C o a l A g e
A McGr a w- Hi l l Pu b l i c a t i o n— Es t a b l i s h e d 1911
D EV O TED TO THE OPERATING, TECHNICAL, AND BU SIN E SS PROBLEM S OF THE COAL MINING INDUSTRY
Neic York, May, 1 9 3 0
Vo l u m e 35 Nu m b e r 5
On Their Way!
P R O G R E S S in m echanized mining is out
stripping the forecasts of cautious optimists.
P re lim in a ry figures f o r 1929 show an in
crease o f 75.6 p e r cent over the preceding year in the to nnage of bituminous coal loaded mechanically o r semi-mechanically. Since 1926, the first y e a r f o r which complete d a ta a re available, the quantity of bituminous coal so h a n d le d has grow n fro m 10,545,000 tons to 3 7 ,8 5 4 ,0 0 0 tons— an d the industry is h a rd ly on the th re sh o ld o f mechanical loading.
M E C H A N I Z E D M I N I N G in fo u r states accounted f o r 74.9 p e r cent o f this tonnage last year, and these same states increased the q u antity o f c o d load ed by machine and con
v eyo r 83.4 p e r cent over th eir 1928 totals.
N e i th e r non-mechanized o perations in these states n o r districts in o th e r states which have been reluctant to em b a rk upon m echanization can dodge the impact o f such a development.
I t is only a question o f how soon they, too, will be compelled to join the m arch.
T H E im m ediate incentive f o r such a step on th e ir p a r t undoubtedly will be a desire to re
duce production costs. Eventually, however, the la b o r situation, now a secondary con
sideration, will take first place. T o d a y there
is little surplus of h a n d loaders. As know l
edge of the machine spreads, the nu m b er of men willing to sweat out a living u n d e r old m ethods will steadily shrink. E m p lo y e rs w ho think they can m atch m achine-production costs with lower w age scales or with g r e a te r efficiency in hand loadin g will be driven to m echanization to recruit labor.
W I T H these changes and the concentration which is p a r t of a m o d ern m echanization p r o g r a m will come benefits n o t yet fully a p praised. M in e s will be in a position to com
pete with factories f o r the services of the second g e n e ra tio n o f workers, whose a le r t
ness and new viewpoints can contribute much to the industry. C o ncentration will open the way to a g r e a te r employment of equipment, and closer supervision of personnel should make mining safer.
T H I S E X P O S I T I O N o f the fu tu re of m echanization is by no means new. But it is no longer a thesis merely o f possibilities:*.
T h e g ro w th already reco rd ed in the p a s t few years and developm ents ju st a ro u n d the corner have advanced possibilities to p r o b a bilities— tinged with the shadow o f the inevitable.
WHY WILDWOOD?
+ A n Economic Preview
W
IL D W O O D , the new mine of the Butler Consolidated Coal Co., in Allegheny County, Pennsylvania, represents the efforts of our company to convert an unprofitable coal investment into one capable of producing substantial returns under present adverse condi
tions. Convinced that the solution of the industry’s problems lay not in government aid or regulation or in effecting consolidations of existing operations (the continued existence of some of which could not be eco
nomically justified), we determined upon the development of an operation which would apply m anufacturing methods to coal mining as the true solution.
W ith virgin acreage to develop, the engineers selected to plan and super
vise the work were not hampered by any necessity for adapting existing facilities to present or future needs or of recommending the charging off of large investments in equipment made obsolete by recent improve
ments in the industry. In their stud
ies and plans, particular stress was laid upon such factors as :
(1 ) T he most economical life of the property in order to determine annual capacity;
(2 ) Sources and types of labor available, and their relation to the problem of housing facilities and the extent to which mechanical loaders could be employed;
(3 ) N atural mining conditions and the types of cutting and loading m a
chines best adapted for efficient and economical o peration;
(4 ) Physical characteristics of the coal as affecting the equipment neces
sary to prepare it for available m ar
kets ; and
(5 ) T he possibility of multiple- sh ift operation in order to make the greatest possible use of necessary me
chanical devices.
From these studies grew the plan
of making Wildwood the first coal mine in America to be designed for 100 per cent mechanical operation.
Mechanical loading, particularly in the Thick Vein F reeport seam, raised the problem of cleaning on a large scale, since steady operation— so vital where there is a large investment in loading equipment— can be attained only through ability to m arket at competitive prices a coal of uniform quality, comparatively free from moisture, and carrying a minimum of impurities. A fte r careful study of available cleaning systems and actual tests on coal produced at Wildwood during the development period, the Peale-Davis pneumo-gravity process was selected as the one best suited to our operating conditions.
Many coal producers labor under the delusion that mechanization is a panacea for the ills of the industry.
Such is not the case. Mechanical loading doubtless affords opportu
nity for large reduction in production costs, but it also creates new prob
lems which m ust be solved before success is achieved. Problem s of op
erating personnel and of maintenance of machines become more vital. The problem o f m ultiple-shift operation likewise presents difficulties which must be overcome. Prevention of accidents grows more im portant, since we have greater concentration of our operating forces and a wide use of electrically driven machines.
Capital investment and the interest charges thereon, maintenance, and de
preciation charges make full-time op
eration imperative. Intensive prep
aration of coal is required. This is generally synonymous with some form of mechanical cleaning and thus means a large additional capital ex
penditure.
By C. F. HOSFORD, Jr.
President, B utler Consolidated Coal Co., Butler, Pa.
W hile mechanical loading at W ild
wood has had the limelight, the facilities provided for preparing W ildwood coal for m arket should be of even greater interest to both pro
ducers and consumers. Crushers have been installed to break down the en
tire output of the mine to any size required to meet prevailing m arket conditions. Provision also has been made for screening any size de
manded by the consumer and to pro
vide any m ixture or combination of sizes which may be required to ob
tain maximum efficiency in any boiler plant.
Exponents of coal-cleaning m eth
ods have stressed mainly the im por
tance of ash and sulphur reduction.
In our judgm ent, however, even more significance is to be attached to the maintenance of uniform quality as a factor in solving combustion prob
lems. W ithout mechanical cleaning, even the most efficient management cannot entirely eliminate the variable hum an factor and cannot, therefore, guarantee the maintenance of any given analysis. This maintenance of a constant quality is a consideration of first water if successful operation is to be had with the increasingly com
plex combustion units now being in
stalled.
