some 24 ft. apart and tightly wedged by hardwood blocks all round the tube. A box girder of a strong section, and about 25 ft. long was placed upon the ring girders in such manner that it could be pushed completely round the tube, and for this purpose hydraulic rams, one at each end were attached in such a manner that they could lie fleeted forward or backward as desired. Upon the box girder a cylinder was placed facing the tube, being connected to a saddle which was capable of sliding from one end of the girder to the other. The saddle was worked by a ratchet and pinion, the latter working upon a rack winch ran along the whole length of the girder. Finer adjustment could be given by two screws and handwheels on the saddle, like that of a lathe. By this means the centre of the cylinder could be brought opposite any point of the surface of the tube within the length of the girder. On the inside of the tube a mandrel or central hollow shaft was formed of the same length as the outside girder, and this was supported at both ends by frames fixed and wedged to the skeleton frame of the tube. On this mandrel a toothed rack was fixed, and a saddle worked by ratchet and pinion could move from end to end. Upon this saddle a cylinder of the same size as the outside one was placed with a long snap or dolly reaching to the plates. The supply of pressure water was so arranged that both cylinders would be turned on simultaneously, but by means of a check valve placed upon the outside supply pipe, the motion of the outside ram was slightly retarded. The girder outside was now placed in line with a row of rivets in the tube, and the inside cylinder was set opposite the same row. The rivets were heated in small furnaces or forges on the inside and heated to a good yellow heat. As soon as a rivet was put into place the inside dolly was set against it, the outside ram being also set down, a tap given to indicate that all was ready, the pressure was turned on, and the rivet closed. The two cylinders were then moved forward, one pitch to the next hole and so on. When a whole row extending over about 18ft. in length had been done, the girder was raised or lowered as the case might be, to the next row, and this continued until the whole section of the tube was rivetted. The machine was then moved 18 ft. forward and the process repeated. No drawing of these rivetting machines is given here, but a machine similar in construction, and used for rivetting the vertical columns, is shown further on.

Between 600 and 700 rivets could be put in by this machine per day, all rivets here being $1 1/8$ in. in diameter. Only the rivets passing through the thick outside plates and beams were done by the machine. Of these there were about 1550 in each section rivetted, or nearly 100 per foot run. The other rivets, in diaphragms and beam-covers, were done by hand. The least number of hands to work this machine were three men outside and two men and a boy inside, with a lad for heating the rivets and a boy to carry them.

The members of entirely circular form were the vertical columns, the horizontal tubes or bottom members between skewbacks, and the bottom members in cantilevers from the skewback to the end of bay 4. In design and in construction they were all the same—differing only in diameter—in the breadth and thickness of the plates, and in the depth and strength of the longitudinal beams. All plates are made, except in special places or for closing lengths, of a uniform length of 16 ft., and the tube is formed of ten plates, lap-jointed, and therefore consisting of five outer and five inner plates. All plates break joint at 8 ft., or half length, with an absolutely close butt, and the butts are covered inside and outside by plate-covers. The lap-joints are covered on the inside by ten continuous girders of T section, the T head being rivetted in with the lap-joint, and two continuous angles are rivetted to the other end of the web, thus making the girder of double T or H section. (See Figs. 68, 73, and 94.) The beams are made in lengths of about double that of the plates, and break joint in different places, and all their joints are covered both in the webs and flanges. At every butt—that is, every 8 ft.—is placed a circular girder or diaphragm, which consists of ten wings placed between the beams and rivetted to the shell, the wings projecting beyond the beams to the extent of about 3 in. An angle-iron ring running right round takes up all the wings to its web, while its heel is rivetted to each of the beams in succession, and thus combines the whole into a stiffening girder. At considerable distances apart, heavy plate-diaphragms are inserted which only leave a manhole in the centre to pass through. All these diaphragms are primarily for the purpose of preserving the true circular form of the tubes, and to prevent flattening; and it is a curious fact that the stiffest of these tubes, although bolted together in the most careful manner