In mines where the physical conditions do not limit the car size, this is usually determined by the cars use. Anthracite mines employing loading chutes from breasts use high, large capacity cars. Mechanical loading demands the largest possible car consistent with haulage and loading conditions so as to reduce the number of car changes and time delay. When the car must be designed for the colliery, shaft size may limit the car length and width. When a new mine is opened, the car size is often chosen arbitrarily, subject to physical conditions as far as can be determined. The haulage curves, dumping devices, etc. are designed to accommodate the new mine car.
Mine Car bodies may be built of Wood or steel, or may be a combination of both. Wooden cars are readily constructed, often being assembled in the shop by unskilled labor. In addition they have a certain flexibility, which gives them excellent riding qualities, and they are inexpensive to construct. All steel cars cost from 50 to 100 percent more to build than wooden cars but their longer life quite often offsets this initial disadvantage. Maintenance costs are estimated at one fourth of wooden cars. Also, for the same outside dimensions the steel car gives 10 to 20 percent more inside capacity. Steel cars have greater durability and can withstand shock that would crush wooden cars. When skilled labor is available, a bent steel car can be repaired and placed into service at a very low cost. Lower maintenance costs and greater capacity should reduce the cost of haulage when steel cars are used.
Wheel material may be either cast iron or cast steel. Cast iron wheels have a hard flange and tread that discourages turning on a lathe, so the wheels are used until a groove is worn in the tread and then discarded. Wheels of cast steel do not break as easy as cast iron and are one-third lighter in weight but wear more readily. They can be machined when they become grooved and this increases their service life. Manganese steel wheels are hard and tough and will out wear many other types.
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