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SAG Mill Drives: Girth Gears at 18,000 HP - Operation and Performance

This paper was originally presented at the Vancouver SAG mill conference in 1996. It has been updated to include the most recent cost data available. We will trace some of the strategies used in selecting a girth gear to drive the world's largest SAG mill in operation to date. Advances in girth gear technology which made this next logical step in SAG mill drive systems possible, are reviewed along with installation and performance of the girth gear drive system. We will also discuss the drive system costs, efficiencies, and maintenance requirements.

When the 18,000 HP (13.4 MW) girth gear driven SAG mill (34 ft. x 18 ft.) at Escondida copper was installed and began operation in July 1995, it was the largest SAG mill in the world. Prior to the installation of this mill, the convention for mills utilizing powers above 12500 HP (9.3 MW) and / or mill diameters above 32 feet (9.75 m) had been to use wraparound motors.

The discussion to select a girth gear driven mill over what had started to become the conventional drive method for these sizes of mills was a controversial matter. There was a good deal of confusion why mills of this size were not built with gear driven systems as had been the normal convention. The simple fact is that until recently it was not possible to cast and manufacture an accurate girth gear greater than 40 feet (12.2 m) in diameter.

We were given the following reasons why a girth gear driven mill was preferred over a wraparound motor mill for this installation:

• It was obvious that the capital cost of a gear driven mill would be substantially lower than that of a wraparound motor option.
• It was estimated that at least six weeks would be saved by installing a girth gear driven mill.
• Several large mills had been successfully operating at this site with only routine maintenance required. Conventional maintenance personnel could easily handle any maintenance issues which is not necessarily the case with wraparound motors.
• Managers at this mine reported that the girth gear driven mills were operating with 94% availability while other mines in the area operating with wraparound motor driven mills were below 90%. It should be noted that the drop in availability may or may not be due to the wraparound motor. At this time records are not detailed enough to point to the exact cause.

We will begin our discussion with some interesting information gathered at Escondida. This will be followed by a discussion regarding load capacity, improvements in casting and manufacturing technology, erection and commissioning costs, and maintenance. Finally we will address the items needed to perform a complete cost analysis.

The SAG mill at Escondida Copper began operation July 9,1995. The girth gear was ordered in March 1994 and delivered in December 1994 which translates to a 40 week delivery.

The SAG mill was running with about 65% load in September 1995. By May of 1996 the SAG mill had run as high as 25 % over design load.

One of the advantages girth gear driven mills have over wraparound motor mills, besides lower initial cost, is the shorter time required to install the mill. Personnel at Escondida estimated that they saved 6 weeks by installing a girth gear driven mill verses a wraparound motor. That translates into 6 weeks of additional production time. If the SAG mill is producing at it's design capacity of 30,000 tons per day with a copper concentration level of 3 % and assuming that the plant can handle the additional copper, the savings would be over $75 million dollars (valued at $1.00 per Lb. copper).

The delivery and reduced installation time are important in determining the cost of a power plant, however one can not say enough about the importance of reliability. This can be confirmed by the high value placed on this SAG mill's estimated downtime cost of $150,000 / hour (valued at $1.00 / Lb. copper). There is a great deal of emphasis placed on reliability of the girth gear drive train as well as the availability of the mill since the down time is extremely expensive.

This SAG mill has never been shut down because of the drive train. Whenever scheduled shut downs occurred, maintenance personnel could take advantage of them and perform alignment modifications as required.

Due to the push to bring the SAG mill on line quickly, static alignment was rushed, which resulted in requiring more dynamic alignment attempts than normal. After a few alignment adjustments were performed, Operations wanted to wait approximately four months to finish the counter clockwise alignment. They could then maintain the production levels they wanted and schedule the alignment when other items required the mill to be shut down. An interesting side note, the 9000 Hp single pinion driven ball mills that operate in conjunction with this SAG mill only required static alignment. After the mills were brought up to operating temperature, infrared alignment checks were employed and confirmed that no additional alignment was required.

The field cost of erection and commissioning are major factors in performing a complete cost analysis. These costs need to be broken down when comparisons between different drive methods are being analyzed. As stated above, Escondida believed that they saved at least 6 weeks of erection time utilizing a girth gear driven system, which in this scenario translates into 6 weeks of production time. In discussions that we have had with various mine managers, they estimated that a wraparound motor takes approximately 12 additional weeks to erect to the point of testing verses a girth gear driven mill. This will impact the total capital cost if it affects the completion time of the plant and the date when operating revenues are generated. Unfortunately, most mines have not been able to break out the detailed labor costs associated with the additional mill erection time. Everyone we talked with agreed that estimates regarding these additional costs need to be included if a meaningful cost analysis is to be developed. Furthermore accurate cost accounting records need to be kept regarding the break down of these costs in order to verify whether the goals developed in the analysis were attained.