To the engineer, to the coal oper
ator, to the combustion expert and to the consumer, Wildwood, we be
lieve, offers many interesting fea
tures. W e hope it points the way to a possible solution of the economic ills besetting so many operations in the past decade. How successful this approach to solution finally will pi'ove to be, time alone can tell.
May, 1930 — C O A L A G E 267
ENGINEERING OBJECTIVES
+ In Planning and D evelopm ent O f W ild w o o d M in e
By JAMES H. FLETCHER
A llen & Garcia Co.
Chicago, III.
Successful o p e ra tio n of a completely m echanized m ine requires m an a g e m e n t and close supervision; m an a g e m e n t t h a t is in w h o le -h e a rte d sym pathy with the u n d e r
tak in g an d able to inspire subordinates w ith the idea t h a t coal m ining is passing fro m the a r t as practiced by th e ir fa th e rs.
T h e m iner t h a t is willing o r able to p u t in a h a n d cutting in o r d e r to produce lump coal is a m em ory.
D
E S IG N of a complete mine which would justify the ex penditure under present conditions in the coal industry and liquidate a frozen asset was the prob
lem confronting Allen & Garcia Co.
in the development of the W ildwood property of the B utler Consolidated Coal Co. In the spring of 1927, C. F . H osford, Jr., president of the coal company, and his associates asked fo r a report and estimate of costs on a completely mechanized operation to recover 40,000,000 tons of coal of the Thick F reeport seam in a tract located in Allegheny County, Pennsylvania, 15 miles north of P itts
burgh. T he report was presented in June, 1927, and in A pril of the fol
lowing year a contract was awarded to build and develop a mine on a progress schedule calling for comple
tion in 30 months. W ildwood is the result.
M aps, drill logs and coal cores were available, sufficient to establish the quality, seam characteristics, and coal contours. All natural conditions of the coal seam were carefully con
sidered. The property has an approx
imate width north and south of two miles, and length east and west of four miles, w ith a coal dipping from east to west 1 to 1^ per cent, and coal face north 25 deg. east. T he main body of the coal is 7 to 8 ft. thick, w ith an area of lesser thickness along the north and south boundaries.
Several plant sites w ere under consideration, each w ith advantages sufficient to w arrant careful field in vestigation. A plant located at the Wildwood station of the Baltim ore &
Ohio R.R. would have the coal seam 125 ft. below the valley at the ju nc
tion of Willow and P ine creeks, be directly on the railroad and on an improved highway, with several
near-by attractive villages capable of absorbing the employees required at the property, and would be advan
tageously located for underground haulage and drainage. I t was decided that this location should be recom mended, although extensive creek change would be necessary to provide the required building area.
T he excellent quality of properly prepared Thick F reeport is well known. A large part of the field is controlled by steel and power compa
nies operating captive mines. A close check on conditions that could be ex pected from a new operation were available and every courtesy shown by operating companies in perm itting inspection of their properties in the same deposit. T he seam contains a 12- to 14-in. central band of im puri
ties, the bottom 6 in. of homogeneous bone, and top 6 to 8 in. laminated streaks of bone and coal. W hile this constitutes the m ajor im purities in the seam, the slight ash variation p er
missible in high-grade coal has been found difficult to control by the usual methods of hand cleaning.
W ith coal the thickness of this seam the use of heavy equipment is
268 C O A L A G E — Vol.35,No.5
feasible and advantageous. A fte r much study, it was decided to adopt a system of m ining and milling, equipping the underground fo r mass production and the surface plant for mechanical cleaning. T he size of the plant was placed at a minimum of 4,000 tons of clean coal in eight hours, w ith an average life of forty years on single-shift and twenty years on double-shift operation. The plan adopted calls for the recovery of pil
lars.
In designing the plant it was the aim to make it possible for each em
ployee to accomplish the maximum by use of brains in place of brawn.
T he plant was visualized as a straight flow of coal from working face to railroad cars, with machines doing the drudgery under supervision of trained men. Prelim inary calculations established the area to be maintained on advancing rooms and retreating pillars to meet tonnage requirem ents.
Sketches on the coal-seam contour map gave a general idea of the most advantageous main entry system lead
ing to the plant site selected, perm it
ting blocking out the top plant in conjunction with underground devel
opment and surface contours.
W eighing first cost against operat
ing advantages determ ined the general plan of rotary dump on mine bottom, an 18-deg. slope containing belt for 750 tons of coal per hour and tracks for handling equipment and material, a vertical exhaust shaft and mine fan, tipple and cleaning plant unit, two buildings to house other requirements of the plant necessary to locate at railroad track elevation, and the mine office.
C arrying charges on operation of this magnitude, plus the limited time to be in full operation, dictated instal
lation of tem porary equipment to proceed w ith underground develop
ment at the earliest moment. The main lines of the B. & O. pass north and south through the property, and surface contours were such th at it was advisable to parallel them on the
west with the empty and load storage tracks. Location of the shaft ad ja
cent to the empty storage north of the main plant site perm itted utilizing the perm anent tracks for mine develop
ment and separated the surface and mine construction activities. T he tip
ple location was adjusted according to requirem ents of the railroad empty and load storage and the slope portal, with cleaning plant immediately to the- west, bath and lamp house close to the mine portal, shop buildings and storeroom w ith space for mine sto r
age tracks and accessible to trucking supplies, office at the highway eleva
tion adjacent to the local trad e bins, with full view of the mine surface operation and convenient to the mine portal.
As engineers, we were confronted with many pioneering problems pecu
liar to the mine designed for complete mechanical loading, and cleaning one of the most difficult coal seams in a highly competitive field, in a rela
tively brief period. In the room and pillar territory it was desirable to maintain equal length rooms and pil
lars, pillar protection on retreat, standard turnouts for heavy equip
ment, maximum number of rooms on the same entry, minimum num ber of
power load centers, e q u a l' length crosscuts, a direct sweep of fresh air across the break line, and m aintain a minimum depleted area. T he m ax
imum cover of 350 feet and overlying strata were taken into consideration and it is believed the objectives are embodied in the room and pillar plan adopted.
The system of main entries, split
ting the property from east to west, perm its blocking out development that will give a new mine on recovery of each two to three million tons lying between the main entries and p rop
erty boundaries. W ith the exception of the first block, which will be mined on the advance to the north property line, the acreage will be depleted on the retreating system, w ith provisions for a flow of air through regulators
across the mined-out and depleted territory to the exhaust shaft.
