The size of mine cars may be determined by the physical conditions of the mine. Low-coal seams demand low height cars. Poor roof requiring timbering close to the track will often limit the width of the cars. The radius of the smallest curve will limit the length of the car. In low-coal mines, large capacity is obtained by increasing the length and width to counteract the low height. Also by using special designs, which permit low car bottoms. By the use of side and end boards, the miner can remove these boards to shovel coal from the end or side without interference. After which the boards are raised and a topping is placed on the cars. 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.
2. Wood or Steel
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.
3. Wheels and Bearings
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. They cannot be turned on a lathe however, and do not have the scrap value of cast iron or cast steel. Wheel size has a bearing on car height. In low-coal mines, small diameter wheels are necessary. In high coal mines, the common diameter is 14 to 18 inches. Large wheels have considerably less friction on the track than smaller wheels and are preferable. Less friction means less wear for the same distance traveled and this means a longer life for the larger diameter wheel. On the other hand, the larger wheel adds more weight to the car, and gives additional height to the car without giving it additional capacity. Wheels may be termed “loose” or “tight” according to whether they turn freely on the axle or whether the axle turns with the wheel. The latter is railroad practice and demands the track be elevated on the curves to permit the outside wheel to travel around the curve without skidding. Where loose wheels are used, the axle is usually fixed in place on the car bottom. In the case of tight wheels, the axle rotates in bearings placed on the car bottom. Bearings may be of the plain or anti frictional type. In the plain bearing type, the wheel hub fits over the axle with only sufficient clearance to permit the use of a lubricant. Friction in a bearing of this type is sliding friction. Brass is usually used as a bearing to diminish wear to the axle. When the bearing would become worn or sloppy, the brass would be changed. With roller bearings, which are practically the only antifriction type used on mine cars, rollers are supported in a frame in such a manner that the shaft of the axle rotates inside of the bearing and the wheel hub rotates with the bearing. This type of bearing employs a rolling friction, which is up to 50 percent less than that of the plain bearing. Plain bearings are lubricated with heavy oil while roller bearings are lubricated with grease. The latter requires greasing every three and one half months compared to 24 days for the plain bearings. However roller bearings must be kept clean of dust. Dirt is easily attracted to the grease, which fills and damages the rollers and reduce their efficiency.
4. Body Designs
The end gate style of body design consists of a car with a gate hinged on a bar at one end of the car and with means of securing it in place. This type of car requires end tilting to be dumped and may be built either of wood or steel and is the most common in the anthracite field. The gate end is not as strong as the tight end and the sides will tend to spread open after time permitting spillage of fine coal onto the roadbed when the car is in motion. The coal might become crushed under the car wheels and increase wheel friction as well as contribute to the coal-dust hazard. Tight end cars have no gate but are tight on both ends, requiring a rotating car dump, which will turn the car upside down to remove the coal. Their construction is more substantial than the gate end car of the same size and material. They do not “leak” fine coal to any extent unless car maintenance is poor and allows cracks to appear in the bottom and sides of the car. They are likewise constructed either of wood or steel.