GEARED VS GEARLESSDRIVE SOLUTIONS FOR GRINDING MILLS

Evaluating the merits of large geared mills versus gearless driven mills considering:

� Currently available systems in terms of power and mill diameter

� Overall efficiency of electrical power consumed vs mechanical output power� Capital cost� Running costs� Cooling: air vs water cooling� Installation and delivery time

Drive type Present Max Power

Single motor gear drive

8000kW - 10000kW

Dual motor gear drive

16000kW – 20000kW

Gearless motor drive range

＞＞＞＞18000kW ～～～～ 35000kW

GearlessGearless

SynchronousSynchronous

AsynchronousAsynchronous

REDUCED TORQUE PULSATIONS USING 24 PULSE SER DRIVES

USING PWM TECHNIQUES - SINUSOIDAL ROTOR CURRENTS

SMALL HARMONIC CONTENT - NO HARMONIC FILTERS REQUIRED

� Geared and Gearless SAG Mills must consider variable speed ability for process reasons� Issues regarding load sharing & torque pulsation Slip Energy Recovery (SER) and Variable

frequency drives have been resolved through technological advances

SLIP ENERGY RECOVERY DRIVE

SMALL HARMONIC CONTENT - NO HARMONIC FILTERS REQUIRED

LOW TORQUE PULSATIONS - INCREASED GEAR LIFE

� SER mature and ready for use� 40 SER installations world wide� Speed variation required is in the range of +/- 15% of synchronous motor speeds.� Successful operation of 2 x 6.5MW (Phu Kham, Laos) � Three 2 x 7MW units under manufacture (Pascu Lama, South America)� 2 X 10MW dual drive mills are achievable and quoted to various sites world wide� Dual hydraulic inching drives provide safer and convenient operation with the added

advantage of locked charge detection.

CLIENT CONCERNS

� What type of drive system best suits the particular application??

� Real data: efficiency, durability, availability and value for money??

� Lets clear up this confusion using accurate information obtained from recent projects.

� Girth gear drives are the primary method of driving SAG mills, Ball mills and other rotating elements.

GIRTH GEARS

� Enormous improvements over time following the use of:� FEA� MAGMA� Casting techniques (eg full ring risers)� Ultrasonic testing� Special materials� Hardness improvement (up to 325BHN)

AGMA321.05 AGMA6004-F88 ANSI/AGMA6114-A06

1968 1988 2006

GEAR DESIGN

ISO 7

AGMA 10

HB300~350

Gear

ISO 5

AGMA 12Machine precision

HRC57~61Tooth

Hardness

Pinion

� Mr Denecki’s (FALK) paper presented at the ‘96 SAG conference, details the then largest gear drive in operation on a SAG mill (34ft x 18ft) at Escondida 13.4MW (18,000hp) - 1995

Shop test runSINO Iron project

7.9m X 13.6m Ball MillTwin 7,800kW drives

HISTORY

� Historically OEM’s did not have access to larger single piece special alloy castings and larger gear cutters

� To the benefit of consumers CITIC HIC has now overcome this limitation and mine operators can now consider more cost effective large gear driven mills

CITIC HIC 16m CNC gear hobber inproduction (Schiess Brighton)

Australia SINO Iron Ball Mill Gear Pour

OD:11.7m Net Wt:118t

Molten steel Wt:375t

Inspection & testing–International standards � CITIC HIC manufacturers over a hundred gear sets every year� All activities are done in house from casting, forging, heat treatment,

carburizing, machining, gear cutting, shop assembly and transport

Manufacture� Ten gear cutters at a single works: 2 x Ø5m, 3 x Ø8m, 1 x Ø10m, 1 x Ø12m, 1 x

Ø13m and 2 x Ø16m (one in climate control).

