on the size of the fragments; the proper speed for large pieces would be
150-200 ft. per minute, while smaller material could be carried at a
maximum velocity of 700-750 ft. Band conveyors will carry in an upward
direction, up to 24 degrees, without any loss of capacity. They can be
used not only to carry light and heavy bodies, such as grain and coal,
in a continuous stream, but also to convey relatively large bodies such
as sacks of flour, or cement, &c., intermittently. Thus a band 26 in.
wide and 350 ft. long is used at a flour-mill in York to load sacks of
flour into railway trucks; by this means 12 wagons can be loaded by two
men in 1 hour. Band conveyors are not necessarily fixed in one place. A
portable model has rendered good service in tunnel-cutting, mining and
quarrying. This band is mounted in a light steel frame, itself fitted
with small wheels, so as to be readily put in any required position, and
is entirely self-contained, being provided with tightening gear, a small
motor, &c. If required, several lengths can be joined together, or one
band can deliver upon another at a lower level. The same advantages that
attend the use of the band-conveyor for handling grain may be claimed
for this appliance when carrying coal and heavy bodies, namely the
demand for relatively small power, smooth and noiseless work, and gentle
handling of material. On the other hand the feed cannot be withdrawn at
intermediate points except by means of a throw-off carriage. The
numerous bearings of the guide rollers require careful lubrication, and
the rubber bands should be protected as much as possible from changes of
temperature.
The _metal band_ or belt conveyor, a modification of the rubber or
canvas band conveyors, is an endless belt composed of iron plates
connected to endless chains, usually of malleable cast iron, running
under the plates. Such appliances, being obviously more cumbrous than
band conveyors, are only used in handling material of a hard and cutting
nature. They usually deliver only at the end, but if intermediate
delivery be desired a scraper may be so fixed across the band at a given
point, at an angle of 45 deg., as to scrape the whole or part of the feed
into a shoot, or a scraper may be mounted obliquely on a suitable
carriage which can be moved to any points at which delivery may be
required. In some bands of this type supporting rollers are attached to
the links and travel with them, or are fixed to the framing so that the
band runs over them, an arrangement which has the advantage of
economizing driving power and of promoting smooth running. Metal band
conveyors are tightened in the same way as textile or rubber bands, and
may run at a speed of 60 to 120 ft. per minute. The driving gear must
always be placed at the delivery terminal, so that the loaded strand is
in tension. Such appliances are often used as sorting tables or picking
bands, for instance, for coal, cement, minerals, &c.
In another modification of the metal band conveyor, the _travelling
trough_ conveyor, the sides of each plate are turned up so as to form
the conveying surface of the band into a continuous trough. With this
arrangement intermediate delivery is impossible, as the sides of the
trough will not allow the use of a scraper. As compared with push-plate
conveyors (which consist of scrapers mounted on endless travelling
chains that run usually in troughs), travelling trough conveyors are
gentle handlers of material.
A conveyor which is capable of dealing with many different kinds of
material is known as the _vibrating trough_ conveyor. It is so far like
the band and travelling trough conveyor that the material it conveys
from one point to another is conveyed without the use of any stirring or
pushing agent, such as belong to worm, push-plate and cable trough
conveyors. For materials requiring gentle treatment, this type of
conveyor is eminently suitable. There are different kinds of vibrating
trough conveyors. In one type the trough is caused to make a
reciprocating motion by means of a crank and connecting rod, the trough
itself being supported on rollers. In another type the trough is
actuated by a cam, or by cranks with some kind of quick return motion.
In the appliance known as the Zimmer or swinging conveyor the trough is
supported in its reciprocating motion by means of laminated spring legs
set obliquely to the trough. These legs are securely bolted at one end
to the floor or any other solid support, and at the other end to the
trough itself; hence no lubrication is required, as would be the case
with supporting rollers. Moreover the combined action of the
reciprocating motion of the crank and the rocking of the spring legs has
the effect of causing the material to travel faster in the trough with a
given stroke of the crank than would be the case with any other support.
The material to be conveyed is not carried along with its support as in
the case of a band or travelling trough conveyor, but is caused to move
in a series of hops, to use popular language.
[Illustration: FIG. 8.--Swinging or Zimmer Conveyor.]
The action will be sufficiently explained by the appended diagram
(fig. 8), which, however, is exaggerated to give a clearer idea of the
actual movements, which are on quite a small scale. The line AB
represents the bottom of the trough, while CC are two of the spring
legs; the full lines indicate the spring legs at the extreme backward
position of the crank, while the dotted lines show the spring legs and
bottom of the trough at the extreme forward position of the crank D.
The material to be conveyed, represented by E, is thrown forward by
the forward movement of the crank, and describes a short parabolic
curve; it is thrown at about a right angle to the inclined legs CC,
but before it has time to complete its parabolic course, the trough
has been moved by the crank into its original position. As soon as the
material has dropped down, the trough makes another forward movement,
whereupon the material is thrown forward another stage, and this
process, which is continually repeated, as indicated by the letters
E1, E2, E3, has the effect of carrying or conveying the material in
the direction desired. It is important to note that the actual
movement both of trough and material is within narrow bounds; the
horizontal movement of the trough is only about 1 in., while the
vertical or upward movement is about 1/8 in. The material is conveyed
by this vibrating trough with a minimum of friction, as it is evident
that the material is carried forward without any contact with the
trough, while the very nature of the motion precludes injurious
friction between the particles themselves. When the trough is full the
material will move as it were in a solid mass.
An important improvement in this type of vibrating trough conveyor is
the balanced conveyor, in which the trough is made in two sections,
one being placed at a slightly lower level than the other, so that
one-half may deliver into the other half. The two sections are driven
by triple or quadruple cranks set at an angle of about 180 deg. to one
another. In this case one-half of the conveyor will move forward while
the other moves backward, thus balancing each other (fig. 9). At the
same time the material keeps moving in the same direction because all
the spring legs are of the same inclination. It is usual to drive
balanced conveyors at or near the centre of their length, but they may
also be driven from one end, in which case the balancing of the
conveyor would be effected by a powerful volute spring which is
compressed and released by a crank and connecting rod, in place of
being connected to one-half of the conveyor. Two sections of a Zimmer
conveyor can be made to run in opposite directions by merely reversing
the inclination of the spring legs; in such a case the sections of a
trough would be connected by a flexible coupling. Conveyors of this
type have been used in lengths up to 500 ft., and in widths of over 6
ft. The feed can be received or discharged at any desired point in the
length; for drawing off material at intermediate points it is only
necessary to open a slide in the bottom of the trough. If a great
increase be desired in the capacity of this conveyor the connecting
rod may be attached, not to the trough at all, but to the spring legs
at a point of about a third or half-way from the base, so that the
free ends of the legs can swing the trough backward and forward; by
this means the stroke is amplified and consequently the capacity is
increased, while the driving power required is practically the same.
