Download - MILL SELF-DISENGAGEMENT BARRING SYSTEMS
MILL SELF-DISENGAGEMENT
BARRING SYSTEMS
JOHANN J. van RENSBURG
ENGINEERING MANAGER MET. PLANTS
ANGLOGOLDASHANTI
CONTINENTAL AFRICA REGION
Jan 2013
CONTENT
1. ABSTRACT
2. INTRODUCTION
3. BACKGROUND
4. MILL DETAIL
5. PRINCIPLE and OPERATION OF SELF-DISENGAGEMENT BARRING SYSTEMS
6. METHODOLOGY
7. COST MODEL
8. ADVANTAGES OF SELF-DISENGAGEMENT BARRING SYSTEMS
9. DIS-ADVANTAGES OFSELF-DISENGAGEMENT BARRING SYSTEMS
10. INSTRUMENTATION and MEASUREMENTS
11. MEASUREMENT RESULTS
12. RESULTS
13. CONCLUSION
14. FINAL WORD
15. ACKNOWLEDGEMENT
1.ABSTRACT
Grinding Mills are production critical machines in not just Gold mining Process
Plants but in every plant where they are utilised in order to meet business
objectives. They in a sense are the heart of plants and in order to achieve
business objectives are required to be available well above 90 %.
These machines have been in operation for close to a century and in this time
with a diameter scale up of at least five fold. A typical or favoured drive
arrangement feature a wound rotor induction motor driving a girth gear via a
single or dual-stage gearbox. This drive train configuration in itself is about
forty years in use.
However numerous improvements and changes occurred during passed years
at various operations on mills from various manufacturers. Large diameter
Wrap-around motor designs with huge diameters are just one of these
interventions. These designs eliminate the use of gearboxes, pinions and girth
gears.
Evolution of Mill technology
Conventional Milling First generation SAG Milling
Latest Generation SAG Milling
14,7Meter diameter wrap-around motor mill design.
Latest development in drives is sophisticated direct motor drives onto the girth
gear without the use of Gearboxes and pinions.
medium voltage frequency converter with exclusive DTC (direct torque
control).Direct motor drives with frequency torque control.
around motor mill design.
Latest development in drives is sophisticated direct motor drives onto the girth
gear without the use of Gearboxes and pinions. This latest generation is a
medium voltage frequency converter with exclusive DTC (direct torque
control).Direct motor drives with frequency torque control.
Latest development in drives is sophisticated direct motor drives onto the girth
test generation is a
medium voltage frequency converter with exclusive DTC (direct torque
This unfortunately does not exclude us improving and maintaining current
plants with old designs and operating principles. It is appropriate to constantly
apply comprehensive design and manufacturing quality assurance to achieve
reliable mill drives. Further to this plant operating practices mill reliability
should be taken into account.
This paper summarises the investigation into some of these old features in
design and operating principles. The findings indicate that some of our
practices over years have negatively impacted on our reliability without almost
realising it. Almost too small an issue to pay attention to.
Another important factor to be considered is that these machines are all
purpose-built and as such in many cases prototypes. It is necessary to
undertake independent design audits and apply rigorous quality assurance and
to assess operating practises.
This abstract have focused on drives and their improvements as a result of
operating practise audits that were conducted. One of the components of
current drive trains is Barring and the paper summarises the investigation into
Barring specifically and the effect thereof on the reliability and sustainability of
Gearbox life.
2.INTRODUCTION
Focussing on the demands of grinding mills we can classify them into the
following.
Operating demands
Maintenance demands
Protective demands
As per the abstract this paper will focus on the requirements of Operating
demands. One of the critical tasks from an Operational demand identified in
the start-up procedure of grinding mills is Barring.
The Barring drive, also known as Inching drive, “Sunday”-drive or auxiliary-
drive, is an important component of any mill installation. It is used for
maintenance and inspection purposes, as well as an emergency auxiliary drive
to keep the mill rotating when the main motor fails and it is required for the
mill to rotate at certain intervals. Re-lining as a maintenance function cannot
be done without a barring drive.
