—The machines most commonly employed for hoisting purposes in tunnel shafts are steam hoisting engines, horse gins, and windlasses operated by hand. Windlasses and horse gins are rather crude machines for hoisting loads, and are used only in special circumstances, where the shaft is of small depth, when the amount of material to be hoisted is small, or where for any reason the use of hoisting engines is precluded. The steam hoisting engine is the standard machine for the rapid lifting of heavy vertical loads. Recently oil engines and electric hoists have also come to be used to some extent, and under certain conditions these machines possess notable advantages.
The construction of hand windlasses is familiar to every one. In tunnel work this device is located directly over the shaft, with its axis a little more than half a man’s height, so that the crank handle does not rise above the shoulder line. To develop its greatest efficiency the hoisting rope is passed around the windlass drum so that the two ends hang down the shaft, and as one end descends the other ascends. A skip, or bucket, is attached to each of the rope ends; and by loading the descending skip with construction materials and the ascending skip with spoil, the two skip loads tend to balance each other, thus increasing the capacity of the windlass, and decreasing the manual labor required to operate it. Skips varying from 0.3 cu. yd. to 0.5 cu. yd. are used. The horse gin consists of a vertical cylinder or drum provided with radial arms to which the horses are hitched, which revolve the cylinder by walking around it in a circle. The hoisting rope is rove around the drum so that the two ends extend down the shaft with skips attached, as described in speaking of the hand windlass. The power of the horse gin is, of course, much greater than that of a windlass operated by hand, skips of 1 cu. yd. capacity being commonly used. Horse gins are no longer economical hoisting machines, according to one prominent authority, when V(H + 20) > 5000, where V equals the volume of material to be hoisted, and H equals the height of the hoist, the weight of the excavated material being 2100 lbs. per cu. yd. As a general rule, however, it is assumed that it is not economical to employ horse gins with a depth of shaft exceeding 150 ft.
As already stated, the most efficient and most commonly used device for hoisting at tunnel shafts is the steam hoisting engine. There are numerous builders of hoisting engines, each of which manufactures several patterns and sizes of engines. In each case, however, the apparatus consists of a boiler supplying steam to a horizontal engine which operates one or more rope drums. The engines are always reversible. They may be employed to hoist the skips directly, or to operate elevators upon which the skips or cars are loaded. In either case the hoisting ropes pass from the engine drum to and around vertical sheaves situated directly over the shaft so as to secure the necessary vertical travel of the ropes down the shaft. Where the shaft is divided into two compartments, each having an elevator or hoist, double-drum engines are employed, one drum being used for the operations in one compartment, and the other for the operations in the other compartment. Where the work is to be of considerable duration, or when it is done in cold weather, more or less elaborate shelters or engine houses are built to cover and protect the machinery.
Choice between the method of hoisting the skips directly, and the method of using elevators, depends upon the extent and character of the work. Where large quantities of material are to be hoisted rapidly, it is generally considered preferable to employ elevators instead of hoisting the skips directly. In direct hoisting at high speed, oscillations are likely to be produced which may dash the skips against the sides of the shaft and cause accidents. The loads which can be carried in single skips are also smaller than those possible where elevators are used; and this, combined with the slower hoisting speed required, reduces the capacity of this method, as compared with the use of elevators. Where elevators are employed, however, the plant required is much more extensive and costly; it comprising not only the elevator cars with their safety devices, etc., but the construction of a guiding framework for these cars in the tunnel shaft. For these various reasons the elevator becomes the preferable hoisting device where the quantity of material to be handled is large, where the shafts are deep, and where the work will extend over a long period of time; but when the contrary conditions are the case, direct hoisting of the skips is generally the cheaper. The engineer has to integrate the various factors in each individual case, and determine which method will best fulfill his purpose, which is to handle the material at the least cost within the given time and conditions.
Fig. 37.—Elevator Car for Tunnel Shafts.
The construction of elevators for tunnel work is simple. The elevator car consists usually of an open framework box of timber and iron, having a plank floor on which car tracks are laid, and its roof arranged for connecting the hoisting cable ([Fig. 37][7]). Rigid construction is necessary to resist the hoisting strains. The sides of the car are usually designed to slide against timber guides on the shaft walls. Some form of safety device, of which there are several kinds, should be employed to prevent the fall of the elevator, in case the hoisting rope breaks, or some mishap occurs to the hoisting machinery, which endangers the fall of the car. Speaking tubes and electric-bell signals should also be provided to secure communication between the top and bottom of the shaft.
[7] Reproduced from the catalogue of the Ledgerwood Manufacturing Company, New York.