[Illustration: FIG. 14]
=24. Koepe System.=--In its lightest form, a drum requires a large
amount of power to set it in motion, which power is absorbed by the
brake and lost when it is brought to rest again. Furthermore, with deep
shafts requiring long drums, the fleet, or angle that the rope makes
with the head-sheave due to its traveling from one end of the drum to
the other, is not only a disadvantage and possible cause of accident,
but it is a source of wear. To overcome these objections and also the
great cost of large cylindrical or conical drums, the =Koepe system=
of hoisting, shown in Fig. 14, was devised by Mr. Frederick Koepe.
A single grooved driving sheave _a_ is used in place of a drum. The
winding rope _b_ passes from one cage _A_ up over a head-sheave, thence
around the sheave _a_ and back over another head-sheave, and down to
a second cage _B_; it encircles a little over half the periphery of
the driving sheave and is driven by the friction between the sheave
and rope. A balance rope _c_ beneath the cages and passing around the
sheave _d_ gives an endless-rope arrangement with the cages fixed at
the proper points. The driving sheave is stronger than an ordinary
carrying sheave, as it has to do the driving and is usually lined with
hardwood, which is grooved to receive the winding rope, the depth of
the groove being generally equal to twice the diameter of the rope.
Instead of being placed parallel, the head-sheaves are placed at an
angle with each other, each pointing to the groove in the driving
sheave, thus reducing the side friction of the rope on the sheaves.
The system has been in successful operation since 1877, and experiments
made on it have determined that, with a rope passing only one-half
turn around the drum sheave, the coefficient of adhesion with clean
ropes is about .3. If the ropes are oiled, the adhesion becomes less,
and sometimes slippage occurs, producing not only wear of the driving
sheave lining but giving an incorrect reading of the hoist indicator
and thus possibly producing overwinding, unless the position of the
cage is indicated by marks on the rope, or unless the engineer can see
the cage.
At the end of the hoist, if the upper cage is allowed to rest on the
keep, its weight and the weight of the tail-rope are taken from the
hoisting rope, and there is then not enough pull on the hoisting rope
to produce sufficient friction with the drum sheave to start the next
hoist. To prevent this trouble, the keeps are dispensed with, or the
rope is made continuous and independent of the cage. To do this,
crossheads are placed above and below each cage and connected by ropes
or chains outside of the cages. The bridle chains are then hung from
the top crosshead, and when the cage rests on the keeps, the weight of
the winding and tail-ropes remains on the driving sheaves.
=25. Advantages and Disadvantages of the Koepe System.=--With this
system, only one driving sheave is necessary for the operation of two
compartments, and it is light, inexpensive to build, and very narrow,
admitting of a short sheave shaft and small foundations. This system
permits a perfect balance of rope and cage, so that the work to be done
by the engine is uniform, except for the acceleration, and consists
only in lifting the material and overcoming the friction. There is no
fleeting of the rope between the driving sheaves and the head-sheaves.
The system has the following disadvantages, which prevent its being
used to any considerable extent: Liability to slippage of the rope
on the drum; if the rope breaks, both cages may fall to the bottom;
hoisting from different levels cannot be well done, for, since the
cages are at fixed distances from each other, the length of the rope
is such that when one cage _A_ is at the top, the other cage _B_ is
at the bottom. If hoisting is to be done from the bottom, this is
satisfactory, but if hoisting is to be done from some upper level, cage
_B_, which is at the bottom, must be hoisted to that level to be loaded
before it can go to the top. Then, when cage _B_ goes to the top with
its load, cage _A_ must go to the bottom, wait there while cage _B_ is
being unloaded, and then be hoisted to the upper level to receive its
load. For each trip, therefore, the time required for a cage to go from
the bottom to the upper level and be loaded is lost; and two movements
of the engines are necessary for a hoist instead of one.
=26. The Whiting System.=--This is a system of hoisting with
round ropes, in which two rope wheels placed tandem are used in
place of cylindrical or conical drums. As shown in Fig. 15, for a
two-compartment shaft the rope passes from one cage _a_ up over a
head-sheave _c_, down under a guide sheave _d_, and is then wound three
times about the rope wheels _e_ and _f_, to secure a good hold, then
around a fleet sheave _g_, and back under another guide sheave _h_, up
over another head-sheave _i_, and down to the other cage _b_. When the
system is to be used for a single-compartment shaft, one end of the
rope carries the cage and the other end carries a balance weight, which
is run up and down in a corner of the shaft. A balance rope below the
cages, as shown, is generally used, though it is not essential to the
working of the system, as it is in the Koepe system. When sinking a
shaft, a balance rope cannot be used as it interferes with the work at
the bottom of the shaft.