Probably the most important function of the Barring drive is to dislodge Frozen
or Lock charge thus as frozen or lock charge protection.
Locked or Frozen charge is a common occurrence with mill applications and is
therefore it is an advantage that the Barring drive can offer protection
againstthis condition. Frozen charge can occur when the mill has beenstopped
long enough for the product to solidify. If the mill is startedagain this solidified
product may fall and damage the mill lining. This is achieved by the several
slow rotations (1turn per minute) by the Barring drive to slowly dislodge the
mill charge.
The inching drive components include a prime mover, speed reducer, and a
connection - engaged by hand or automatically, between the inching reducer
and the main drive. Also included in the system, is a brake or backstop to hold
the equipment when it is stopped in an unbalanced position as well as other
appropriate safety devices.
Starting a mill with a locked load will not necessarily causes visible damage to
the shell or any other of the mill components, but can cause drive-shaft
misalignment and will impact on the long term integrity of the mill. Locked
charge start-ups will cumulatively reduce the overall mill life.
As a general rule of thumb all tumbling mills are to be barred for at least 2
revolutions prior to start-up if they have been standing for more than 6 hours.
How do they work:The inching drive for the mill motor consists of a small
motor with reducer (gearbox) and clutch/brake package. During normal
operation, the clutch/brake is disengaged. However, if forsome reason it
becomes necessary to turn the motor slowly, or inch the motor, the main
motor isstopped and the clutch is engaged on the inching package. Using the
inching drive package, themain motor now can be turned or inched at a much
slower rate. This task is accomplished by havingthe inching package supply a
pulsing feature to provide rotational movement through the motorpinion to
the main motor drive. To return to the main motor, the clutch is disengaged
and the main motor is energized.
Typical Barring System
3.BACKGROUND
At present, most mills are started by the main drive motor from an initial rest
position. This results in large torque outputs and consumption of high torque
dependant electric current. Even when Barring Systems are fitted this is the
case as at the end of the traditional Barring operation the drive train comes to
rest for the system to be dis-engaged. The main drive motor again is at rest
and for milling needs to overcome initial inertias from its rest position.
In a mill configuration with self-disengagement coupling systems this is not the
case as the main drive motor does not start from initial rest position as the mil
is rotated by the barring motor before start-up. At start-up the drive motor
does not start from rest, but from the barring rotation speed.An automatic
coupling ensures speed dependant uncoupling of the barring system after
start-up of the main drive motor.
Siguiri, one of the Anglogoldashanti’s Continental Africa Region business units
have a Self-disengagement system installed on a large mill and due to its
successful history and operational simplicity it was decided to evaluate this as
possible implementation on other business units.
4.MILL DETAIL
• Ball Mill : 6.1m diam. × 9.0m (Polysius)
• Motor Rating : kW 6000 , 994 r.p.m
• Gearbox Rating : 57kN.m
• Gearbox Type : Flender H3H12, Double Reduction Combiflex
5.PRINCIPLE AND OPERATION OF SELF-DISENGAGEMENT BARRING SYSTEMS
With the new baring gear coupling installed, the mill motor starts up whilst
baring is taking place, and the mill load is in motion. The baring gear coupling
slides back in a keyway and a limit switch simply stops the baring motor. The
self-disengagement clutch is a directionally actuated freewheel clutch.
Part A is mounted on the driver gear unit shaft with axial movement by means
of a key and part B on unit to be driven and is fixed.
This imply that A is driving B in the Barring mode.
The overrunning clutch is engaged in the stationary condition by shifting partA
axially to engage with B.
Once the speed of clutch part B is higher than that of clutch part A (Starting
point of main motor), independent dis-engagement caused by the angled faces
of the engaging dogs on clutch part A and clutch part B takes place. Motor and
driven machine unit are then dis-engaged mechanically. A is then locked in the
dis-engaged position.
