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.


gear without the use of Gearboxes and pinions. This latest generation is a


control).Direct motor drives with frequency torque control.


test generation is a


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


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


Tim

e [

s]

020

4060

80

100

120

-2.5-2

-1.5-1

-0.50

0.51

1.5


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


Tim

e [s

]

020

4060

8010

012

014

0-1

-0.50

0.51


Tim

e [s

]

020

4060

8010

012

014

0-3-2-1012


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.

Documents

MILL SELF-DISENGAGEMENT BARRING SYSTEMS