Mechanical Equipment

CONDITIONS BEARING ON MINE EQUIPMENT; WINDING APPLIANCES; HAULAGE
EQUIPMENT IN SHAFTS; LATERAL UNDERGROUND TRANSPORT; TRANSPORT IN
STOPES.

There is no type of mechanical engineering which presents such
complexities in determination of the best equipment as does that of
mining. Not only does the economic side dominate over pure mechanics,
but machines must be installed and operated under difficulties which
arise from the most exceptional and conflicting conditions, none of
which can be entirely satisfied. Compromise between capital outlay,
operating efficiency, and conflicting demands is the key-note of
the work.

These compromises are brought about by influences which lie outside
the questions of mechanics of individual machines, and are mainly
as follows:--

1. Continuous change in horizon of operations.
2. Uncertain life of the enterprise.
3. Care and preservation of human life.
4. Unequal adaptability of power transmission mediums.
5. Origin of power.

_First._--The depth to be served and the volume of ore and water
to be handled, are not only unknown at the initial equipment, but
they are bound to change continuously in quantity, location, and
horizon with the extension of the workings.

_Second._--From the mine manager's point of view, which must embrace
that of the mechanical engineer, further difficulty presents itself
because the life of the enterprise is usually unknown, and therefore
a manifest necessity arises for an economic balance of capital
outlay and of operating efficiency commensurate with the prospects
of the mine. Moreover, the initial capital is often limited, and
makeshifts for this reason alone must be provided. In net result,
no mineral deposit of speculative ultimate volume of ore warrants
an initial equipment of the sort that will meet every eventuality,
or of the kind that will give even the maximum efficiency which
a free choice of mining machinery could obtain.

_Third._--In the design and selection of mining machines, the safety
of human life, the preservation of the health of workmen under
conditions of limited space and ventilation, together with reliability
and convenience in installing and working large mechanical tools,
all dominate mechanical efficiency. For example, compressed-air
transmission of power best meets the requirements of drilling,
yet the mechanical losses in the generation, the transmission,
and the application of compressed air probably total, from first
to last, 70 to 85%.

_Fourth._--All machines, except those for shaft haulage, must be
operated by power transmitted from the surface, as obviously power
generation underground is impossible. The conversion of power into
a transmission medium and its transmission are, at the outset,
bound to be the occasions of loss. Not only are the various forms
of transmission by steam, electricity, compressed air, or rods, of
different efficiency, but no one system lends itself to universal
or economical application to all kinds of mining machines. Therefore
it is not uncommon to find three or four different media of power
transmission employed on the same mine. To illustrate: from the
point of view of safety, reliability, control, and in most cases
economy as well, we may say that direct steam is the best motive
force for winding-engines; that for mechanical efficiency and
reliability, rods constitute the best media of power transmission
to pumps; that, considering ventilation and convenience, compressed
air affords the best medium for drills. Yet there are other conditions
as to character of the work, volume of water or ore, and the origin
of power which must in special instances modify each and every one
of these generalizations. For example, although pumping water with
compressed air is mechanically the most inefficient of devices,
it often becomes the most advantageous, because compressed air may
be of necessity laid on for other purposes, and the extra power
required to operate a small pump may be thus most cheaply provided.

_Fifth._--Further limitations and modifications arise out of the
origin of power, for the sources of power have an intimate bearing on
the type of machine and media of transmission. This very circumstance
often compels giving away efficiency and convenience in some machines
to gain more in others. This is evident enough if the principal
origins of power generation be examined. They are in the main as
follows:--

_a_. Water-power available at the mine.
_b_. Water-power available at a less distance than three
or four miles.
_c_. Water-power available some miles away, thus necessitating
electrical transmission (or purchased electrical power).
_d_. Steam-power to be generated at the mine.
_e_. Gas-power to be generated at the mine.

_a_. With water-power at the mine, winding engines can be operated
by direct hydraulic application with a gain in economy over direct
steam, although with the sacrifice of control and reliability. Rods
for pumps can be driven directly with water, but this superiority
in working economy means, as discussed later, a loss of flexibility
and increased total outlay over other forms of transmission to pumps.
As compressed air must be transmitted for drills, the compressor
would be operated direct from water-wheels, but with less control
in regularity of pressure delivery.

_b_. With water-power a short distance from the mine, it would
normally be transmitted either by compressed air or by electricity.
Compressed-air transmission would better satisfy winding and drilling
requirements, but would show a great comparative loss in efficiency
over electricity when applied to pumping. Despite the latter drawback,
air transmission is a method growing in favor, especially in view
of the advance made in effecting compression by falling water.