[Illustration: FIG. 9.]
The power absorbed by the Zimmer conveyor is comparatively small; a
length of 100 ft. conveying a load of 50 tons per hour takes 8.75 h.p.
With a speed of 300-370 revolutions per minute of the conveyor, the
material will traverse 40-70 ft. per minute. The gentle action of this
appliance has caused it to be largely used in dealing with friable
materials, such as coal. The simplicity of the mechanism leaves little
to get out of order, and the entire absence of travelling gear, such
as supporting rollers, is a valuable feature. The capacity of the
conveyor may be sensibly increased by running it on a downward
gradient, while the capacity will be correspondingly diminished by
working in an upward direction. Among many purposes for which this
type of conveyor has been found suitable is that of a drainer in
connexion with coal-washing plants. A perforated plate at the head
will allow the water to escape, while the coal is carried to the other
end. A slight upward slant permits the water left with the coal to run
back and escape. In colliery work this conveyor makes a suitable
picking table. The motion of the trough, while not so fast as to
baffle the pickers, has the advantage of uniformly spreading the lumps
of coal. This apparatus also lends itself to the grading of coal. All
that is necessary is to fit the trough with a sieve which divides it
into an upper and lower deck. The coarser material passes along the
top of the sieve, while the finer coal, sifted out by the
perforations, travels along the bottom of the trough till discharged.
In spite of the gentle propelling action of this conveyor, it has a
thorough sifting action; a perforated plate from 10 to 12 ft. long is
usually sufficient to separate any desired grade, and at a certain
Belgian colliery a conveyor of this type fitted with grading sieves
feeds seven trucks standing in a row, but each on a different siding,
and each taking coal of a different size. This conveyor has been found
useful both as a drying and cooling appliance. Several substances of a
sticky nature, such as moist sugar, which are difficult to deal with
mechanically, can be efficiently handled by the swinging conveyor.
The _gravity_ or _tilting bucket_ conveyor can be used as a combined
elevator and conveyor. It consists essentially of two endless chains or
ropes held at fixed distances apart by suitable bars which are fitted
with small rollers at each end. Every link, or second link, carries a
bucket, and the whole forms an endless chain of buckets. But these
buckets, unlike elevator buckets, which are bolted on to a band or
chain, are free to move on the axis on which they are suspended above
their centre of gravity. When the conveyor is at work the buckets will
always be in an upright position, whether the motion be vertical or
horizontal. Each bucket carries its load to the point at which delivery
is required, where an adjustable tippling device is ready to catch and
tilt the bucket, thus emptying it. This type of conveyor is chiefly used
in connexion with coal stores and boiler houses, where it has undeniable
advantages. For instance, in feeding overhead bunkers a well-designed
gravity bucket conveyor may do the work of (1) a horizontal conveyor in
bringing coal from the railway siding, (2) a vertical elevator in
raising it to the bunkers, and (3) a horizontal conveyor in distributing
it to the respective bunkers. In some cases the returning empty strand
of buckets is used to clear the ashes from under the boilers.
Conveyors of this type run at a mean rate of 40 ft. per minute, and if
it be desired to attain a given capacity the size of the buckets must be
adapted to the increased load as an increase of speed for a higher
capacity is impracticable. The power absorbed is not great, the heaviest
demand on the motive force being made by the elevating operation. Such
conveyors have the merit of handling the material gently, while feeding
and discharging can take place at any point. There are many journals to
be looked after, but in the most approved systems their lubrication is
effected automatically. Whilst such a plant has the advantage of
requiring only one driving gear, a breakdown at one point of the
installation means the stoppage of the whole.
Among typical conveyors on this system is the Hunt conveyor (fig. 10),
which consists of a double link carrying a series of pivoted buckets
which are free to revolve on their axes at all points, except at that
point at which they discharge. This operation is effected by a cam
action, the buckets on their release righting themselves and becoming
ready for refilling. The driving gear propels the chain by means of
pawls which engage with the cross studs of the chain and have a
central thrusting action. Another well-known appliance of this type is
the pan bucket conveyor. This consists of a continuous trough built in
sections and supported on axles and guide wheels running on suitable
rails. There is one axle to each section, and in each section of the
trough a bucket is pivoted to the sides. There are several other
conveyors of this type, amongst which the "Tipit" should be mentioned.
For the Bousse gravity conveyor it is claimed that it will go round
any curve backwards or forwards in both planes, and is therefore
adaptable for installations when the typical gravity bucket would be
useless. The buckets of this conveyor are coupled together by a link
in the middle, which obviously allows more latitude in negotiating
curves than the double chain of most of the other types.
_Pneumatic Grain Elevators_ have been employed with good effect in
loading and unloading grain from ships. This method of conveying grain
falls under three systems: (1) the blast system; (2) the suction system;
and (3) the combined blast and suction system.
In the first system a barge, known as a machinery barge, is fitted with
a steam boiler, a set of air compressing engines, and a length of
flexible piping long enough to reach from any part of the barge to the
farthest corner of the ship to be loaded. A small pipe, known as the
nozzle, is inserted at the inlet end of the piping, where the grain is
taken in, and communicates with the air compressor at the other end.
Compressed air can be admitted to the nozzle or shut off by a valve. The
inlet end of the flexible pipe is pushed into the grain in the barge,
while the other end is led over the hatches of the vessel to be loaded.
As the compressor is set to work and the valve of the compressed air
supply pipe opened, the air naturally rushes up the pipe and escapes at
the other end which is lying over the ship's hatchway. If the inlet
nozzle be immersed in the grain to the depth of 12 to 18 in. the induced
atmospheric air will follow the lead of the compressed air, and drawing
the grain around into the inlet nozzle will carry it up the pipe and
deliver it into the hold of the vessel loading.
[Illustration: FIG. 10.--Travelling Bucket Elevator.]
In the suction system, which is identified with the name of F. E.