In an operation using only day- labor employees, it is necessary to obtain the maximum efficiency of equipment installed. T his required minimum movement of equipment and concentration of several hundred horsepower direct current in limited areas with a rapidly shifting load cen
ter. T he standard method of p er
manently located pow er-generating stations required a prohibitive num ber of large copper feeder wires. The problem was solved by designing a portable substation.
Tests dem onstrated cutting of the kerf in the top section of the band of im purities located near the center of the seam would free a percentage of the coal and perm it removing the bone sections in large blocks. T he lowest m aintenance and large tonnage pro duction could be expected only by preparing the coal suitable for load
ing and not requiring digging from standard shots. T his could be ac
complished by center cutting and shearing each room and entry. The tu rre t type of cutting machine was selected and trackage was required in all places to the working face. M in
imum am ount of track laying was then required when using track- mounted loading machines.
Detail figures were compiled on the assumption that as a minimum te rri
tory would be open at one time, belt conveying would be the most eco
nomical method of handling coal from loading machines to the cleaning plant. B ut the detail work required in carrying a face on the room -and- pillar system in advancing and re
treating 9 ft. per day of single shift, with each room tracked for the cut
ting and loading equipment, prohib
ited the installation of belt conveyors in place of mine cars.
T he nearest approach to a belt sys
tem developed as a combination of portable, track-m ounted conveyors 18 ft. long. These are a multiple of the 9-ft. cutting and are to tran sfer the coal direct from the loading ma
chine into mine cars, handled in trains of sufficient capacity to hold the en
tire fall of coal. T he installation cost is permissible when operating with a loading machine norm ally re
quiring two locomotives to maintain car change. T he mine development plan has been projected with room necks and crosscuts on 72-ft. centers for uniform ity in trackage and power wiring, and the portable conveyors which have been designed for this mine can be installed.
N ot being necessary to weigh each
May, 1930 — C O A L A G E 269
car of coal as it comes from the mine, it was decided the mine bottom could be handled by one man. If it was not necessary at times to judge between rock and coal, it could be made en
tirely mechanical, and we undoubtedly neglected television. B efore laying out the slope bottom, the area was closely drilled and seam floor con
tours established. By careful plan
ning, extensive grading was avoided, while a full gravity bottom was ar
ranged with' no interference between incoming and outgoing trips. The rotary dump placed in the center of the V form ed by branches from the m aterial slope is outside of the main ventilating current. Loads enter from the east on three-quarters of one per cent downgrade to the car haul, pass through the rotary dump, and leave to the west empty storage on a one and one-half and one per cent grade.
The entire output to the belt ca
pacity of 750 tons per hour is han
dled by one man at the control board of the rotary dump. Cars passing in trips discharge coal into a 25-ton coal hopper feeding onto the belt going direct to the preparation plants, and by pressing a button a gate is opened which permits dum p
ing the occasional car of rock into a 50-ton assembling hopper to be fed onto the belt and discharge into a rock hopper at the surface when the belt is not in service for handling coal. T he entire quota of mine cars can be loaded during the night shift from development entries, stored on the main-line load haulage, and passed through the rotary dump without confusion.
Designing the cleaning plant re
quired an engineering study of the raw material and underground con
dition by all departm ents to deter
mine w hat was to be received, what was wanted as final product, and how the product was to be handled m e
chanically to best fit in with the whole project. Coal discharge by belt from the mine would have three classes of m ateria l: clean coal, coal with im purities and material of no value. Experience had dem onstrated
that loading machines did not spe
cialize in one-man lumps and there would be a large range in sizes. To meet the demands of a specialized m arket w arranted by the quality of the coal, close sizing and rem ixing were required.
T he splitting point between hand picking and mechanical cleaning was made at 4-in. size, the tonnage above 4 in. to l)e split over two picking tables, each of ex tra length and de
sign, to allow* adequate opportunity to pick and inspect the coal as it was loaded into railroad cars. T here was to be no necessity for the pickers to hesitate in removing a lump contain
ing some im purities on account of total loss to the company. Coal with im purities had to • pass to a crusher, then join the flow of minus 4 in.
entering the mechanical cleaning plant. T he lump m aterial of no value was to pass direct to the refuse.
The 4xf-in. rejects- for the cleaning plant would contain coal that had to be liberated by finer crushing and recle'aning. T he final results were to be only clean coal to the m arket, with minimum merchantable coal left in the mine or tran sferred to the spoil bank. To obtain knowledge that the above results were being obtained required installation of adequate sampling equipm ent and a chemical laboratory.
Cleaning Thick F reeport is recog
nized as a most difficult problem.
W hile we believed either the wet or dry process could be employed suc
cessfully, the sales organization felt th at in the particular m arkets to be served there would be less sales re sistance with a dry cleaned coal. The design of the cleaning plant was delayed until coal prepared and loaded according to the method to be followed, in norm al operation was available from development work at the W ildwood mine. Carload lots were passed through the test plant until sufficient data were accumulated to w arrant us in inform ing our clients we were prepared to proceed with the design of the cleaning plant now in operation at Wildwood.
T he material slope perm itted han
dling all m ajor repairs and overhaul
ing of equipment at a surface shop where equipment and light would be available to do a first-class job at lower cost than it could be done in the mine. T he equipment to be maintained would require machine and electrical work as well as black
smith service, with parts from the
■supply room. By placing these four departm ents under one roof, the greatest operating efficiency was pos
sible and lower initial construction costs were achieved.
Successful operation of a com
pletely mechanized mine depends on the hearty co-operation of employees.
As the w orkers would be required to travel some distance, it was believed they w ere entitled to adequate pro
visions for changing clothes and a bath. T he grouping of change room, bath, lamp room, first aid, mine m an
ager’s room, safety engineers, in
struction room, and heating plant under one roof, with the bulding close to the entrance of the mine, perm it
ted economical construction, adequate accommodations for each division, and fitted into the ground plan.
W hile accommodations provided un
der this roof are om itted at many mines, we feel large indirect returns are received.
T he equipm ent was to comply with the laws and regulations of the State of Pennsylvania. E very electrically operated unit underground th at would o r m ight leave the main-line haulage was to be equipped with gas
proof permissible m otors and con
trol. T he type of equipm ent and m aterial best suited fo r the problem at this particular mine were specified and where obtainable from more than one source, were subm itted for competitive bids, and contract let in accordance with the best interests of the owners.