_c_. In the situation of transmission too far for using compressed
air, there is no alternative but electricity. In these cases, direct
electric winding is done, but under such disadvantages that it
requires a comparatively very cheap power to take precedence over
a subsidiary steam plant for this purpose. Electric air-compressors
work under the material disadvantage of constant speed on a variable
load, but this installation is also a question of economics. The
pumping service is well performed by direct electrical pumps.

_d_. In this instance, winding and air-compression are well accomplished
by direct steam applications; but pumping is beset with wholly
undesirable alternatives, among which it is difficult to choose.

_e_. With internal combustion engines, gasoline (petrol) motors
have more of a position in experimental than in systematic mining,
for their application to winding and pumping and drilling is fraught
with many losses. The engine must be under constant motion, and
that, too, with variable loads. Where power from producer gas is
used, there is a greater possibility of installing large equipments,
and it is generally applied to the winding and lesser units by
conversion into compressed air or electricity as an intermediate
stage.

One thing becomes certain from these examples cited, that the right
installation for any particular portion of the mine's equipment cannot
be determined without reference to all the others. The whole system
of power generation for surface work, as well as the transmission
underground, must be formulated with regard to furnishing the best
total result from all the complicated primary and secondary motors,
even at the sacrifice of some members.

Each mine is a unique problem, and while it would be easy to sketch
an ideal plant, there is no mine within the writer's knowledge
upon which the ideal would, under the many variable conditions,
be the most economical of installation or the most efficient of
operation. The dominant feature of the task is an endeavor to find
a compromise between efficiency and capital outlay. The result is
a series of choices between unsatisfying alternatives, a number of
which are usually found to have been wrong upon further extension
of the mine in depth.

In a general way, it may be stated that where power is generated
on the mine, economy in labor of handling fuel, driving engines,
generation and condensing steam where steam is used, demand a
consolidated power plant for the whole mine equipment. The principal
motors should be driven direct by steam or gas, with power distribution
by electricity to all outlying surface motors and sometimes to
underground motors, and also to some underground motors by compressed
air.

Much progress has been made in the past few years in the perfection
of larger mining tools. Inherently many of our devices are of a
wasteful character, not only on account of the need of special
forms of transmission, but because they are required to operate
under greatly varying loads. As an outcome of transmission losses
and of providing capacity to cope with heavy peak loads, their
efficiency on the basis of actual foot-pounds of work accomplished
is very low.

The adoption of electric transmission in mine work, while in certain
phases beneficial, has not decreased the perplexity which arises
from many added alternatives, none of which are as yet a complete or
desirable answer to any mine problem. When a satisfactory electric
drill is invented, and a method is evolved of applying electricity
to winding-engines that will not involve such abnormal losses due
to high peak load then we will have a solution to our most difficult
mechanical problems, and electricity will deserve the universal
blessing which it has received in other branches of mechanical
engineering.

It is not intended to discuss mine equipment problems from the
machinery standpoint,--there are thousands of different devices,--but
from the point of view of the mine administrator who finds in the
manufactory the various machines which are applicable, and whose
work then becomes that of choosing, arranging, and operating these
tools.

The principal mechanical questions of a mine may be examined under
the following heads:--

1. Shaft haulage.
2. Lateral underground transport.
3. Drainage.
4. Rock drilling.
5. Workshops.
6. Improvements in equipment.

SHAFT HAULAGE.

WINDING APPLIANCES.--No device has yet been found to displace the
single load pulled up the shaft by winding a rope on a drum. Of
driving mechanisms for drum motors the alternatives are the
steam-engine, the electrical motor, and infrequently water-power
or gas engines.

All these have to cope with one condition which, on the basis of
work accomplished, gives them a very low mechanical efficiency.
This difficulty is that the load is intermittent, and it must be
started and accelerated at the point of maximum weight, and from
that moment the power required diminishes to less than nothing
at the end of the haul. A large number of devices are in use to
equalize partially the inequalities of the load at different stages
of the lift. The main lines of progress in this direction have
been:--

_a_. The handling of two cages or skips with one engine
or motor, the descending skip partially balancing
the ascending one.
_b_. The use of tail-ropes or balance weights to compensate
the increasing weight of the descending rope.
_c_. The use of skips instead of cages, thus permitting of
a greater percentage of paying load.
_d_. The direct coupling of the motor to the drum shaft.
_e_. The cone-shaped construction of drums,--this latter
being now largely displaced by the use of the tail-rope.

