Chilled 7ree/s.—The most approved system of making cast-iron wheels is that in which the body of the wheel is allowed to cool slowly, so as to retain as much of its toughness as possible, while the rim, which is subject to wear, is made as hard as possible by various methods of rapid cooling called " The chilled cast-iron wheel is in common use in the United States on both passenger- and freight-cars.
Composite Irlecels.—Wheels constructed in part of wroup.-,ht iron are well adapted for their intended service. The earliest forms of composite wheels were those of Stephenson and Losh (ISI6). In these, the spokes were made of wrought iron, and the hub and rim were cast upon them. A wrought-iron tire was then shrunk upon the rim, and secured in place by a dove-tail channel cast in the rim, or other convenient artifice. A wheel of this pattern is shown in Fig-ure 3. When the tire is in place, the periphery of the wheel is turned down to the proper dimensions in the lathe. Disk-wheels (As,,s. 2, 4) have almost entirely superseded the spoke wheels, as they afford a more uniform support to the wearing-surface of tlie wheel. The diameter of the wheels in use on American railways varies from 3o to 33 inches for passenger- and freight-cars, and from 26 to 28 inches for coal-cars. This measurement is the least diameter from tread to tread. Respecting the wear of the chilled wheel in general use in the United States, Trautwine gives fifty thousand miles as the average run.
PaPer 1171CC/S.—Aii interesting novelty in construction which has lately attracted attention is the use, with composite wheels, of wood and paper as a packing between tires and hubs, to absorb jar (p. 3T, fig. 15). The so-called " paper car-wheel " is formed of a cast-iron or steel hub and a tire of rolled steel, in the centre or main body, of which straw-board is tightly pressed as a packing between two circular plates of rolled iron, which are very strongly secured by bolts. Paper wheels have come largely into use on locomotive-trucks and on passenger-, parlor-, and sleeping-cars, and have made an unusually good record in practice. Some allowance should be made, however, as Trantwine says in speaking of their wearing qualities, for the fact that they are -used principally under the best class of passenger-cars on through trains that make few stops; hence they are sub jected to less of the destructive action of the brakes than the common wheels. The greater number of the latter, furthermore, are running under freight-cars, where they are subject to the hardest kind of usage, not only on account of the frequent stops, but also on account of the inferior charac ter of the springs in use with cars of this class.
Sleet Tires.—The material of the tires, where compound wheels are used, is commonly steel—a material which, notwithstanding its greater cost, demonstrates its value by much greater durability in service than the common chilled cast-iron wheels. Compound wheels of cast and wrought
iron with steel tires are coining into general use under first-class passenger cars. The modifications in form and construction are numerous.
Car-axles are constructed of wrought iron or steel, with the wheels of the cars fastened to them by shrinking or pressing, so that they may better withstand the strain to which they are exposed by the lateral movements of the car in swinging around curves or by reason of irregularities of the roadway. The appearance of the car-axle is shown in Figure 3 (pl. 31). The severe torsional strain to which the axle, made of a sing-le piece, with the wheels firmly attached to it, is exposed in turning curves has caused the invention of many ingenious artifices to obviate the difficulty, but none have come into general use. The axles run in bearings of metal made of anti-friction compositions of various kinds. The bearings, in turn, are contained in so-called " axle-boxes," which contain, likewise, the various contrivances employed for lubrication. The axle-boxes vary greatly in details of construction, but should be as simple as possible, and should afford proper protection against the entrance of dust and grit and against the loss of lubricants, and should be readily accessible for examination.
Cerr-springs.—The axles bear the entire weight of the car, and, that the jar of the wheel shall not be transmitted directly to the car, it is the uni versal practice to interpose some elastic or yielding material between the car and the axle, to receive the shock and moderate its effect. For this purpose springs of a great variety of form are employed (figs. 7-11), cast steel and rubber being the materials commonly selected. Figure 5 (pi. 3o) exhibits a method of supporting the car-body upon springs in the absence of trucks, the provision for the vertical play of the spring being very clearly shown. Figure 6 exhibits another device for securing the frame to the ends of the springs to insure the requisite flexibility of the combination. Figure 13(7 shows a double-spring system employed on German railway passenger-carriages, on which one set of springs is placed between the longitudinal supporting frame of the carriage and the axle, and the other between the frame and the carriage-body. In American practice, in which, as previously noted, the car-body rests upon trucks, it is the custom to interpose two sets of springs between the car and the axle. This is shown on Plate 31 (fig. 2b), from which it will be observed that the end of the car rests upon the swinging bolster, beneath which are elliptical springs, the entire weight being suspended from a frame which, in turn, rests upon springs of inclia-rubber carried ou a so-called " equalizing-bar," whose ends rest on the axle-boxes. Figure 7 05/. 3o) exhibits certain details of the running-gear and Figure 12 the under-frame of the car-body of German railway-carriages.