Tests on the cutting performance of acontinuous miner

by F. F. ROXBOROUGH*, B.Sc., Ph.D., F.I. Min. E., M. Aus. I.M."".,P. KINGt, B.Sc. (Hon.), F.I. Min. E., F.S.A.I.M.M, C. Eng., andE.J. PEDRONCELLI:j:, B.Sc., M.S.A.I.M.M.

SYNOPSISThis paper describes tests on the performance of a continuous miner in cutting South African coal. Although coal

cutting has been investigated extensively overseas, the results have not led to significant changes in machine designbecause these machines find little difficulty in cutting the softer overseas coals at a fast rate.

In general, the results of the tests show that cutting efficiency improves markedly as the depth of cut per revolutionof the drum increases, and that the best spacing of picks on the drum is twice the depth of cut. Pick shape is of secon-dary importance in terms of cutting efficiency, only small differences having been found between the performances ofconical- and chisel-shaped tools.

SAMEVATTINGHierdie referaat beskryf toetse in verband met die werkverrigting van 'n deurlopende myner vir die sny van

Suid-Afrikaanse steenkool. HoeNel die sny van steenkool op groot skaal in die buiteland ondersoek is, het dieresultate nie tot beduidende veranderinge in die ontwerp van die masjien gelei nie omdat hierdie masjiene dit niemoeilik vind om die sagter oorsese steenkool teen 'n vinnige tempo te sny nie.

Die resultate van hierdie toetse toon oor die algemeen dat die snyrendement merkbaar verbeter namate diediepte van die sny per omwenteling van die trommel toeneem, en dat die beste spasiering van die pikke aan dietrommel twee maal die diepte van die sny is. Die vorm van die pikke is van sekondere belang wat die snyrendementbetref, aangesien daar net geringe verskille tUssen die werkverrigting van keelvormige en beitelvormige pikkegevind is.

Introduction

One feature common to all mechanical methods of coalwinning, whether involving a hand pick, a coal drill, or acutting machine, is that each depends for its action onthe penetration of a wedge of some shape or form intothe coal face. In the case of a hand pick, it is a singlewedge being repeatedly struck at the face, the force,frequency, and position of the blow being left to theexperience and judgement of the mineworker. Theintelligence of the miner and his inherent physicalflexibility provided him with additional variables in theuse of his wedge. It is certain that no mining machine isas efficient as a man in terms of the coal producedper unit of work done1.

However, mining machines can concentrate vastlymore power in the confines of a coal face than can beobtained from manpower. A machine can deploy alarge number of high-powered, fast-moving wedges toattack the coal, cutting prodigiously, but doing so in a'non-thinking' repetitive fashion unresponsive to the typeof opportunity for ease of extraction that could so ablybe recognized and exploited by a coal hewer.

From the earliest intoduction of coal-cuttingmachines, their efficiency has left considerable room forimprovement. However, this was not highlighted untilefforts were made to broaden the application of the firstmechanized longwall installations in Europe by the useof machines such as ploughs and shearers. In particular,the introduction of the German-invented coal plough intoBritish mines after World War II revealed serious

*School of Mining Engineering, University of New South Wales,Australia.

tGold Fields of South Africa Limited, P.G. Box 1167, Johannes-burg 2000.

tChamber of Mines Research Laboratories, Carlow Road,Emmerentia, Johannesburg 2195.

~1981.

inadequacies in its ability to handle the generallystronger British coals. 2

Reports on the coal-cutting and associated workundertaken by the Mining Research Establishment ofthe National Coal Board (N.C.B.) have been publishedwidely over the years. Perhaps the most important singleaccount of the research is the monograph by Evans andPomeroy3. However, there are several other key publica-tions, each of which represents a significant contributionto our knowledge of the mechanics of coal breakage bypick. Evans's model of wedge penetration into coalprovides a good theoretical understanding of the effectsof coal strength, depth of cut, and pick geometry on theforces required to cut4, and the results of Pomeroy'slaboratory coal-cutting experiments established certainprinciples that are claimed to be fundamental to thedesign of an efficient coal-cutting system 5,6.

Despite the widespread availability of such usefulinformation, surprisingly little practical use has beenmade of it. The N.C.B. designed a large pick drum forlongwall shearers based on these principles 7, but ithad only limited application and success. Similarly,parallel work undertaken at The University of New-castle- upon -Tyne in England 8 led to the developmentof a novel type of longwall plough 9, which also hadlimited but, in this case fairly successful, application 10.

The main, but by no means only, reason for thelimited application of known principles is that modernlongwall machines, particularly the shearer, have beenmade progressively more powerful and have therebyfound little difficulty in cutting coal, even if ineffi-ciently, at a much faster rate than can be loaded ontoand carried away by the conveyor. Cutting has thereforebecome of secondary importance to the loading functionof the machine. Since the drum of a shearer has thedual and simultaneous roles of cutting and loading,

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1981 9

designs have tended to cOncentrate on loading efficiency,which has requirements that are, from several stand-points, in conflict with the maximization of cuttingefficiency. This dichotomy remains largely unresolved.The increasing levels of output and production raterequired from expensive longwall installations, beingachieved by an increase in the size and power of theequipment rather than by an improvement in its effi-ciency, are often at the expense of high coal degradationand unacceptably high airborne dust levels at the face.

In South Africa, progress in the application of fullymechanized continuous mining systems will depend ontheir ability to cut at maximum efficiency. Here, thehigh strength and abrasiveness of the coals are majorfactors inhibiting the use of full face extraction bymachine. Equipment designed overseas, principally foruse in the D.S.A. and Europe, where coal seams aregenerally much weaker than here, is usually technicallyand economically inferior, in terms of productivity, toconventional South African working methods, whichmake use of explosives to break the coal.

In the medium term at least, the continuous miner isone of the most appropriate machines for South Africa'spredominantly bord-and-pillar coal mines, since it doesnot involve any major change in the traditional methodof mining. The conventional cyclic operation of drill,cut, blast, and load is replaced by the continuous miner,which is able to undertake the equivalent of all of thesefunctions simultaneously. To be able to deal with hardcoal, however, and to improve significantly on the per-formance of conventional methods, the continuous minerhas to be used to its maximum potential. In this context,one of the most important considerations is to establish,In the field as well as in the laboratory, criteria for thedesign of cutting drums so that the available power ofthe machine is directed towards achieving the maximumpossible cutting and production rate consistent withminimum downtime and machine maintenance require-ments.

Experience with the 34 continuous miners operatingin South Africa in December 1978 showed that, on average,40 per cent of the total downtime for these machineswas due to mechanical failures associated with cuttingll.This is double the figure for continuous miners in theD.S.A.12. In addition, the instantaneous cutting ratefor the South African machines was appreciably lowerthan that found either in the D.S.A. or Australia. Thesefacts bear testimony to the difficult cutting conditions

e =breakout'" =rake

-: w !- angle'"

angle direction

~Ie ~ of cutting

. . d "'. P=clearance

'", anglerc~

FN

~

rlj ~-distance cuI (I)

Fig. I-Notation and definitions for coal cutting by pick

10 JANUARY 1981 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

prevailing in South African coal mines, and emphasizethe urgency of finding ways of improving the applica-bility, performance, and reliability of continuous minersin this country.

To achieve this general objective, the Chamber ofMines Research Organisation has embarked on a majorresearch programme on the assessment of coal cuttingand the cuttability of coal seams. One of the majorcomponents of this programme is the evaluation, in themine and using an actual continuous miningm achine,of those principles determined mainly in the laboratoryby Evans, Pomeroy, and others4-6. As far as is known,this is the first time that a full-scale controlled fieldinvestigation of the principal variables involved in thedesign and application of a machine cutting drum hasbeen undertaken. The project was much encouragedby the experience with a similar experimental continuousminer in the D.S.A. that was used in an assessmentof the validity of the deep-cut principle13 - one of theconcepts originally proposed by Pomeroy.