Duckham, the process is somewhat different. An air-tight tank or
receiver, 8 to 10 ft. in diameter and 10 to 20 ft. high, is fitted with
a hopper bottom, and is erected, if floating, on a barge, at a
sufficient height to allow grain falling from the hopper bottom, and
passing through an air lock, to be delivered by gravity through a shoot
into the vessel being loaded. A pipe connects the vacuum tank with the
exhaust pumps. Several flexible pipes of sufficient length to reach any
corner of the ship to be unloaded, may be connected with the vacuum
tank. As the air pumps are set working a partial vacuum is formed within
the tank, and as the nozzle end of the pipe is immersed into the grain
to the depth of a few inches, the air and grain are drawn in at the
mouth of the nozzle and carried along the pipe to the vacuum tank. The
natural expansion of the air then lets the grain drop to the hopper
bottom, whence it issues from an air-lock valve, while the air is drawn
away by a pipe communicating with the pumps and is thence discharged
into the open.
In the third system, or blast and suction combined, the grain is sucked
into a vacuum tank, as just described, and drops from this through
valves into a second receptacle, whence it is conveyed to any desired
point by flexible pipes. This second tank is divided into two sections
and provided with valves so that the two sections will alternately be
under the influence of blast or suction. Alternatively the grain is
discharged by an automatic valve from the vacuum tank into the second
air-tight chamber which communicates with the compressed air chamber.
From this section the grain is discharged by an outlet pipe by the
agency of compressed air. A similar system was introduced by Messrs
Haviland & Farmer, who have, however, since abandoned it on account of
difficulties connected with the application of the blast, which was
found to abrade the grain rather severely, especially at the bends in
the pipes. An even greater objection was the delivery of dust with the
grain, which made it impossible for trimmers to remain in the hold while
the elevator was at work. Messrs Haviland and Farmer now work on the
suction system, in which they claim to have introduced several
improvements, notably in regard to the purification of the air between
the vacuum chamber and the exhaustors, and in devising a new automatic
air trap.
The first pneumatic suction elevator in Great Britain was erected at the
Millwall docks (London) under the Duckham patents. At Sulina, on the
Lower Danube, a pneumatic elevator erected on the Haviland-Farmer
system, which has undergone one or two reconstructions, has been proved
capable of elevating 160 tons of grain per hour with 375 i.h.p.
The only objection to pneumatic elevators appears to be that of expense.
The cost of installation is relatively heavy, and the power required for
working is large. But in dealing with vessels carrying heavy cargoes of
grain the saving of labour and demurrage is sufficient to justify the
large outlay of capital required in ports where there is sufficient
grain traffic.
_Hot Coke Conveyors._--Hot coke is admittedly one of the most difficult
materials to handle by mechanical means, and though it might be too much
to say that all difficulties have been surmounted by the engineer, it
has, since the end of the 19th century, been more or less satisfactorily
handled by machinery. Even in a dry state coke is a troublesome material
to handle by machinery. It is of a gritty and rasping nature, and is at
the same time very friable. Unless it is gently handled, breakage is
bound to occur and to result in the making of a certain proportion of
fine dust known as "breeze." Apart from the depreciation in the value of
the coke, this breeze is a sharp, cutting material, calculated to do
considerable injury to the working parts of the conveyor, such as
chains, and to the bearings, if it can get inside. Of course the
conveying of the coke in an incandescent condition is another serious
difficulty, as this glowing material must be quenched by water, a
sufficiently delicate operation in itself. The chief use for hot coke
conveyors has been found in connexion with gas works, but attempts have
also been made to provide efficient machinery for the service of coke
ovens of great capacity.
The justification of any kind of machinery must rest on its relative
efficiency and economy. As compared with some other materials the
mechanical handling of hot coke does not realize such a striking
economy; a hot coke conveyor is expensive to build--on account of the
great wear and tear it must be very solidly constructed--and it is
costly in upkeep. Still in large gas works the use of machinery for
treating glowing coke is economically advisable. Exact calculations are
not very easy to make, because while the cost of hand labour in this
department of a gas works is accurately known, the efficiency of
different hot coke conveyors varies. G. E. Stephenson, of the Gathorn
gas works, estimated that a saving of 4-3/4d. per ton had been realized on
each ton of coke conveyed to the yard from the retort house, as against
the same material wheeled in barrows. This saving represented the
difference between the cost of twelve men, who formerly handled the hot
coke with shovels and barrows, and the cost of one conveyor with the
wages of one man to look after it. In an ordinary way one man would rake
out the coke from the retort mouthpiece into a barrow placed underneath,
while a second man quenched the glowing coke with buckets of water, or
better still with a hose. Then the barrow would be wheeled out into the
yard. Obviously this is a slow and relatively expensive method, apart
from the deleterious fumes arising from the quenching of the coke. Some
improvement was effected by the substitution for the old hand-barrows of
cage-like tipping trucks; these are run on narrow gauge rails out of the
retort house and the red-hot coke they contain is quenched by a copious
spray, the truck being placed the while over a grating through which the
surplus water is drained away, under an inverted funnel with an uptake
to carry away the fumes and vapours. These trucks have been hauled, in
lieu of human arms, by endless ropes or even small locomotives.
The earlier hot coke conveyors were of the _pushplate_ type. The
trough, some 27 in. wide, consisted of cast iron sections, while the
pushplates, formed of malleable castings, were attached at a pitch of
24 in. to a central chain and were pulled along on a wrought iron bar,
which could be renewed when necessary. These conveyors with a speed of
48 ft. per minute, had a capacity of some 20 tons per hour. A conveyor
constructed on these lines was installed at the Gathorn works in 1903.
The wear and tear was very great; moreover the chain, being central,
suffered severely from the hot coke, to the action of which it was
directly exposed.
The New Conveyor Company's conveyor consists of a water-tight trough
through which pass closely-fitting tray plates, attached to a single
chain. These plates are joggled down at one end to receive the flat
front part of the succeeding plate, with the aim of excluding the
breeze from the under part of the carrying plate. The chain is made
entirely of steel with side rollers attached to every third plate, the
plates, 1/4 in. thick, are dished in the shape of a tray, which is less
liable to distortion (from heat) than a flat plate. The speed of
travel is about 45 ft. per minute, while the capacity when handling
coke from 20 ft. retorts is some 30 tons per hour.
A conveyor made by Messrs Graham, Morton & Co., consists of a
travelling tray, the sections of which are joined together by steel
spindles provided with a roller at each end, the latter running on
suitable rails. These sections consist of steel castings with a number
of lateral slots; thus the tray has the appearance of a travelling
grating. To receive the quenching water that escapes through the
grating a trough is placed beneath, and a scraper is used to free the
trough of the dust escaping through the grating.
