moderate grades the driving wheel adhesion is sufficient, on the steepest grades there is a rack rail between the two running rails and with it engage gears keyed on the driving axle, thus giving a positive tractive effort.

Q. How are rack locomotives prevented from running away down grade?

A. They have powerful air, hand, and water brakes. Q. What is the purpose of freight helper locomotives? A. Increasing the capacity of an up-grade or a heavily taxed track, balancing the power on the division, reducing the number of locomotives and crews required to handle heavy freight tonnage over a busy mountainous district.

Q. What are the prime requisites of a steam locomotive for freight helper service?

A. Maximum adhesion for tractive and braking power, minimum rail pressure per driving wheel, and short rigid combined with a long flexible, driving-wheel base, to adapt it to a mountain line having considerable curvature and gradient.

Q. To accomplish this, what solution is adopted?

A. Americanizing foreign railroad practice by making use of the Mallet articulated feature, which permits of the use of a maximum effective wheel base, with materially reduced rigid wheel base, and provides for developing an average through-freight train speed without excessive wear. Subdividing the power through the use of four independent cylinders, pistons, main rods, crank pins and frames, in place of two, and better balancing of the reciprocating parts, result in less strain on all parts, and reduce the liability for breakage and failure.

CHAPTER III

THE BOILER AND ACCESSORIES

Q. What is the most important part of a locomotive? A. The boiler.

Q. What are its essential parts?

A. They are usually six (sometimes seven) in number; cylinder, main shell, or barrel, waist (in many cases), shell or outer fire-box, dome, inner fire-box or fire-box proper, tubes or flues, combustion chamber, smoke-box or arch, and stack or chimney.

Q. What materials are used?

A. Wrought iron and mild steel; the latter now coming into use to the exclusion of the former.

Q. What are the advantages of soft steel for locomotive boiler construction?

A. Great tensile and compressive strength, ductility, and uniformity of structure; thus enabling a boiler to stand more pressure for given weight, or to be lighter for given pressure.

Q. What is the peculiarity of the locomotive boiler, as distinguished from the stationary?

A. In having but one shell for both furnace and boiler. Q. How are locomotive boilers classified?

A. By their shape, as "straight top," having the cylindrical shell of uniform diameter from the fire-box to the smoke-box; as "wagon top," having a conical or sloping course of plates next to the fire-box, tapering to the cylindrical courses; "extended wagon top," having one or more cylindrical courses between the fire-box and the sloping course which tapers to the main shell diameter. They are further classified as "wide fire-box," "narrow fire-box," Belpaire, Wootten, etc.

Q. Describe a locomotive boiler and explain the use of its various parts?

A. The locomotive boiler is usually in form a cylindrical shell joined to a rectangular one, the rear or rectangular end containing the fire-box, usually also of rectangular shape to conform to the shape of the outer shell. The fire-box is supported at the bottom by the mud ring, or foundation ring, and at the sides, front, and back by stay-bolts screwed through the outside sheets and firebox sheets, and hammered over. The space between the fire-box and outer boiler sheets varies from 3 to 9 inches. The fire-box roof or crown sheet is supported by crown bars in some forms of boilers, and radial stays (long stay-bolts) in other types. To each side sheet near the bottom are bolted the grate rests, which support the grates. The fire-box is entirely surrounded by and covered with water, and forms the most efficient heating surface in the boiler. The cylindrical part of the boiler to which the fire-box end is jointed contains the tubes, or flues, which vary in number according to the size of the boiler. They are usually in locomotive practice from 2 to 214 inches in diameter. These are also surrounded by water and form the greater part of the heating surface, all the hot combustion gases distilled passing through them.

To the upper part of the boiler, and usually ahead of the fire-box, is riveted the cylindrical dome, which is a storage reservoir for the steam generated. Its object is to obtain drier steam. It also contains the throttle and stand-pipe connecting to the dry pipe, used to convey the steam from the boiler to the cylinders.

Riveted to and forming an extension of the cylindrical part of the boiler is the smoke arch, to which the cylinder saddles are bolted, and which also contains the steam pipes that convey the steam from the dry-pipe to the cylinders, the exhaust nozzle, through which the steam escapes to the stack, the diaphragm sheets, the petticoat pipe or draft pipe, used to regulate the draft on the fire, and a netting to prevent spark-throwing. At the top of the smoke arch is the smoke stack, while at the bottom

in some types is a hopper for the discharge of the sparks that may accumulate in the front end.

Under the fire-box is bolted the ash-pan (to catch the ashes), either or both ends of which are fitted with a damper to prevent ashes or coals from falling out, and to regulate the admission of air to the fire-box.

Q. What are the chief requirements of a boiler?

A. That it shall be strong enough in all parts to stand the maximum pressure which will be put on it; deliver steam enough for the cylinders under maximum duty; be economical of fuel, and able to use various sorts of fuel advantageously; be readily inspected and readily and cheaply repaired; shall not foul, and not be readily lessened in steaming power by scale.

Q. What is an extended wagon-top boiler?

A. A locomotive boiler having a shell made of one or more cylindrical plate courses next to the fire-box, a conical course tapering down to smaller diameter, and one or more adjoining cylindrical courses of reduced diameter next to the smoke-box.

Q. Why put the cylindrical course next to the fire-box? A. To provide a place for the steam dome, thus doing away with crown-bar staying over the crown-sheet as in an ordinary wagon-top boiler.

Q. What name is given to the cylindrical part of a locomotive boiler?

A. The waist or barrel.

Q. What about the diameter of straight boiler-shells as compared with those of the wagon-top type?

A. With the straight shell the waist is about two inches greater in diameter than with the wagon-top, for a given steam-space and water-room.

Q. This being the case, which type gives, with an equal number of flues, the more circulation-room for water between flues?

A. The straight shell type, by reason of its larger diameter.

Q. What is another advantage of the straight shell? A. It is stronger than where there is a wagon-top.

Q. How many pounds per square inch should good steel boiler-plates stand?

A. 60,000 pounds per square inch of cross-section, lengthwise with the fiber; 54,000 across.

Q. To how much of this pressure is it proper to subject a steel boiler in use?

A. To about one-fifth, so that any strains which may be applied to it will not make it permanently stretch or otherwise change its form or dimensions.

Q. What is the strength of wrought-iron boiler-plate as compared with mild steel?

A. About one-sixth less.

Q. What is the test of a good wrought-iron or steel boiler-plate, stay or rivet?

A. It should stand not less than 50,000 pounds per square inch of cross-section without breaking, and stretch about one-eighth of its length before breaking; if not over an inch thick should be capable of being bent double when hot, without cracking. If under one-half

inch it should be capable of being bent double when cold, without cracking. A hot rivet-shank when flattened to half its diameter should stand having a hole punched through it without tearing at the hole.

Q. Of what kind of steel should rivets be made? A. Of the very softest or mildest, to lessen the danger of their getting hard and brittle in working and in use.

Q. What is the reason that metal of the highest tensile strength is not desirable for steel boiler-plates and rivets?

A. It is apt to be hard and brittle, and the soft ductile metal is safest for such work.

Q. How strong is a rivet-seam between two plates of equal thickness and strength, as compared with the plates which it fastens together?