Indian 10urnal of Engineering & Materials Sciences
Vol. 7, April 2000, pp. 77-81
Cycle time and idle time analysis of draglines for increased productivityA case study
P Rai, Ratnesh Trivedi & R Nath
Department of Mining Engineering, Institute of Technology, Banaras Hindu University, Yaranasi 221 005, India
Received 28 Jlln e 1999; accepted 2 November 1999
In Indi a, massive opencast mines are being planned, with dragline as an excavator, to strip out large vo lumes of
overburden (O/B ) at a high stripping ratio. The increased use of draglines is primarily because the draglines have
tremendous fl exibility of operation at varying OIB depths. Besides, it is capable of giving hi gher rate of OI B removal at
relatively low operating cost. However, at the same time, it must be distinctl y borne in mind that the dragline is single most
expensive equipment to be procured by a mine. Hence, anal ysis of cycle time and idle time is of tremendous significance
with such capital intensive equipment. Such analysis paves the way for improvement in the utilizati on, production and
productivity of thi s eq uipment. The present study is based on tield investi gation conducted on draglines in one of the most
presti gious opencast coal mines of Coal India Limited. The study criticall y analyses the cycle time and the idle time
distribution of the draglines, as operating in the field .
The increased demand of coal, as a source of fuel and
as a source of power, owing to rapid industrialization
and population exp losion, has forced the open cast
coal mines to tum towards hi gher mechanization for
extraction of coal and removal of OIB as well. To
cope up with the higher demand s, deeper opencast
mines, involving the handling of huge bulk of OIB at
a high stripping ratio are currently being planned l .
These deep and large open cast mines have led to
several positive developments IJl the state of
mechanization . It has definitely paved the way for
deployment of large sized heavy earth moving
machines.
Presently, in open cast coal mines, large number of
heavy earth moving equipment are being used to
achieve required target. T he system of OIB removal
must be efficient and economical. The choice of the
machine fo r OIB removal is usually governed by the
geomini ng conditions like thickness of OIB layer and
coal seam, gradient of the seam, pit geometry, strike
length, the rate of OIB removal, life of mine,
geological disturbances and capital availability .
Draglines are commonly deployed to remove the OIB
material that is dug, conveyed and unloaded in
relatively long and narrow open pit excavated as a
part of a series of adjacent and parallel pits 2 . Owing
to its long reach and absence of auxiliary haulage
equipment,
dragline
has
gained
tremendous
significance. It strips as well as reclaims the OIB
hand in hand . The present paper discusses the various
aspects of the operation of dragline in form of case
study of an Indian opencast coal mine.
Dragline Productivity
When considering the method of O/B removal to
be employed several factors need to be taken into
account which include': (i) total volume of overburden (OIB) to be removed and the ratio of OIB to
coal by volume, (ii) rate of OIB removal necessary to
achieve the targetted coal output, (iii ) the geology of
the site with regard to seam dip, the significant
faulting and the depth of interburden between seams,
and (iv) total owning and operating costs.
The objective of deploying dragline is to remove
maximum possible amount of OIB in the shortest
possible time. To evaluate the performance of a
dragline of particular bucket capacity, the cycle of
operation and average cycle time must be known. It
can be seen that decrease in cycle time and increase
in availability and utilization of the machine can
improve the overall productivity of the dragline.
Dragline cycle of operation comprises a number of
time elements like scooping, sw inging, dumping and
4
sw inging back time . Scooping time is the time taken
in filling the dragline bucket and maneuvering.
Swinging time is the time consumed in sw inging the
bucket from the end of scooping time till it starts
releasing the material from the bucket. Dumping time
78
INDIAN 1. ENG . MATER. SCI., APRIL 2000
involves the time taken in releasing the material from
the starting of re leasing the materi al till the bucket
di sch arges compl etely and swinging back time is the
time taken since completion of dumpin g the mate rial
till the buc ket touches the bank. If we are abl e to
reduce onl y a few seconds in a cyc le time of a
draglin e, the re would be treme ndous improve ment in
producti on as we ll as profit~.
A bas ic approac h to estimatin g dragline producti on
in vo lves use of a standard cyc li c exca vator equ ati on
suc h as th e one be low th at has been tail ored to
6
monthl y drag line output .