BASIC MECHANICS OF COAL CUTTING

When a pick cuts across the surface of a block of coal,it is seen always to produce a groove that is much widerthan the width of the pick. Also, the depth of the grooveis sometimes greater than the depth of the pick. Asillustrated by Fig. I, the excess lateral breakage occursas sidesplay and, although its surfaces are usuallyirregular, it can be represented by an equivalent angle ofinclination to the vertical. This is termed the 'breakoutangle', and for a given coal it remains fairly constant forall depths of cut. The production of excess coal is vari-able and depends mainly on the direction of cutting inrelation to the cleat and bedding of the coal, pick shapealso being of some significance in this context.

The cutting of coal is characterized by a rapid linearincrease in the force acting on the pick as it penetrates.Eventually this force exceeds the strength of the coal,and a coal fragment or chip is produced with an attendantand instantaneous reduction of pick force. The coal chipextends ahead of the pick, the latter then advancingunder zero or negligible force until it re-engages a freshcoal surface, after which the chip-formation process isrepeated. This leads to a typical saw-tooth shape offorce-distance diagram for a pick, as shown in Fig. I,with peaks that are irregular in magnitude and frequencyowing to the heterogeneous nature of coal. Althoughcoal is known to have time-dependent stress-strainproperties, these are of no practical significance whencutting, even at the slowest speeds. From this stand-point, coal can be regarded as a brittle material. Evans'smodel provides a valuable analytical insight into themechanics of coal-chip formation by a wedge4.

The recognition that breakout angle 8 remains con-stant with depth of cut is significant, since it leads towhat is probably the most important fundamentalprinciple of coal cutting. Evans's theory, which is fullyconsistent with laboratory experience, shows that thecutting force acting on a pick is linearly proportionalto the depth of cut. A low specific energy (Le., the workdone or energy consumed to produce unit volume ormass of coal) implies a high efficiency. Also, as shown

components: Fe the cutting force that acts in the ~direction of cutting, and FN the normal force acting -dperpendicular to the direction of cutting. The mean - -

cutting force ~Fc ~u~tiplie~. by the ~i~tance :;ut giv,es i. s

ick at a constant interaction:)le displacement in -

t contribute to the- ~ S = 2d tan e

ng "linecoal. 1.Jepenalllg on pICK snape :-. - p-ctors, a sideways or lateral force can

I Ipicks too

rated. When it does exist, it is always closeand can therefore safely be ignored ,

dbting depth on the work done to cut - ,

.-!-ionship to the quantity of coal pro- it'1ws. Fig. 2-lnteraction of cutter picks in arrayt coal = Fc.l = KI.d.l, . (1)re Fe = mean cutting force

d = depth of cut >. dl = distance cut ~ ~KI = a constant (depending on ~ 2d

the shape of the pick and (A) ~ ~: /' 3d

u- -- --- "..-, -- ---u-r' - ,-,where 8 = breakout angle

W = pick width.Since, for a given situation, pick width, W, and break-

out angle, tan 8, are both constants (say K2 and K:respectively), the combination of equations (1) and (2:gives

S .fiF c.l

pecI c energy = (W.d+d2.tan8)l

- KI.d- d(K2+Ka.d)

=K~do(3:

where Ko = Kl/K2and K = Ka/ K2.

Equation (3) indicates that specific energy decreases(i.e., the efficiency improves) as the depth of cutincreases. It also shows that as d tends to zero, thespecific energy approaches a maximum finite valmof Ko/K.

These considerations relate to a single pick cuttingunrelieved; a mining machine uses an array of picksdisposed on some form of cutter head, drum, or jil:in which the picks are required to interact. The effectof spacing between picks on their cutting efficiency isimportant.

If two adjacent picks in the array shown in Fig. 2are placed a large distance apart, 8, they cannot interact,and, effectively operating as unrelieved cutters, they

- - -- -

~ - -reached at which they start to interact, the groove cutby the leading pick providing relief for the followingpick. It is found that such relief will cause a reduc-tion in the specific energy requirements for the followingpick. If the spacing between picks is further reduced, the

~l ~spacing

~~ 2:(B)

1- -

~~

spaci ng I dept h of cutFig. 3-Hypothetical relationship between pick spacing and

specific energy

specific energy will continue to fall but not indefinitelyso. Indeed, as the spacing tends to zero, the depth ofcut of the following pick tends to zero since it is thencutting exactly in the 'shadow' of the leading pick.At that position, the specific energy is at a maximum,as indicated by equation (3). The effect of spacing onspecific energy therefore, as shown in Fig. 3A, indicatesa value of spacing at which the specific energy can beexpected to be minimized. In fact, a family of curves canbe drawn, each showing a minimum specific energy for adifferent depth of cut and consistent with the generallevel of specific energy, being lower at the high depthsof cut; also, that a wider pick spacing is appropriatewhen the depth of cut is larger.

If, as indicated in Fig. 2, the interaction starts whenadjacent grooves just touch (i,e., 8 = 2d.tan8), thegeometrical similarity implicit in the family of curves

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 198' 11

In Fig. 3A can be normaIlzed it the spacIng,s, IS dividedby the depth of cut, d. Now, on the basis of spacingexpressed as a multiple of cutting depth, interactionbetween the grooves will occur at 2 tan6, which is thesame for all depths of cut. Similarly, if geometricalsimilarity persists, the BId ratio at which the specificenergy is minimized will be the same for all depths ofcut. Fig. 3B shows how specific energy is expected tovary with BId ratio, the value of the minimum specificenergy reducing at the higher cutting depths.

THE EXPERIMENTAL CONTINUOUS MINER

Since the continuous miner has a drum that is usedexclusively for cutting, it is probably the most suitableof all production machines to feature in coal-cuttingexperiments. For the same reason, it is the machinewhose performance is likely to be most influenced bychanges in the design of its cutting system.

The following objectives were chosen for the experi-mental work conducted by the Chamber of Mines:

(i) the establishment of the cutting principles thatcan be incorporated into current machine designswithout need for major modification,

(ii) the evaluation, under practical operating con-ditions, of the benefits that will accrue from theapplication of those principles,

(iii) a determination of whether any improvement thatfollows can be sustained in practice and be ofsufficient magnitude to significantly improve the

performance of the present generation of con-tinuous miners in South Africa,

(iv) the provision of field data and other appropriateinformation relevant to the design of futuremachines and components for South Africanconditions,

(v) the establishment of a yardstick of performanceagainst which associated laboratory cutting testscan be related and compared, and the provisionof a similar link with techniques being developedconcurrently for the assessment of seam cut-tability.

For these controlled field experiments, the Chamber ofMines purchased a Lee-Norse HH 456 (Fig. 4), which isan adaptation of the conventional HH 455 productionmodel. The selection was influenced by the performanceof the HH 456 in the U.S.A., where it was used success-fully for research on the generation of airborne dustduring cutting and on the deep-cut principle in thiscontext13.

Several modifications were made to the machine,which is shown schematically in Fig. 5.

(i) Provision of higher and variable Bumping force.This was achieved by an integrated hydraulic'goalpost' anchor structure at the rear of themachine. When jacked between roof and floor,the machine can be thrust forward from it by theuse of two hydraulic rams. These provide amuch greater sumping force than can be obtainedfrom the crawlers, which are now allowed to

12 JANUARY 1981

Fig. 4-A general view of the H H 456

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

'free-wheel'. A wide range of sum ping forces canbe generated by variation of the hydraulicsupply to the rams, and this enables the sumpingrate andc orresponding depth of pick penetrationper drum revolution to be controlled and varied.

(ii) Enhanced shearing force. After generating therequired depth of sumping, which is normallydone at roof level, a conventional productionmachine shears the face by hydraulic rams pullingthe cutting boom down. To increase the magni-tude of the available shearing force, a shear-assist assembly was mounted above the boom,consisting of a heavy cantilevered arm connectedto the boom by a double-acting hydraulic ram.When activated, the ram forces the arm to theroof and thereafter provides a downward forceon the boom additional to the normal arrange-ment. Hydraulic flow and pressure to the totalsystem are controlled so that varying rates ofshearing and depth of cut per drum revolutioncan be achieved.

(iii) Variation of cutting-drum speed. The rotationalspeed of the cutting drum can be varied by thechanging of gear boxes. Each of the two drivetrains between the motor and the cutting drumincludes four gear boxes, three of which can beinterchanged or replaced to provide differentgear ratios. In addition to the normal productionspeed of 66 r/min, four other speeds of approxi-mately 42, 35, 15, and 7 r/min are available.