270 C O A L A G E — Vol.35,No.5
MACHINE CONSIDERATIONS
+ Determ íne U nderground Planning A t W ild w o o d
T
H O U G H the underground layout and methods of working at Wildwood are those of room and pillar mining, as a whole they have no counterpart in any other coal- producing plant in the country. The layout is different, because complete mechanization, upon which the op
erating program is based, demands change from the orthodox. Depend
ent w orking methods are different for the same reason. N either the en
gineers in charge of construction and management, nor the holding com
pany has allowed whims or fancies to guide decisions. T he ideas incor
porated in the plan which at first may appear to be new or radical are merely tried and proved ideas applied in a different way.
T he great degree of concentration encompassed within this mine is un
matched by any other plant where m ining is by rooms and pillars. Con
versely, the mine is different from others chiefly with respect to those factors that contribute to or are in
fluenced by concentration. B ut 89 working places, in a single working section with an over-all area of, roughly, 80 acres, are planned to pro
duce nearly 5,000 tons of coal in eight hours.
In the schematic sketch, Fig. 1, are presented the pertinent factors which influenced the mine layout. This sketch shows the directions and rela
tions of the dip, the coal cleats and the entries with respect to each other and with respect to the general outline of the property. T he tract is rec
tangular in general outline and meas
ures approxim ately 2x4 miles, with the long axis running east and west.
The seam being mined is the Thick Freeport, which is 7 to 8 ft. thick over the main body of the tract. It
By ALPHONSE F. BROSKY
outcrops at the eastern extrem ity of the property and dips generally west
ward on a prevailing gradient of 1 to 1^ per cent. T he cover for the most part varies in thickness between 200 and 350 ft.
Location of the plant at the W ild
wood station, on the Baltim ore &
Ohio Railroad, is more favorable to.
the underground layout than would have been any other possible site.
W ith the plant at this point, the direc
tion of the main entry is along the long axis of the property and the grade in main haulage is in favor of loaded cars, as is also, to a large de
gree, the grade of room entries.
T hrough a fortunate combination of circumstances—which has been taken advantage of— the butt cleat of the coal and the strike of the bed are practically parallel. Face entries and rooms, consequently, are generally level. F urtherm ore, as the prim ary
Associate E ditor, Coal A g e
direction of development and extrac
tion is up the rise, or eastward, d rain age is facilitated and w ater will not generally accumulate at the faces.
Incidentally, with the plant located at Wildwood, advantage is taken of a geological factor, in the erosion which form ed Pine Creek Valley and al
lowed the sinking of a slope through only 125 ft. of cover. The construc
tion of this slope for the lifting of coal to the surface by a belt conveyor- and for the handling of m aterials and supplies enabled straight-line and continuous movement of the coal from face to railroad car. T he economic advantages of the chosen plant site with respect to the outside of the mine have been mentioned in a p re
vious article.
W hile the prim ary direction of coal extraction is eastward, with the slope Fig. 1— Schem atic S ketch Show ing Characteristics o f Coal Seam
and T h eir Relation to the Layout
r r
! Preparation i p /a n t\
P illa r fin e
£ x tra c te d re tre a tin g -/'
/
Mine section—/-*.
/ /
1,368ft: y z : * . ~ y
A / Á /F
G o a f-^ /^ y . ,-P rim a ry d ire c tio n o f
:ne~~-p l 7~~ ~f J ¡ mm,'n9 /s advancing
(6 0 ° to /m e T o f advance
h -4 ...— L_Í________________
M a in -e n try lin e
4 m iles -
--- _ j*1
M ay, 1930 — C O A L A G E 271
bottom as the base, recovery within a mine section will be on a full re
treat away from the boundary, as in
dicated within the inclosure A B C D in Fig. 1. T he adjacent mine section B C E F , on the advance side, will re
main untouched (except for the driv
ing of blocking-out entries),, until a fte r practically the whole of section A B C D has been mined out.
The layout which will be followed in m ining a section, such as A B C D , is reproduced in Fig. 2. It will be noted that in this plan, the section is shown as being mined advancing from south to north, instead of retreating from north to south in accordance with the intended standard. T his de
parture from the standard for the mine will be applied only to the ex
traction of one or perhaps two of the first sections to be mined. By this procedure the mine will be brought up to full production in the shortest time. Meanwhile, development can be pushed toward the boundary in preparation for mining rem aining sec
tions retreating tow ard the main entry.
Right-angled turns are avoided in the mine layout so fa r as possible.
Practically all rooms and entries are being turned at an angle of 65 deg.
at the juncture. Right-angled turns are used only in the driving of stub rooms and entries where the shape or restricted size of the area being tapped does not perm it the utilization of an easing turnout. All crosscuts, w hether between rooms or headings, are turned at an angle of 60 deg. to the line of advance.
The use of track-m ounted equip
ment for cutting, drilling, and loading the coal, governed the adoption of the off-the-perpendicular turnout. These machines are large and heavy and of long wheelbase, as also are the mine cars, and require easy tu rn s fo r quick and safe tran sfer from one place to another. T o illustrate the necessity for the wide-angled turnout, one of the tw o types of loading machines in use, the Oldroyd, m easures 57 ft. in length and weighs 30 tons.
E very place in the mine is laid off and driven on sights, including cross
cuts. T he reason for this practice should be readily apparent. M ining of the coal in the W ildwood property is considered simply a problem com
bining only two m ajor processes, ex
cavation and transportation. As such, the timing, sequence and rate of ex
traction m ust be regulated by engi
neering standards as precise as those applied to the removal of overburden and coal in a m odern stripping op
eration.
H eadings are driven 12 ft. wide, on 50-ft. centers. T he main entry, running east and west across the property, is on the six-heading system flanked by barriers 300 ft-, through.
M ain face entries are driven in a multiple of five headings protected by 150-ft. barriers. Room entries are driven in a multiple of four headings, all of which serve as intakes when room and pillar m ining commences.
T hree headings would be sufficient for ventilation p u rp o ses; four are driven to assure economy in entry driving by machines.
Intervals between crosscuts in the dividing pillar between intake and re
tu rn headings are made large to m in
imize the short-circuiting leakage of air from one to the other. O n the main entry these crosscuts are driven on centers of 360 f t . ; on face entries they are driven on 180-ft. centers.
All other crosscuts, both off rooms and headings, are turned on 72-ft.
centers.