The first and third of these are absolutely essential for anything
like economy and speed; the others are refinements depending on
the work to be accomplished and the capital available.

Steam winding-engines require large cylinders to start the load,
but when once started the requisite power is much reduced and the
load is too small for steam economy. The throttling of the engine
for controlling speed and reversing the engine at periodic stoppages
militates against the maximum expansion and condensation of the
steam and further increases the steam consumption. In result, the
best of direct compound condensing engines consume from 60 to 100
pounds of steam per horse-power hour, against a possible efficiency
of such an engine working under constant load of less than 16 pounds
of steam per horse-power hour.

It is only within very recent years that electrical motors have
been applied to winding. Even yet, all things considered, this
application is of doubtful value except in localities of extremely
cheap electrical power. The constant speed of alternating current
motors at once places them at a disadvantage for this work of high
peak and intermittent loads. While continuous-current motors can
be made to partially overcome this drawback, such a current, where
power is purchased or transmitted a long distance, is available
only by conversion, which further increases the losses. However,
schemes of electrical winding are in course of development which
bid fair, by a sort of storage of power in heavy fly-wheels or
storage batteries after the peak load, to reduce the total power
consumption; but the very high first cost so far prevents their
very general adoption for metal mining.

Winding-engines driven by direct water- or gas-power are of too rare
application to warrant much discussion. Gasoline driven hoists have a
distinct place in prospecting and early-stage mining, especially in
desert countries where transport and fuel conditions are onerous,
for both the machines and their fuel are easy of transport. As direct
gas-engines entail constant motion of the engine at the power demand
of the peak load, they are hopeless in mechanical efficiency.

Like all other motors in mining, the size and arrangement of the
motor and drum are dependent upon the duty which they will be called
upon to perform. This is primarily dependent upon the depth to be
hoisted from, the volume of the ore, and the size of the load.
For shallow depths and tonnages up to, say, 200 tons daily, geared
engines have a place on account of their low capital cost. Where
great rope speed is not essential they are fully as economical as
direct-coupled engines. With great depths and greater capacities,
speed becomes a momentous factor, and direct-coupled engines are
necessary. Where the depth exceeds 3,000 feet, another element
enters which has given rise to much debate and experiment; that
is, the great increase of starting load due to the increased length
and size of ropes and the drum space required to hold it. So far
the most advantageous device seems to be the Whiting hoist, a
combination of double drums and tail rope.

On mines worked from near the surface, where depth is gained by
the gradual exhaustion of the ore, the only prudent course is to
put in a new hoist periodically, when the demand for increased
winding speed and power warrants. The lack of economy in winding
machines is greatly augmented if they are much over-sized for the
duty. An engine installed to handle a given tonnage to a depth of
3,000 feet will have operated with more loss during the years the
mine is progressing from the surface to that depth than several
intermediate-sized engines would have cost. On most gold mines the
uncertainty of extension in depth would hardly warrant such a
preliminary equipment. More mines are equipped with over-sized
than with under-sized engines. For shafts on going metal silver mines
where the future is speculative, an engine will suffice whose size
provides for an extension in depth of 1,000 feet beyond that reached
at the time of its installation. The cost of the engine will depend
more largely upon the winding speed desired than upon any other
one factor. The proper speed to be arranged is obviously dependent
upon the depth of the haulage, for it is useless to have an engine
able to wind 3,000 feet a minute on a shaft 500 feet deep, since it
could never even get under way; and besides, the relative operating
loss, as said, would be enormous.

HAULAGE EQUIPMENT IN THE SHAFT.--Originally, material was hoisted
through shafts in buckets. Then came the cage for transporting mine
cars, and in more recent years the "skip" has been developed. The
aggrandized bucket or "kibble" of the Cornishman has practically
disappeared, but the cage still remains in many mines. The advantages
of the skip over the cage are many. Some of them are:--

_a_. It permits 25 to 40% greater load of material in
proportion to the dead weight of the vehicle.
_b_. The load can be confined within a smaller horizontal
space, thus the area of the shaft need not be so great
for large tonnages.
_c_. Loading and discharging are more rapid, and the latter
is automatic, thus permitting more trips per hour and
requiring less labor.
_d_. Skips must be loaded from bins underground, and by
providing in the bins storage capacity, shaft haulage is
rendered independent of the lateral transport in the
mine, and there are no delays to the engine awaiting
loads. The result is that ore-winding can be concentrated
into fewer hours, and indirect economies in labor
and power are thus effected.
_e_. Skips save the time of the men engaged in the lateral
haulage, as they have no delay waiting for the winding
engine.