We define commissioning costs as those costs associated with taking a mill through the load testing stage to full production. With a gear driven mill, these would be the costs to infrared align the mill gear and set up the motor controls. Of course the costs associated with gear alignment are minor since they will have little effect on production. In the case of Escondida, production was basically not affected. This may not be the case in every situation. However if the static alignment is done properly up front, adjustments required from dynamic alignment checks should not exceed 3 to 5 days.

When a wraparound motor is used, commissioning costs would be the cost associated with adjusting the air gap, and debugging the control systems.

An important factor that must be identified and incorporated into any cost analysis is the manufacturers field supervision costs. These costs should be stated by the manufacturer at the time of bidding.

Once erection and commissioning are complete, maintenance costs for a girth gear driven mill are relatively small in the context of actual production values.

A specific maintenance routine needs to be adopted using proven inspection methods. These proven methods do not include a wash down of the gears.

The specific methods are as follows:

• Stroboscopic viewing while the girth gear is operating - This gives the operator a good picture of the condition of the gear.
• Infrared temperature measurements while gear is operating - This monitors load distribution and provides the operator with a method of scheduling and planning the specifics of any alignment that may need to be done due to, foundation shifts for example.
• Visual examination of the teeth when the mill is stopped for other reasons - A good example of this would be when the mill is shut down for liner replacement.

These items should be conducted and recorded on a scheduled basis and will normally be found to be stable after the final dynamic alignment is completed.

There was one main item that required clarification, gear load capacity. After all, mill loads increased from 12,000 to 18,000 horsepower which resulted in a 50 percent increase in load. Initially, many thought that this increase would be too much for a girth gear to accommodate. Since girth gear teeth are affected by actual tooth load we were able to prove that the loading required by this mill was well within normal design limits by using a simple load intensity index called the K factor.

The load intensity index, K factor is defined as

W_t = Tangential Tooth Load (pounds)

hp = Horsepower
d = Operating pitch diameter of the pinion (inches)
N_p = Pinion speed
Face = Face Width (inches)
Mg = Gear Set Ratio

Table I offers a listing of girth gears operating with motors over 5000 HP. This table effectively demonstrates that the specific tooth loading of the 18000 HP SAG mill was actually lower than the loading experienced on the mills already operating at the Escondida mine facility.

Table I - 'K' Factor listing for mills with pinion motor exceeding 5000 HP

Pinion technology has changed dramatically over the past 10 to 15 years. Today pinion forgings use improved ladle treatments, vacuum degassed and vacuum arc melt materials. The forgers follow ASTM standards and ultrasonically inspect pinion forgings.

We have gone from AGMA Q - 8 through-hardened, as cut pinions, to AGMA Q - 12 case hardened and ground pinions. The surfaces have improved drastically. As - cut, through-hardened pinions had finishes of 125 to 250 Ra while the current generation of ground pinions are achieving 24 to 40 Ra finishes.

Another advantage to grinding the pinion teeth is the ability to apply tooth modifications to the lead (helix angle) and tooth profile. Proper lead and profile modifications can reduce the Hertzian contact stress by 30 percent (see Figure 1). The benefits derived from the tooth modifications are even greater when the pinion is misaligned. Reductions as high as 50 % can be realized in a misaligned condition.

Girth gear materials have made several advances of there own.

• Castings are enhanced using full ring risering techniques.
• Simulation programs are employed to verify that proper solidification has taken place. Various examples are run to confirm that the gates, wedges, and risers are properly sized. This improves the casting quality, reduces foundry costs, and virtually eliminates casting failures.
• New material developments which allow designers to take advantage of increases in hardness and, therefore, increased ratings.

It was not until recently that gear manufacturers were able to accurately generate tooth forms on gears larger than 40 ft. (12 m). Form cutting machines existed, however their accuracy, or tooth quality levels, did not meet the necessary standards for the loads imposed.

Another advancement in girth gear technology is finite element analysis. The stresses in a girth gear's structure can be calculated but, more importantly, the gear's deflection can be analyzed and the proper pinion tooth modifications can be determined.