[Illustration: FIG. 11.--Bronder Hot Coke Conveyor.]
An interesting conveyor is that of G. A. Bronder, of New York (fig.
11), which has some affinity with the gravity bucket conveyor. It runs
in a water-tight trough which is filled up to a certain height, the
water being slowly circulated by mechanism which resembles a water
wheel. The chain of buckets runs in the trough, the sides forming the
rails for the supporting rollers. The conveyor is covered in along its
whole length, and forms a sort of flue which is connected at each
bench with a number of shoots through which the coke drops into the
conveyor buckets. A pipe of large diameter is connected with an
exhaust fan, which draws away the fumes created by the quenching
process, and sends them into a chimney discharging into the open. The
chain and buckets, being carried on rollers which run on the outer
edge of the trough, cannot come in contact either with the hot coke
or with gritty particles. The chain of buckets is connected by
horseshoe-shaped brackets extending upwards beyond the sides of the
buckets and connected with the links of the driving chains. When the
conveyor is at work the covers of the mouth-pieces are opened and the
coke is fed into the buckets; simultaneously the water valves are
opened and the glowing coke is quenched. Any breeze which may have
fallen between the buckets is collected by a scraper and delivered
into a tank at one end, while the propeller wheel draws the water from
this tank and drives it back to the other end of the trough. The top
strand is the working strand and delivers its load at the terminal.
One important difference between an ordinary gravity bucket conveyor
and this apparatus is that the buckets are here rigidly connected to
the supporting wheels.
The West hot coke conveyor consists of a strongly-built trough in
which a single wide chain partly carries and partly drags the coke. In
the trough is a false bottom, the plates of which are loosely fixed
and kept in position by angle irons on which the chain drags. By two
arm-like extensions the links of the chain are widened right across
the trough. The pitch of the chain is 12 in., so that all the large
pieces of coke are more carried than dragged. The speed of travel is
about 40 ft. per minute.
[Illustration: FIG. 12.--Wild Coke Conveyor.]
The Wild conveyor (fig. 12) consists of a cast iron or steel trough 24
to 30 in. wide by 9 in. deep, supported by cast iron brackets to which
the rails that support the strands of the chain are secured. Both
chains run outside the trough, and are secured on either side to the
pushplates, so that only the scraper comes in contact with the hot
coke. Every second link of the 12 in. pitch chain carries a push or
scraper-plate, as shown in illustration.
The De Brouwer hot coke conveyor, which is much used in gas works both
in Great Britain and on the continent of Europe, was invented by a
Belgian engineer. Its construction has undergone many modifications
which experience has shown to be desirable. It consists of a trough of
cast or wrought iron, or mild steel, 20 to 36 in. wide and 3 to 6 in.
deep. Double endless chains run in the corners of the trough, the two
chains being connected together by round cross bars set 30 in. apart,
so as to form a sort of ladder. The hot coke is carried or dragged
along by these bars. One end of the trough is closed and the other is
bent upwards with a view to retaining the quenching water. As the hot
coke is dragged along it is subjected to the action of jets of water.
The conveyor bars, which act as scrapers, sweep the water and the coke
along the trough till the point is reached where the latter curves
upwards. Then the water flows back like a small cascade on the
half-quenched coke, which is thus thoroughly extinguished.
Considerable inclines can be negotiated with this conveyor; in some
installations on the continent of Europe angles of 30 deg. to the
horizontal have been surmounted. In a modification of the De Brouwer
conveyor, installed at the Cassel gas works, the bars which form the
rungs of the conveyor were replaced by cast iron rakes. In another
modified form, the work of F. A. Marshall, to be found in the
Copenhagen gas works, sluices are provided for withdrawing an excess
of water at any point in the trough.
In Great Britain a hot coke conveyor has been designed on similar
lines by Messrs R. Dempster & Sons, Ltd. (fig. 13). The chains are
parallel from end to end, and are composed of identical and
interchangeable malleable cast links. Instead of the chains carrying
the rollers, as is often the case, the chains are themselves carried
and guided by flanged rollers supported from the framework. This
arrangement has the advantage of decreasing the weight of the chain,
as neither the rollers nor the lubricators have to be conveyed, being
stationary. The scrapers are of cast steel and have a rake-like shape
with a view to minimize the breakage of coke.
[Illustration: FIG. 13.--Dempster Coke Conveyor.]
The essential features in a hot coke conveyor are strength and
simplicity, a minimum of wearing parts, interchangeability of wearing
surfaces and of worn and broken parts, protection of wearing and working
parts from contact with the hot coke, and facilities for keeping the
temperature of the conveyor as even as possible, so as to avoid
distortion of parts through sudden changes. To attain these latter
conditions, it appears essential to construct conveyors of the pushplate
type. In these the hot coke is kept continually moving, and thus the
good effect is secured of heating the conveyor from end to end uniformly
and gradually. This applies particularly to gas works conveyors.
For the service of coke ovens the plate or tray conveyor might be
suitable because more gentle. It must be remembered that coke oven
conveyors must be of large capacity, and moreover in this case there is
more scope for cooling the coke in front of the oven before it is
removed to the conveyor, the work being all effected in the open.
_Elevators._--This term is here confined to its proper meaning (in
English engineering treatises) of a device for raising material in a
vertical or slanting direction by means of buckets attached to endless
belts or chains. Lifts for passengers are also sometimes termed
elevators (q.v.), and in America the term is also currently applied to
the granary or warehouse in which grain is stored (see GRANARIES).
In the bucket elevator, an endless belt or chain runs over terminal
pulleys which are fixed at different levels, the distance from centre to
centre of these pulleys beings known as the length of the elevator. The
design and construction of the elevator will be varied to suit its
purpose. Grain elevators are invariably cased in wooden or iron trunks,
and the head and foot are also of wood or iron, iron trunks being
particularly used in so-called fire-proof buildings. The trunk of the
grain elevator (fig. 14) is almost always vertical whilst the band to
which the buckets are attached may consist of leather, cotton, hemp,
webbing or other suitable substances. When an elevator is intended for
lifting heavy materials, such as coal, coke or cement, it is usually set
at a slant (figs. 15 and 16), and the endless belt is replaced by one or
two strands of endless chain which support the buckets and run over the
terminal sprocket wheels. The buckets are attached to the links of the
chains, and to prevent these heavy buckets and chains from sagging in
their inclined position, rollers or more often short skidder bars are
fixed to each bucket, sliding on well-oiled angle bars on each side of
the elevator frame.
