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Grinding Mill Gear Drives for the Future

Introduction

Grinding mill capacities have continued to increase which translates into larger grinding mill sizes and horsepower requirements. Until recently the only method of driving mills over 12,500 HP was the gearless drive.

Traditionally the drive method of choice has been the girth gear. The girth gear has been preferred over the trunnion coupled reducers or gearless drives due to their lower initial cost, simplicity to install, operate, and maintain.

Girth gears have gone through enormous improvements over the past 15 years. This paper will describe the present gear manufacturing technologies. The advantages and disadvantages between the gearless and girth gear driven systems will be discussed.

Rating Girth Gears

Initially mill builders, operators, and consultants questioned the ability of girth gears to drive mills with power requirements over 12,000 horsepower. To date the highest rated girth gear is driving a grinding mill rated at 12,500 HP. However, a girth gear is only effected by actual tooth load. A brief examination of rating girth gears will clear up the question of high horsepower capability.

Girth gears are sized by two AGMA (American Gear Manufacturers Association) rating criteria, tooth surface durability which is a Hertzian contact fatigue stress and strength rating which is a bending fatigue stress.

Another simple method used for evaluating helical gears is K factor which is a convenient index to measure the intensity of tooth loads. This allows an easy comparison between tooth load intensity levels of various applications.

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

A listing of grinding mills that have girth gears rated for motor horsepowers of 4500 and above can be found in Table 1. You will note that prior to 1994 the largest grinding mill was 12,500 HP or 6250 HP per pinion. However, this is not the most highly stressed girth gear in operation.

Table 1 shows the most highly stressed gear sets have K factor as high as 232.8 and many with values over 200. Typically for grinding mills an AGMA service factor of 1.50 to 1.75 on Durability and 2.25 to 2.50 on Strength have been applied.

In 1994 an 18,000 HP mill (9000 HP per pinion) was sold to operate in Chile. This gear set has service factors of 1.88 and 2.45 for durability and strength respectively. The gear hardness is 300 BHN and meshes with a surface hardened and ground pinion which results in a K factor of 204. This is 50% less than the value used in catalog speed reducers. Again this factor is extremely conservative.

Processing and Quality Improvements

The process starts in Engineering and Design. Here the girth gear is analyzed using Finite Element Analysis. Computer analysis of all rating and cutting geometry is performed followed by the gear drawings being developed on a CAD system.

Once drawings are completed, a detailed list of all quality checks is assembled and forwarded to the customer.

Material Technology for forged pinions has improved with ladle treatments, vacuum degassed and vacuum arc remelt materials. Pinion forgings are processed to ASTM material standards and ultrasonically inspected.

Girth gear castings are enhanced by risering techniques, such as full ring risers. Solidification and cooling is verified by computer simulation programs and proven through rigid quality controls. The girth gears are ultrasonically tested to verify their integrity before turning and tooth cutting.

Finally, the part integrity is checked throughout the machining process and then through successful field application after application.

The cutting and verifying of the final tooth quality has seen some of the greatest improvements. High quality gear teeth start with quality inspection equipment for the hobs and shaper cutters. Girth gears up to 46' (14 meters) in diameter are cut to AGMA Quality 10 levels and pinions are capable of being surface hardened and ground to AGMA 12. These tolerance levels must be verified using precise certified measuring instruments. These inspections should be closely monitored using ISO 9001 guidelines.

Gearless vs Girth Gear Drives

The greatest advantage of the girth gear driven system over the gearless system is the initial cost. This is particularly true if fixed speed motors are used with the girth gear system. An 18,000 HP girth gear drive system may be $3 to $5 million less expensive than the gearless drive system which translates into a 20% to 30% savings. This savings compares a variable and fixed speed gear driven system to a variable gearless system. These cost advantages include items such as motors, clutches, gears, and installation costs.