As with pinions, girth gear machining has also seen improvements. New cutter materials are constantly applied and tested to improve accuracy and surface finish. Once the girth gear and pinion are through the manufacturing process, you need the proper equipment to measure the quality attributes necessary to meet pitch (tooth spacing), profile, and lead angle. In order to measure the girth gear's profile a profile measuring instrument is required, and the same is true for the other attributes. The only attribute that can not be measured is the girth gears lead angle. However this attribute can be verified by checking the pinions lead angle on the proper checking instrument and verifying the girth gear's lead by applying a fixed center contact check. This check incorporates the verified pinion mounted in a fixture that is brought into mesh with the gear while the gear is still on the cutter. Both profile and face or lead contact are verified.

When evaluating a girth gear supplier, it is important to assure that they have the facilities to properly perform the above checks. This will ensure that the gear set will function properly at startup. Unfortunately, when separate suppliers are used for the pinion and the girth gear, you can not be assured that the two items will mate properly when installed at the mine site. The only way to address this situation would be to have the pinion forwarded to the gear manufacturer's facility for final testing but this could result in an unacceptable schedule delay.

It is also important to ensure that the girth gear supplier has the capacity to turn the diameters, accurately machine the splits and locate the alignment dowels.

In order to perform a complete cost analysis, both the equipment and operating costs must be analyzed.

As SAG mills became larger, interest in variable speed drives heightened. Ore bodies with high variability in hardness make variable speed mills attractive. However ore bodies with low variability may find that the cost of variable speed mills is excessive and not necessary. Regrind circuits are more common in this case.

The motor type selected does not materially effect the gear set design or price. Girth gears perform equally well on both fixed and variable speed systems. The girth gear driven system needs only to limit the starting torque to approximately 200 % of full load torque during acceleration of the mill.

Several electrical systems are reviewed below. For simplicity however, we have omitted dual wound rotor motors and systems using primary reducers.

The mill data we will use for our example is as follows:

Recently available plant operating studies show no significant difference between the production availability of girth gear driven mills or wraparound motor driven mills. Most of the inspection requirements are performed either while running or during downtime for reasons not related to the drive. Therefore, production availability is not a valid factor in the analysis of the girth gear verses wraparound motor cost.

Equipment Costs

The following cost estimates have been provided by motor and mill manufacturers. However the actual electrical loss values should be provided by the electrical vendor. The use of more, or less, copper in their system effects both capital costs and efficiency and need to be known.

Table 1. Various Drive Package Costs ( X $1000)

Electrical components:

As can be seen, electrical components for the variable speed girth gear driven packages have only a relatively small variation in cost.

A harmonic filter is needed to protect the electrical grid with variable frequency motors. A filter suitable for the LOAD COMMUTATED INVERTER (LCI) is less expensive than that required for cycloconverters (CCV) . Again the electrical industry should provide this data.

The issues of variable speed, and whether the mill is to be gear driven or to use a wraparound motor are separate . The girth gear driven package is suitable for either variable or fixed speed as long as the acceleration torque's are controlled.

Gear Components:

The girth gear drive costs include the girth gear, two pinions and necessary bearings, clutches, gear guard and lubricating system.

Mill extra for gearless:

There is an extra cost for preparing the mill shell for mounting a wraparound motor which is estimated at $250,000.

The cost elements are:

• The mounting ring for the rotor, generally about 10" or 12" increase on the diameter.
• A similar ring at the other end of the mill for the caliper brake.
• A possible small increase in mill shell plate thickness.
• A change in trunnion bearings to accommodate the increased weight of the wraparound motor and to axially restrain the wraparound motor.

Depending on the mill configuration, the actual bearing diameter may not need to be increased.

Erection Costs:

These costs are based on actual finished job records of mills of similar size. The cost to erect the girth gear components is approximately $90,000 which translates into 1800 man hours at a rate of $50.00 /hr. The difference between the erection costs provided in the table 1 and the girth gear component erection costs are the additional costs to install the drive motors and their corresponding controls. The $50/hour is a value which must be evaluated for each project as location, altitude, labor effectiveness and local rates all need to be considered.

The costs for the wraparound motor breakdown as follows, construction crew needed to assemble motor and brake will require 15,000 hrs. at $50 /hr is $750,000. The factory support we will estimate at $800,000 with expenses. The actual job records that we reviewed demonstrated that the actual factory support charges were nearly double our estimate.