By referring to Fig. 2, a good idea 'may be had of the plans for concen
tration which will enable the getting of nearly 5,000 tons from a single mine section. T his production, to gether with that coal which comes from m ajor development elsewhere in the mine, will meet the norm al shift capacity of the preparation plant. T he designed capacity of the plant is 750 tons an hour, or 6,000 tons of mine- run in eight hours.
T he width of a section from which the large block of tonnage will be gotten is roughly 1,500 ft. In the section, but five room entries will be completely or partly open at any one time. W hile pillars are being ex
tracted off that room entry nearest the goaf, rooms will be driven from the adjacent room entry and the three rem aining room entries will be in stepped stages of development. All pillars off the first entry and all rooms off the second entry will be worked sim ultaneously on m ining fronts parallel to the room -entry line.
A s th e pillar ir o n ^ A -A (see Fig. 2) retreats from the goaf and advances tow ard solid coal, the room fro nt B -B will advance toward the goaf, the re
F ig. 2— L ayout o f W o rkin g Places in a M ine Section W hich W ill Y ield N ea rly 5,000 T o n s a S h ift
kK r-ra ce h eadings on SO c e n te rs
These headings a re k e p t a d v a n c e d f a r enough a h e a d to accom m odate three p a n e / e n tr ie s u n d e r d e v e /o p m e n t - "
Shotholes A re Placed by a T ruck-M ounted D rilling M achine W hich I s Self-P ropelling
spective movements of the two fronts being so timed that they meet on the line C-C. Face entries, too, will be completed only as needed. By adher
ence to this schedule, neither places nor the track in them will stand idle fo r any appreciable time, if at all.
A compilation of the num ber of tons to be produced by each and by all of the places in a section in one working shift is presented in Table I. These tonnages are based on the taking of one cut from each place.
As indicated in Fig. 2, along the line A -A , an attem pt will be made to draw the pillars by open cuts. P ro tection is afforded machines and men by the projecting points of the pil
lars. It is believed that this method will be practicable because of the speedy rate of extraction made pos
sible by machine loading and because of the comparatively shallow cover (m axim um of 350 ft.) over the coal.
T he experience thus fa r in mechanical mining at Wildwood points to the probability that the rooms and pillars com prising a panel will be completely mined in 140 working shifts.
Above the Thick Freeport seam is the Conemaugh group of rocks, con
sisting of various shales, some lime
stone, and several sandstones of con
siderable aggregate thickness. O f these strata, that which is- most diffi
cult to break in the process of taking pillars is the M ahoning sandstone.
This stratum ranges in thickness from 20 to 35 ft. It is of coarse-grain tex tu re and hard. T he elevation of this sandstone above the coal is not constant. In places it replaces much or all of the so fter measures between it and the coal. Directly over the coal is 6 to 10 ft. of slate which, above
the Wildwood property, exhibits un
usual strength contrary to the usual experience of other mines in the field.
Below the coal are a 6-ft. bed of fire
clay and a 3-ft. stratum of limestone.
The waters in this mine, incidentally, are alkaline.
Little or no timbering will be re
quired in first mining, which repre
sents about 30 per cent of total extraction. T he few rooms that have already been driven— they are 18 ft.
wide— required practically no tim ber
ing. Even at turnouts, where the open roof span is comparatively wide, sup
ports have not been needed under normal conditions. Occasionally, however, an area of roof of subnor
mal strength is encountered. Such roof areas are usually supported by 70-lb. re-used rails, set as crossbars.
T he taking of pillars by open cuts Will require an unsupported span of roof 17 to 18 ft. wide between solid coal and props in the goaf area. Nine feet of this span will be taken up by the cut in the coal. T he open span of roof will be supported on the goaf side by three rows of tim bers on 3- to 4-ft. centers. As the cover over the Thick Freeport seam is made up of a considerable thickness of sandstone, pressure-relieving roof breaks occur only at intervals of hundreds of feet.
In pillar mining, therefore, standing stumps of coal or timbers cannot be abandoned in the goaf.
If the area in the wake of a retreat
ing pillar line is not cleared of these obstructions to free roof falls, the natural balance and other tendencies of the roof will be so disturbed as to cause trouble. Because of these con
ditions, it is more m anifestly neces
sary in this seam than in most others
Binc/er B o tto m coaf
Firec/ay
to move roof supports in step with the retreat of the pillar line. This has suggested to the management an investigation of the possibility of using mechanical supports.
As extraction approaches a cross
cut in a pillar, the support given to the roof by coal will be decreased.
T he last cut of the pillar between a crosscut and the goaf may have to be taken from the crosscut. I f this last cut is taken from the goaf side, the crosscut m ust first be timbered.
Owing to the occurrence of sand
stone strata in the cover, over the Thick F reeport seam, m ajor roof breaks do not take place frequently.
Generally the breaks come at inter
vals of 500 to 600 ft. A fte r the oc
currence of a break, relief from pres
sure naturally is experienced fo r a time. T hen the pressure gradually builds up again until relief is again obtained from the next break. In the W ildwood layout, the width of room panels, including headings, is 576 ft.
It is believed that falls will occur at intervals approxim ately equal to the width of the panels.
It will be noted by reference to Fig. 2 th at the room crosscuts are at an obtuse angle to the room headings.
These crosscuts are turned, as they are to provide a pronounced saw
tooth point on each room pillar for protection of men and machines in
F ig. 3— R epresentative Cross-Section o f the Coal Seam at IVildwood
<0
Bony coaf
Bone
Coal 30"to 36"
R o o f slate
Coal L am inated binder
with coat
Tabic I — E xpected Tonnage F rom One Scction in E ig h t H ours 20 R o o m s a d v a n c in g , IS ft. w id e = 51 to n s e a c h o r 1,020 to n s to ta l 19 C ro s s c u ts , 12 ft. w id e = 35 to n s e a c h o r 665 to n s to ta l 19 R o o m p i l l a r s r e t r e a ti n g , = 120 to n s e a c h o r 2,280 to n s to ta l 3 R o o m e n tr ie s , 7 p la c e s e a c h = 245 to n s e a c h o r 735 to n s to ta l 1 M a in f a c e e n tr y , 8 p la c e s e a c h = 280 to n s e a c h o r 280 to n s t o ta l
4,980 to n s to ta l
the open area between pillars. Owing to this divergence of directions of room crosscuts and headings, the greatest potential trouble in pillar draw ing would seem to lie in carrying the pillar line across the room head
ings. If m ajor breaks occur at in ter
vals of 500 to 600 ft., in accordance with the general experience in this field, it may be possible to control them and cause them to take place near the heading closest to the goaf.
