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SPACING RIVETS, WIDTH OF LAPS, ETC.

On a previous page where we mentioned the pitching of rivet holes, we referred mainly to steel sheets, and wholly to drilled holes. Where punching is the system used in construction, the rivet (pitch remaining the same) should be increased fifteen per cent. for steel sheets, and twenty per cent. for iron sheets.

Arranging the rivet holes for double riveting, and adopting the triangular form, or zig-zag pitch, the sides of the triangle should never be, equal, but the base on lower row must be that of two right-angled triangles, forming, of course, a triangle whose perpendicular is one-half the whole base. Therefore in the case of" steel plates (96" shell) using 34 rivets (double riveted joint) the pitch according to standard before mentioned, measured on the hypothenuse, will be 2," while the whole pitch at the base will be 3". The perpendicular distance between rows, of course, will be 11⁄2", and the lap or total width of joint can be now fixed, as the distance from rivet hole to edge of sheet will be one and one-quarter times the diameter of rivet; so the joint or lap will be 4" wide. Now a wide joint adds strength to a boiler, and requires but very moderate caulking when closed up with a hydraulic riveter.

The great object to be reached when putting plates together is to have the shearing strength

of a double row of rivets equal to the remaining section in the sheets. The tendency to rupture at the sheet is most at the section under the rivet, or from the hole to the edge of sheet. Many boiler makers fix this distance less than the diameter of rivet. This method is wrong, and is liable to split or rupture at this point, as the rivet is acting like a blunt wedge to force the metal apart. The excessive driving of steel drifts to "line" the holes has a very weakening effect at the point between the hole and edge of sheet, and is one of the worst and "wickedest" tools of a boiler maker's "kit." This, and the old sharp cornered caulking tool, should be abolished, as there are more ways than one to make rivet holes "line."

In fastening stays for crown sheets and large internal flues or flat surfaces, the riveting of holding lugs or crow feet must be well distributed. The strains must not be concentrated on the shell of any boiler. Instead of lugs and crow feet, bars of T-iron should be used, rivited with tolerably fine pitch to the roof of shell, so that the whole arch will receive the strain, and the shell consequently be strengthened by the T-iron.

Now while a boiler is under a static force, constant pressure, it is all right; but when that force is turned into a dynamic current, and that current is intermittent, with pulsations ranging

in number fifty to four hundred and fifty per minute, then the conditions are vastly different. As we note these pulsations indicated on the steam gauge by a fall, twice in the revolution, of from one to five pounds, we pass the poor old boiler without even a glance while it is going through the same range of pulsations as the steam gauge pointer; the only difference being that while it does no harm to the gauge, it on the other hand has a very injurious effect on the shell. Take the case of the locomotive that exploded a few years ago on the Morris and Essex Railroad at Hoboken. Allowing this engine to have run five hours each day, with an average revolution of drivers of 150 per minute, this would subject the shell to 180,000 pulsations per day, and if there was bad bracing the laws relating to the fatigue of metals must certainly have asserted their rights. We believe the sheet ruptured, in the explosion mentioned here, was near the crown sheet, a little to one side of centre, and received the crow-feet lugs in a straight horizontal line, just below where was a double riveted side seam very stiff in itself. The action then was to buckle the sheet at each pulsation as the cylinders took steam, between the side seam and crow feet; the stays and double seam below being the two fulcrums. The parting of sheet in two places, one very near the crow feet and one near the double seam, establishes the correctness of this theory.

HEATING SURFACE.

The evaporative power of all boilers must be measured, of course, by the efficiency of their surface exposed to the fire in transferring the heat from without the plates to the water within them. Heat reaches the plates by radiation and contact. The contact comes from the solid fuel, the radiation from the flames and gases.

Water has a weak conducting power, and, if heat is applied to its upper surface, it does very little toward warming the water beneath, and, for this reason, the heat should be applied to the bottom of any vessel to produce the most effect as to evaporation. In case the enclosure is surrounded by water, the heat being applied from within, the greatest amount of evaporation takes place at the top of casing. Where the water is within and the fire without, the best surface is the curved style of crown sheet of a locomotive boiler. The next best surface is the horizontal plate, though seldom used on account of its weakness in design. The next in efficiency is the large cylindrical tubular boiler shell (the under side), where the heat first strikes it. The top inside surface of tubes and flues has also high evaporative power. The bottom of a tube or flue evaporates very little water, as the water will not leave the under side of the tube unless driven away by forced circulation. With a 31⁄2" tube there is six times more water evaporated

directly over the tube than at a point directly under it, while the sides of the tube will evaporate a little more than one-third of the quantity evaporated at the top of tube. We are now speaking of boilers that pass the heat through the tubes. Then, again, the heat does not travel in true parallel lines through the tubes, but has wave curves, with a decided tendency to hug the upper side of tubes.

Where the water is in the tubes, and the fire and heat around them, the order of things is reversed, and the bottom side of tubes is the most powerful generator, the top side yielding a very small evaporative duty.

Sectional boilers, with water tubes inclined to induce circulation, give the greatest evaporative power, but the life of them is much shorter than the shell and fire tube system. A properly proportioned and constructed fire tube boiler will evaporate from 934 to 10 lbs. of water per 1 lb. of coal, and a good sectional water tube boiler will evaporate between II and 12 lbs. of water per one pound (1 lb.) of coal, both systems working under the same conditions as to grate surface, firing, feeding, etc.

The shell fire tube boiler will last, with ordinary care and fair feed water, twenty-five years, and very little repairing will be needed during that time; but a sectional boiler of same capacity will fail, with the best of care and feed