All devices above the datum line must, obviously, be power driven, the actual power expenditure depending upon the height to which the material has to be raised, plus power necessary to overcome friction. If it be assumed that all the devices shown on the diagram are of the same capacity, i.e., carry the same load in a unit of time, the driving power needed will decrease gradually until the conveyor parallel with the datum line is reached, where only frictional resistance has to be overcome. Below the datum line gravity enters into the calculation, so that at a downward gradient of about 71° gravity alone is sufficient to overcome fric tional resistance and the conveyor will run by itself. Finally, there are positions which would need a retarding conveyor or inverse elevator. As the down gradient increases, gravity assists further, pro rata, and if suitable gear is applied to one of the conveyor terminals energy may be collected to actuate other machines, to be converted into heat by a brake, or to be dissipated.
The usual type of conveyor for moving material both hori zontally and vertically is known as the bucket conveyor. It resem bles the bucket elevator very closely, but the buckets are rather different.
Another type is the V-bucket conveyor, the buckets of which are V-shaped and attached to two endless chains, one at each side. The portion of such a conveyor which runs horizontally, constitutes a scraper conveyor, and that which runs vertically, a bucket elevator. At the junction of the horizontal and vertical runs the material drops automatically into the buckets. The path of the conveyor may be L-shaped or in the form of an oblong, but the machine must always discharge on a horizontal run if ever so short.
If the buckets are not rigidly connected to the chains but sup ported on trunnions and above their centre of gravity they con stitute what is known as the gravity bucket conveyor. Devices of this kind are generally so disposed in power-houses that the lower horizontal run passes along the basement floor, ascends at one end of the building, traverses the top floor above the bunkers and descends at the other end, thus forming an oblong. Owing to their method of suspension the buckets always remain in a perpendicu lar position, except when they are tipped and thus emptied by coming in contact with a tripping device at the various points of discharge. They are filled in the basement by a rather complex feeding device while the chain of buckets is under way. Gravity bucket conveyors were at one time largely used in power-houses, where they sometimes handled coal on the ascending and upper horizontal run, discharging the coal into the bunkers, while the lower run collected the ashes. They are slow-running, rather ex pensive machines, but very reliable. As, however, they are now but rarely installed, no further comment is necessary. Pneumatic
handling is dealt with in a separate article. (See PNEUMATIC CON VEYING.) Intermittent Handling Devices.—The following types are the most important under this heading : ropeways and aerial cable ways; mono-rails and telphers; finger-tray elevators ; and skip hoists. The last two devices only are dealt with here, the others form the subjects of separate articles.
Finger- or Swing-Tray Elevator.—This device serves the same purpose as a bucket elevator, but handles larger intermittent loads such as packages, cases, baskets, sacks, barrels, etc. It can re ceive goods on any one floor and set them down on any other. The loads are picked up on the ascending strand and set down on the descending one. The device consists of one or two endless chains with corresponding upper and lower sprocket terminals.
The Skip Hoist.—This device may be likened to a bucket ele vator having only one bucket, which, instead of running contin uously in the same direction as the former, works intermittently up and down a vertical or steeply inclined rail-track. With a twin installation one skip is in the loading position while the other is discharging; usually the discharge is over the top, like a "monkey on-a-stick." The size of the skip is determined by the tonnage to be handled and the depth of the pit. The skip may be of either the top or bottom discharge type. In the former instance the skip is provided with a bail, to the base of which it is hinged. At its upper end are trod wheels, which follow at the discharge point along an approxi mately horizontal path, whereby the skip is tipped and emptied. As a rule there are also a pair of trod wheels which guide the lower end. With bottom-discharge skips, as the name implies, a door is provided at the base of the skip, which at the same time forms a shoot, and which is kept closed during travel, generally by means of a pair of wheels which engage in a certain way. At the discharging point these rails are interrupted, when the weight of the contents forces open the door and thereby effects discharge.
In operation the initial speed of travel of a skip hoist is slow, increasing to a maximum of 5 ft. per sec.; decreasing when ap proaching upper delivery point. The speed may be controlled by the form of the drum on which the wire rope is wound, this being lemon-shaped; or in large installations by automatic speed varia tion of the motor. Skip hoists are in use for twin installations, with maximum handling capacity of soo tons per hour and skips holding 6 cu. yds. (See MASS PRODUCTION.) See H. Broughton, Electrical Handling of Materials (1921-23) ; H. V. Hetzel, Belt Conveyors and Belt Elevators (1922, 2nd ed., 1926) ; G. F. Zimmer, Mechanical Handling and Storing of Material (1916, 3rd ed., 1922). (G. F. Z.)