(iv) Pick spacing pattern. A specially modified drummakes provision for variations in the lateral andradial spacing of picks in the cutting array. Theposition and number of detachable pick boxescan be changed to give 4 levels of lateral spacing(50, 100, 150, and 200 mm) using picks of eitherchisel or conical profile. The centre line of a pickbox makes an angle of 36,5° with the drum radiusat the pick tip. The type and location of gaugecutters were not varied but remained the sameas for a normal production machine. The role ofthe gauge cutter is somewhat different from thatof the line cutter. Because they account for notmore than 9 per cent of the coal cut by the drum,it was considered preferable to maintain con-stant gauge-cutting energy and vary only themain pick array, rather than to attempt anassessment of the effects of different types anddispositions of gauge picks.

shearheight

Fig. S-Schematic diagram of the experimental continuousminer

The machine, a frontal view of which is shown inFig. 6, has the following general specifications:

Mass, kg . 55 600Maximum/minimum cutting height, m . 3,48/2,12Cutting drum width, m . 3,15Cutting drum diameter, m . 1,13Maximum drum torque, Nm . 162 700Nominal drum speed (production), r/min 66,0Nominal drum speeds (experimental),

r/min . . 41,734,514,77,0

Nominal pick tip speeds (experimental),m/s . 2,47

2,040,870,41

50100150200

. 2 X 224

. 224

.248

Line pick spacings, mm .

Cutter motors, kWPump motor, kWGround pressure, kN /m 2

INSTRUMENTATION AND DATA RECORDING

An initial 'shake-down' period, during which theHH 456 was used as a normal production machine,proved to be invaluable in indicating the type ofinstrumentation and the method of data recording to beused, and in the planning of the experimental programme.

As a consequence of such experience, the machine wasplaced in a dedicated heading under strictly controlledoperating conditions, so that the effects of a change inanyone variable at a time could be evaluated with agood degree of confidence.

Continuous Remote Recording

Electrical signals from various instruments placed onthe machine were fed to chart recorders located in aremote recording station. Several quantities weremeasured and recorded for control purposes; those pro-viding information on machine performance were asfollows.

(i) Cutting-drum revolutions - from a tachometergiving a voltage output proportional to the drumspeed.

(ii) Thrust and shearing velocity - from a tachometeractivated by a wire attached to the shear-assistarm via the cutting boom.

(iii) Cutting-drum power - from a power transducergiving an analogue trace at the recording station.

(iv) Thrust pressure - by remote recording, from theoutput of a pressure transducer, of the pressurein the cylinders thrusting from the goal-postanchor frame.

(v) Shear pressure - from a pressure transducer withoutput fed to the remote station for recordingthe pressure in the shear-assist cylinder and thenormal boom cylinders (which were the sameduring shearing).

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY "ANUARY 1961 13

Direct-reading Instrumentation

Several instruments were placed on the machine toprovide the operator with immediate control and opera-tional information for spot checks. These includedmotor ammeters on the cutting head, a voltmeter, anhour meter on the cutting head, an inclinometer on thebody, and pressure gauges on all the main hydrauliccircuits.

Adjustable relief valves enabled pressures in thehydraulic circuits to be pre-set to provide the requiredforces for each test.

External Measurements

In addition to the several readings and recordingsobtained from the various instruments, the followingmanual measurements were made: seam height, finalsumping depth achieved, shearing distance, machine tiltangle (by use of the inclinometer), angle of thrustmechanism (goal-post thruster rams), horizontal distancebetween machine and nearest survey peg, time taken tosump, and time taken to shear.

In addition to these measurements, note was alsotaken during each test of the cutting-head operatinghours, the quantity of dust-suppression water consumed,and the extent of we:1r on the cutting picks. In addition,samples of coal were taken from selected cuts for deter-minations of size distribution and calculations of thedegree of coal fragmentation based on the coarsenessindex14. Since local variations might occur in the coallithology and strength over the distance covered by a

series of tests, samples representing the main verticalsubdivisions of the seam were recovered at regularintervals for the monitoring of such changes.

Calibration and Resolution of Machine Forces

The pressure transducers were calibrated to provide adirect rei:,ding of absolute thrust and shear forces by theuse of a hydraulic jack and load cell. For thrust calibra-tion, the jack and cell were placed between the faceand the cutter drum to provide a known and incremen-tally increasing horizontal force, and thereby a corres-ponding readout on the thrust-ram transducers. Similar-ly, a known vertical force was applied in increasingincrements to the drum and the response on the shear-pressure transducer was measured.

A complete analysis was also made of the distributionof the forces acting on the machine. When cutting in sumpor in shear, a continuous miner is subject to a number ofexternal forces, and their distribution, direction, andmagnitude influence the performance and stability ofthe machine. In addition, internal forces and momentsare generated during its operation, and their natureand relationships must be determined to provide a basisfor the analysis and interpretation of experimentaldata. This analysis, which is too lengthy to include here,was found to agree fairly well with the resolved com-ponents of force measured during calibration15.

The shear-assist assembly is shown in Fig. 6.Other quantities, such as sum ping and shearing rates,

cutting-hc:1d power, speed, and torque, were measurable

14 JANUARY 1981

Fig. 6 Shear-assist assembly showing relationship to cutter-head boom

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

(A) sump ..

J;~c instantaneous~depth of cut

\ .I- maximum6"'1 depth of cut

II

""Adrum position

after one rev In

~ = drum contac1

(B)Sh."\

J6/

",,< instantaneousdepth of cut

\\\

drum Position-> jrafter one revln. /

maximumdepth of cut

Fig. 7-Configuration of depth of cut and contact arc forsumping and shearing

to a high level of accuracy. By use of the characteristiccurves for the motors, in conjunction with the recordeddata, the power absorbed and the total work performedby thc head motors could bc detcrmined.

EXPERIMENTAL PROGRAMMEAlthough the experimental continuous miner is able

to operate at a number of different head rotationalspeeds, the experiments described here were all under-

taken at a head speed of 41,7 r/min. The results of testsmade at other speeds will be reported elsewhere.

Drum Desi~n and Operational Variables

The main variables and the levels at which each wasinvestigated in the experiment were as follows:

No. of

Variable levels Levels

Pick type 2 Chisel: ConicalPick spacing 4 50mm, 1O0mm, 150mm, 200mmDepth of cut * Range 0 to 120 mm

*The depth of cut taken by the machine cannot be predeterminedsince its value depends on the hydraulic pressure and supplyto the thrust and shear rams. It can be set to provide a roughdepth, but a precise value for the average depth of cut must becalculated. In this context, it has to be recognized also thatthe depth of cut taken by a pick varies along its arc of contactwith the coal face as illustrated by Fig. 7.

The pick geometry and the numbers used at each ofthelevels of spacing were as follows:Pick Width Tiptype angleChisel 29 mm 600Conical 600

Rakeangle100

6,50

Clearanceangle20023,50

The rake and clearance angles presented to the faceby each of the above pick types depends on the angleof the pick box, which throughout these experimentswas fixed at 36,50.

Spacing Number of picksmm Face picks Gauge picks Total

50 51 24 75lOO 26 24 50150 18 24 42200 14 24 38

The number and disposition of face picks was the samefor both the chisel and the conical types at any givenlevel of spacing. The position of the gauge cutters,which were always of conical shape, was not varied.

Because the continuous miner cuts with a rotatingdrum, a pick does not take a constant depth of cut as itpasses through coal. When sumping, as shown in Fig.7A, the pick enters the coal tangentially at a shallowcut near the roof, gradually increasing its depth as itrotates, achieving a maximum at about mid-drum level.After that the depth of cut diminishes until the pickleaves the coal tangentially again at a shallow cut. Inshear, illustrated in Fig. 7B, the pick also enters the facetangentially at about mid-drum height. Its depth thenincreases to a maximum as the pick leaves the coal face.

Values of depth of cut, calculated from the rotationalspeed of the head and from the sumping and shearingrates, are therefore maximum or near-maximum valuesin shear, but are average values of the mid-drum depthin sump, since, as the sump deepens, so the arc of cuttingcontact lengthens and the depth of cut decreases.