[Illustration: FIG. 14.--Grain Elevator.]
Both grain and mineral elevators are usually fitted with tightening
gears to keep the belt or chain taut; these are generally placed at the
lower or well end so as not to interfere with the position of the upper
terminal, which is almost invariably the driven one. The tightening of
the band at the bottom terminal in the elevator well necessarily alters
the space between the terminal pulley and the bottom of the well. This
is of little consequence in grain elevators, but for elevators intended
to handle coal or any material of varying size the ordinary tightening
gear is unsuitable. In such a case the best plan is to attach the
elevator-well to the terminal in such a way as to go up or down with the
sprocket wheel when the chain is loosened or tightened, while the foot
bracket which supports the well and terminal spindle remains a fixture.
In order to tighten elevator chains without interfering with either of
the terminals, adjustable jockey pulleys at some suitable point may be
used, and the desired effect can thus be attained by pressing against
the chains and thereby taking up the slack without any interference with
either the feed or delivery end.
Elevator buckets must be proportioned to the size and nature of the
material they are intended to carry, and care must be taken to
maintain a uniform feed. This may readily be effected by adjustable
outlets and spouts for grain and the like, and by certain feeding
devices for handling minerals of uneven size. For instance, an
oscillating feed shoot making from 30 to 60 oscillations per minute
can be installed in such a case, and adjusted to deposit at each
backward and forward stroke the exact amount of material adapted to
the capacity of the elevator. The speed of the shoot will naturally
vary with the size of material to be fed. For small coal 60
oscillations would be about the correct speed; for large coal the
speed might be reduced to 30 or less. Speaking generally, care should
always be taken to prevent an undue rush of feed, that is, more than
the elevator can take up, and if tenacious materials are handled,
feeding devices should be employed provided with stirrers or agitators
that will effectually keep the material moving and prevent any larger
lumps from arching over the feed spout, and thus producing chokes.
Elevators should always be fed from that side on which the buckets
ascend, that the stream of material may meet the elevator buckets on
their upward journey. This will prevent the material from filling up
the elevator well and spare the buckets from dredging through an
accumulation of feed. Elevators erected at an incline are best fed at
a point several feet above the well into the chain of ascending
buckets, as under such conditions little will miss the buckets and
drop into the well.
The reason why grain elevators are set vertically, whereas elevators
intended to carry heavy bodies such as coal and ore are generally
inclined at an angle, is that the former can be run at a much greater
velocity than the latter. Grain, for instance, would be uninjured by a
velocity at the delivery end which would fracture coal and seriously
reduce its value, to say nothing of the dust production and the damage
which would be done to the receiving spouts and shoots. Elevators
carrying a light material can be run at a circumferential velocity of
250 to 350 ft. per minute, and if vertically set, will throw the
grain, &c., clear of the elevator into the shoot for its reception. On
the other hand, elevators handling heavy material must be set at an
angle in order to give a clear delivery at a much lower speed of 50 to
60 ft. per minute; in other words, the elevator is so inclined that
the shoot for the reception of the material can be put underneath the
delivering buckets which slowly disgorge their load. To obtain good
results, without taking up too much space, an elevator carrying heavy
material should be set at 40 deg. to 60 deg. to the horizontal. The
same results can be obtained if the main portion of the elevator is
vertical and only the upper portion inclined, or so curved as to bring
the delivery over the shoot. The speed at which vertical elevators
should be run will depend on the diameter of the terminal pulley, that
is, the pulley over which the buckets and bands pass. The centrifugal
force of pulleys revolving at the same speed is in direct proportion
to their diameters, and this is twice as much in a 2 ft. as in a 1 ft.
pulley. It may be taken that the centrifugal force of a pulley will
increase in proportion to the square of its velocity; hence the
centrifugal force of a pulley 2 ft. in diameter running at 50
revolutions per minute will be four times the centrifugal force of a
pulley of the same diameter making only 25 revolutions per minute. It
must not be forgotten that to effect a clean discharge of the buckets
of a vertical elevator, the centrifugal force must be sufficient to
overcome the gravity of the material, because the material thrown off
the delivery pulley in a horizontal direction will be more rapidly
deflected into a parabolic curve the higher its specific gravity. It
follows that for a specifically heavy material a greater centrifugal
force will be required; that is to say, the elevator will have to be
higher speeded than in dealing with a lighter material.
[Illustration: FIG. 15.--Mineral Elevator, upper terminal.]
Elevator buckets must be varied according to the nature of the
material; for instance, shallow buckets will be found best for a soft
and clinging material such as flour, moist sugar, sand, small coal,
&c., while for a hard or semi-hard body such as wheat, coal, &c.,
deeper buckets are preferable. On account of their lower speed,
elevators for specifically heavy material require much larger buckets
and chains than grain elevators of the same bulk capacity. The most
economical form of elevator is fitted with a continuous chain of
buckets. Such elevators may be constructed to carry either grain or
minerals. The advantages are greater capacity than an ordinary
elevator of the same dimensions and a more uniform delivery; moreover,
smoother running is secured, since the buckets being close together
need not plunge intermittently through the contents of the
elevator-well.
_Intermittent Conveyors._--The elevators we have been considering,
whether used for carrying and distributing coal or grain, have this in
common, that they raise material from a lower to a higher level, so to
speak, in a continuous stream, the continuity being broken only by the
short spaces between the buckets. In the continuous bucket type indeed
the stream of material is practically, if not absolutely, continuous. In
all these cases the elevator is fed with the material in a continuous
stream, and by some mechanical means; whether by band, worm or shoot, is
immaterial. Elevators of a somewhat different and more substantial
construction may be and are often used for handling filled sacks,
barrels, carcases of animals and other bulky objects, which cannot be
delivered in a uniform stream, but may have to be conveyed by the
elevator intermittently. The ordinary buckets used for grain or coal are
replaced by other appliances for gripping and holding the object to be
raised from a lower to a higher level, but in principle these appliances
are essentially elevators.
[Illustration: FIG. 16.--Mineral Elevator, lower terminal.]
Another kind of elevator, known as a _lift_ or _hoist_, is used in mines
and quarries and in serving blast furnaces. This is an elevator with one
or two buckets. Essentially a heavy load lifter, it is intended for
material of too large a bulk to be handled economically by ordinary
elevators, and is employed for lifting in either a vertical or, more
often, an inclined direction.
