The Mechanical Handling and Storing of Material

66 THE MECHANICAL HANDLING OF MATERIAL The advantage of this type of conveyor is that it can be used at a considerably steeper incline than an ordinary push-plate conveyor. A further advantage is, that there is no friction between the sides of the channel and the material being conveyed, the sides not being stationary, but travelling with the push-plates, as described. For material of a non-cutting nature, such as wood chips, tan, peat, and other light goods, the usual order is sometimes reversed, and the chain fixed below the scrapers, running in a groove in the bottom of the trough. T. he push-plates in these cases are often made of hard wood. Some forms of push-plate conveyors, e.g., when the chain is attached in the centre of the plates, can be used for double service by directing the return strand through a second trough either above or below the original one. When sufficient headroom is not available the terminal wheels may be arranged horizontally instead of veitically, so as to have both strands in the same plane, and in this case also the conveyor may be made a double service machine by adding a second trough for the return strand. This construction is, however, confined to single chain conveyors. Such conveyors are used among other purposes for stock pile installations (see Fig. 918, page 654). Fig. 81. Push-Plate Conveyor for Ice. Ice Conveyors.—Scraper conveyors, or perhaps more correctly elevators for ice- houses, are used particularly on the Continent. They were perhaps more often used before the introduction of refrigerators, but are still in general use in the vicinity of the Baltic Sea. These appliances very much resemble push-plates. The trough in which the ice slides up into the ice-house is made of wood, while the scraper plates are attached in the usual way to the chain, and drag the ice up the incline. Fig. 81 shows a cross section through such an ice-house. The pieces of ice are floated into the lower end of the trough, where they are brought into contact with' the plates, pushed up the inclined trough, and finally deposited in the store. Capacity of Push-Plate Conveyors.—This depends on the size of the trough, and the pitch and speed of the plates. The pitch of the plates ranges from 18 to 36 in., and the speed of travel from 50 to 180 ft. per minute. For coke and other friable substances the speed should not exceed 50 to 100 ft. per minute, whilst in cases where the breakage of the material is of minor importance, the maximum speed of 180 ft. per minute may be employed. For example, a conveyor with a trough 24 in. wide, having the plates 24 in. apart, and running at a speed of 100 ft. per minute, would convey 67 tons of coal per hour. A push-plate conveyor for coke with a 27-in. trough, and with malleable-iron plates