The depth of cut could not be predetermined with anyprecision, but a required high, medium, or low valuecould be arranged by variation of the pressure and flowto the appropriate sum ping and shearing circuits. Thefollowing pressures were available to the machineoperator:

AppliedPressure force Resultant force at drum

MPa kN kN20 (max.) 995 872 (along boom)

4 (min.) 199 96 (along boom)20 (max.) 1224 338 (normal to boom)

3 (min.) 184 137 (normal to boom)

Machine Performance CriteriaTo give as complete and useful a basis as was possible

for a comparison of the effects of changes in the principalvariables, the following performance criteria were used.

Force along the boom. This is the total force actingalong the cutter boom required to cause the drum tosump into the coal face. It is a measure of the aggregateforward force acting on all the cutter picks as they aremade to penetrate the coal face (i.e., EFN in Fig. 7A).

Force perpendicular to the boom. The aggregatenormal force acting on all the active picks as they aremade to penetrate the coal face during the shearingoperation (i.e., EFN in Fig. 7B).

Cutting-head torque. The torque required for thecutting head to cut the coal face in either the sumpingor shearing operations. It is an aggregate of thecutting force acting on each of the active picks (Le.,EFc in Fig. 7).

System

Sump

Shear

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1981 15

Subsection and Impact Hardgrove Uniaxial Specificthickness Description Strength Grindability compressive Energy

Index Index strengthm MPa MJ/t

1-0,25 Dull lustrous 57 69 15 112-0,17 Mainly bright 72 44 20 203-0,43 Dull lustrous 64 74 18 114-0,15 Mainly bright 67 77 17 125-0,55 Mainly dull 72 58 30 246-0,30 Mixed 76 57 23 257-0,30 Mainly dull 69 59 15 88-0,20 Mainly dull 70 62 12 149-0,20 Mainly dull 69 60 21 15

10- 0,3.5 Mainly bright 74 55 18 1411- 0,60 Dull lustrous 63 92 11 9

'fOTAL -3,50

- - - - - ventilation ducting- recorder cable

31 32 33 34

continuousminer

datarecording

irlock

forcefan

Fig. 8-Layoutof test site

Specific energy. The amount of work expended incutting unit mass (kJjt) or unit volume (MJjm3) ofcoal. It is computed from the cutting-head torque, thecutting speed, and the quantity of coal cut in a giventime. Specific energy is the most widely acceptedcriterion of cutting efficiency, a lower specific energyindicating a higher efficiency.

Coarseness index. This is a dimensionless number onan arbitrary scale that reflects the size distribution ofcut coal. It is a summation of the cumulative masspercentages obtained from a conventional size analy-SiSI4.It should be noted that, when the machine is operating,

only a proportion of the picks on the drum are actuallycutting. In sump, at any instant, the number of picksengaging the coal is rarely more than half the totalnumber. In shear, the number is usually between onequarter and one third of the total.

Data Reduction and Analysis

An elaborate procedure was used in the treatment ofthe data obtained from the experiments. This includeddigitizing of the strip-chart traces and subsequent

computer analysis of these and other information fromthe data sheets. Certain standards and other scientificcriteria were laid down for the acceptance or rejectionof data.

Owing to factors like coal heterogeneity, variations infriction coefficients, and lack of precise control in thecutting depth, it was inevitable that the final resultsshould show an appreciable amount of scatter. It wastherefore an essential aspect of the data analysis thatthe experimental results should be subjected to variousrigorous statistical treatments so that they would showthe required empirical relationships and so that thestatistical level of confidence could be established.

THE TEST SITE

"The experiments were undertaken in the No. 4 Seam

at Anglo Power Collieries (Kriel Division), which issituated in the Witbank Coalfield approximately 160km to the east of Johannesburg.

The test site, which is shown in Fig. 8, was at a depthof 50 m in a fully reserved area remote from othermining activity. Since it had been developed exclusivelyby continuous miner, it was free of the possibly dis-rupting effects of blasting. The seam at this location isnearly horizontal and well clear of faults and dykes. Aparting in the seam provided a good natural roof, givinga face height of 3,5 m. Rooms were driven 6,5 m wide,leaving 9,5 m square pillars and giving a factor of safetyof 2. Thero was no significant spalling or other indicationof high roof pressure, and no roof supports were requiredor set. All the tests were undertaken in heading B toavoid possible effects from changes in the direction ofmining.

At Kriel, the No. 4 Seam (Table I) has an averagethickness of 4,5 m, although only the lower 3,5 m isextracted. The seam roof is composed of shale, and theimmediate floor is sandstone. The stratification of theseam is regular and, although it is relatively cleat freeand compact, there is evidence of bedding with the coalsubsections showing significant variations in strengthbetween roof and floor. However, there seems to be littletendency for the seam to part along the bedding planes,except at the distinctive horizon 3,5 m above the floor.

'fABLE IUTHOLOGY AND STRENGTH PROPERTIES OF THE NO. 4 SEAM AT THE TEST SITE

16 JANUARY 1981 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

1 - coni cal 200 mm spacing2 - 150

I 3 - 100Il1 - 4- 50

"I 5 - chisel 150,,-

6- 100';;-11 A-B limit of exptl data

20 40 60 80 100 120depth of cut (mm)

1'he weighted average values are as fonowsvarious coal-strength parameters:

HardgroveGrind-abilityIndex

for the

Uniaxialcom-

pres31vestrength

MPa MJItSumping section 65 69 18 13Shearing section 72 58 25 IH

The values of uniaxial compressive strength weredetermined from cylindrical specimens, 45 mm indiameter and 90 mm in length, cut axially and loadedperpendicular to the bedding. The values of specificenergy were obtained by use of the Chamber of Mine'scoal-face penetrometer, which is designed for use in thefield. The results from this instrument are not expectedto equate with those from an actual mining machinebecause of differences due to scale and cutting confine-ment. However, the information it provides on hard-ness, strength, and cuttability gives a useful qualitativeassessment of the strength of the seam and its sub-sections.

Specificenergy

ImpactStrengthIndex

EXPERIMENTAL RESULTSEarly experience showed that no meaningful data

would be obtained for chisel picks operating at spacingsof 50 and 200 mm. A 29 mm-wide chisel spaced at 50 mmleaves a gap of only 21 mm between picks, thus requiringvery little breakthrough of coal between adjacent picks.Conversely, the very high stresses imposed on chiselsat a spacing of 200 mm caused unacceptable damage topicks and pick boxes, and this part of the test pro-gramme proved to be unpractical.

200

Clz-'"~150E00

~

g',000<i..u.E 50

00

Fig. 9-Force along the boom during sumping

Performance during Sumping

The maximum forward advance of the machine duringsumping was 0,635 m, which gave a total sumpingdepth of the same value providing no slip occurred atthe goal-post anchor. The actual sum ping depthachieved was always measured and, if it fell below 0,57m, the test was abandoned. To avoid end-effect prob-lems, the data obtained over the first 50 mm of sumpingwere discarded, and all the results given here relate to asum ping interval between 50 mm and at least 570 mmor at most 635 mm.Force along the Boom during Sumping

The variation in force acting along the boom as the

Fig. 10- Typical shear cut, showing face coring and break-through on ledge

,JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1981 11

Spacing Depth of cut Boom force No. of Av. no. of Normal forcemm at A, mm at A, kN picks picks cutting per pick, kN

200 40 163 38 12,7 12,8150 30 112 42 14,0 8,0100 20 91 50 16,7 5,550 10 58 75 25,0 2,3

depth ot cut was Increased is shown In Fig. 9 tor thetwo types of pick at their respective levels of spacing.In each case, the A-B region is the range to which theexperimental data apply.

Three significant factors emerge from these results.(i) There is, in all cases, a linear increase in sumping

force with depth of cut.(ii) The gradient (rate of increase of force with depth)

is lower when the pick spacing is higher.(iii) Interaction between picks in the array occurs

only in the A- B region. In each case a cuttingdepth below that appropriate to point A isunpractical since the picks in the array are thencutting too shallow to interact.