MANUFACTURE

carburizing, machining, gear cutting, shop assembly and transport � Inspection and test plans follow international ASTM & ISO standards with a large

team of NDT and inspection supervisors for all equipment

Gear Castings � MAGMA software utilizing full ring risers for solidification and cooling.� Actual metal pour is approximately 2.5 times the net weight of the ring gear. (eg a

molten pour of 375t result in a 118t gear)� The full ring gears at CITIC HIC are normally a single pour even though they may

be in halves or quarters. � Ensuring uniform chemical analysis and even hardness after heat treatment � Pouring capacity is up to 600t and hence net gear weights of up to 250t.

� Presently gearless drives are more costly than the complete mechanical portion� Customer/consultant needs to deal with two parties through project� Interface between the mill vendor and the gearless motor supplier may require mediation

for any issues � After award to both parties (at the same time ) a 6 weeks delay exists in getting certified

data from Gearless drive supplier before the mill vendor can start the FEA & basic design, taking a further 6-8 weeks.

� Geared Mills can commence engineering immediately

Gearless drive rotating elements are

GMD CONCERNS

Gearless drive rotating elements are heavier. The foundation and load data on gearless drives are higher. Air gap variation is a source of magnetic pull imbalance.

All these add significant cost to the overall cost for the client when compared to gear driven mills.

MECHANICAL INTERFACE

� Total weight of the poles are approximately 20% heavier than the gear and need to be individually installed after the rotating element is in place on the foundations

SAG Mill Ball Mill

Mill Diameter (m) 12.2 8.5

Motor Power (MW) 20 20

Motors 2 x 10 MW 2 x 10 MW

Pinion Speeds (rpm) 160 150

Mill Speed (rpm) 9.24 11.09

Pinion Gear Pinion Gear

Number of teeth 21 344 21 288

Width of teeth (mm) 1080 1070 1080 1070

Normal Modulus (mm) 42 42

GEAR DESIGNSAG Ø12.2m (40ft) x 6.1m (20ft) – 20 MWBall Ø8.5m (27.9ft) x 12.8m (42ft) – 20 MW

Normal Modulus (mm) 42 42

Tool normal tooth profile angle (degrees) 25 25

Helical Angle (degrees) 7.5 7.5

Accuracy Level Number ISO 5 8 5 8

AGMA 12 9 12 9

Hardness of tooth face (BHN) Brinell Hardness 310 300

Rockwell hardness 57 57

AGMA 6114(min S.F)

Durability ≥ 1.75 ≥ 1.75

Strength ≥ 2.5 ≥ 2.75

Actual Safety Factor to AGMA 6114

Durability 2.75 1.81 3.20 1.94

Strength 2.52 2.52 3.20 2.62

MECHANICAL Motor Girth Gear Reducer Total Mechanical Efficiency

Gearless Drive 97% N/A N/A 97.0%

LS Synchronous 97% 99% N/A 96.0%

HS Asynchronous 96.5% 99% 98.5% 94.1%

ELECTRICAL Starter/VSD VSD TransformerHarmonic

FilterSupply

TransformerTotal Electrical Efficiency

Gearless Drive 99% 98.5% 99.5% N/A 97.0%

Fixed Speed Low Speed Sync (LCI)

99% 99% 99.5% N/A 97.5%

Variable Speed Low Speed Sync (VSI)

98.5% 99% 99.5% N/A 97.0%

Fixed Speed High Speed 99.9% N/A N/A 99% 98.9%

RESULTS

Fixed Speed High Speed Asyn (LRS)

99.9% N/A N/A 99% 98.9%

Variable Speed High Speed Asyn (SER)

99.7%99.8% (rotor circuit only)

N/A 99% 98.5%

All the above figures are based on the averages of documented figures provided by vendors in datasheets or are derivedfrom published papers (referenced in this paper)

OVERALLMechanical Efficiency Electrical Efficiency Total Overall Efficiency

Gearless Drive 97% 97% 94.1%

Fixed Speed Low Speed Sync 96% 97.5% 93.6%

Variable Speed Low Speed Sync (VSI) 96% 97% 93.1%

Fixed Speed High Speed Asyn (LRS) 94.1% 98.9% 93.0%

Variable Speed High Speed Asyn (SER) 94.1% 98.5% 92.7%

Approx. relative costGearless Drive Highest - 140%Fixed Speed Low Speed Sync 125%Variable Speed Low Speed Sync 130%Fixed Speed High Speed Asyn (LRS) Lowest - 100%Variable Speed High Speed Asyn (SER) 110%Variable Speed High Speed Asyn (VSI) 115%

Capital cost comparison (excl Installation & Commissioning) 2x 10MW or 20 MW.

