The Works Of Messrs. Schneider And Co.

144 MESSES. SCHNEIDER AND CO.’S WORKS. so that the checks can be separated. The separating of tlie sheaves is effected automatically by the trucks; these carry a spindle which, in engaging in the groove of the sheaves, insures the separation of the cheeks while the truck passes ; two counterweights afterwards bring the cheeks into contact. (Figs. 438 to 463, Plate XCIL, are details of the érection gear.) The sections of the ribs are deposited either on the suspended flooring or on the wood centres of the service bridges. They are then adjusted, this being a very delicate operation. They are first lowered by using tlie steam winch brakes, on oak wedges placed at tlie ends of each voussoir, the wedges serving to give an approximate position, final adjustment being obtained by means of small plates that bear on the oak wedges. Sledge-hammers can- not be used, as the suspended flooring is not adapted to withstand violent shocks ; screw-jacks are not available, as the flooi’ing is not strong enough to afford abutment. A special device has therefore been designed by which the direction of the sections can be regulated in a vertical plane, according to the position they have to occupy in the rib, the angle formed with the horizontal diminishing progressively to the springing to the key. The voussoirs are held by four hooks under the top flange (Fig. 445, Plate XCIL) ; these hooks are united in pairs transversely to the voussoirs by small stays held by a rod. The position of the cable with relation to the transverse centre line of the rod is made to vary by means of a screw, so that the centre of gravity of the voussoir can be altered at will with relation to the suspension point. When suspension is effeeted at a greater or less distance from the centre, the voussoir is more or less inclined to the horizontal, and this enables the foreman to détermine in advance the required angle by his ordinate and abscissæ table. This same table is also used for the vertical adjustment of the ribs, before striking the centres, as follows : Two sights placed at each end of the roller bridge détermine the horizontal plane, according to which the heights of the rods laid on the four corners of each voussoir is traced, to obtain the required height. When necessary, the level is corrected by slightly lifting the voussoirs by means of levers, small iron wedges being inserted between the voussoirs and the top plate. In order to obtain a slight lateral displacement, the lower plate is drilled with holes in which levers are inserted to shift the voussoirs to their final place. The horizontal spacing of the sections is constantly verified by means of steel gauges placed between the voussoirs of the arch last erected, and the one in course of érection. The difficult part of the transverse adjustment of the ribs lies in the perfect adjustment of the first segments. Each half rib is inspectée! and passed at the works on a full-sized centre, on which all the voussoirs are fitted together previous to delivery, so aa to verify that they are of the exact dimen- sions and that they join perf eetly; consequently, if the work on the spot is started coirectly, each half arc rights itself on its whole length. Each two adjacent sections are joined by means of ten 36-millimetre in.) bolts. When all the sections are fitted together, the centre is struck, and this nécessitâtes a fresh adjustment of the whole rib, as the temperature influences in a marked degree the variations in the height of the key ; it is therefore necessary to adjust all tlie ribs at a medium temperature, which is taken as a basis for the operations. In order to facilitate corrections due to varia- tions in temperature or to undetected imperfections in manufacture, a special joint has been made at the key, the extent of which is made to vary by the interposition of plates of different thickness. The segments of two ribs of a group being adjusted, they are stayed both horizontally and vertically, by pro- visional wood bracing placed so as to withstand compres- sion, and by steel tie-bars to maintain the exact distance apart, the wedges are then removed. Special care has had to be taken during these operations to ascertain that none of the screw-jacks works to a greater extent than its neigh- bour, as a fracture of the suspension rod might ensue Recording screw-jacks are therefore used to enable the foreman to check their working. These Jacks (see Figs. 449 to 454, Plate XCIL) consist of a cylindrical casing formed of a steel plate which rests through two sleepers riveted to it on the steel string-pieces of the suspended flooring. The action of the loads on the head of the screw-jacks is transmitted through a screwed rod fitted with a nut to a series of Belleville springs that rest on the bottom part of the envelope. Each variation in weight gives a corresponding variation in the com- pression of the springs, by which the rods can be graduated up to 16 tons. When the wedges are driven all the screws-jacks are operated simultaneously, the effort being inereased by a half-ton each time. When the two ribs are raised suffi- ciently to free all the oak Blocks, iron wedges, &c., the joints are put in place ; the jacks are tlien eased with equal précautions, the level of the two joints being maintained constant by means of a water-level, which enables the foreman to ascertain that the motions of the two ribs are simultaneous. A pair of ribs can also be lowered by a series of successive operations, one gang of men working alternately on the up-stream and on the down-stream ribs, provided that the difference in the level by lowering, does not exceed 10 millimétrés (f in.) to 15 millimétrés in.). When the two first ribs are self-supporting and are well stayed, the service bridge is shifted on its rails, to be used in the érection of a second pair. When in working position, the roller bridge flooring suspension bars are arrangée! so as not to involve taking down the flooring for shifting, but simply the removal of one of the bars in every two of each group. When the first two groups of ribs were set free, the up-stream set of piles were removed and brought down-Htream ; the bank centres also were shifted, to be used in erecting the third group ; this shifting is very easily effected, owing to the interposition of oak sole- beams between the heads of the piles already driven, and