The taking of successive shallow cuts at a high spacingdoes not cumulatively lead to a breakage patternequivalent to a large single cut. A series of shallowsuperimposed cuts produces deep narrow channels inthe coal face, inhibiting lateral breakout. If this 'coring'of the coal occurs, it prevents the machine fromadvancing when the depth of core exceeds the reach ofthe pick. Such coring is often seen at the roof duringsumping, and at the face during shearing, owing to theshallow depth of cut as the picks enter the coal tangen-tially. Fig. 10 shows this clearly for shearing, but itshould also be noted that coring is absent on the shelfof the shear, at which point the picks are taking theirfull depth of cut and thereby producing good lateralbreakout.

It is significant to note (for the reasons outlinedabove) that no experimental data could be expectedwhen the depth of cut was less than that appropriate topoint A on each of the graphs of Fig. 9. The positionof point A is in each case equivalent to an sld ratio of 5,the implication being that interaction between thegrooves ceases when the sld exceeds 5. Such was indeedfound in practice, with minimal operation data showingat sld values greater than about 5. As an adjunct tothese observations, a series of laboratory pilot-cuttingexperiments was undertaken at various cutting depthsup to 30 mm, and it was confirmed that interactionbetween adjacent grooves in a block of No. 4 seam coalceased when the sld exceeded about 5.

Based on the foregoing considerations, and the know-ledge that pick forces are linearly proportional to cuttingdepth, it is reasonable to propose that, in each case,point A should be linearly connected to the origin.Being equivalent to unrelieved cutting, this O-A sec-tion provides, by inference, data on the performance ofa set of single unrelieved picks. The average normal forceper pick, FN, can be determined if the ordinate value is

rABLE II

taken at the point A tor each graph, which IS the limitof unrelieved cutting, and is related to the sld ratiovia the depth of cut (abscissa) and the known value ofspacing for the graph, and if the average number ofpicks cutting at anyone time throughout the sum pingoperation is calculated. Data derived in this way forconical picks are given in Table H.

When plotted, as in Fig. 11, the normal force per pickis seen to be linearly proportional, through the origin,to the depth of cut. This result is wholly consistent withthe results obtained from unrelieved cutting tests in thelaboratory on a wide range of coals throughout the world.

~14......~u'0.12

0

"-CIIQ.10

~u5 8-

'ii 6E"-0c: 4

~(71.E 2=:;)u

00 10 20 30

depth of cut (mm)Fig. I I-Average normal and cutting forces per pick for an

sld of 5 (limit of relief) during sumping

40

Torque during BumpingWhereas the force acting along the boom is related to

the normal force acting on the picks, the torque is ameasure of the pick cutting force. It is torque thatsupplies the work to cut the coal.

The torque-penetration curves for conical and chiselpicks are shown in Fig. 12. The data available for eachgraph, as in Fig. 9, are restricted to a depth-of-cutrange A-B over which interaction between the pickswas occurring. Again, the A-B section is substantiallylinear, with the gradient generally decreasing withincreased spacing. However, there is one departure fromthis trend: the 150 mm spacing gradient is somewhatlower than for 200 mm spacing. Statistically, a decreasinggradient with spacing relationship is still possible for the200 and 150 mm spacings without transgressing anacceptable estimated standard error of their difference.

AVERAGE NORMAL FuRCE FOR PICKS CUTTING UNRELIEVED IN SUMP

18 JANUARY 1981 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Depth of Torque/drum I Av. no. of Cutting forceSpacing cut at A radius at A

I

pick" cutting per pickmm mm kN kN

---,----,-- - -----200 40 89,9 12,7 7,1150 30 67,3 14,0 4,8100 20 43,3 16,7 2,6

50 10 26,0 25,0 1,0

'1'he general argument presented. in the section onforce along the boom on the interaction ceasing at pointA, and the several reasons for this and its implications,are precisely the same for torque. Values for the averageunrelieved cutting force for conical picks wore deter-mined on a similar basis and are presented in Table 111.These values, also plotted in Fig. 11, show a linearincrease of cutting force with depth of cut.

Some preliminary laboratory experiments with aninstrumented cutting rig on Kriel No. 4 seam coalprovided some average cutting force values for cuttingdepths of 15 mm and 30 mm using a conical pick ofsimilar geometry to those on the continuous miner. Theaverage values obtained of 1,7 kN and 4,1 kN respec-tively are shown superimposed on Fig. 11. However, theangle of attack of the pick to the coal in the bboratorytests was 45°, as opposed to the 36,5° pick-box angleused on the continuous miner. In the laborJ,tory thiswould produce a front rake angle 8,5° larger, resulting insomewhat lower cutting forces. Despite this, the labora-tory values show excellent agreement with the unre-lieved cutting data abstracted from the torque-pene-tration graphs, and adds considerably to the confidenceand practical significance that can be attached to coal-cutting data obtained in the laboratory.Variation in Specific Energy with Depth of Out duringBumping

It must be emphasized, in the context of these results,that depth of cut refers to the advance per revolution ofthe cutting drum and not to the total depth of sumpachieved.

The effect of depth of cut on specific energy for the4 levels of conical-pick spacing and the 2 levels of chisel-pick spacing is shown in Fig. 13. From these results thefollowing general observations can be made.

(i) In all cases, the specific energy was seen to fallsharply as the depth of cut was increased. Thisemphatic trend and the general shape of thecurves obtained are generally consistent with thetheoretical model advanced earlier that led toequation (3).

(ii) There seems to be little difference in the per-formance and efficiency of chisel- and conical-shaped picks except at high penetration, whenconical picks appear to have a slight advantage.

80

E' 60z

-'"..::J0-405 1 -conical 200mm spacing

2 -" 1503 - 1004 - 50

':"'.1 5-chisel 150.,,' 6- 100"'I A-B limit of exptl data

E::J~ 20

20I

40 60 80depth of cut (mm)

100 120

Fig. 12- Torque during sumping

TABLE litAVERAGE PICK FORCE CUTTING UNRELIEVED IN SUMP

The respective curves at 100 mm spacing areeffectively the same. Although laboratory workconsistently shows the chisel pick to be a moreefficient tool than a pick of any other shape, it isevident that such differences as may exist are ofsecondary importance, and that the dominantvariable in terms of cutting efficiency is depthof cut. Indeed, because of the high concentra-tions of stress at its tip, the conical pick canachieve a larger depth of cut for a given level offorce than a chisel pick.

(iii) It could not be expected that the curves in Fig.13, which are for relieved cutting, would exactlyfollow the shape prescribed by equation (3),which relates to unrelieved cutting. Consequently,the curves for the two narrower spacings of 50 and100 mm reach a minimum specific energy, whichthen increases. This means that, for these spacings,the picks are cutting too deeply, resulting in an8/d ratio that is below the optimum (i.e., operatingon the left side of the minimum in Fig. 3). Simi-larly, the largest depth of cut achieved in theexp3rimsnt was not sufficiently high for the 150and 200 mm spacing to approach the minimumspecific energy.

1800

1600

~ 1400---.-'" \ 0

'<00>.Cl'Q; 1200cG> co.50

"" - - - -ch.l00-c" co.l00

".150¥'0

~ 1000en

800 - co. coni cal picksch. chisel picks

6000 20 40 60

depth of cut (mm)80 100

Fig. IJ-Effect of depth of cut on specific energy duringsumping

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1981 19

0110> E~ E~ ~x15.'-8~

X'X-""'"-.-".., x .0

TO/:..

\'

// 0

\/'

:\, "', X,./~'~'

'0 "-," '.'"",(,','"

'. ",'"/, ," '. ..

",!I'"".0 ':"':",".- '..,'." -x~- .-xx..

~4Ob

2200

2000

-~ 1800,.,0>~ 1600c011u

:;: 1400'u011C.III 1200

'\:

1000

800

6000

0 conical picks.chisel picks

2 3 4spacing to depth of cut ratio

5 6

Fig. I4-Variation in specific energy with ratio of spacing todepth during sumping (all data combined)

(iv) At a spacing of 150 mm and more, few resultswere found for depths less than about 30 mm and,at a spacing of 100 mm, there are no data belowa cutting depth of 20 mm, This is consistentwith the observation made elsewhere that inter-action does not occur when the sld ratio exceeds 5.