The overrunning clutch is suitable only for horizontal arrangement
B B A
COUPLING ENGAGED
COUPLING DIS-ENGAGED
A
B
6.METHODOLOGY
The methodology was to develop a model and quantify the effect of Self-
disengagement barring systems on key performance indicators of a mill with
special attention to life time reliability.
The purpose of the investigation was to compare the following key
performance indicators for operation of a ball mill with and without a self-
disengagement barring system. The following was taken as critical evaluation
factors.
• Total cost.
• Fatigue life.
At present, most mills are started by the main drive motor from an initial rest
position. This results in large torque outputs and consumption of high torque
dependant electric current.
The following strategic questions were addressed:
• How can the effect of the barring & automatic smooth coupling be
modelled on the mill life cycle cost?
• How does a model correspond to reality and why is the correspondence
justified?
• What is the lifecycle cost saving realised by changing a typical existing
system into this automatic coupling system?
• What is the effect of the barring system with smooth coupling on the
fatigue life of the gearbox, motor and the rest of the drive train? How
will it influence reliability and availability of the system?
• Is it feasible to modify an existing ball mill system into the automatic
coupled system given remaining service life and other client specified
parameters?
This report is complementary to the Financial Excel model and contains the
workflow of the design process, results and findings, and, user manual thereof.
7.COST MODEL
As total costs was one of the critical factors and a cost model was developed to
determine this. The requirement was a model that can be used by the different
plants to establish the total cost of ownership of its ball mill. The output of the
model must also enable to quantify the savings achieved by installing a barring
system on the ball mill.
The financial model is a representation of all activities related to the ball mill
that influence its cost of ownership. Such models provide clarity regarding the
financial consequences of past activities and allow for educated decisions
regarding future activities. The financial model will also serve as a knowledge
base to capture important information and statistics.
The following steps were followed in creating the model:
• The ball mill components and financial variables were identified and
any relationships quantified.
• Historic and operating data were obtained for all costs associated
with the identified components and variables. The data included
sufficient information to determine:
o Capital costs.
o Operating costs.
o Product costs.
o Repair and maintenance costs.
Lost production due to unplanned failures
These user requirements imply that:
The model must be able to project costs over a fixed period (for example
20 years) to enable valid comparison of results from different mines.
There must be a function that allows the user to determine predicted costs
for the system when a barring system is installed.
The primary requirements of the model can be summarised as follows:
The model should run on a platform that is available at all sites to avoid any
unnecessary expenditure of new software. Excel would be a suitable
platform.
Data entry can be manual but must be user friendly.
• The model must be able to store data that can be easily retrieved for
calculation purposes, i.e. database capability.
• The graphical user interface (GUI) must be user friendly and easy to
understand.
• Report writing must be quick and easy and the report may be in Excel or
Word format.
• The model must be able to calculate Net Present Value for all costs
accrued to date.
• The model must be able to calculate predicted Cost of Ownership over a
period of 20 years.
• The model must be able to calculate average operating cost of a ball mill
per year.
• The model must allow for an option to calculate the effect of installing a
barring system at a point in time.
• Some calculations will require user input for inflation and interest
values.
• The model must be applicable to all sites.
• The model must be easy to install and maintain with the use of a user
manual.
• The model must not interfere with any other software applications that
are currently run by the client.
• The model must be stable.
To quantify the financial consequence of a barring system, the following tasks
were conducted:
• Key measured performance indicators were compared between the
system during start-up with and without the automatic coupling system.
• The performance indicators were utilized to aid in the construction of a
fatigue model for the ball mill system.
• A life expectancy model was constructed based on the fatigue model, to
establish the relationship between the system with and without the
automatic coupling. This relationship is used to give an indication of the
expected life of a system without barring, which is then used in the
construction of the financial model.
Examples of Cost model Input-sheets.
General Mill detail
Gearbox Detail
Maintenance detail
Operational Detail
8.ADVANTAGES OF SELF-DISENGAGEMENT BARRING SYSTEMS
• Smooth torque transfer through the drive train because backlash is
taken up by the barring drive.