0== B x BFx HS x A x l x 3600
(I +S )xC x( 1+ R)
where,
0== output (bcy/month )
B== bucket size (yd ' )
BF== bucket fill factor
HS== hours schedul edlmonth
A== mainte nance avail ability (% of schedul e time
mac hine is availabl e for strippin g/l 00)
1== job fac tor (% of time th at mac hine is availabl e
fo r strippin g)
S== swe ll pe rcentage/lOO
C== ave rage cycle time (s)
R== re handl e pe rcentagell 00
It is e vide nt fro m above equ ati on that hi ghe r level
of producti vity can be obtained by reducing the
a mount of re handle and cycle time .
Another important index is a producti on index,
whi ch can be de fin ed as the bank measure o f O/B
vo lume moved per pe riod per rated bucket volume .
Thi s measure is quite he lpful in comparing the
produ cti vity of differe nt drag lines of diffe rent make
and capac ities. The inde x can also be useful in the
machine se lect ion procedure w he n used to ca lcul ate
required buc ket capac ity per peri od g iven the O/B
vo lume pe r pe ri od . 'Producti on Rate' can be defin ed
as the bank measure of O/B per pe ri od. Thi s bank
meas ure is an index he lpful in produc ti on forecasting
and schedulin g.
In order to reduce the bucket cycl e time, global
efforts are be in g put to reduce a porti on of buc ket
cyc le time consumed in scoop and swing ing
operati ons 7 . Al ongw ith reduc ti on in bucket cycle
ti me, an apprec ia bl e inc rease in buc ket fill fac tor is
necess itated. With the advent of computer aided
des ign (CAD) and finite el e ment method (FEM )
analys is, severa l inn ovati ons have bee n achi e ved in
the bucket design8. Research and development efforts
have been concentrated toward s the innovative bucket
des igns which has resulted into increased bucket fill
factor, greater breakout force, reduced bucket cycl e
time and increased bucket life. Tod ay' s buckets are,
hence, a careful bal ance of capability vis-a-vis
durability l.
Swing an gle is an important aspect of dragline productivity whi c h, gene rally , vari es from 30 0 to 180 0 .
Once a dragline has been pos itioned, th e swin g angle
becomes important. Inc rease in sw in g ang le inc reases
the swin ging to time and sw ing in g bac k time.
Besides the studi es on cyc le time, it is also
essenti al to in vesti gate the idling time of these
equipment. Thi s investigati on throws li ght on the
potential reasons that lead to the id lin g under the
actual fi e ld ope rating condition. For hi gh rate of
re moval of O/B by excavati ons and for optimum
utilization, idlin g time mu st be ke pt minimum.
In orde r to ensure bette r pe rformance and cost
9
effecti veness of th e dragline syste m, F ishl e r has
stated th at it may pay to ope rate w ith two smaller
machines in tande m rather than one large mac hine .
Pe rcentage of re handlin g is also an important
parame ter affecting th e draglin e producti vity.
10
Sharma ex pressed that percentage of re handlin g
increases with the decrease in boom le ngth and angle
of repose of mate rial. Othe r pa rameters whi ch affect
the dragline performance a re O/B depth , swell fac tor,
angle of repose of spoil pil e, pit configuration , benc h
height, mac hin e c harac te ri st ics a nd operators
efficie ncy .
Case Study
To accompli sh meanin gful results field stud y was
conduc ted in one of the maj or ope n cast coa l min es of
Coal Indi a Limited . On the bas is of f ie ld observati ons
the performance e valuati on of the e xistin g dragline
syste m operating in O/B , has been undertake n. F ie ld
Table I- Dragline techno-operation al parameters
(Co urt esy : No rth ern Coalli eld s Ltd. )
Dragline type
: Walkin g
Bucket ca pacit y
24 m'
Boom length
: 96 m
Boom angle
. 35°
Diggi ng reach
: 85 m
Dumpin g reac h
: 85 m
Digging height
: 43 m
Dumping height
: 40 m
79
RAI e/ a t.: CYCLE TIME AND IDLE TIME ANALYSIS OF DRAGLIN ES
T able 2- Dragline time study (dragline type==walking, 24/96; bench height==30 m)
Swing angle
Scooping time
(s)
Swinging time
(s)
Dumping time
(5)
Swinging back time
(s)
Total time
(s)
60 0
21.75
20.42
07 . 10
20. 12
68.54
90°
22.70
23.10
06.40
22.47
74. 67
1200
23.78
25.10
06.85
24 .62
80.35
150 0
25.72
26.75
06.95
25.85
85.27
180 0
25.60
28.58
07 .20
27.45
88.43
Table 3-Performance indicators fo r dragline for
the peri od April 1997 to March 1998
Table 4-Producti vi ty ind ices of dragline
(Cou rtesy: Northern Coalfields Ltd)
Percent of rehandlin g
: 37%
Working hou rs
: 6702
Rate of OIB removal
: 425.24 m' /working h
Maintenance hours
: 1088
Rate of OIB removal
: 584.60 m' /worki ng h
Back-down hours
: 445
(including reh andling)
Idle hours
: 525
Production index
Avai labili ty hours
: 83 %
Util isatio n
:77%
OIB Removed
: 28.5 Lac m'
Reh andling
: 10.68 Lac m'
3
: 17.72 m3/work i ng hlm bucket
capacity
3
: 24.36 m3/working h/m of
Production index
bucket capaci ty
(including rehandling)
I Swing Qngle
I
~
i
19
Scooping time ts)
>-
~:~i~
~
i
17
i
'
I
t:= 0
18
23
28
Swlng lnglo1imc (s)
>-
~4 01
A-
~20
~
22
27
~coopln!l11me(l)
()
..