(v) In general, Fig. 13 shows that deep-cutting,wide-spaced picks produce a considerable im-provement in cutting efficiency. Comparison ofthe extremes shows that a shallow-cutting,narrow-spaced array of picks required a specificenergy in the region 1600 to 1700 kJ It, whereasa wider-spaced, deeper-cutting combination re-quired about 900 kJ/t. This represents almost ahalving of specific energy, with strong indicationsthat there would be a further significant improve-ment in efficiency at even greater depths of cut,

Variation in Specific Energy with Pick Spacing duringSumping

Because there was not precise control over the depthof cut with the continuous miner, it was not possible fora series of curves to be constructed to show how specificenergy varies with the sld ratio at different depths of cut.Comparisons with the hypothetical curves in Fig. 3,which were verified in principle by laboratory cuttingtests, cannot therefore be made.

It is logical, nevertheless, for all the results to becombined, irrespective of depth of cut, and to be present-ed graphically on the premise that the collective datashould define an envelope similar to that bounded bythe curves for smallest and largest depth of cut shown inFig. 3B, and from which an optimum sld ratio mightbe seen. As the evidence indicated that pick shape is ofsecondary significance in terms of cutting efficiency, theresults for conical and chisel picks were plotted together.

The results (Fig. 14) give a strong indication that theoptimum ratio between pick spacing and depth of cutfor maximum cutting efficiency is 2. When the median

20

was used, the highest specific energy was found to beabout 1800 kJ/t, which improved to a best value ofapproximately 1l00 kJ/t when the sld ratio was 2.

An alternative method used in the analysis of theseexperimental data is based on the now reasonably well-authenticated hypothesis that the optimum sld ratio isthe same for all depths of cut. A mathematical model isproposed that describes the part of the curve that appliesto relieved cutting, this being the region to which thedata on the continuous miner apply, The equation isas follows:

S.E. = a +a S2+(bo+bls4)

0 I d2'

where S.E. = specific energy (kJ/t)s = pick spacing (mm)

d = depth of cutao, aI, bo, bl = constants.When this model is used, contours of equal specific

energy can be constructed for different pick spacingsand depths of cut. The constants determined from ananalysis of the experimental data are as follows:

Conical picks Chisel picksao = 1140 1 176

al = 0,01116 -0,01116bo = 154 245 103 571bl = 0,001l8 0,001l8

If these values are substituted in equation (4),differentiating with respect to s and equating to zeroshow that the minimum specific energy occurs, for alldepths of cut, at an sld ratio of 2,17, This is very closeto the value of 2, which was assessed direct from Fig, 14.

.(4)

Performance during Shearing

Shearing tests were undertaken immediately aftersumping had been completed. Sumping was, in all cases,done at the same horizon immediately beneath the coalroof. The total depth of sum ping achieved before

JANUARY 1981 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Depth of cut Boom force Total Estlm. no, of Av. normalat A, mm at A,kN no. of picks picks cutting force per pick

kN

40 811,1 38 9,11 9,030 62,5 42 10,5 6,020 50.1 50 12,5 4,010 17,8 75 18,8 0,9

20' 1 coniCAl!200mm spac'ing2 1503 1004 505 chisel 1506 100

A-a limit of expt1 data

~ 150

-'"E0

~ 10.8..E(; 50c..u~

A

00 20 40 60 80

depth of cut (mm)100

Fig. IS-Force normal to the boom during shearing

shearing commenced was consistently in the narrowrange 0,570 to 0,635 m, giving a shearing thickness ofthe same amount.

Force Normal to the Boom during ShearingThe force required to move the boom down the face

and shear the coal is shown in Fig. 15. This includesdata for the conical picks at four levels of spacing, andfor the chisel picks at two levels of spacing. The down-ward force on the boom during shearing is seen in allcases to increase linearly with depth of cut. The graphsare very similar, inshape and order of sequence in termsof pick spacing, to those for sumping, which are shownin Fig. 9. The rate of increase in force is again found todepend on pick spacing, with the rate decreasing as thepick space increases.

With the same line of reasoning as advanced for forcealong the boom, and with the experimental data againbeing confined to the A-B section of each graph, asimilar analysis can be undertaken to show the averagenormal force acting on the picks. As found previously,point A in all cases occurs at the spacing-to-depth ratioof 5, which has been shown to be the limiting value forinteraction between picks and the value above which itis unpractical for the machine to function.

The data for pick normal forces at the depths of cutappropriate to point A are given in Table IV. Thesedata are plotted in Fig. 16, which shows a reasonablygood linear correlation of force with depth of cut. Inthis case, the values for normal force are somewhatlower than those found during sum ping. This is to beexpected since, when shearing, the picks are cutting to afree face, which is not available during the sum pingoperation.

120

iJ'orque during Shear~ngThe relationship between torque and depth of cut for

the two types of pick at the different spacings is shownin Fig. 17. The linear increase in torque with depth ofcut is evident once again, the effect of spacing on gradientand the range of experimental data confined to theA-B section of each graph being as persistent as thosefound in :Figs. 9, 12, and 15. Point A once more repre-sents an sld pick interaction limit of 5 in the case ofeach graph.

The torque, appropriate to point A in each case, wasmeasured on the ordinate, and is tabulated with otherrelevant data in Table V.

The results for cutting force per pick that are plottedagainst depth of cut alongside the normal force valuesin Fig. 16 are of similar magnitude and gradient to thecorresponding relationship for sumping, which is shownin Fig. 11.

It might be expected that both the cutting and thenormal forces for a pick operating in shear would beappreciably less than for a pick operating in sump,owing to the lower arc of drum contact in shear and thepresence of an additional free face to assist breakage.It should be noted, however, that the strength of thesection of the coal seam at the test site in which theshearing experiments were undertaken is significantlyhigher than the strength of the coal section wheresum ping was carried out. (The relevant data on seamstrength are given in Table I.)

Z.::t:

:;'12u

'5.L-l04>a.

~ 8uL-E 6

0

-;u

E 40c

w

,f 2

a 00

.(+/

lab, test

+- - - - - -data for Fe

10 20 30depth of cut (mm)

40

Fig. I6-Average normal and cutting forces per pick for an sjdvalua of S (limit of relief) during shearing

Spacmgm

AVERAGE NORMAL FORCE FOR PICKS CUTTING UNRELIEVED IN SHEAR

TABLE IV

200150100

i)()

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1981 21

ep 0 orque rumSpacing cut at A radius at A

mm mm kN

200 40 80,0150 30 52,0100 20 35,0

50 10 21,4

~60Ez=-..:>[: 40.2

E:>{;

20

~0

0 20

Variation in Specific Energy with Depth of Out duringShearing

The effect of depth of cut on specific energy duringshearing is shown in Fig. 18 for both types of pick attheir various spacings. The conclusions drawn from theseshearing results are virtually identical to those foundin sump.

(i) For each type and spacing of pick, the specificenergy reduces rapidly as the depth of cut isincreased. The shape of the curves obtainedconforms well with that prescribed by the theore-tical predictions of equation (3). Because ofinteraction effects, the results for both 50 and 100mm spacings (Fig. 18) approach minimumvalues of specifiCIJenergy. This was also observedduring sumping, and the reasons outlined earlierare equally valid here. The data for the curvesat 150 and 200 mm spacing in Fig. 18 do notextend to a sufficient depth of cut for the mini-mum specific energy to be reached.

(ii) The effect of pick shape on cutting efficiency issubordinate to the effect of cutting depth.Despite theoretical and laboratory experimentalevidence to the contrary, the conical pick wasfound in practice to perform at least equally aswell as the chisel pick. The intrinsically betterpenetration of the conical pick, which enablesit to achieve a given depth of cut with less forcethan the chisel pick, is evidently of overridingsignificance in this context. Having achieved suchpenetration with less force, the almost optimallyspaced array of conical picks would produce acoal yield similar to that produced by chisel picks.This observation on the relative performance of

TABLE VAVERAGE CUTTING FORCE FOR PICKS CUTTING UNRELIEVED

IN SHEAR

D th f T Id

I

Estim. no. of

I

Av. cuttingpicks cutting force

k:-pick

9,5 8,410,5 4,912,5 2,818,8 1,1

80

A

200mrr. spacing150100

50150100

B limit of exptl data.