Loads equivalent to those from skips are obtained in some mines
by double-decked cages; but, aside from waste weight of the cage,
this arrangement necessitates either stopping the engine to load the
lower deck, or a double-deck loading station. Double-deck loading
stations are as costly to install and more expensive to work than
skip-loading station ore-bins. Cages are also constructed large
enough to take as many as four trucks on one deck. This entails a
shaft compartment double the size required for skips of the same
capacity, and thus enormously increases shaft cost without gaining
anything.

Altogether the advantages of the skip are so certain and so important
that it is difficult to see the justification for the cage under
but a few conditions. These conditions are those which surround
mines of small output where rapidity of haulage is no object, where
the cost of station-bins can thus be evaded, and the convenience
of the cage for the men can still be preserved. The easy change
of the skip to the cage for hauling men removes the last objection
on larger mines. There occurs also the situation in which ore is
broken under contract at so much per truck, and where it is desirable
to inspect the contents of the truck when discharging it, but even
this objection to the skip can be obviated by contracting on a
cubic-foot basis.

Skips are constructed to carry loads of from two to seven tons,
the general tendency being toward larger loads every year. One
of the most feasible lines of improvement in winding is in the
direction of larger loads and less speed, for in this way the sum
total of dead weight of the vehicle and rope to the tonnage of
ore hauled will be decreased, and the efficiency of the engine
will be increased by a less high peak demand, because of this less
proportion of dead weight and the less need of high acceleration.

LATERAL UNDERGROUND TRANSPORT.

Inasmuch as the majority of metal mines dip at considerable angles,
the useful life of a roadway in a metal mine is very short because
particular horizons of ore are soon exhausted. Therefore any method
of transport has to be calculated upon a very quick redemption of
the capital laid out. Furthermore, a roadway is limited in its
daily traffic to the product of the stopes which it serves.

MEN AND ANIMALS.--Some means of transport must be provided, and
the basic equipment is light tracks with push-cars, in capacity
from half a ton to a ton. The latter load is, however, too heavy
to be pushed by one man. As but one car can be pushed at a time,
hand-trucking is both slow and expensive. At average American or
Australian wages, the cost works out between 25 and 35 cents a
ton per mile. An improvement of growing import where hand-trucking
is necessary is the overhead mono-rail instead of the track.

If the supply to any particular roadway is such as to fully employ
horses or mules, the number of cars per trip can be increased up
to seven or eight. In this case the expense, including wages of
the men and wear, tear, and care of mules, will work out roughly
at from 7 to 10 cents per ton mile. Manifestly, if the ore-supply
to a particular roadway is insufficient to keep a mule busy, the
economy soon runs off.

MECHANICAL HAULAGE.--Mechanical haulage is seldom applicable to
metal mines, for most metal deposits dip at considerable angles,
and therefore, unlike most coal-mines, the horizon of haulage must
frequently change, and there are no main arteries along which haulage
continues through the life of the mine. Any mechanical system entails
a good deal of expense for installation, and the useful life of
any particular roadway, as above said, is very short. Moreover,
the crooked roadways of most metal mines present difficulties of
negotiation not to be overlooked. In order to use such systems it
is necessary to condense the haulage to as few roadways as possible.
Where the tonnage on one level is not sufficient to warrant other
than men or animals, it sometimes pays (if the dip is steep enough)
to dump everything through winzes from one to two levels to a main
road below where mechanical equipment can be advantageously provided.
The cost of shaft-winding the extra depth is inconsiderable compared
to other factors, for the extra vertical distance of haulage can
be done at a cost of one or two cents per ton mile. Moreover, from
such an arrangement follows the concentration of shaft-bins, and of
shaft labor, and winding is accomplished without so much shifting
as to horizon, all of which economies equalize the extra distance
of the lift.

There are three principal methods of mechanical transport in use:--

1. Cable-ways.
2. Compressed-air locomotives.
3. Electrical haulage.

Cable-ways or endless ropes are expensive to install, and to work
to the best advantage require double tracks and fairly straight
roads. While they are economical in operation and work with little
danger to operatives, the limitations mentioned preclude them from
adoption in metal mines, except in very special circumstances such
as main crosscuts or adit tunnels, where the haulage is straight
and concentrated from many sources of supply.