I
[::1 I ~,; , i::tL
[~~tLi
:- l
>-40
~201~
In degra e
!
at
~
10
10
'::0
O
[::rT::'
I
18
23
28
Swinging .q IIme'( s)
~30
5. 5
6 .5
~ 10
La
:
18 23
7.5
28
":i~'·'-"'m.
~ 10i. . .-.".6. .". . - .f,.'. . ,.- -~ 11~,
~
20
0
0
Dumping time (s)
>-
,., ,"r;:.::
55
65
75
",d" ,,'
12:Lil-
18 23
28
Sw inging b"ck .ime (I)
i
60·
:
60 70
80
To',,1 cyc Ie time (5)
>-4° L L 120·
~201
~1 i• ~~tLl
~:gl ~
i:: Jl'
~
/ !'
I: ,
10
"-0
17
21
2 5 27
Scoop ing time (s) ,
~ 1 .../I
cl O
!1:,/
17
!
2'2
27
Scooping time (s)
OJ
~~ ' 31010~
!
"
~
0
I
i i ,
17
22
27
Scooping . ime (s)
/
18
30
".
23
~ 10
I
""0
5.5
7.5
6. 5
!::r~~'
~
It
I
10
i'
/
0
8 .5
5 .5 6.5
7. 5
Dumping tI
18
I
&to
I
23
28
I
65
75
85
{"~""('~ "1
~
! \ ~~o /'-1'!
~O
To."t cycle . ime
10
/
(S)150.
20
I
1
IS
23
28
Swinging to time (I)
.,0
~O
I
28
i:;r'~"
,. ,
"10
~O
8 .5
18 23
S I I
18
70
S'O
';0
[::1 ,;-, i::1 (f."~ [::f /,"
," ~.'" ,,- (i~:l '~". ".. (" ,,,'
~
/
o
18
~3
1
a
Swinging to time (s)
~
()
~ 10
I
0
5 .5
6.5
...
7.5
8 .5
Dumping t ime (I)
:
0
~ 10
...
18
23
28
Swinging back timc (s)
Fig. I-Frequency di stsribulion of a dragline cyc le
I
0
i
75
85
'5
105
total cycle time (s)
INDIAN 1. ENG. MATER. SCI., APRIL 2000
80
observations are based on the time and motion study
of all segments of cycle time like scooping, swinging,
dumping and swinging back times and swing angle.
These field observations have been analyzed and are
presented in Tables 1-4 . The frequency di stribution of
a dragline cycle is shown in Fig. I .
Besides the segmental cycle time studies, a
detailed field investigation was carried out to reveal
the potential areas that lead to the idling of different
draglines . Towards this end, the draglines, viz., 15/90
and 24/96 ( 15m' and 24 m 3 bucket capacity and 90 m
and 96 m boom length) operating in the field were
studi ed. The results of the in vesti gation for each
equipment type, viz., 15/90 dragline and 24/96
dragline are shown in Figs 2 and 3.
Results and Discussion
Cycle time analysis
Based on the values obtained from the field
studi es, the tables and graphs are prepared and
illustrated . From Tables 1-4 and Figs 1-3, it may be
noted that the variation in sw ingin g to time and
sw inging back time is little for constant swing angles.
As sw in g angle varies these two time segments also
vary. Dumping time is more or less constant. It is the
scoopin g time which varies from cycle to cycle.
Scooping time mainly depend s upon the degree of
fragmentation of OIB material that is to be handled .
Hence, optimum blasting practices need to be
practiced. However, in the present study, the effect of
degree of fragmentation on th e scooping time
could n't be studi ed since thi s factor didn 't vary
throughout the study .