• Angular momentum at point of start-up of the main drive. The system
does not start from rest that could result in lower torques.
• Bearings remain lubricated during barring.
• Small and cheap motors and drive keeps the mill in rotation.
• Safe and non-complex system to operate and operator friendly
• Very low and basic maintenance intensive system
• Non-complex retrofit on current systems
• Time saving operation
• Relative small capital layout
• Eliminates the possibility of bridging out limit switches as it form part of
the Control Philosophy.
9.DIS-ADVANTAGES OF SELF-DISENGAGEMENT BARRING SYSTEMS
• Capital requirements.
• Retrofit to current systems that could require a specific design.
• In certain cases space can be a problem to retrofit depending on existing
design.
• Production loss time to install.
• Culture change to long existing practise.
• Resistance to change – “the not invent here syndrome”
10.INSTRUMENTATION and MEASUREMENTS
The gearbox fitted with the Self disengagement Barring system was
instrumented as follow:
Top view of ball mill layout, showing the accelerometer positioning.
1. Two 90° degree bi-axial shear strain gauges on the barring gearbox
output shaft. Wired to measure torque output – One full-bridge channel
(position ε1)
2. Two 90° degree bi-axial shear strain gauges on the main gearbox’s input
shaft. Wired to measure torque output – One full-bridge channel
(position ε2)
3. Three accelerometers on the main gearbox housing on the input shaft
end measuring tri-axial acceleration of the main gearbox.
4. Three accelerometers on the main gearbox housing on the barring
coupling end measuring tri-axial acceleration of the main gearbox.
This totals to 2 strain signals and 6 acceleration signals. The position of the
accelerometers and strain gauges is shown in the above figure.
Photo of instrumented barring output shaft and part of main gearbox system.
11.MEASUREMENT RESULTS
ACCELERATION
Acceleration on Gearbox Casing atBarring end. UNCOUPLED(Fig 1)
60
80
100
120
140
160
180
-3-2-1012A
ccel
era
tion in t
he o
rthogonal
directions o
n t
he b
arr
ing e
nd w
ith t
he b
arr
ing s
yste
m u
ncouple
d
Acceleration [m/s2] in x direction
Tim
e [
s]
60
80
100
120
140
160
180
-3-2-1012
Acceleration [m/s2] in y direction
Tim
e [
s]
60
80
100
120
140
160
180
-3-2-1012
Acceleration [m/s2] in z direction
Tim
e [
s]
Acceleration on Gearbox Casing atMain drive end. UNCOUPLED (Fig 2)
Acceleration onGearbox Casing atBarring end. COUPLED(Fig 3)
6080
100
120
140
160
180
-3-2-1012A
ccel
erat
ion
in t
he o
rtho
gona
l dire
ctio
ns o
n th
e dr
ive
end
with
the
bar
ring
syst
em u
ncou
pled
Acceleration [m/s2
] in x direction
Tim
e [s
]
6080
100
120
140
160
180
-3-2-1012
Acceleration [m/s2
] in y direction
Tim
e [s
]
6080
100
120
140
160
180
-3-2-1012
Acceleration [m/s2] in z direction
Tim
e [s
]
Acceleration onGearbox Casing at Main drive end.COUPLED (Fig 4)
020
4060
80
100
120
-0.6
-0.4
-0.20
0.2
0.4
0.6
0.8
Acc
eler
atio
n in
the
ort
hogon
al d
irectio
ns o
n t
he b
arr
ing
end w
ith t
he b
arr
ing
syste
m c
oup
led
Acceleration [m/s2] in x direction
Tim
e [
s]
020
4060
80
100
120
-2.5-2
-1.5-1
-0.50
0.51
1.5
Acceleration [m/s2] in y direction
Tim
e [
s]
INPUT SHAFT TORQUE
COUPLED(Fig. 5)
020
4060
8010
012
014
0-1
-0.50
0.51
Acc
eler
atio
n in
the
ort
hogo
nal d
irect
ions
on
the
mai
n dr
ive
end
with
the
bar
ring
syst
em c
oupl
ed
Acceleration [m/s2] in x direction
Tim
e [s
]
020
4060
8010
012
014