Analysis has shown that initial cost savings found in a girth gear package can not be offset over a 10 to 20 year span operating with a gearless drive because of substantial increases in efficiency and the absence of spare parts cost such as pinions. Let's address each item:

• Efficiency: Girth gear efficiency of 99 to 99.5% is reasonable to expect with gears now being supplied. Currently, instrumentation is not accurate enough to directly measure the high efficiency of girth gear meshes. However, tests on large reducers and computer simulation indicate the above efficiency is reasonable. This efficiency is substantially higher than girth gear efficiencies that have been published in previous articles.

 

To compare the gear system inefficiencies, one needs to add the losses in the high speed motor to drive a girth gear verses the total losses found in a gearless ring motor. The high speed motor can be selected with a leading power factor as compared to the lagging power factor of a ring motor to lower cost of power or eliminate associated equipment to increase power factor.

 

• Pinion life expectancy: Presently with surface hardened and ground pinions, several mills have been operating for over 5 years with negligible pinion wear. With such small amounts of wear, expected life will be over 8 years per flank which results in about 15 to 20 years of total pinion life.

 

• Gear life: The overall life of the gear set is dependent upon continuing proper lubrication and alignment.

 

• Alignment: To obtain proper performance both static and dynamic alignment techniques need to be performed. The static alignment needs to be performed during initial alignment only. The dynamic techniques should be employed on a continuing schedule in order to prevent any damage from occurring due to foundation shifts or system wear. The infrared alignment method was pioneered and presented by Falk personnel at the IEEE Cement Conference in 1979 and today is used all over the world. An advantage of this procedure is that it allows evaluation of a fully loaded gear mesh without shutting down the mill. This method is particularly helpful in dual rotation alignments.

   

  Alignment and maintenance of the proper air gap is required on a gearless ring motor.

   

• Lubrication: The majority of lubricants used today are asphaltic types or greases which utilize a soap and oil blended to obtain the viscosity necessary to protect the gear flanks. Methods are now being developed to use fluid lubricants (gear oils) similar to the types used in speed reducers. These methods will eliminate the disposal problems associated with asphaltic and grease lubricants.

Gearless ring motor have the following advantages:

1. Unlimited size capability.
2. Variable speed capability available.
3. No load sharing problems due to dual motor drive.

And the following disadvantages are:

1. High initial capital expenditure of motor, controls, and installation.
2. Extensive and complicated electrical controls that require a clean, dry environment.
3. Expensive ring motor and electrical spare parts.
4. Requires a very complicated water and air-tight seal between rotating mill and fixed stater assembly compared to that used on a girth gear seal.
5. Alignment and maintenance of proper air gap is critical to proper operation and effects motor efficiency.
6. Any required maintenance and repair is typically beyond the scope of conventional maintenance personnel.
7. Difficult liner bolt accessibility in some circumstances.
8. Unexplainable electrical control reactions in other plant equipment.
9. Larger trunnion bearings required because of the additional weight of the motor.

The advantages of a geared drive are:

1. Initial cost is much less the cost of a gearless system.
2. Spare parts are less costly.
3. Proven reliability with thousands of drives operating successfully.
4. Present size capabilities to 20,000 Hp without exceeding normal gear design allowables.
5. Equipment serviceable by conventional personnel.
6. Installation is much simpler and less expensive than a gearless system.
7. Inching operation is extremely simple to perform and cost effective.

And the disadvantages are:

1. Variable speed only reduces total savings.
2. Auxiliary equipment required for inching operation.
3. Proper lubrication critical to operation.

Conclusion

A detailed cost analysis which includes initial capital, net present value of true projected operating costs, and maintenance costs will be required to prove that the girth gear drive is more cost effective on a case by case basis. We believe that the girth geared drive has cost advantages over the gearless driven systems with the technology that is used at present. In total, the girth gear driven mills are substantially advantageous when one compares initial cost (including installation), proven reliability, maintenance personnel requirements, and ease of maintenance and operation.