[Illustration: FIG. 15]
The drums or wheels _e_, _f_ are light, inexpensive, and narrow, thus
permitting short sheave shafts and small foundations. They are lined
with hardwood blocks, each lining having three rope grooves turned
in it. The main wheel _e_ is driven by a hoisting engine, which may
be either first or second motion. The following wheel _f_ is coupled
to the main wheel by a pair of parallel rods, one on each side, like
the drivers of a locomotive. As the rope wraps about the wheels _e_,
_f_ three times, there are six semi-circumferences of driving contact
with the rope, as compared with the one semi-circumference in the
Koepe system, and there is no slipping of the rope on the wheels. The
following wheel _f_ is best tilted or inclined from the vertical an
amount equal, in the diameter of the wheels, to the pitch of the rope
on the wheel, so that the rope may not run out of its groove and may
run straight from one wheel to the other without any chafing between
the ropes and the sides of the grooves.
The capacity of the wheels _e_, _f_ is unlimited, while grooved
cylindrical drums, conical drums, and reels will hold only the fixed
length of rope for which they are designed.
As shown by the dotted lines, the fleet sheave _g_ is arranged to
travel backwards and forwards, in order to change the working length of
the rope from time to time to provide for an increased depth of shaft,
and for the changes in the length of rope due to stretching and when
the ends are cut off to resocket the rope. The fleet sheave _g_ is
moved a distance equal to half the change in the length of rope.
=27=. Hoisting from intermediate levels can be readily done with the
Whiting system; for instance, if the cage _a_ is at the top and cage
_b_ at the bottom, and hoisting is to be done from some upper level, it
is only necessary to run the fleet sheave _g_ out, and thus shorten
the working length of the rope until cage _b_ comes up to the upper
level. It can then be loaded and go to the top. While cage _b_ goes to
the top, cage _a_ descends to the same level, where it can be loaded
while cage _b_ is being unloaded, and can then go directly to the top
without any of the lost time, as is the case in the Koepe system.
The system permits a perfect balance of rope and cage, so that the work
to be done by the engines is uniform, except for the acceleration, and
consists only in lifting the material and overcoming the friction.
There is no fleeting of the rope, so the rope wheels can be placed as
close to the shaft as may be desired.
=28.= This system was tried as early as 1862 in Eastern Pennsylvania,
but it was not used extensively because hoisting from great depths was
not necessary, since, for depths of less than 1,000 feet, cylindrical
and conical drums are quite satisfactory. In the Lake Superior copper
region, there are now three Whiting hoists, two of which are probably
the largest hoisting plants in the world. Each plant consists of a pair
of triple-expansion, vertical, inverted-beam engines, driving direct
a pair of 19-foot drums. The high-pressure cylinders are 20 inches in
diameter, the intermediate cylinders 32 inches, and the low-pressure
cylinders 50 inches, and all six of them have a 72-inch stroke. The
rope used is a 2¼-inch plow-steel rope and hoists 10 tons of material
at a trip, in one case from a depth of 4,980 feet, the deepest shaft
in the world. Several plants on the Whiting system have been built in
England, and two or more are working in South Africa.
=29. Modified Whiting System.=--A modification of the Whiting system
is sometimes used in which a large drum keyed to the crank-shaft
replaces the small tandem drums, and even the slight probability of
the rope slipping in the Whiting system is thus obviated. One rope is
fastened to one end of the drum, and the other rope to the other end
in such a way that while one is winding on the other will be winding
off the drum. One rope passes directly to the head-sheave while the
other passes first around a fleet sheave, similar to that used for the
Whiting system, but preferably placed horizontal, and thence to the
head-sheave. This system possesses the same advantages as the Whiting
system except that the depth of hoist is limited by the size of the
drum, and that there is a fleet of the rope. Up to the limiting depth,
as determined by the size of the drum, this system can be used with
equal economy for any depth. This hoist, as well as the Whiting, is
therefore especially suitable for a place where one mining company
operates several mines, for it enables the company to select one size
for all their permanent work, with all the advantages that come from
duplicate machinery.