T hus the pressure would be removed and no trouble encountered in taking the panel-entry pillars. The first pil
lar line is to be started under 200 ft.
of cover.
The face and butt cleats of the coal are well marked. In Fig. 3 is re
produced a representative section of the coal bed. T he occurrence of bone and bony-coal partings in the Thick Freeport seam causes this coal to be one of the most difficult to mine and prepare for an exacting m arket.
P rio r to the completion of the mechanical cleaning plant, which was put into full operation on A pril 21, the bone and bony coal in the middle of the bed had to be mined separately and dumped as refuse on the outside, in order that the shipped coal would meet the company’s m arketing speci
fications.
T his practice, though necessary, was naturally wasteful of coal re sources and labor. T he 6-in. band of bony coal contains about 60 per cent of merchantable coal recoverable by mechanical cleaning. A nd as face preparation involved shearing also, and as all cutting necessarily had to
be done at one time, clean-coal cut
tings from the shearing cut were removed with the refuse. T he aggre
gate of the cuttings disposed of as refuse amounted to 18 per cent of the tonnage produced. T his system of face preparation not only complir cated mining methods but required the employment of about 60 men for shoveling cuttings and for cleaning up during the machine-loading opera
tion in a daily mine output of 3,000 tons gross. Mechanical cleaning has eliminated these men.
T he cutting machine adopted as standard is the track-m ounted Old- royd which is equipped with a uni
versal cutter bar for both horizontal and vertical cutting. U nder the old system of cutting, this machine took two cuts horizontally and one ver
tically, each cut being 9 ft. deep. The first horizontal cut was made in the bony coal and the second in the bone band. T hen the cutter bar was turned and the shearing cut'w as made in the center of the place. One of these machines with a two-m an crew cut 7 to 9 places, or 200 to 300 tons, depending on w hether the places were headings or rooms.
In the method of face preparation now being followed, only one h ori
zontal and a shearing cut are taken.
T he cuttings are left for loading with the lump coal, and so all of the seam extracted goes to the preparation plant. T he horizontal kerf is made in the bony coal, which is easier to cut than the massive bone occurring directly beneath it. T he selection of this cutting horizon has a tw ofold ad
vantage : F irst, it sends the . bony coal to the preparation plant in a finely divided state, with im purities largely broken from clean particles of co a l; second, it leaves the bone for blasting and loading with the lower bench of coal. T his bone is massive and when removed by blasting it breaks up into large pieces which can be removed readily from the lump coal on the picking tables. F o r these two reasons, the cutting method re
moves a big burden from the cleaning plant.
Two types of loading machines, Joy and Oldroyd, are in use. Six of the first type and three of the second are now in operation. As is generally known, the Joy is a caterpillar mounted machine, while the O ldroyd stays on the track while loading coal.
The broad plan of mechanization adopted fo r this mine calls for the use of machines which operate en- T h is T ype o j M achine I s to Be Used in M ining
lÄ/irlp F nrpx—R n n mc m td P ill nr*
274 C O A L A G E — Vol.35, No.5'
tirelv on track, so fa r as feasible.
F o r this reason, the Oldroyds will be used in room mining and in pillar ex
traction as tonnage machines, and the Joys will be confined to development work. All machines have been con
fined almost entirely to development work thus far, as few rooms have been driven. F u rth er discussion of the mechanized mining operations apply strictly to production by the old method of face preparation, whereby cuttings were loaded by hand shovel
ing. A s the new method of face preparation was only recently insti
tuted, related perform ance data are not available. A shift is of eight hours’ length.
Each of the Joys was assigned ten places, and with a crew of 16 men, including three hand shovelers who loaded out the cuttings, produced 200 to 230 tons per shift, double-shifting.
T he crew proper was composed of the following men : One loading ma
chine operator and one helper; two cutters and one scraper; one driller:
one shotfirer; two trackm en and the half-tim e services of a third ; three hand shovelers ; two haulage men ; the half-tim e services of one man for pum ping and hanging brattice; and one foreman. Each of these m a
chines was served by one locomotive, one cutting machine and the half
time attendance of another, and one hand-held electric drill. One of these Joy machines established a record of driving a total of 171 lin.ft. of entry in three consecutive shifts.
Only one Oldroyd loader was in operation when these data were col
lected. This machine was assigned twenty places, and with a crew of 23 men, including 6 hand shovelers for loading out cuttings, produced 350 to 450 tons per shift, double-shifting.
The crew proper was composed of the following m en : One loading m a
chine operator and one helper; four cutters; two drillers; one shotfirer;
three trackm en; six hand shovelers;
three haulage m e n ; one m an for pumping and hanging b ra ttic e ; and one foreman. T his machine was served by two locomotives, two cut
ting machines and one truck - mounted drilling rig.
Fig. 4— Show ing Position o f Shotholes in H eadings and in Room s
H ead in g
This last machine is the Sullivan, type CD4, and is self-propelling.
T hree of these drills are now in
stalled. W ith inexperienced crews each has been drilling 75 to 100 holes per shift.
The placement of holes for blast
ing in rooms and in headings is illus
trated in Fig. 4. Drilling precedes cutting, and holes with a diam eter of 2 in. are drilled to a depth of 9 ft.
Each of the upper holes is charged with one to two sticks of duPont Monobel 9-A and each of the lower holes with three to five sticks of the same explosive. T he average charge per hole is 0.3 lb. of explosive, and blasting yields 8^- tons of coal per pound of explosive. All holes are drilled horizontal.
W hile the Wildwood mine is yet in the development stage, the results thus far achieved therein are favor
able to mechanized methods. Ground was broken for this plant July 5, 1928, and the first coal was mined Dec. 5, 1928. U p until April 1, 1930, 27 miles of headings and crosscuts had been driven. A t the end of this period the rate of development had reached 3 miles per month, and the daily mine output of merchantable coal about 3,000 tons. T he output per man at the plant per day was 12.1 tons inclusive of rejected cuttings.
The Caterpillar Type o f Loading M achine, A ccording to Plans, W ill Be Confined to N a rro w W o r k -
R oom
TRANSPORTATION
+ Geared to High-Speed Production A t W ild w o o d
T
r a n s p o r t a t i o n , of prim ary im portance in any mine, becomes a vital factor at a fully mechanized, highly concen
trated operation, such as the W ild
wood mine of the Butler Consolidated Coal Co. H ere, where a large amount of equipment m ust have access to the face at all times, the design of the transportation system m ust incorpo
rate every provision that will prevent loading machines, coal drills, cutting machines, gathering locomotives, and cars from interfering w ith each other and that will m aintain a steady and non-fluctuating flow of coal to the surface.