1 conical2345 chisel6

40 60 80depth of cut (mm)

100 120

Fig. 17- Torque during shearing

1800

~ 1600::;.><

>.i:'~ 1400Cl>

<):;:

<)

! 1200UI

- co.- conical pickch. chisel pick

10000 20 40 60

depth of cut (mm.)

Fig. IS-Effect of depth of cut on specific energy duringshearing

80. 100

conical and chisel picks in array at depth is notnecessarily in conflict with the basic theoreticaland laboratory conclusions, which strongly favourthe chisel pick.

(Hi) As with sum ping, the results in shear show thebenefit to the cutting efficiency that can bederived from the use of deep-cutting, wide-spaced picks. A compitrison of Figs. 13 and 18shows that the general levels of specific energyfor shearing are not very different from thoserequired for sumping.

Comparison of Specific Energy with That during SumpingData for a depth of cut of 80 mm abstracted from

Figs. 13 and 18 gives the values listed in Table VI.For each type of pick, the specific energy during shearis higher than during sump. This is contrary to theexpectation that the shearing operation would be ableto exploit the free face produced by sumping. It appearsalso to be in conflict with the results of earlier experi-ments with an instrumented continuous miner in theD.S.A., which indicate a 40 per cent reduction in specificenergy requirements during shearing as opposed tosumping13.

The reason for this discrepancy almost certainly liesin the fact that the strength of the coal at the Krieltest site is significantly less over a thickness of about 1 madjacent to the roof than it is over the remainder of theseam section (Table I). Since sum ping is carried outwholly in the top 1 ill of the seam, the specific energyrequirement would be appropriately lower at that hori-zon. Some average measures of coal strength over thesumping and shearing sections of the face are given inTable VII.

As a low Hardgrove Grindability Index corresponds to ahigh coal strength, Table VII shows, in all cases, asignificantly lower cuttability of coal at the sump-insection. In fact, the values for penetrometer specificenergy and average grooving force (obtained from thesame instrument) reflect the difference in cutting resis-tance probably more accurately than the other testmethods, since they are measured in situ and involveactual cutting of the coal. The tests for Hardgrove

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY22 JANUARY 1981

50 1130 1190Conical 100 1060 1150

150 940 1090____m-- ----

Chisel 100 1100 1270150 1040 1230

Degree of higherMeasure of Sumping Shearing strength of shearing

coal strength section section section%

------Hardgrove Grind-

ability Index 69 58 16Impact Strength

Index 65 72 11Penetrometer, Jig 13 19 46Av. grooving force,

kN 12 17 42Unconfined compres-

sion strength, MPa 18 25 39

TABLE VISPECIFIC ENERGY OF CONICAL AND CHISEL PICKS

Specific energy, kJ It---------

Type of pick Spacingmm

Sumping Shearing

~ ---------

TABLE VIIRELATIVE STRENGTH OF SUMPING AND SHEARING SECTIONS

OF NO. 4 SEAM

Grindability Index and Impact Strength Index are bothindirect methods, which are carried out on coal fragmentsin the laboratory.

Of interest and obvious significance is the 46 per centand 42 per cent higher cutting resistance in the shearingsection with the specific energy found in the D.S.experiments, which is 40 per cent lower in the shearingthan in the sumping section.

Effect of Pick Spacing on Specific Energy during ShearingThe lack of precise control over the depth of cut was

the same in the shearing as in the sumping experiments.The experimental data were therefore again combinedand, because of further supporting evidence found duringshearing that pick shape is of secondary significance, theresults for conical and chisel picks were plotted together(Fig. 19). The magnitude and trend of the results arevery similar to those found for the sumping tests, asshown by a comparison of Figs. 14 and 19.

Specific energy reaches a minimum when the ratio ofspacing to depth of cut is approximately 2. The depth-of-cut range over which the experimental data span is39 to 110 mm compares with the depth-of-cut rangeduring sumping, which was between 13 and 95 mm.

Although the family of curves lying within the envelopedefined by the upper and lower boundary curves cannotbe drawn, a median curve was constructed as was donefor the sum ping data. This gives a maximum specificenergy of about 1550 kJ It, falling to approximately1150 kJ/t at a spacing-to-depth ratio of 2. The corres-ponding values during sumping were 1800 kJ/t and1100 kJ/t.

The same mathematical model that describes the

reHeved cutting section of the curve, which IS expressedby equation (4), was applied to the shearing data.When the following constants were used, it was found toprovide an excellent fit with the contours of equalspecific energy constructed from the experimental datawhen the depth of cut was used as the vertical axis andspacing as the abscissa;

Conical pick Chisel pickao 1173 1316al -0,00930 -0,00930bo 260413 193286bl 0,001140 0,001140

The substitution of these constants in equation (4),differentiating with respect to s, and equating to zeroshow that specific energy is minimized when the ratioof spacing to cutting depth is 2,02. This agrees wellwith the value of 2,17 found from the same equationbased on sum ping data. Similarly, both these calculatedvalues show good agreement with the optimum ratio ofspacing to depth of cut of approximately 2, which wasobtained direct from the experimental data for sum pingand shearing and is shown in Figs. 14 and 19 respec-tively.

Size Distribution of Coal

Controlled laboratory cutting experiments invariablyshow a clear relationship between size distribution of thecut material (coal or any other rock material) and thespecific energy. Generally, the experimental variablesthat lead to a minimum specific energy also produce themaximum size distribution and vice versa. The curve forcoarseness index is usually found to be the inverse of thecurve for specific energy. This is quite understandablesince it can reasonably be argued that an excess ofenergy over the minimum required to cut the materialis absorbed in the production of a greater degree of coalfragmentation. From this thesis it would be expectedthat inefficient coal cutting would lead to the productionof small coal and a large amount of dust.

Measurements of airborne dust were not made duringth£ se experiments for several technical reasons butmainly because experiments in the D.S.A. with a similar

1700

1600\

\

31400»Cl

~1300..

,.. -Cl E.. K-~ E

K'4'""

~ 0

,/£;::

/ ~~/, ."I

". , I\ .. /\ . . /, .: '.' /,.' ' '11,

0 -::~t'", :

1500 K'

u

"=1200u

..c-l/)

1100

1000 "conical picks.chisel picks

9000 1 2 3

spacing to depth of cut ratio "5

Fig, 19-Variation in specific energy with ratio of spacing todepth during shearing (all data combined)

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1981 23

machine had shown that the production of dust is indeedclosely related to cutting efficiency13. However, samplesof coal taken during the present experiments were analysedto provide a coarseness index, which was determined bya summation of the cumulative mass percentages offive size fractions: plus 50 mm, 25 to 12,5 mm, 12,5to 6 mm, (j to 3 mm, and minus 3 mm. The coarsenessindex ranged from 600 to 100, the upper limit beingappropriate to all the material larger than 50 mm, andthe lower limit to all the material smaller than 3 mm.

Unfortunately, considerable problems were experien-ced in the finding of a reliable sampling technique, andvalues of coarseness index proved to be very inconsis-tent. Disappointingly little use could therefore be madeof the data, and no conclusive trends could be estab-lished. Fig. 20, which shows coarseness index plottedagainst pick spacing for all the depths of cut, givessome indication of the wide scatter of the data. However,it provides some evidence, although tenuous, that coalsize increases as pick spacing is increased.

The use of the size distribution data proved to besomewhat more revealing than the use of coarsenessindex. This involved the expression of the minus 6 mmmaterial as a percentage of the total c::>al cut during atest. In Fig. 21, which shows this plotted against pickspacing, there is a fairly strong indication that thepercentage of small coal decreases significantly as thepick spacing increases.

Continuous miners in South Africa typically producealmost double the percentage of minus 6 mm coal thanwas produced on average by the experimental machine.A likely explanation for this is that most productionpenetration rates are substantially less than those in theexperimental programme, an observation that lendssupport to the belief that coal size improves with cuttingefficiency.

Conclusions

Specific conclusions on how pick forces and cuttingenergies are influenced by pick shape and spacing, and

480 ,"""",.,,, I'

".,, .

""","""

/'" X

"....,,'

. conical picks)( chisel picks x

)(

~c

460IC t":(\

"",0""~e'?IC

'e;./ <'c;

"'~,~"'0<'"'c;.. '-,-it

'"

4...(;:)

'"'" ''"

'". '".'"(

VI

~ 440cCIIVI'-1110U

420

;.