Commissioning Costs:

The girth gear drive costs are based on one factory technician, one factory senior engineer for one week plus a construction crew of four men for 12 hrs/day times 6 days which we will round up to $35,000. Again the costs to commission the motors and controls is the difference between the total commissioning costs and $35,000.

The commissioning cost of $650,000 for the wraparound motor, is less than one half of the value that was reported. Based on discussions with another motor manufacturer it was determined that our estimate would more closely represent the actual cost picture. The actual job records for commissioning the wraparound motor covered factory support personnel and we were concerned that these figures may have been based on unusual circumstances. Therefore we did not believe that it would be correct to list what may be an exaggerated figure into the analysis. Furthermore, there are more wraparound motor manufacturers in this field and commissioning levels higher than these would probably not be allowed.

Total Cost & Cost Savings:

Total cost is the summation of each column. Cost savings is the total cost of each girth gear driven mill subtracted from the wraparound motor total cost. Therefore the cost savings is the saving incurred by purchasing and erecting the girth gear driven system. The values shown were carefully researched early in 1996 and updated in early 1997. The arrival of a second source for wraparound motors has produced a newly competitive situation which is depressing their cost. It will be advantageous for the buyer to complete a drive system capital cost analysis similar to our example, for future projects due to the current market volatility.

Operating Costs

We will consider the same motor options as we have with the equipment cost analysis. The drives shown under the synchronous motor column allow power factor correction when connected across the line, as does the twin LCI drive when bypassed and operating at fixed speed.

Table 2. Various Drive Package Annual Operational Costs

The best overall efficiency is shown by the fixed speed (95.6%) as would be expected. However this option precludes the use of the benefits of variable speed.

The bottom line of actual electrical operating costs for the mill shows that the values are similar, with only the fixed speed option showing any appreciable savings.

Only the electrical motor manufacturers can provide accurate estimates for efficiency losses since these losses are related to design and the amount of copper used, as noted earlier.

In summary, the efficiency analysis suggests that annual costs will not be a driving consideration in the selection of a drive system, particularly if the KW / hour costs are low. Similarly, the production availability of either drive system is almost identical and is not an issue which will drive the decision between a girth gear driven and wraparound motor driven mill. Also, maintenance costs have been proven to be insignificant in evaluating the operational cost of either drive system. Therefore, the decision is solely related to capital costs and the time taken before revenue service can commence.

The marvels of new technological change are always interesting, however, companies are in business to make money and therefore the commercial aspects can not be ignored. The goal of this paper was to present a mix of new technology and to inform users, OEM's, and consultants about the additional pieces of information needed to assess various mill package options. The additional pieces of information that need to be included are erection costs, commissioning costs, and the lost revenues incurred because of the additional time needed to erect a mill drive.

We addressed the operational costs and demonstrated that the only real difference that exists is between fixed speed verses variable speed. The availability of a girth gear driven system verses a wraparound motor system has never resulted in any appreciable difference for or against girth gears. Maintenance costs are another piece of the operational puzzle. Actual maintenance requirements do not require the additional costs that some have erroneously suggested. Standard inspection such as infrared, stroboscopic, and visual are all that is needed. Wash downs are not necessary.

Rating technology has only recently taken advantage of surface hardening technology. Previously rating standards did not adequately handle the advantages of surface hardened pinions running with through hardened girth gears. Even today we question whether the rating practice is not too conservative when surface hardened pinions are driving through hardened girth gears. We need to investigate this question and develop further rating criteria.

Knowing how to apply technological advances to the benefit of the end user is the essence of the mill drive supplier's expertise. Operators can not afford break downs nor should they expect them. Based on their mechanical simplicity and proven performance, properly applied and maintained girth gears will never be the reason for any shut down.

References

Danecki, C. , 1996, "SAG Mill Drives: Girth Gears at 18000 HP - Operation and Performance", SAG Conference 1996, Vancouver, B.C., 425 pp.

Danecki, C. and Kress, D. , 1995, "Grinding Mill Gear Drives for the Future", IEEE Technical Conference, San Juan, Puerto Rico, June 1995., 381 pp.

Kress, D. and Hanson, D. , 1989, "Girth Gear Design Concept Through Operating Criteria", SAG Conference 1989, Vancouver, B.C., 609 pp.

Antosiewicz, M., "The Use of Mill Gear Operating Temperatures for Alignment Evaluation", IEEE Technical Conference, Tarpon Springs, Florida, May 1979., Section: Drives and Related Products, Paper no. 5