The Forth Bridge/Rivetting
Rivetting.
An early estimate has fixed the number of rivets in the Forth Bridge at 5,000,000, but this was evidently insufficient and the figure has risen to 6,500,000. It is, however, doubtful whether even this covers the total amount, for on the central or Inchgarvie pier, where an exact record of rivetting was kept, the number closed there amounts to near upon 2,700,000 alone, and a very large quantity of material was sent across from the shops and the field which had already been rivetted up, and such rivets are not included in the above total. The rivets varied as to diameter from 11⁄8 in. for the heavy tubes and the skewbacks down to 3⁄4 in. for the buckle-plates; and, as to length, from 111⁄4 in. (measured without the head) down to 111⁄4 in. The greatest thickness of plates rivetted together was 9 in., and this occurred in the top junctions at the head of the vertical columns on Inchgarvie—the least thickness was 1⁄2 in., in the flooring of the viaduct.
The various hydraulic rivetting machines, by which about one-half of the rivetting was done, have already been described in the places where they carried on their work, and need not be further enlarged upon here.
At the commencement, ordinary furnaces fired with coal were used for heating, but it soon became evident that these could not be taken on the outlying platforms owing to their weight, the weight, of fuel, and last, though not least, to the danger of fire caused by hot ashes left on timber staging.
Various kinds of furnaces were designed and tried, all heated by oil, and in the end the difficulty was solved by turning the burner of an ordinary Lucigen lamp, in a somewhat modified form, into a small furnace and setting fire to it by a piece of burning waste drenched in oil. This was tremendous advantage, for these little furnaces, though made of iron and brick-lined, weighed little more than half a ton, and could be handled and shifted about with the greatest ease. All they required was a small pipe-lead to supply compressed air, and a small tank with oil, and a crane would pick them up and put them into any place where they were most handy. A boy could work them and turn out 200 rivets an hour easily, all heated evenly to a bright yellow heat in perfect condition for the hydraulic machines, three or four of which were fully kept going by one of these little furnaces. They were taken inside the tubes, and the smoke, if care was taken in adjusting the burner, could not molest any one.
Larger furnaces were set up in No. 2 shed for heating angles and tees, and they were very successful owing to the regularity and evenness of the flame and the facility with which they could be lighted and kept up.
For the hand-rivetters in the struts and lattice-girders, ordinary small forges were in use, with bellows worked by a treadle or by the hand. But here also the oil-furnaces came in usefully, for not only were the rivets pre-heated in the furnace if one happened to be anywhere near, but the boys contrived to make a connection somewhere with the compressed air pipes, and thus obtained a constant blast for their forges and saved themselves the trouble of working the bellows.
In places where so large an amount of timber staging was required, and where a fire on such staging might have been accompanied by the most disastrous results to portions of the permanent work, such furnaces as above described are of inestimable value, for there is nothing left behind that could cause a fire after the men left work. As soon as the supply of air and oil are turned off all flame at once disappears, and in five minutes the inside is black and cold.
The ordinary small furnaces used in the tubes and on the staging were about 11 in. wide by 8 in. high and 4 ft. long on the inside, with two doors for charging and drawing of rivets. They consumed about two gallons of oil per hour, with which they could heat 200, and on an emergency 250 rivets per hour, or even 300, if of smaller size.
The actual size of this kind of furnace is however a matter regulated entirely by circumstances, and it is not possible to lay down hard and fast rules for their construction. The furnaces and fittings shown in the illustrations are:
A furnace for heating the ends of angle bars, tees, or narrow plates. (See Figs. 144 to 146.) An early form of oil furnace with grate at bottom to keep a body of glowing coal to assist combustion. This however is not necessary. (See Figs. 147 to 149.) The latest form of small rivet furnace as used on the stagings and inside the tubular members. (See Figs. 150 to 152.)
Figs. 153 and 154 show the disintegrator, or spray producer, the action of which explains itself. In its present form it is much simplified, and if any obstruction in the small jet occurs, it is easily cleared by screwing back the mouthpiece.