November 6, 1914. THE COLLIERY GUARDIAN. 969 with tanks and bunkers empty. In the ease of 6 w.c. locomotives this proportion is usually 5'2 to 1 and 4-5 to 1 respectively. In any condition, however, having due regard to our English climate, this ratio should never be less than 4-25 to 1. With 6 w.c. engines there is, as a rule, no difficulty in obtaining sufficient adhesion, as they of necessity must be much heavier than 4 w.c. engines of the same theoretical power. Moreover, supposing a 6 w.c. and a 4 w.c. locomotive to be of equal theoretical power and weight, the 6 w.c., though theoretically less efficient owing to greater mechanical friction, etc., would be found to be capable of hauling a greater load if the rails were in a greasy condition, consequent upon having six instead of four points of contact with the rails. When considering the purchase of a suitable locomo- tive, it is advisable to choose one at least 20 to 30 per cent, more powerful than the current requirements demand, having in view the desideratum that the out- put will be an increasing one, demanding more loco- motive power in the future, and also that after a few years running the working pressure of the boiler will be reduced. Boiler Shell. — The boiler barrel and firebox casing plates are almost invariably of steel, Yorkshire iron plates being now only used in rare cases of exceptionally bad water. The barrel is best made in one plate in the case of small boilers, and in two plates for the larger ones, thus keeping down as far as .possible the number of riveted joints. Great care should be exercised in selecting these plates, which should be of a very mild nature. The barrel and firebox casing plates are bent cold in rolls, and the tube throat and back plates should cement. A good brand should be used, ground up with water to the consistency of a thick paint, and applied with a stiff brush- This is allowed to set as a thin coat- ing to the barrel plates. If without cracks, and the pre- caution has been taken to have the plates, rivet heads, and joints thoroughly cleaned before application, pro- tection can be relied on for a considerable time, how- ever bad the water may be, as the film is practically impervious until broken away—which, how’ever, is very liable to occur during the process of cleaning out the. boiler. Another supposed preventative is a mixture of tar and plumbago; but as there is a danger of this boil- ing off in time, the author cannot recommend it. Paint- ing the surfaces of the boiler with a special carbon com- pound is stated to possess the merits of hindering pitting and of preventing the adherence of scale to the surfaces. It is used locally in some cases with success, and as this substance forms a minutely thin film, the firebox can also be coated with it, which is not the case with the before-mentioned preventatives. The cement would crack owing to the repeated expansions and con- tractions, whilst the tar and plumbago would keep the water away from the firebox, and thereby prevent the formation of steam. The joints and riveting should receive great consider- ation, because they are subjected to severe strains as well as to continual vibration. The longitudinal and throat plate joints (and in the case of the larger boilers the barrel ring) are double riveted, the remainder being single riveted. The longitudinal joint of the barrel should be well out of the water space, but in very, small boilers where this is not possible, it is better to have the joint totally submerged than half and half. may be feamered out, plugged and re-drilled, thus form- ing a bush. Smaller tubes than the original, or tubes with smaller ends, are then refitted, after the bushes have been beaded over to cover the cracks and prevent leakage. Should a' number of bridges be cracked then it is advisable to renew the tube plate, or at any rate the upper portion. With boilers carrying high working pressures, the bulging may be so severe as to cause the plate to be pressed over the tube ends. If this occurs, the plate should be straightened and longitudinal stays fitted between the tube plates. There is also a tendency to expand sideways, which is resisted by the stays. This expansion assists to produce the bulges, and in time grooving in the radius of the flange may become notice- able. Also the parts of the side flange in contact with the fire generally waste, and ultimately the lap may be burnt away and the flange crack through the lap to the rivet holes. When this occurs, if the remainder of the plate is good, it may be patched, but if deterioration has set in, it is advisable to replace the plate. Experience proves that the higher the working pres- sure the shorter is the life of this plate, and this remark applies generally to the boiler as a whole, in spite of the same being designed for the higher working pressure. With regard to the firebox generally, the majority of troubles such as cracks, bulges, leaky stays, and tubes, are in almost every case caused by the accumulation of scale. Some locomotive builders, with the idea, of fitting a well-stayed box, make the mistake of having these stays too close to the flanges, and on the curves of the wrapper plate, which, together /with the plates flanged to too Fig. 9.