Page:EB1911 - Volume 18.djvu/562

 scale work or temporary service, such as raising the material blasted in sinking shafts. They hold from a few hundred pounds up to 1 ton. In hoisting from great depths the weight of the rope, which may exceed that of the cage and contents, produces excessive variations in the load on the engine difficult to deal with.

Moreover, the limit of vertical depth at which rope of even the best quality will support its own weight only, with a proper margin of safety, is, say, 10,000 to 12,000 ft.; and with the load the safe working limit of depth would be reached at 7000 to 8000 ft. A number of shafts in South Africa, the United States and elsewhere, are already approximating depths of 5000 ft., a few being even deeper.

Ropes of tapering section may be used for great depths, but are not satisfactory in practice. Stage hoisting is applicable to any depth. Instead of raising the load in one lift from the bottom of the shaft, one or more intermediate dumping and loading stations are provided. Each stage has its own engine, rope and cage. The variations in engine load are thus reduced, and incidentally hoisting time is saved.

In shallow mines the men use the ladder-way in going to and from their work. This is sometimes the case also for considerable depths. It is more economical to save the men’s strength, however, by raising and lowering them with the hoisting engines.

At mines with vertical shafts this is a simple operation. Cages of the size generally used in metal mines will hold from ten to fifteen and occasionally twenty men. The time consumed in lowering the men is shortened by the use of cages having two or more decks. These are common in Europe, and are sometimes employed in the United States and elsewhere in mines where the output is large and the shafts deep and of small cross section. While a shift of men is being lowered the miners of the preceding shift are usually raised to the surface in the ascending cages, the entire shift being thus changed in the time required for lowering. Nevertheless, in very deep and large mines the time consumed in handling the men may make serious inroads on the time available for hoisting ore. At a few mines special man-cages are operated in separate compartments by their own engines for handling part of the men, and for tools, supplies, &c. For inclined shafts, where the mineral is hoisted in skips, the operation of raising and lowering men may not be so simple, Even a large skip will hold but a few men, the speed is slower, and more time is required for the men to get into and out of the skip than to step on and off a cage. Moreover, skips are rarely provided with safety attachments, so that the danger is greater. When the shafts are deep and the number of miners large man-cars are sometimes employed. These are long frames on four wheels, with a series of seats like a section of a theatre gallery. Ordinarily 4 or 5 men occupy each seat, the car accommodating from 20 to 36 men. Such cars are in use at a number of deep inclined shafts in the Lake Superior copper district, where the depths range from 3000 to 5000 ft. or more. At a few mines (since safety catches cannot be successfully applied to man-cars) these conveyances are raised and lowered by separate engines and ropes. To replace the ore-skip expeditiously by the man-car when the shifts are to be changed a crane is often erected over the shaft mouth. At the end of a shift the ore-skip is lifted from the shaft track—the hoisting rope being uncoupled—and the man-car put in its place and attached to the rope. This change may be made in a few minutes.

Formerly, at many deep European mines, and at a few in the United States, men were raised by means of “man-engines.” A man-engine consists of two heavy wooden rods (like the rods of a Cornish pumping plant), placed parallel and close to each other in a special shaft compartment, and suspended at the surface from a pair of

massive walking beams (or “bobs”). The rods are caused to oscillate slowly by an engine, one rising while the other is falling. Thus they move simultaneously in opposite directions through a fixed length of stroke, say from 10 to 12 ft. At intervals on the rods are attached small horizontal platforms, only large enough to accommodate two men at a time. As the rods make their measured strokes one of the miners, starting from the surface, steps on the first platform as it rises to the surface landing and is then lowered on the down stroke. At the end of the stroke, when his platform comes opposite to a corresponding platform on the other rod, he steps over on to the latter during the instant of rest prior to the reversal of the stroke, descends with the second rod on this down stroke, steps again at the proper time to a platform of the first rod and so on to the bottom. The men follow each other, one by one, so that in a few minutes all the rod platforms in a deep shaft may be simultaneously occupied by men stepping in unison but in opposite directions from platforms of one rod to the other. Meantime, the men quitting work are ascending in a similar way, as there is room on each platform for two men at a time when passing each other. Man-engines were long used,