Fig. 45. Bolster, Links and Spring Plank.
Fig. 46. Steel Tire Wheel.
There are a number of swivel trucks made which have departed considerably from M. C. B. lines, but nearly all retain the features of a bolster mounted by springs on a spring plank, a spring plank hung from a transom, a transom rigidly fastened to the rectangular truck frame of which it forms a part; and a truck frame with one or more sets of spiral springs between it and the journal boxes. Maximum Traction Trucks. A type of swivel truck that once was very popular but has largely been superseded by the type just described is the “maximum traction truck.” This truck has two large wheels on an axle which carries 60 to 70 per cent of the weight on the truck, and two small wheels carrying the balance of the weight. The motors are on the large wheels.
Car Wheels. The car wheels most commonly used are of cast iron. In order to make a tread and flange upon which the wear comes, hard enough to give a good mileage, the tread and flange are chilled in the process of casting. Around the periphery of the mould in which the wheels are cast, is a ring of iron instead of the usual sand. When the molten cast iron comes in contact with this ring of iron, which is called a “chill,” the iron is cooled so suddenly that it becomes extremely hard. The balance of the wheel, cooling more slowly since it is surrounded by sand, has the hardness of ordinary cast iron. A steel tire wheel is shown in [Fig. 46].
Fig. 47. Elevated Car Axle.
Wheels with steel tires are coming into use for elevated and interurban cars because their flanges are not so brittle as those of cast-iron wheels. In wheels of cast metal there is always a liability that the flanges and tread will chip and crack. On high-speed cars the falling-out of pieces of flange may be a serious matter and result in a wreck. Steel-tired wheels have a hub and spokes either of cast or forged steel or iron. On to this wheel a steel tire is shrunk. The tire is heated in a furnace built for the purpose, and is then slipped over the wheel. It is made just such a size that it will slip over the wheel when hot, and when it is cool it will shrink enough to make a very tight fit. When the tire is to be removed after it is worn out, it is heated until it has expanded sufficiently to drop off.
An axle for elevated car is shown in [Fig. 47].