Figure 52, on [p. 145], represents a section of the tread and flange of a chilled wheel, showing the peculiar crystalline appearance of the chilled iron.
Fig. 51.—Cast-iron Car Wheels.
In making cast-iron wheels the quality of the iron used is of the utmost importance. The difficulty in making good wheels lies in the fact that most iron which is ductile and tough will not chill, whereas hard white iron, which has the chilling property in a very high degree, is brittle, and wheels which are made of it are liable to break. There are some kinds of cast-iron produced in this country which have the two qualities combined, in a very remarkable degree; that is, they are ductile and tough, and will also chill. Wheel-founders also mix different qualities of irons to produce wheels with the required strength, and which will resist wear; that is, they use a certain amount of hard white iron which will chill, with that which is ductile and soft. By changing the proportions, any required amount of chill can be produced. The danger is that iron which has little strength or ductility will be fortified with hard chilling iron, and a very weak wheel will thus be the result. Thousands of such wheels have been bought and used because they are cheap, and many lamentable accidents are undoubtedly due to this cause. To guard against this, car-wheels should always be subjected to rigid tests and inspection.
In Europe wheels are made of wrought-iron, with tires which were also made of the same material before the discovery of the improved processes of manufacturing steel, but since then they have been made of the latter material. Owing to the breakage of a great many cast-iron wheels of poor quality, steel-tired wheels are now coming into very general use on American roads under passenger-cars and engines. A great variety of such wheels is now made. The "centres" or parts inside the tires of some of them are cast-iron, and others are wrought-iron constructed in various ways.
Fig. 52.—Section of the Tread and Flange of a Car Wheel.
What is known as the Allen paper wheel is used a great deal in this country, especially under sleeping-cars. A section and front view of one of these wheels is shown by Figure 53. It consists of a cast-iron hub, A, which is bored out to fit the axle. An annular disk, B B, is made of layers of paper-board glued together and then subjected to an enormous pressure. The disk is then bored out to fit the hub, and its circumference is turned off, and the tire C C is fitted to it. Two wrought-iron plates, P P, are then placed on either side of it, and the disk, plates, tire, and hub are all bolted together. The paper, it will be seen, bears the weight which rests on the hub of the axle and the hub of the wheel.