RESULTS

Running cost comparison.20 MW Fixed Speed Ball Mill

WRIM/LRS SYN/LCI GEARLESSSystem efficiency 93.0% 93.6% 94.1%Power extra cost pa(due to inefficiencies) $963,802 $875,542 $802,851Gear Lubricant $130,000 $130,000 $0Gear Lubricant $130,000 $130,000 $0Lost interest due to extra capital cost 0 $368,000 $592,000Total pa $1,093,802 $1,373,542 $1,394,851

Total EXTRA pa over cheapest $0 $279,740 $301,049Over 20 years $0 $5,594,800 $6,020,980

20 MW Variable Speed SAG Mill

WRIM/SER SYN/VSI GEARLESS

System efficiency 92.7% 93.1% 94.1%

Power extra cost pa(due to inefficiencies) $1,008,361 $949,013 $802,851

Gear Lubricant $130,000 $130,000 $0

Lost interest due to extra capital cost $0 $296,000 $440,000

Total pa $1,138,361 $1,375,013 $1,242,851

Total EXTRA pa over cheapest $0 $236,652 $104,490

Over 20 years $0 $4,733,040 $2,089,800

Cooling requirements� Air cooling is the most common form of cooling for motors and drives. � Gearless motors and large cyclo convertors use water cooling � The heat dissipation represents the motor and drive losses (approx 4%)� A 20MW drive requires approx 800 kW of heat dissipation� In a GMD this equates to approx 2lpm for each kW representing approx 1600lpm of cooled

water � Typically a cooling tower or chiller is required in a closed loop system due to the lack of fresh

water � This adds approx $1million for 20MW and requires power to run, adding to running costs

(Approx 480kW for 20MW), or conservatively 1600lpm of water on a continuous basis.

OTHER ISSUES

(Approx 480kW for 20MW), or conservatively 1600lpm of water on a continuous basis.

Installation and delivery time for mills 15MW and above� The girth gear is the longest lead time item on a geared mill at approximately 9 to 14

months� On a gearless mill the Gearless drive is the longest lead time item at 12 to 18 months. � The installation time for a geared drive is approx. 12 to 16 weeks (approx. 8000 man hrs) � For a gearless drive it is approx. 10 to 12 weeks for the mill and 10 to 15 weeks for the

Gearless drive making a total of 18 to 24 weeks (minimum 20,000 man hours).� This increases the capital cost of the installation by a minimum US$2 million and causes and

additional 2 to 3 months to the project schedule.� The additional time and costs associated with commissioning of a gearless drive vs geared

drive has not even been considered

CONCLUSIONS

� For mills above 12.2m in diameter or over 20MW of power there is currently no viable alternative to a gearless drive.

� For mills 12.2m or less in diameter and 20MW or under in power, gear driven systems offer a more viable alternative to gearless drives, delivering nearly equivalent efficiency, much lower capital cost, easier installation, simpler cooling, shorter installation time and quicker (2-3 months) project overall startup schedule.

SAVINGS ON 20MW BALL MILL� Capital cost approx. $8 million

Running cost approx. $6 million� Running cost approx. $6 million� Cooling approx. $1 million� Installation approx. $2 million� Approx. saving over 20 years $17 million

SAVINGS ON 20MW SAG MILL� Capital cost approx. $6 million� Running cost approx. $2 million� Cooling approx. $1 million� Installation approx. $2 million� Approx. saving over 20 years $11 million