0-1
-0.50
0.51
Acceleration [m/s2] in y direction
Tim
e [s
]
020
4060
8010
012
014
0-3-2-1012
Acceleration [m/s2] in z direction
Tim
e [s
]
COUPLED(Fig 6)
INPUT SHAFT TORQUE
UNCOUPLED(Fig 7)
0 20 40 60 80 100 120 140-15
-10
-5
0
5
10
15
20
25
30
35
X: 29.12
Y: 32.92
Time [s]
Torq
ue [
kN
.m]
Torque on main gearbox input shaft with barring system coupled
4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6-5
0
5
10
15
20
25
30
35
40
45
X: 5.051
Y: 32.09
Time [s]
Torq
ue [
kN
.m]
X: 5.152
Y: 28.13
Detail section of initial torque peaks on main gearbox input shaft
UNCOUPLED(Fig 8)
12.RESULTS
Acceleration results
60 80 100 120 140 160 180-15
-10
-5
0
5
10
15
20
25
30
35
40Torque on main gearbox input shaft with barring system uncoupled
Time [s]
Torq
ue [
kN
.m]
X: 85.68
Y: 34.65
61.3 61.4 61.5 61.6 61.7 61.8 61.9 62-5
0
5
10
15
20
25
30
35
40
45 X: 61.65
Y: 38.9
Detail section of initial torque peak on main gearbox input shaft
Time [s]
To
rqu
e [
kN
.m]
X: 61.73
Y: 1.557
X: 61.7
Y: 24.37
X: 61.71
Y: 9.866
From Fig. 1 to Fig. 4 the accelerations measured on the main gearbox housing
is shown. When comparing the two sets of acceleration data the initial
acceleration magnitude is larger on the gearbox housing (clearly visible on the
drive end accelerations) when the barring system is uncoupled, which supports
the statement above.
Comparing Fig.2 with Fig.4 the acceleration measured on the main drive end
on the y direction recuses from 0.8 mm/s.s uncoupled to 0.25 mm/s.s coupled.
Reduction of 68%.
Torque results
The rated power of the main motor output of the ball mill is 6MW, with a
rotational speed of 994rpm.
ω
PT =
where:
P = rated power of the motor, [Watt].
ω = angular velocity, [radians].
T = rated torque, [kN.m].
Using this equation the rated torque for the gearbox input shaft is thus
calculated as being 57kN.m. The average operational torque calculated from
the measured data, over the operational section of the data for the main
gearbox, was calculated as being 21.2kN.m. This operational torque of the
main gearbox input shaft is 37% of the rated torque of the system.
When considering the start-up of the ball mill with the barring system
uncoupled, (Fig.7&8) impact loads is seen by the shaft and other components
of the gearbox. Impact loads could create stresses that are significantly higher
than the stresses created when similar loads are applied gradually. The torque
data for the main gearbox input shaft shows two significant impact loads, one
at initial start-up, as shown in the red section and the second at the sorting of
the soft start system, shown in the green section of Fig. 5 and Fig. 7. These
impacts will have no influence on the fatigue life of the shafts; however the
compressive force seen on the gearbox internals is significant.
The results from the measured data for the ball mill with barring, included:
• When the mill drive motor is started, it uncouples the barring system
clutch. To uncouple this clutch the torque on the barring system changes
due to directional changes that will cause a peak (32.09kN.m).However, the
peak shown in Fig.8 (38.9kN.m) where the mill is started without barring is
caused by the backlash in the mill gearbox and is transferred through the
gearbox. Therefore, for damage comparison on the mill gearbox, the peak
on the system started with barring (32.09kN.m) may be ignored and the
peak on the system started without barring (38.9kn.m) shall be included.