For elevating materials, such as large coal, iron ore, limestone, &c.,
which are too large to be fed into ordinary elevators, and must
therefore be handled intermittently, the single bucket elevator or hoist
may be used with advantage. But as the essential use of mechanical
appliances for handling material is to save human labour as far as
possible, that hoist will prove the most economical the operation of
which is as automatic as possible. The Americans seem to have been
pioneers in the construction of _furnace hoists_, which form the
principal elevators of this class, but some excellent examples of the
modern furnace hoist are now to be found in Great Britain and elsewhere
in Europe. Generally speaking, a furnace hoist consists of an inclined
iron bridge girder set at an angle to the upright shaft of the furnace.
On this incline are laid rails for the ascent and descent of the bucket,
which in this case is known as a skip and is provided with suitable
wheels, while the hoisting gear manipulating the skips by a steel rope
is erected on or near the ground level. The rails when they approach the
upper terminus are usually bent in a more or less horizontal position so
as automatically to tilt and thereby unload the skip. To attain the same
end, the rails supporting the back wheels of the skips may be bent at
the terminus, or the back wheels may have additional wheels of a larger
diameter on the other side of their flanges, so that during the ascent
and descent the skip runs on its four normal wheels, while at the upper
terminus the outer and larger back wheels engage with short lengths of
extra rails and thus tilt and effect the automatic clearance of the
skip. The dead weight of the skip may be balanced by a counter weight,
or double tracks may be laid, so that the empty skip descends on one
track whilst the loaded skip is being raised on the other. In this case
the distributing hopper at the top of the furnace has an elongated shape
so as to take the charges alternately from buckets on either track.
Again, the two tracks may be laid one above the other, so that one skip
runs on the upper rails and the other on the lower. The two buckets will
pass each other at about the centre of the framing, where there will be
plenty of room for clearance.
The capacity of the skip will of course depend to some extent on the
capacity of the furnace, but an average charge may be put down at 2 tons
of ore and lime, or 1 ton of coke. To raise such a charge to a furnace
80 ft. high would require, assuming no counter weight were used, a motor
of about 100 h.p. On account of the great speed at which the hoist
works, the time taken in raising the charged skip, discharging it, and
returning it empty would be only 30 to 40 seconds. The hoist cable runs
over guide pulleys placed at the top of the furnace, and the cable is
often manipulated by an electrically driven winch in a cabin below. The
descent of the empty skip in more modern installations is utilized to
effect an even distribution of the feed from the hopper to the furnace
by causing the hopper to revolve. To this end the latter is provided
with an ingenious mechanism which only comes into operation as the car
descends. After every charge shot into the hopper the latter is revolved
a few degrees, and this has the effect of giving the delivery of the
next load in another direction, so that the charges of the skip are in
turn distributed over the whole area of the surface. This is deemed a
most essential point in furnace-charging, and it is not one of the least
recommendations of this mechanical system of furnace-charging that it
can give an even feed without any hand labour whatever. A double hoist
has been designed which has the advantage that if one elevator breaks
down the work of the furnace is not interrupted. In this system two
furnaces are connected at the top by a gantry or bridge, against which,
between the furnaces, two inclined elevators are set, so that each can
serve either furnace. The skips are on wheels and detachable from the
elevator, and are loaded from the ore pockets at the lower terminal and
drawn up on a cradle; as this reaches the top where the rails on the
gantry correspond with the gauge of the skip or car, the latter is
carried by its own weight down a slight incline to either furnace,
discharging its contents as it passes over the conical mouth. Another
advantage claimed for this system is that the rails of the cradle, when
in its lowest position, correspond with the rails which lie parallel to
the furnaces and run right under the store bins from which the skip is
loaded. The economy to be realized from a furnace hoist will be in
direct proportion to the use made of mechanical means of feed
conveyance. For instance, the store bins in connexion with such
elevators might be economically fed by suitable conveyors, or the
material might be brought in self-unloading hoppered trucks into
conveniently placed bins, ready to be drawn into the skips.
_Ropeways._--A ropeway has been defined as that method of handling
material which consists of drawing buckets on ropes, and by means of
ropes, such buckets being filled with the material to be handled and
being automatically or otherwise discharged. At what period of history
ropeways were first used it is impossible to say, but the fact that
pulley blocks, and even wire ropes, were known to the ancients, renders
a pedigree of 2000 years at least possible. In more modern days, an old
engraving shows a single ropeway in working order in 1644 in the city of
Danzig. This, the work of Adam Wybe, a Dutch engineer, was a single
ropeway in its simplest form, consisting of an endless rope passing over
pulleys suspended on posts; to the rope were attached a number of small
buckets, which evidently carried earth from a hill outside the city to
the rampart inside the moat. The rope was probably of hemp. Modern
ropeways worked with wire ropes date from about 1860, when a ropeway was
erected in the Harz Mountains. Since then several systems have been
evolved, but in the main ropeways may be divided into the single and
double rope class.
The ropeway is essentially an intermittent conveyor, the material being
carried in buckets or skips, and practice has proved it an economical
means of handling heavy material. The prime cost of a ropeway is usually
moderate, though of course it varies with the ground and other local
conditions. Working expenses should be low, because under the
supervision of one competent engineer unskilled labour is quite
sufficient. A ropeway may be carried over ground over which rails could
only be laid at enormous cost. To a certain extent ropeways are
independent of weather conditions, because their working need not be
interrupted even by heavy snowfalls. Their construction is very simple,
and there is little gear to get out of order. Sound workmanship and good
material will ensure a relatively long life. As an instance, a certain
rope in a Spanish ropeway tested new to a breaking strain of 29-1/2 tons
was shown after carrying 160,000 tons (in two years' incessant work)
still to possess a breaking strain of 27-1/2 tons. The power absorbed by a
ropeway is relatively moderate, and under special conditions may be nil.
The only demand it makes on the superficial area of the ground traversed
is the small emplacements of the standards, which in modern ropeways are
few and far between. Wayleaves, or the permission to erect standards and
run the line over private land, may of course mean an item in the
capital outlay. This circumstance may have checked ropeway construction
in Great Britain, but it must also be borne in mind that a large portion
of that country is comparatively level and well provided with railways.