IC

4000 50 100 150

pick spacing (mm)200

Fig. 20--Effect of pick spacing on coarseness index

24

22

.20EE

(0

I

CII 18.~VI

, .,-, , , ., ,

.I'-?O. 1t, ,,;;-;

'<:'0

"~I"..',/0.:;. ~I)k <:'~11 ~ r

'~9.'-,foZ!

.)C

-0 ,....' ....

....... )C....

"-)t "-~ ...

" ... :'11

". ""-" " ..

~ 16uGICl111C~ 14'-CIIa. . conical picks

It chisel picks12

0 50 100 150pick spacing (mm)

Fig. 2 I-Effect of pick spacing on the production of small coal

200

by depth of cut, have been drawn throughout this paper,and only the major conclusions are listed here on a moregeneral basis.

(i) For maximum cutting efficiency, the spacingbetween adjacent picks in an array should beapproximately twice the pick penetration perrevolution of the drum (i.e., depth of cut).

(ii) The depth of cut per revolution is the dominantvariable in terms of cutting efficiency.

(iii) Provided that a large depth of cut is achieved, theeffect of pick shape on cutting efficiency is of littlesignificance.

(iv) At comparatively great depths of cut (92 to 103mm) in what is by any standards a very hard coal,the torque availability of the HH 456 was well inexcess of requirements.

(v) The sump force along the boom to produce thegreatest depth of cut exceeded 200 kN or approxi-mately 20 t.

(vi) The downward force on the boom to give thegreatest depth of cut in shear was more than150 kN (approximately 15 t).

(vii) The maximum nominal instantaneous cuttingrates occurred at the highest depth of cut and thewidest pick spacing.

(viii) Although the experimental results in this pro-gramme are far from conclusive, there is someevidence to support the laboratory and fieldconclusions of other workers that thc size dis-tribution of coal improves with cutting efficiency.

(ix) There is some good evidence to suggest thatcarefully designed and controlled cutting experi-ments undertaken in the laboratory can providedata that are both qualitatively and quantitative-ly relevant to machine-cutting operations in amine.

JANUARY 1981 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

(x) The general principles for efficient coal cuttingthat have been established through systematiclaboratory investigation and theoretical studywere shown, by and large, to be valid in practice.

It needs to be stressed that the results presented hereand the conclusions reached are based exclusively on afixed drum speed of 41,7 r/min. This paper is therefore aprogress report: the project is intended to include astudy of the performance of the same machine in thesame seam at drum speeds of 34,5 r/min, 14,7 r/min,and 7,0 r/min. It is not expected that the resultsobtained during further experiments will lead to con-clusions that are different from or in conflict with thosepresented here, but the further work will quantify theeffects of cutting speed on pick forces, cutting energies,and productivity rates.

Acknowled~ements

The permission of Dr M. D. G. Salamon, ResearchAdviser to the Chamber of Mines of South Africa, topublish this paper is gratefully acknowledged, as is thevaluable assistance given by the following: Lee NorseCompany, Ingersoll Rand Company (S.A.) pty. Ltd,Anglo American Corporation of South Africa, theManager and underground staff at Anglo Power Col-lieries (Kriel Division), Boart International, Mr J.Wisner of the Ingersoll Rand Research Institute, andMr P. A. Boardman, Mr E. Ribbink, Mr D. Moore, andMiss M. 1. Watson of the Chamber's Coal Mining Labora-tory. Considerable assistance was also provided by manyothcr members of the Coal Mining Laboratory.

References1. BAILEY, J. J., and DEAN, R. C. Rock mechanics in the evo-

lution of improved rock cutting methods. Proc. 8th Symp.Rock Mech., Univ. of Minnesota 1966, A.1.M.M. & P.E.

2. FOREMAN, H. M., and ADAMS, T. H. F. Problems in theoperation of a cutter plough at Thrislington Colliery. Min.Soc. J., 1953.

3. EVANS, 1., and POMEROY, C. D. The strength, fracture andworkability of coal. London, Pergamon Press, 1966.

4. EVANS, 1. A theory of the basic mechanics of coal ploughing.Proceedings of International Symposium on Mining Research,London, Pergamon Press, 1962. vo!. 2.

5. POMEROY, C. D., and BROWN, J. H. Laboratory investigationsof cutting processes applied to coal winning machines.J. Strain Analysis, vo!. 3, no. 3, 1968.

6. POMEROY, C. D. Mining applications of the deep cut principle.Min. Engr, Jun. 1968.

7. BARKER, J. S., POMEROY, C. D., and WHITTAKER, D. TheM.R.E. large pick shearer drum. Min. Engr, Feb. 1966.

8. PUTTS. E. L. J., and RoXBOROUGH, F. F. Further studies onthe ploughability of coal seams. Min. Engr, Jan. 1961.

9. PUTTS, E. L. J., RoXBOROUGH, F.F.,and WHITTAKER,B.N.Experiments with the automatic variable geometry coalplough. Min. Engr, May 1967.

10. PUTTS, E. L. J. Production research and development - auniversity contribution. Min. Engr, Dec. 1969.

11. KING. P., and GRANT, I. L. Some aspects of the performanceof continuous miners in South African collieries during theyear 1978. Johannesburg, Chamber of Mines of South Africa,Research Report 12/79. Feb. 1979.

12. BENDIX CORPORATION. Logistic performance of continuousminers. Report SO 144087 to U.S.B.M. Jan. 1976.

13. BLACK, S., and ROUNDS, L. Deep cutting continuous miner.Ingersoll-Rand Res. Inc., Report HO 122039 to U.S.B.M.154/77, Prince town N.J., Jun. 1977.

14. RoXBOROUGH, F. F., and RISPIN, A. The mechanical cuttingcharacteristics of the Lower Chalk. Tunnels and tunnelling,Jan./Feb. 1973.

15. KING. P., ROXBOROUGH, F. F., PEDRONCELLI, E. J., andBOARDMAN, P. A. Investigations into coal cutting using acontinuous miner - progress report No.!. Johannesburg,Chamber of Mines of South Africa, Research Report 23/79.Jun. 1979.

International mining, GermanyBergbau 81, the world's largest mining fair, to be held

at Diisseldorf, West Germany, from 11th to 17th June,1981, will provide the most complete coverage of miningequipment, services, related products, and technologyever presented at one location. In addition to the hun-dreds of exhibits, Bergbau 81 will be the scene of threeinternational conferences.

International Mining Congress

During this five-day Congress, developments andtrends in mining and their economic and environmentalimplications will be discussed. Specifically, the newly-emerging importance of the world's coal resources willreceive a major emphasis in the Congress programme.Bergbau 81 participants will be able to attend numerousseminars on the overall role of the mining industry innational economies and the international community,on environmental aspects of mining, and on manage-ment and operation of mines. Technological advancesthat will result in more economical mining methods willbe presented. Additionally, future educational andtraining requirements for mining engineers will beoutlined.

Interocean 81The International Congress for Ocean Mining will be

held on 15th June, 1981, at the Diisseldorf Fairgrounds'Exposition Congress Centre. Interocean 81 is sponsoredby the German Committee for Marine Research and

Technology e.V. and the Association for IndustrialMarine Technology e.V. The topics will cover all import-ant technical and economic aspects of ocean mining,including specific scientific and industrial activities inthe exploration and exploitation of marine minerals.The programme will cover prospecting, mining, andprocessing methods and technology for

- manganese nodules containing manganese, copper,nickel and cobalt;

- hydrothermal muds with high zinc, copper, andsilver contents;

- phosphorite nodules known to contain 22 to 29per cent phosphorite;

- mineral sands consisting of eassiterite, titanium,and monazite.

Tunnel 81A special conference on tunnel construction, Tunnel 81,

will offer seminars on innovative tunnel constructionmethods, technology, and new research findings specifi-cally related to transportation and sewage-disposalprojects in urban areas. This three-day conference,11th to 13th June, 1981, will be of particular interest totransportation and civil engineers and to planningconsultants.

More information about Bergbau 81 and the relatedconferences can be obtained by contacting the S.A.German Chamber of Trade and Industry in Johannes-burg.

.JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1981 25