in diameter, with a stroke of a little over 12 in., the cylinder and ram passing through the upper box girder. The head of the ram acted by means of a swivelling cap immediately upon the cross-girder C C passing from column to column. The working of these girders and rams was as follows:—The lower box girder was held by a sufficient number of pins to carry the total weight placed upon it.
As soon as a small amount of pressure water was allowed to enter the ram, the latter lifted the cross-girder C, and with it the box girder B, to a small extent. The pins could then be withdrawn from the girder B, and the girder lifted 6 in. or 12 in., as the case might be. The pins were then driven through the sides of the girder B into holes newly exposed, and the beams and the weight let down upon that girder. The pressure was then allowed to pass on the top of the ram, and the cylinder was drawn up to the ram, bringing with it the lower box girder A. When the holes in girder and beams had come opposite, the pins were driven in, and the same process could be gone over again.
The arrangement as just described was placed in each of the four vertical columns, and in the case of Inchgarvie into the central ties as well, thus giving six points of support there, instead of four. The longitudinal girders were now placed upon slides on the top of the cross-girders passing from side to side, and the tops of the girders covered over with cross-timbers, and decked with planking 3 in. thick. Thus there were two platforms about 25 ft. wide by 200 ft. long, or 350 ft. long in the case of Inchgarvie, embracing completely and projecting to some distance beyond the vertical columns. Figs. 103, 104, and 105 show various positions and details of platforms.
_Figs._101_and_102,_Page_45.png)
HYDRAULIC RAMS AND GIRDERS FOR LIFTING RAMS IN CENTRAL TOWERS
At the starting-point just above the skewbacks these platforms were some 110 ft. apart, centre to centre, approaching each other, of course, with every few feet of lifting to the extent of the batter of the vertical tubes, which is 1 in 71⁄2 about. (See Plate IX.)
With every few feet of lift these platforms had to be pushed together, which was done by hydraulic rams also, and this accounts for the number of slide-blocks mentioned above.
The ordinary routine in lifting was as follows:
From a level position of the platforms the cross-girder in the north or south columns was lifted first 6 in; next the other end was lifted 1 ft.; then the opposite end 1 ft.; and so on until sixteen lifts had been made, when a last lift of 6 in. at the end which commenced the lifting made the platform level again.
When the platforms had in the first instance been lifted to a sufficient height, the rivetting cages were suspended from the under side of the platforms, and in subsequent lifts they were always drawn up with the platforms. (See Plate IX., rivetting cages on Inchgarvie.) This cage simply consisted of a stout circular wire cylinder being placed round a rivetting machine, similar in construction to the one described for the horizontal tubes, but hung vertically. Inside this cage the men worked with perfect safety as regards falling themselves or dropping things down on those working below. (See Figs. 106 and 107.)
The portions of the rivetting machine inside the vertical column were not drawn up by the lifting of the platform, but were lifted separately subsequently. The length of the plates in the vertical columns being 16 ft., the lifts of the platforms were of the same length.
The vertical columns were always built above the platforms to any height which could be reached with the cranes or other means at disposal, while the diagonal struts were at times built above, at times below the platforms.
On the inside of the platforms at each end were hoists worked by wire ropes from winches placed on deck, and all plates, beams, and other material was brought up by means of these hoists and distributed over the platforms. (See Figs. 104 and 105.) Two-ton and 3-ton derrick hand cranes were found the most kandy for the erection of the different members, but a traveller, or Goliath, worked by hydraulic power, was also used.
When a lift had first been made, the first work, as a rule, was to put in the two closing plates to each column which had been left out to allow for the passage of the cross girders. The inner part of the rivetting machine was then drawn up, the whole machine fixed, and rivetting was at once commenced ; while above the platform a section was added to the vertical tubes and the diagonal struts. The platform on Inchgarvie being of so much greater length, and weighing fully 700 tons, had to be provided with two additional supjKirts, and these, of course, were carried upward the same as the vertical columns.
The water pressure for lifting was supplied by a special set of pumps which produced a pressure of 35 cwt. to the square inch. No accumulator was used, but the pumps simply forced up the rams gradually. Under this great pressure the cup-leathers in the hydraulic rams and the leather washers in the pipe-joints, frequently gave way and caused much delay. Frosty weather also did a great deal in the way of freezing pipes to hinder the work, but after once the men had got into the way of working the plant, things went more smoothly. The first lift of the Inchgarvie platform took, with one thing and another, nearly eighteen days in the months of January and February, 1887; while the last lift, on the 9th of August of the same year, was accomplished in five hours. With the batter which the vertical columns and other side members