In building a ropeway it is essential to take as straight a line as
possible, because curves generally necessitate angle stations, which
mean extra capital and working cost. On the other hand, ground that
would be difficult for the railway engineer, such as steep hills, deep
valleys and turbulent streams, has no terror for the ropeway erector.
There is a case of a ropeway of a total length of 5400 ft. with a total
difference in altitude of 2000 ft.; it is claimed this ground could not
be covered by a railway with less than 15 m. of line graded at 1 in 40.
Perhaps the simplest type of a single rope system is an endless
running rope from which the carriers are suspended, and with which
they move by frictional contact. Or the carriers may be fixed to this
rope and move with it. The ropeway itself would consist of an endless
rope running between two drums, one, known as the driving drum, being
provided with power receiving and transmitting gear, while the drum at
the opposite terminal would be fitted with tightening gear. The
endless rope is carried on suitable pulleys which themselves are
supported on standards or trestles spaced at intervals varying with
the nature of the ground. The rope runs at an average speed of 4 m.
per hour, a speed at which the bucket or skip can automatically unload
itself. In the double ropeway the carrier runs on a fixed rope, which
takes the place of the rails of a railway. The carrier is fitted with
running heads furnished with grooved steel wheels. The load is borne
by a hanger pivoted from the carrier, and is conveyed along the rail
rope by an endless hauling rope at an average speed of 4 to 6 m. per
hour. The hauling is operated by driving gear at one end, and
controlled by tightening gear at the other end just as in the single
rope system. Double ropeways have been carried in one section over 18
to 20 m., and will transport single loads of 6 cwt. to a ton or more.
Broadly speaking, the single ropeway is not so suitable for heavy
loads and long distances as the double, but in this connexion the work
of Ropeways Limited should be noted, which favours a single rope
system. Their engineer, J. Pearce Roe, introduced multiple sheaves for
supporting the rope at each standard. Thus the rope may pass over one,
two or four sheaves, which are provided with balance beams that have
the advantage of adjusting themselves to the angle caused by the rope
passing over the sheaves, thus equalizing the pressure over a number
of sheaves. A ropeway erected on this system in Japan spans 4000 yds.
of very broken ground; yet only 17 trestles are used, and as each
support is placed as high as possible, no one is of great height. An
altitude of 1130 ft. is reached in a distance of 1200 yds. The ropeway
has a daily carrying capacity of 60 tons in one direction and of 30
tons in the other. Another installation on this system, which serves
an iron mine in Spain, spans 6500 yds. of very rough country, so steep
that in many places the sure-footed mule cannot keep on the track.
This ropeway can deal with 85 tons per hour. The greatest distance
covered by this system, on one section, is 7100 yds., or about 4 m.,
and the carrying capacity is 45 tons per hour.
The motive power required for a ropeway will vary with the conditions.
In cases of descending loads the power generated is sometimes so
considerable as to render it available for driving other machinery, or
it may have to be absorbed by some special brake device. In a ropeway
in Japan of 1800 yds., which runs mostly at an incline of 1 in 1-1/2,
the force generated is absorbed by a hydraulic brake the revolving fan
of which drives the water against fixed vanes which repel and heat it.
In this way, 50 h.p. is absorbed and the speed brought under the
control of a hand brake.
_Aerial Cableways._--The aerial cableway is a development of the
ropeway, and is a conveyor capable of hoisting and dumping at any
desired point. The load is carried along a trackway consisting of a
single span of suspended cable, which covers a comparatively short
distance. The trackway may either run in a more or less horizontal
direction, i.e. the terminals may be on the same level, or it may be
inclined at such an angle that the load will descend by gravity. The
trackway or rail rope rests upon saddles of iron or hard wood on the
tops of terminal supports, usually known as towers. These towers may be
constructed either of wood or iron, and if the exigencies of the work
render it desirable, they may be mounted on trolleys and rails, in which
case the cableway is rendered portable, and can be moved about,
sometimes a great advantage in excavating work. The motive power may be
either steam, gas, or electricity. The motor is situated in what is
termed the head tower, which is sometimes a little higher than the other
or tail tower. Sometimes, but not frequently, the latter is also fitted
with a motor. The span between the two towers sometimes extends to 2000
ft., but this is exceptional. Very heavy loads are dealt with, sometimes
as much as 8 tons in a single load. The load, which may be carried in a
skip or a tray, is borne by an apparatus called the carrier, which is a
modification of a running head, consisting of pulleys and blocks and
running along the main cable or trackway. The carrier is also fitted
with pulleys or guides for the dump line. The carrier is drawn along the
main cable by an endless or hauling rope which passes from the carrier
over the head tower and is wound several times round the drum of the
winding engine to secure frictional hold, then back over the head tower,
to the tail tower, returning to the rear end of the carrier. The
hoisting rope passes from the engine to the fall block for raising the
load. The dump line comes from the other side of the winding engine drum
and passes to a smaller block attached to the rear end of the skip or
tray. The whole weight of the skip is borne by the hoisting rope, while
the dump line comes in slack, but at the same rate of speed. Whenever it
is desired to dump the load, the dump line is shifted to a section of
the drum having a slightly larger diameter, and being thus drawn in at a
higher rate of speed the load is discharged. The engine is then
reversed, and the carriage brought back for the next load.
This is in outline the mode of operating all cableways. This appliance
has rendered great service as a labour saver in navvying, quarrying and
mining work; in placer-mining, for instance, cableways have been found
very useful when fitted with a self-filling drag bucket, which will take
the place of a great number of hands. Cableways can be worked at a great
speed, but a good mean speed would be 500 to 750 ft. for conveying and
200 to 300 ft. for hoisting. A cableway used in excavating work in
Chicago was credited with a capacity of 400 to 600 cub. yds. per day at
a total cost of 2d. per yard, including labour, coal, oil, waste, &c.
_Coaling Ships at Sea._--In the coaling of ships at sea the cableway has
rendered great service. The conditions under which this operation has to
be carried out present many difficulties, especially in rough water. One
of the chief obstacles is the maintenance of the necessary tension on
the cable used in conveying the coal from the collier to the ship. The
first test in coaling ships at sea, made by the British admiralty, took
place in 1890 in the Atlantic at a point 500 m. south of the Azores in
water 2000 fathoms deep. Ten ships of war were coaled, each vessel
taking enough coal to enable it to steam back to Torbay, 1800 m. away.
