rope decreases. A similar equalizing effect is obtained by the
use of flat rope and reel, the rope winding on itself like a ribbon.
Tapering ropes, tail-ropes suspended from the cages, and other
means of equalization, are also employed. If, for a two-compartment
shaft, a pair of drums (or a single wide drum)
be keyed to the engine shaft, with the ropes
wound in opposite directions, the hoisting
is “in balance,” that is, the cages and
cars counterbalance each other, so that the
engine has to raise only the useful load of
mineral, plus the rope. This arrangement
allows no independence of movement:
when the loaded cage is being hoisted the
empty must be lowered. Independent
drums, on the contrary, are loose upon
their shaft, and are thrown on or off by
tooth or friction clutches. The maximum
load on the engine is thus greater and more
is required than for fixed drums.
Steam consumption is economized, whenever
possible, by throwing in the clutches
of both drums and hoisting in balance.
Fixed drums are best for mines in which
the hoisting is done chiefly from one level;
independent drums when there are a number
of different levels. Hoisting engines are
provided with powerful brakes and frequently
with reversing gear. In deep
shafts hoisting speeds of 3000 or 3500 ft.
per minute are often attained, occasionally
as much as 5000 ft.
Formerly hemp and also fibre ropes were commonly used. Except in a few instances these were long ago superseded by iron-wire ropes, which in turn have been replaced by steel because of its greater strength. For hoisting in deep shafts, and to reduce the weight of rope, Hoisting Ropes. tempered-steel wire of very high tensile strength (up to 250,000 or 275,000 ℔ ultimate strength per sq. in.) is advantageously employed. A 1-in. ordinary steel rope has a breaking strength of about 32 tons, which, with a factor of safety of six gives a safe working load of 514 tons. A 1-in. plow-steel rope has breaking and working strengths respectively of at least 48 and 8 tons. Standard round rope (fig. 13) has six strands of 19 wires each and a hemp core.
| Fig. 13.—Standard round Rope. | Fig. 14.—Flat Rope. |
Flat rope is in favour in some districts. It is composed of several four-stranded ropes, without hemp centres, laid side by side, and sewed together by wire (fig. 14). It is not as durable as round rope and is heavier for the same working strength. As the sewing wires soon begin to break, a flat rope must usually be ripped apart and resewed every six or eight months. Numerous patent ropes, some having wires and strands of special shapes, have been introduced with the idea of improving the wearing properties. Such, for example, are the Lang-lay, locked-coil and flattened strand rope. Hoisting ropes are weakened by deterioration and breakage of the wires, due to corrosion and, repeated bending, and should be kept under careful inspection. To prevent excessive bending stresses the diameter of drum and sheave must bear a proper ratio to that of the rope. A ratio of 48 to 1 is the minimum allowable; better 60 to 75 to 1, and for highly-tempered steel ropes ratios of 150 to 1 or more are desirable. To prevent corrosion the rope should be treated at intervals with hot lubricant. With proper care a steel rope should last from two to three years.
A frame of wood or steel, erected at the shaft mouth, and carrying the grooved sheaves over which the hoisting ropes pass, is known as the head-gear (fig. 15). In Great Britain and her colonies it is also called the poppet-head or pit-head frame; in the United States head-frame or gallows-frame. Though it is small and simple in construction Head-gear. for light work, for heavy hoisting at high speeds massively framed towers, often 80 to 100 ft. in height, are built. Steel frames are more durable than those of wood, and have become common in nearly all mining countries, especially where timber is scarce. A German design is shown in fig. 16. The head-gear is often combined with ore-bins and machinery for breaking and sizing the lump ore previous to shipment to the reduction works.
| (From The Colliery Engineer, May 1897.) |
| Fig. 15.—Head-gear. |
Cages, running in guides in the shaft, are used for raising the cars of mineral to the surface (fig. 17), They may have one, two or more decks, usually carrying one or two cars on each deck. Multiple-deck cages are rarely employed except for deep shafts of small cross-section or when the mine cars (tubs) are small, as in many Cages and Skips. parts of Europe. In many mines the mineral is raised in skips (fig. 18), filled from cars underground and dumping automatically on reaching the surface. Skips are sometimes of very large capacity, holding 5, 7, and even 10 tons of ore; such are used, for example, in several shafts at Butte, Montana, in the Lake Superior copper district, and in South Africa. Fig. 18 is a small skip; the upper illustration showing position for dumping. The lower cut is of a skip for either ore or water; note valve in bottom. Hoisting buckets or kibbles are employed for small