Compressed-air locomotives are somewhat heavy and cumbersome, and
therefore require well-built tracks with heavy rails, but they
have very great advantages for metal mine work. They need but a
single track and are of low initial cost where compressed air is
already a requirement of the mine. No subsidiary line equipment is
needed, and thus they are free to traverse any road in the mine and
can be readily shifted from one level to another. Their mechanical
efficiency is not so low in the long run as might appear from the
low efficiency of pneumatic machines generally, for by storage of
compressed air at the charging station a more even rate of energy
consumption is possible than in the constant cable and electrical
power supply which must be equal to the maximum demand, while the
air-plant consumes but the average demand.

Electrical haulage has the advantage of a much more compact locomotive
and the drawback of more expensive track equipment, due to the
necessity of transmission wire, etc. It has the further disadvantages
of uselessness outside the equipped haulage way and of the dangers
of the live wire in low and often wet tunnels.

In general, compressed-air locomotives possess many attractions
for metal mine work, where air is in use in any event and where
any mechanical system is at all justified. Any of the mechanical
systems where tonnage is sufficient in quantity to justify their
employment will handle material for from 1.5 to 4 cents per ton
mile.

TRACKS.--Tracks for hand, mule, or rope haulage are usually built
with from 12- to 16-pound rails, but when compressed-air or electrical
locomotives are to be used, less than 24-pound rails are impossible.
As to tracks in general, it may be said that careful laying out
with even grades and gentle curves repays itself many times over in
their subsequent operation. Further care in repair and lubrication
of cars will often make a difference of 75% in the track resistance.

TRANSPORT IN STOPES.--Owing to the even shorter life of individual
stopes than levels, the actual transport of ore or waste in them is
often a function of the aboriginal shovel plus gravity. As shoveling
is the most costly system of transport known, any means of stoping
that decreases the need for it has merit. Shrinkage-stoping eliminates
it altogether. In the other methods, gravity helps in proportion to
the steepness of the dip. When the underlie becomes too flat for
the ore to "run," transport can sometimes be helped by pitching
the ore-passes at a steeper angle than the dip (Fig. 36). In some
cases of flat deposits, crosscuts into the walls, or even levels
under the ore-body, are justifiable. The more numerous the ore-passes,
the less the lateral shoveling, but as passes cost money for
construction and for repair, there is a nice economic balance in
their frequency.

Mechanical haulage in stopes has been tried and finds a field under
some conditions. In dips under 25 and possessing fairly sound
hanging-wall, where long-wall or flat-back cuts are employed, temporary
tracks can often be laid in the stopes and the ore run in cars to
the main passes. In such cases, the tracks are pushed up close
to the face after each cut. Further self-acting inclines to lower
cars to the levels can sometimes be installed to advantage. This
arrangement also permits greater intervals between levels and less
number of ore-passes. For dips between 25 and 50 where the mine
is worked without stope support or with occasional pillars, a very
useful contrivance is the sheet-iron trough--about eighteen inches
wide and six inches deep--made in sections ten or twelve feet long
and readily bolted together. In dips 35 to 50 this trough, laid
on the foot-wall, gives a sufficiently smooth surface for the ore
to run upon. When the dip is flat, the trough, if hung from plugs
in the hanging-wall, may be swung backward and forward. The use of
this "bumping-trough" saves much shoveling. For handling filling
or ore in flat runs it deserves wider adoption. It is, of course,
inapplicable in passes as a "bumping-trough," but can be fixed to
give smooth surface. In flat mines it permits a wider interval
between levels and therefore saves development work. The life of
this contrivance is short when used in open stopes, owing to the
dangers of bombardment from blasting.

In dips steeper than 50 much of the shoveling into passes can be
saved by rill-stoping, as described on page 100. Where flat-backed
stopes are used in wide ore-bodies with filling, temporary tracks
laid on the filling to the ore-passes are useful, for they permit
wider intervals between passes.

In that underground engineer's paradise, the Witwatersrand, where
the stopes require neither timber nor filling, the long, moderately
pitched openings lend themselves particularly to the swinging iron
troughs, and even endless wire ropes have been found advantageous
in certain cases.

Where the roof is heavy and close support is required, and where
the deposits are very irregular in shape and dip, there is little
hope of mechanical assistance in stope transport.



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