In this case the collier was lashed alongside the battleship it was
feeding, thick fenders being interposed to prevent damage, but
nevertheless as the colliers got light they pitched considerably, and
one or two sustained dents in their sides. The ships did not roll, being
kept bows-on to the swell, which became heavy before the coaling was
completed. The coal was taken in by derricks at the main deck ports. It
is clear that had the sea been really rough coaling in this fashion
would have been impossible.
The most practicable method of coaling at sea yet devised is the marine
cableway of Spencer Miller, which has been tried with some success in
the American navy. It is intended for use between vessels 350 to 500 ft.
apart. The ship being coaled takes the collier in tow, steaming at the
rate of 4 to 8 knots; it has been found that a speed of five knots in
moderately rough water will keep the cableway taut and maintain a
sufficient distance between the crafts. The collier is fitted with an
engine having double cylinders and double friction drums, which is
placed just abaft the foremast. A steel rope 3/4 in. in diameter is led
from one drum over a pulley at the mast head and thence to a pulley at
the head of shear-poles on the vessel being coaled, and brought back to
the other drum. The engine moves in the same direction all the time and
keeps on winding in both the strands of the conveying rope. Should the
two vessels increase the distance between them during the operation of
conveying the coal bags, of which two, weighing 420 lb each, may be
fastened to the carrier, the extra rope called for is obtained by
slipping the upper strand from the drum; this increases the speed of the
upper cable. On the other hand should the distance between the vessels
be reduced, this operation is reversed, the speed of the upper strand
being reduced. To keep the carriage steady on its return empty, a rope,
known as the sea-anchor line, is stretched above the two strands of the
conveyor line, and under a pulley on the carriage. This cable is
attached to the vessel, resting on a saddle on the shear head, whence it
leads through the carriage over pulleys at the head of the foremast and
mainmast of the collier, running on astern several hundred feet into the
sea. A drag or sea-anchor, usually made of canvas and cone-shaped, is
attached to the end of this rope. This anchor is used to support the
empty carriage on its return to the collier. The diameter of the cone's
base is graduated to the speed of the vessels. Thus in a smooth-water
test, with a ship steaming at 6 knots, one 7 ft. in diameter was used,
while the same anchor answered its purpose very well with a ship doing 5
knots in rough water.
The results given by this system of coaling at sea are relatively
satisfactory. Tests made in the United States navy showed that 20 to 25
tons of coal per hour could be delivered by a collier to a war-vessel
during a moderate gale. As the ship was under steam all the time and
consumed 3 to 4 tons of coal per hour, the balance of the coal bunkered
amounted to between 16 and 20 tons per hour, or say 384 tons in 24
hours. It has been suggested that under service conditions the speed of
the towing vessel might be increased to 8 or 10 knots an hour; this
would of course increase the coal consumption unless the collier
proceeded under her own steam. But in such a case the space between the
two crafts might be diminished, which would have the effect of causing
the cable to sag and of stopping the work, since the conveyor cable to
act properly must be kept taut. In Great Britain the Temperley
Transporter Company have taken up this method of coaling at sea, working
in collaboration with Spencer Miller, and have introduced several
improvements in detail. Their system has been tried by the British
admiralty.
The coaling of a large vessel by this appliance has the advantage of
economizing hand labour. One man is required to work the hoist on the
collier, while 20 men will be in the hold filling the bags and
delivering them to the deck, where 15 or so will transfer the bags to
the lift. One or two men suffice for the overhead work; their station is
in the trestle trees. On board the receiving ship a few men will be
stationed at the shear head to empty the bags into a canvas shoot, and
then return them, while there will be the usual force of bunker
trimmers. A ton of coal per minute has been transferred from the collier
to the vessel, but for this capacity the ships must not be too far
apart, else the rope would not remain taut under such loads. During the
Russo-Japanese War, many of the Russian battleships were coaled by means
of aerial cableways. The coaling of vessels in this manner seems a
success, but it would be desirable to increase the carrying capacity of
the cableway or to duplicate the installations.
_Telpherage._--A telpher ropeway or cableway may be defined as a ropeway
or cableway worked and controlled electrically, only a rail rope being
required besides the live rail or wire from which the electric current
is taken. Telpherage was devised by Professor Fleeming Jenkin in 1881,
and developed by him in conjunction with Professors W. E. Ayrton and J.
Perry. The telpher itself consists of a light two-wheeled truck,
carrying the driving motors, which, to avoid gearing or other
complicated mechanism, are usually coupled directly to the axles of the
telpher. Thus the telpher is a self-propelled electric carrier running
on a mono-rail, which, according to the conditions, may be a steel rail
or a steel cable. From the telpher are suspended carriers which can be
adapted to any kind of material. In many cases the whole load may be
suspended from the telpher, or the load, especially if of some length,
may be supported at one end by a telpher, and at the other end by what
is known as a trailer, or again, two telphers may be installed, one at
each end of the load. The telpher carries a small trolley sheave or bow
which serves to collect the current from a trolley wire stretched a
little above the rail. Frequently the telpher is accompanied by an
attendant who manipulates it, but by dividing the trolley wire into
sections any system of telpherage may be constructed to work
automatically, and by switching off the current from the section in
which the telpher is required to stop it can be brought to a standstill
at any required point. The speed of the telpher may be readily regulated
by the introduction of a resistance between any section of the line and
the supply of electricity. The speed may be high, as much as 1500 ft.
per minute over the straight portions of the line, but slackened at
curves and loading stations, or when approaching a terminus. The
required power may be obtained from the mains of an ordinary electric
supply with either direct or alternating current, but the former is
preferable. The mean expenditure of power in a working day is said to
average (including electrical hoisting) 1 H.P. per ton of average load.
The uses of telpherage are many and various. In factories and
warehouses, where the buildings are scattered, it has been installed
with excellent results. Being essentially an overhead system, there is a
saving of floor space, the ground not being obstructed by trucks or
trolleys. The same reasons which render ropeways an economical means of
handling such material as coal, ore, stone, slate, &c., between the mine
or quarry and the rail or barge, may be adduced in favour of telpherage.
For the unloading of railway trucks in a crowded goods-yard it is
undoubtedly applicable. Any kind of tipping or hoisting operations can
be automatically effected by its aid, and any sort of grab may be used
in dealing with such materials as sand, clay or gravel. Telpherage is
clearly a labour-saving method of handling materials, but of course the
exact conditions under which any system is to be used need careful
study, while the economy to be effected by the installation of a telpher
line must to a great extent depend upon the available supply of
electrical energy. (G. F. Z.)