The production index and rate of OIB remo val
measured in terms of OIB m' per working hour per m'
of bucket capacity and OIB m' per working hour are
not so satisfactory . Percentage of rehandling is also
large. The efficiency factor that can be expressed as
mu ltiplication of availability and utilization is quite
low. For these breakdown hours , idle hours are to be
minimized, and as it is proposed by many other
researchers introduction of four shifts with an overlap
is a good compromise for improving avail ability and
utilization of dragline operations .
Idle time analysis
A close perusal of the bar charts (F igs 2 and 3),
reveals that for the 15/90 dragline as well as for 24/96
dragline, there are five important reasons responsible
for machine idling. These are non availability of
power, dozing operation, blasting operations, idle
marching and mi scellaneou s reasons .
In case of 15/90 dragline 24.87 % of total idle hours
and 26.5% of total idle hours is due to no power
availability. In case of 24/96 dragline, 26.5 % of
dozing operation s, constitute loss of 21 .98 % and
23 .83 % respective ly , of total idle hours. Third maj or
operation is blasting. In case of 15/90 dragline, it
accounted to 20.94% of total idle hours . In 24/96
dragline, however, the blasting operations, which
accounted for 16. 11 %, stood fourth . Mi scellaneous
operations like inspection by officers, operators not
available, losses during tiffin hours and shift
changeovers constituted 21.76% of total idle hours in
case of 24/96 dragline and 18.58% in 15/90 dragline.
Previous studi es in thi s direction have also
revealed a significant loss in available hours of these
HEMM due to blastin g operations, dozing operations,
tiffin periods and shift changeovers, etc. II - I ,.
Hence, it is imperative at this stage that after
analyzing the pertinent reasons for machine idling,
the responsibility of idling may be systematicall y
LEGEND
I
\I
NO POWER
OOZING
11\ BLASTI NG
IV MISCELLANEOUS
V MACHINE MARCHING
~
1~'
NO POWER
I
\I OOZING
\II MISCELLANEOUS
IV BLASTING
V MACHINE MARCHING
2~,
21.7 6" ,
-
~
13.61 " ,
---,--
~
\I
III
IV
V
\I
Fig. 2-Break-up of total idle hours ( 15/90 dragline)
III
IV
v
Fig. 3-Break-up of total idle hours (24/06 drag line)
RAI et al.: CYCLE TIME AND IDLE TIME ANALYSIS OF DRAGLINES
divided among the managerial group, technical group
and the mining group. Say, for instance, the losses
due to extended tiffin periods, shift changeovers etc.
can be attributed to the 'managerial group'. Similarly
losses due to non availability of auxiliary
equipment/services can be attributed to the technical
group and the losses due to dozing operations,
machine marching, blasting operations can be
assigned to the mtntng group. This clearcut
demarcation of accountability of machine idling can
substantially reduce the idling losses by giving forth
to a feeling of healthy competition.
Conclusions
The detailed analysis of dragline productivity
studies indicated that:
I
With the increase in swing angle, cycle time
increases that can be reduced by proper
positioning of dragline keeping in view that
rehandling should be minimum.
2
There is a great variation in scooping time.
However, improved bucket design and proper
blasting practices can reduce scooping time.
3
Idling of dragline should not be tolerated even for
a few seconds as it leads to depreciation and
production losses.
4
Idling hour distribution of draglines has thrown
sufficient insight into the reasons primarily
responsible for machine idling in the field
operating conditions.
5 Non availability of power appears to be one of
the major bottlenecks in dragline performance.
Hence, power availability in mines must be given
attention.
81
6
The idea of assigning the responsibility of idling
hours to the appropriate group may increase the
accountability and thereby minimize the machine
idling hours.
7
Productivity of the dragline can be improved by
increasing utilisation of the machine by reducing
the cycle time and idle time through proper
planning.
References
I
2
Rai P & Nath R, Indian Min Eng J, 37 ( 1998) II.
Rodriguez R, Berlanga J M & Ibarra M A, Math ematical
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3 Jameson I J & Thomas M D, Min Teclll1ol, Feb. ( 1992) 36.
4
Reddy Y R & Dhar B B, Dragline performance in openpit
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5
Dhar B B & Reddy Y R, Optimal utility of drag lin es in
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6
Hrebar M J & Cook Jr, IIlI J Stllf Min Reclam Environ,
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7
Sanda A, Coal, 96 ( 1991 ) 30.
8
Fiscor S, Coal, 97 ( 1991 ) 35.
9
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