Steel mine cars carry the coal from the loading machines to the under
ground dum p at the slope bottom, where a belt conveyor takes over the job of putting it on the tipple. W hen the full production of 6,000 tons per sh ift of eight hours is attained, at the end of this year, eight cable-reel locomotives, seven storage-battery locomotives (w ith one extra battery ), and two haulage locomotives will be in constant use between the working places and the slope bottom. A ver
age length of haul for the gathering locomotives will be 1,100 ft., and each will handle 80 loads per shift, or a total of 400 tons o f run-of- mine product. T he 250 cars in use—
each holding an average of 5 tons will be dumped five times a shift.
The two haulage locomotives in use will each haul about 50 24-car trips per shift. W hen the average length of haul becomes greater than 1^
miles, the haulage locomotives will be run tandem, pulling trips of 4S cars.
M axim um length of the main-line haul at the end of the life of the property will be 10,000 ft. W hen
full development is reached, at the end of the present year, the average length of main-line haul will be about 4,000 ft.
As tw o-thirds of the daily produc
tion will be obtained from tw o room entries, one advancing and one re
treating, in which there will be a total, aside from the mine cars, of five loading machines, five coal drills, eight cutting machines, and ten gath
ering locomotives in continuous operation, track layout becomes an item of importance. O f greatest weight is the problem of supplying facilities for the speedy movement of cars to and from the loading machine.
In rooms, 30-lb. track in all openings, as in Fig. 1, to rem ain until the pil
lars are removed, supplies the answer.
T hus the track in the rooms and crosscuts serves the triple purpose of developing the panel, facilitating car movement, and transporting the coal mined in pillar robbing.
In entry driving, changing facilities are equally im portant if the machines are to be kept producing at a m axi
mum. W hen main entries are ad
vanced, track is not left in all the passageways, however. T he practice is to keep steel in two crosscuts back of the faces being worked, one of which is used by the gathering loco
motive for car changing and the other as a roadway for equipment which m ust go to the back entries. T rack also is necessary in all openings while driving room entries, though later a part of this is removed, as shown in Fig. 1.
Tw o gathering locomotives attend each of the Oldroyd loading machines to cut to a minimum time losses in changing. U nder present conditions, only one car can be loaded at a time, a fte r which it is pulled out by one locomotive and stored in a crosscut while the other locomotive places an
einpty under the loading conveyor.
T o enable the machines to load a com
plete trip at a time, it is proposed to use portable belt conveyors back of each of the loaders, which will em pty the coal into cars at a crosscut and al
low full trips to be loaded without changing.
A s loading out a face at W ildwood is a m atter of m inutes rath er than days, and as the cutting and drilling machines ordinarily m ust have im mediate access to the cleaned-up places to complete their alloted tasks w ithout delay, ordinary tracklaying has been supplanted by a method em
bodying the use of portable turnouts and portable track sections. Both are made of 30-lb. rail laid on steel ties. T he portable turnout, shown in Fig. 3. is built in two sections, both of which are approxim ately the same weight. Also, the two sections have one dimension not exceeding 6 ft.
3 in. Such division enables turnouts to be transported or handled w ith a minimum of effort, and the assembled unit can be placed in the center of a 12-ft. entry, clearing the rib on the curved side before a side-cut is made to start a crosscut.
Portable sections are furnished in lengths of 9 ft. for the straight sec
tions, approxim ately the advance of the coal face in one cut, while the curved sections are of different lengths and curvature to correspond to the angles at which crosscuts are turned. Rails are welded to the ties to prevent the tracklayer from tear
ing them apart, w ith consequent de
struction of the units. M ethods of installing the portable turno uts and track sections in driving main entries, room entries, and rooms are shown in F ig 4.
N aturally, the use of preconstructed sections precludes any haphazard method of tracklaying. Spads are set for driving all openings, and the turnouts are placed in die center of the entry, with the point of the frog
276 C O A L A G E — Vol.35, Xo.5
■72' /& ¥ .
A
...2 0 room s.1J 6S '...>h r/<?£?' / £ ) / IK
.XjT T T T T T T T t _ T T T T t ' T T ? T T " " " A laoaaanaonnnnnrt rnms// m -' 'onnananmonanaHtn&mTr
.
/ VA .&
w
C ' V y-sc
'crosscuts 'a d v a n c in g o o ' l b r a i i
..places_______________ y ^ Ib.rci,!-^ y , „ ^ -f /;
IQOrjlircjnpfÿçur sidinrr^-Z-'tr’TT //
'si'c/ing'^x
Fig. 1— Track Installation at the W ildw ood M ine 4 | in. back of the spad. On the
curve side, one or two curves are placed ahead of the frog and straight sections are added until the crosscut is broken through, when a turnout is laid in the entry ju st broken into as shown in Fig. 4. Curves with a radius of 38 ft. were adopted as standard to allow the mine cars, which have an extra long wheelbase, and other equipment to operate smoothly and with the least chance for wrecks.
Loaded and em pty trips operating over the main haulage roads are kept
separate until the room entries are reached by the use of two entries, as shown in Fig. 1. On roads over which outbound or loaded trips operate, 60-lb. rail is used. Inbound or empty trips travel through a sepa
rate entry over 40-lb. steel. The 60- lb. steel also is extended into the room entries where loaded trips stand and heavy machinery and loco
motives must run around the cars, as shown in Fig. 1. T o insure smooth operation over main haulage routes,
curves are put in with a radius of 150 ft.
Rail weights on both main and secondary haulage roads were dic
tated by the character of the traffic.
Thus, 60-lb. steel was picked to w ith
stand the stress generated by the operation of 15-ton haulage locomo
tives pulling loaded trips of 24 cars when operating singly or 48 cars when in tandem, while the absence of the coal load on the return trip per
mits the use of 40-lb. steel on the empty haulage routes. In rooms, ability to hold up under heavy equip
ment along with the m aximum of portability were the factors consid
ered in the choice of 30-lb. steel.
W ood ties are standard on all main haulage routes and under room entry track. W here the latter is concerned, though the life of track is short, wood ties are used under the 30-lb.
rail, because of the large quantity of coal that will pass over it before it is torn out. Main roads are graded where necessary for even operation of haulage equipment.
