American locomotives differ from European locomotives in the follow ing general arrangement: (I) in their frames, which are built up of bars of rectangular section instead of being made of plate iron; (2) in their cylinders, which are invariably outside the frames; (3) in the application of equalizing beams for distributing the weight; (4) in the absence of buffers, and in the use of a pilot or cow-catcher in front to throw obstruc tions from the tracks, which arc less carefully guarded in America than in Europe; (5) in the use of a large headlight in front of the stack instead of two small ones over the buffers; (6) in the use of an enclosed and com paratively comfortable house-cab; and (7) in carrying a large bell. Amer ican engines use the dome almost universally; and those which burn wood have a large flaring stack which is unfamiliar to European engineers and travellers. The subjects of frames and outside-connected engines receive separate treatment in the following pages (pp. 287, 288), where they are considered especially with reference to their evolution in America; and equalizing beams are treated at length on page 291.
Adhesion and frictional adhesion of the treads of the driving-wheels to the rails constitutes the tractive power of locomotives. That this adhesion may be as great as possible it is necessary to load the driving-wheels with as great a portion of the locomotive's weight as it is safe to concentrate on such small areas of the rails. By a suitable arrange ment of the springs, carrying the structure and resting on the axle-boxes of the wheels and of equalizing bars, nearly any desired fraction of the total weight can be thrown on the driving-axle; but this cannot be varied while running. The truck-axles—that is, those not rotated by the motor, but only sustaining the carriage—bear only a small proportion of the weight. If the driving-axles or axle (p1. 97, figs. 2, 3, recognizable by their bearing large wheels) were loaded with the entire weight, and the running axles in front and back had no load, the entire weight of the locomotive could be utilized in producing tractive adhesion, and the undesirable fric tion of the running-axles would be avoided. But in this case the locomo tive would be suspended on the driving-axle in unstable equilibrium, and as it could turn horizontally on the rails it could easily be derailed. For this reason, and because with•a too greatly concentrated load, which would' endanger the substructure of the railroad, injure the ties and the rails, and destroy the fish-plates or other joints, the above-mentioned most favorable application of the weight for tractional adhesion is not made, and the form of construction in Figures 2 and 3 is employed, this being nearly identical with that in Robert Stephenson's first type of locomotives, which for fast trains require less tractive power than for heavy trains.
evils of concentrated loads are overcome by applying several driving-axles, so that the running-wheels are entirely or partially dispensed with by being converted into drivers. The addi tional driving-axles have their crank-pins so connected with those of the main driving-axles by parallel rods or "side rods" that one pair of cylin ders rotates all the driving-axles. In Figures 3, 5, 6, 8, and II (pl. 98) there are two, in Figures 2, 7, and io there are three, and in Figure 4 there are four driving-axles coupled together.
It might be asked, "Why not have all the axles coupled as drivers and have no running-axles?" The principal objection to such an arrangement is due to the curves of the track. A locomotive on a sharp curve, on which a chord equal to the distance between driving-wheel centres subtends a greater angle on the inside rail than on the outer, the rigid union of sev eral driving-axles would cause a forcing apart of the rails and the conse quent derailment of the machine. Hence the position of the wheels, as
shown in Figures 2 and 3 (p1. 97), is retained for locomotives for fast trains, enormously large driving-wheels having been first used by Cramp ton. And hence also the use of coupled driving-axles for locomotives for freight trains, which run more slowly, but exert more tractive power.
Mountain railroads, which naturally have sharp curves, require espe cially powerful locomotives to overcome their up-grades. The utilization of the entire weight for tractive adhesion here seems eminently desirable, because the dead load has to be raised to the top of the mountain. Special efforts, therefore, have been made to use on these roads locomotives with coupled wheels, and to overcome the difficulties arising from the curves. A further advantage of coupled wheels is, that as on the down grade the locomotive must aid in checking the train, which would otherwise acquire too great speed by its weight, the more coupled axles there are where the track is imperfect the more effectively the steam is employed to reverse the engine. (The arrangement of American locomotive-wheels receives special treatment on page 289, and the evolution of the American driv ing-wheel is detailed on page 29o.) disadvantages of curves was first sought to be overcome by the use of trucks (with two or four wheels), upon which rested the front, and sometimes the back, part of the boiler, in such a manner, however, as to enable the truck to turn and accommodate itself to the curves. For this purpose the truck is connected with the locomotive by a pin or bolt, placed as in Figure 8 Nowotny's system), or over the centre of the truck or outside of it further to the rear, as in Figure to (Norris's system), in which the bolt does not serve as both fulcrum and centre of motion, but acts merely as a pivot, the fulcrum being- a circular rail placed on the truck. If, as in the Norris system, the truck has but one axle, it is called a "Bissel truck ;" and if it has two, a "Baessen truck," after their respect ive inventors. (American trucks are considered on page 29o; the Bissel truck is made a special topic on the same page.) Enscrik's Locomotive.—More completely to utilize the dead load of the locomotive as well as of the tender (p1. 97, ; pi. ji 5)—which, being as unprofitable as a heavy unpaid-for freight-car, but which never theless has to be transported—not only all the locomotive-axles, but also those of the tender, may be coupled together, thus converting all into driving-axles. An example of this system is Engerth's Semmering-Rail way locomotive, which in external appearance resembles that shown in Figure 4 (pt. 98), with the difference that it has three coupled locomo tive-axles and two coupled tender-axles. By the peculiar construction of the tender, on which the back portion of the boiler rests, and to which it is secured by bolts, one of its driving-axles is brought close to the rear locomotive-axle in such a manner that the tender-axle is driven by a cog wheel fastened to it, a second gear-wheel being fastened on an inter mediary axle, and a third fastened ou the loconlotive-axle. The first tender-axle transmits the power by means of coupling-rods to the others. This mechanism has been replaced by an intermediary crank shaft and coupling-rods.