• The first torque peak to be considered for the main gearbox input shaft was
calculated as being 28.13kN.m. This is about a 32.6% increase from the
average operational torque (21.2kN.m) calculated for the shaft and is 49%
of the rated torque of the gearbox. (57kN.m).
The results seen in Fig. 7andFig. 8 for the ball mill with no barring included:
• The first initial torque peak for the main gearbox input shaft was calculated
as being 38.9kN.m. This is about a83.4% increase from the average
operational torque (21.2kN.m) calculated for the shaft and is 68% of the
rated torque of the gearbox (57kN.m).
• Fig 8 showsthe torque peaks with two distinct high torque values. Also, a
second peak is seen at 24.37kN.m. The initial torque peak was considered
in the fatigue assessment as this is the highest value calculated and the
second peak is similar to what was seen by the system with barring and
would not change the fatigue life when comparing the two systems.
From the above calculated results the second torque peak in both the barring
data and the no barring data is almost the same magnitude and the difference
would have no effect on the operational life of the system, when comparing
the barring and the no barring data. The result that would have an effect on
the operational life of the system is the difference in the first torque peak
(38.9kN.m) in the system uncoupled and the second torque peak (28.13kN.m)
in the system coupled. This difference equates to a 38% increase in torque
experienced by the system compared to starting up from the barring speed.
13.CONCLUSION
During the assessment on the data measure Siguiri mine the following was
determined:
• The torque on the main gearbox input shaft, included:
o An initial torque peak on both the sets of data, with barring and no
barring. The initial torque peak in the data with barring could be
neglected as this torque is not transferred to the gearbox, as
supported by the acceleration data. The acceleration data showed a
peak not at the initial start-up.
o The torque peaks on the input shaft were 38.9kN.m for the system
with no barring and 28.13kN.m for the system with barring.
o A second torque peak is seen when the soft start system is
disengaged. This peaks was not considered in the life assessment of
the system as this peak was similar in both sets of data and would
have no effect during comparison of the data.
• The torque on the barring gearbox output shaft reached 7kN.m.
The fatigue life assessment included the design of a fatigue model for the
gearbox system along with a model for system availability. The results from
the effect of barring on fatigue life included:
• The contact stresses in the pinions and gears increase by about 17% for no
barring compared to barring.
• The fatigue life assessment showed that the shafts are designed for an
infinite life time and no difference was seen between barring and no
barring on the fatigue life of this component.
• There is a significant change in operational life cycles for the pinions and
gears of the gearbox, which are most likely the components to fail. A ration
of 9.5 x 10-4
for life cycles of no barring compared to barring was
determined.
• A reduction in operational life of 31.5% for the gearbox is calculated when
the barring system is not used on the ball mill system.
A financial model was constructed according to user requirements to enable
the comparative analysis of cash flows for a Ball Mill with a barring and
automatic coupling mechanism and a Ball Mill without it. Utilising this model in
general indicated a payback of less than a year on a typical 3 Mw Mill.
14.FINAL WORD
It is my absolute opinion that this is what is called a “no – brainer”.
Just go and do it.
From a safety perspective and with specific attention to Stored Energy this
system is almost in comparable to the normal practice. Just the elimination of
force by hammer or lever to engage the wheel speaks for itself .If you consider
sometimes the physical condition of the engagement wheel you would prefer
not to be present at this operation during specifically at night time.
This is also a system that as far as “bridging-out” safety systems is concerned
can be promoted as not possible. This becomes part of the Mill Control
Philosophy and wired into the starting sequence.
It is my absolute belief that this is a Step Change in Mill starting procedures
with many other benefits and advantages not quantified in this paper. This
does not eliminate other factors to be considered that will hinder this change
but in no doubt I believe we can overcome that.
I sincerely hope this paper will inspire either an individual, a team or in
whatever way to embark on this route.
15. ACKNOWLEDGEMENT
Dr. Michiel Heyns from Investmech for the in depth study conducted on the
field.
AMRE for the request to write this paper.