Gathering locomotives haul cars loaded by the machines to the loaded- car sidings shown in Fig. 1, where they are made into trips of 24 cars.
From there they are taken to the dump by the haulage locomotives.
Fig. 2— M ain Haulage R outes and Slope B ottom Plan
A irs h a ft-
T W e iF '2 -West
•¿-¡Vest
R o ta r y dump-
/}
office
WasMouse-pCO f 7'Right. 1/
6-RighK\ /
~ ± E l3 Ù tI L C
t-H o is t house
IfJ^ipripp/e
'Cleaning p ia n i
... . - ./¡’-¡Of... -...
7 Z ~6$ (Pol!m g w ith sp rin g s compressed)
‘IWheel housing
18 ]5l0%"
Portable S w itch Construction Fig.
H aulage locomotives use the ru n around track to get in fro nt of the loaded trips, and this track also is used by other equipment which finds it necessary to leave the room panel.
By using three tracks for load and empty storage and for run-around purposes, the fourth is reserved for the use of gathering locomotives and other equipment working in the rooms or on pillars.
T he slope bottom is designed for free access from either side of the mine. A s shown in Fig. 2, loads from both the east and west sides of the mine are dumped from the east side of the bottom and empties go out to the sections from the w'est.
Loaded trips from the east come di
rect to the dump on C -E ast entry.
Loads from the west side of the mine are hauled through B -W est entry to the east side of the dump, and are placed on C -E ast entry for dumping.
H aulage locomotives cut off on C -East entry and travel to the empty side of the dump through D -E ast and D -W est entries. Em pties for the east side of the mine are pulled through the cut-off entry to the west of the dum p and travel out 3-East entry. Em pties for the west side of
the mine travel directly back over 3-W est entry.
Cars are dumped at the bottom by a one-car ro tary clump with a capacity of 6,000 tons per shift. From the dump, the coal goes to a hopper hold
ing six cars, and by opening a gate in the chute, the rock goes to a sepa
rate bin of the same capacity. Both bins are equipped with feeders for loading the material on a belt con-
setting the brakes a necessity. Cars are controlled on th e bottom by a 30- ft. car feeder with a capacity of 50 loaded cars, equipped with a hydrau
lic brake for stopping the trip. In dum ping trips, two loaded cars al
ways are kept on the incoming side of the dump and incoming trips are coupled to them.
Coal is transported to the tipple by a 54-in., 12-ply rubber belt, 900 ft.
between centers of the head and tail sheaves. T he inclination of the con
veyor is 3 to 1, its capacity is 1,000 tons per hour, and it operates at a speed of 350 ft. per minute.
All-steel cars, of A. & G. design, built by the W a tt Car & W heel Co., w ith semi-automatic swivel couplers, spring d ra ft gear, and spiral spring suspension are used for transporting the coal from the loading points to the dump. O w ing to the fact that the coal at W ildw ood thins out around the edge of the property, the mine cars were designed with a height of 36 in., with an additional 12-in.
top extension for use where the coal
Fig. 4— Use o f Portable Turnouts and T rack Sections. L e ft, in M ain E n tries;
Center, in R oom E n tries; R ig h t, in R oom s veyor for transportation to the tipple.
The loaded track leading to the dump and the empty storage track are laid to grades of f , 1-| and 1 per cent, respectively, sufficient to help the car feeder but not enough to make
Fig. 5— Construction D etails, A ll-S te e l Car
is high. T rack gage is 42 in., and the capacity, level full with 12-in. top extension, is 4.66 tons. T he weight of the car w ithout load is 5,700 lb.
Principal dimensions are shown in Fig. 5. W hen low coal is encoun
tered, the extension can be removed by cutting a few rivets, making a car 36 in. high with a capacity level full of 3.08 tons.
Locomotive speeds and weights are as follows : M ancha storage battery (g a th e rin g )— weight, 9 tons;
speed, 4£ m.p.h. ; Goodman cable reel (g a th e rin g )—weight, 8 to n s; speed on trolley, 5 m .p.h.; W estinghouse haulage—weight, 15 to ns; speed, 9 m.p.h. Cable reel gathering locomo
tives are used in entry and room development but, as the Pennsylvania regulations governing operation of trolley locomotives in a gaseous mine precludes their use in pulling pillars, storage-battery locomotives will be used fo r this purpose exclusively.
278 C O A L A G E — Vol.35,No.5
NEW FEATURES IN
+ Electrical and M echanical Design A t W ild w o o d
By J. H. EDWARDS
Associate E ditor, Coal A g e
O
P E N IN G a brand new mine to load mechanically 6,000 tons per shift, and to separate this product on the surface into clean coal, and reject in the approxim ate ratio of five to one, presented an opportunity for considerable pioneering in electrical and mechanical de
sign. Accordingly, W ildwood em
bodies several new features. The underground conductors of 2,300- volt power are buried in trenches and installed in sections with plug- ging-in receptacles for convenience in making taps and extensions; in
side substations are truly portable;
an electrical differential replaces the conventional mechanical differential on the drive of the main-slope belt conveyor; and new principles have been utilized to substitute electricity satisfactorily in place of air for op
erating the rotary dump.
Installing 2 J0 0 -V o lt B anded-A rm or Cable in Trench D ug by Cutting M achine
Utility, safety, reliability, and economy of operation were weighed carefully in the entire electrical and mechanical design of the plant. On the outside, every conductor of the
Illustrating A ssem bly o f P lug B o x Betiveen Section P lu g s and o f Feeder P lug to Portable Substation
power, signal, and telephone sys
tems is carried in a trench or conduit, and the m otors and controllers of all inside equipment are of the ex plosion-tested type.
P ow er is purchased from the Duquesne Light Co. at a present net cost of less than one cent per kilo
w att hour. T his unit cost will de
crease as the mine is developed to full capacity. A generating plant to utilize the bone coal as fuel was given serious consideration, but was ruled out in the final analysis. T he demand portion of the monthly power bill is computed from a measure of the maximum kilovolt
amperes registered in a 15-minute interval.
A t the power company’s step- down station, which is within about
1,000 ft. of the tipple, the coal com
pany erected a small brick building containing a 2,300-volt main panel and three distributing panels. In ad
dition to oil switches with overload trips, the latter panels include m eter-
May, 1930 — C O A L A G E 279