—4-W.O. Side-tank Engine Cut Down in Height. Fig. 10.—6-W.C. Saddle-tank Engine with Inside Cylinders. Fig. 11.—6-W.C. Saddle-tank Engine with Outside Cylinders. 1 Fig. 12.—4-W.C. Saddle-tank Engine Cut Down in Height. be flanged by hydraulic power, at a heat above cherry red, after which they should be annealed and left to cool slowly. The barrel is usually connected to the smokebox tube plate by a steel angle ring. The smoke- box tube plate is a place where deterioration frequently sets in. It is subjected to such influences as chemical action on both sides, galvanic action in the water side, and mechanical action due to its connection to the engine frame. On the smokebox side grooving often sets in around the bottom of the plate, owing to the fine ash lying in the bottom of the smokebox and the moisture caused by the exhaust steam, and any rivet heads in this locality will waste off, necessitating new rivets. The fine ash and dampness act very quickly, and in the hope of affording protection to the tube plate some engineers have an iron casting fitted around the bottom part; but this is of little utility, as the ash and moisture still gain access to the plate. The author would sug- gest Portland cement sloped to drain the moisture away from the plate as the most effective precaution. On the inside of this plate corrosion sometimes occurs around, the tube holes, being accentuated by some kinds of water which are good conductors of electricity, aiding a galvanic action between the brass, tubes and the steel plate. A common trouble in some localities is the pitting action of the water on the barrel plates. With a view of preventing the same, numerous compositions have been tried, with varying degrees of success. Some engi- neers claim as a successful precaution the coating of the barrel plates below the water line with a thin layer of The foundation ring forming the bottom of the water space and connecting the inner firebox to the casing is usually a mild steel forging, though some engineers insist on this being of Yorkshire iron, as there are at least two scarf-joint welds to make good. The corner joint is often a troublesome one, and the setting of the plates requires to be very exact to make certain of sound work. The radii of the comers should not be too sharp, as it then becomes extremely difficult to get rivets through or screws in. For this class of engine single riveted foundation rings are all that is required. Firebox. The internal firebox is almost invariably of copper, as it wears better than iron or steel. Lowmoor iron boxes have been tried, but not adopted to any extent, though the author knows of several rare instances where they are employed with comparative success. Experi- ments in this country and on the Continent have proved entirely unfavourable to steel. When the high scrap value is taken into account, in addition to the fact that the engine is less frequently laid up for repairs, the difference in th’e long run is in favour of copper. In fairness to steel and, as a point of interest, it may be added that a local colliery company is using steel fire- boxes with remarkable success, their unique experience being that steel fireboxes last longer than copper. The firebox tube plate is probably the most severely strained part of the boiler, the strain being caused by the expansion of the tubes, the expansion of the sides of the box, and the resistance to its own expansion offered by the roof stays imparted by the crown sheet. If the cracks are neither numerous or severe, the tube holes small a radius, results in the flexibility of the box being diminished, causing cracked plates and broken stays. It is now becoming the practice with good builders to turn the thread off the centre of these stays to give greater flexibility, and, in addition, some engineers are now having an in. hole drilled up the centre to give warning, when breakage occurs by blowing into the fire- box. This practice is good, and is very much to be desired. As there is generally an unsupported area between the top row of firebox stays and the bottom row of tubes, it is customary to fit palm stays, the number depending on the size of boiler. The top of the firebox being flat, or, at any rate, only slightly curved, requires staying. Some manufacturers fit wrought iron or cast steel girder stays, but the most approved method is by direct stays. The method adopted generally for staying the flat sur- faces of the smokebox tube plate above the .tubes, and the back plate of the firebox, is by longitudinal stays, which should be screwed through both plates, with a solid head on the outside of the back plate, and a nut on the outside at the tube plate 'end—the joints being made with copper washers. The best practice is to have these made of steel from the solid bar and stressed from 8,000 to 9,0001b. per sq. in. The fire hole ring forming the connection between the firebox and the outer casing is usually a plain welded Yorkshire iron or steel ring. It is preferable to have this thinned to about 1| in. thick, with the copper plate dished to suit, thereby giving flexibility to the copper box and affording some protection to the rivet heads which become somewhat shielded from the fire.