Specifying and Maintaining Girth Gears
Mill crushing drive systems can be a "lifeline" for cement producers. With the increased emphasis on operating efficiency, the dependable and reliable operation of these drive systems is essential. No one needs the cost or headaches associated with unscheduled downtime or inefficient machinery operation.
A reliable drive system starts with the proper selection of the ring, or girth, gear and continues through proper maintenance. Selecting the right girth gear ideally brings together the gear manufacturer, the system designer, and the end user. At the outset, the gear manufacturer must know the details of the application, the demands that will be placed on the gear, and the nature of the equipment to be driven.
Similarly, the user and the system designer must be familiar with the variables that affect gear drive performance and service. For example, an application that places a torque load on the drive in excess of its rated capacity will inevitably result in gear tooth distress and, in severe cases, breakage.
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Produced by Rexnord at the geared
products group, the world's largest horsepower-driven girth gear drives a primary ore semiautogenous grinding mill at the Escondida Copper mine in Chile's Atacama desert. The custom-made, steel-cast gear is 13.2 m (43.3 feet) in diameter and weighs 86,180 kilograms (190,000 lbs.)
With two pinions, the ring gear is driven by two 6,714 kW (9,000 hp), 176.5 rpm synchronous motors. With a reduction ratio of 17.48:1, the drive is capable of producing 12.7 million Nm (9.3 million lb-ft) of output torque with an output speed of 10.1 rpm.
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Operating Standards
Girth gears are specified using American Gear Manufacturers Association (AGMA) standards. AGMA presents several basic technical standards for rating (AGMA 2001) and tolerancing (AGMA 2000) of spur and helical gears. This basic standard is then used by separate product committees, staffed by experts who use gears in a particular industry, to set the appropriate product standard for that industry.
These in-house experts can decide questions of reliability, dynamics and quality, and the product standards can be written based on their experience. Other international standards include basic standards for technical gear rating, but they generally do not maintain individual product standards.
The AGMA Mill Gearing Committee consists of approximately 10 mill gear experts. In May 1988, AGMA published a 21-page standard, AGMA 6004.
Critical to the selection and specification of girth gears is the surface durability of the teeth - the Hertzian fatigue stress - and the strength rating of the gear teeth - the bending fatigue stress.
Users should note that the AGMA standard does not include the criteria for the structure design of the gear or the material quality of the gear or pinion. Additional specifications for
these features should be included in the quotation, purchase order, or quality assurance requirements between the seller and buyer.
The current AGMA standard does change the specifications for girth gear ratings from the previous standard (AGMA 321). The old standard, while superseded, is familiar to most users and is still being used by some. However, the current standard is predominately used.
To everyone's benefit, manufacturers are comparing the gears selected by the new standard with those selected by the old to assure that drastic changes in rating have not been made. The new standard includes horsepower ratings for spur gears, which were not in the old standard. Both standards are only applicable to steel gears.
In particular, the current standard includes a bending strength service factor for girth gears. (See Table 1) Previously, the strength service factor was set equal to the durability service factor. While the increase in bending strength service factor seems rather drastic, most suppliers provided service factors beyond the minimum required by the old specification. The higher strength service factor provides the necessary rating to prevent catastrophic tooth breakage from starting or peak shock loads.
These service factors represent a minimum per the standard. The service factor may be increased based on the known operating conditions of the user, reliability requirements of the contract, and the manufacturing experience or design of the gear builder.
Many users are requesting a durability service factor of 1.75, which effectively increases the reliability or the life of the gear set. (See Table 2)
The present standard, a 1.5 durability service factor, is based on a failure rate of one in 100. Increasing the service factor to 1.75 increases the reliability to a failure rate from one in 100 to one in 1,000.
AGMA 6004 also accounts for the ability of manufacturers to build more accurate gears. The old standard did not address quality level, but the resulting rating equated to a quality level for durability of AGMA Quality 6 and for strength of AGMA Quality 9 when compared to the new standard.
Table 3 shows the effects of increasing the gear quality level using the new standard. The specified quality has a major effect on the horsepower rating of a gear set. Therefore, selection and manufacturing to the quality level is extremely important. Using high quality levels for calculation of horsepower without appropriate manufacture, inspection and operation will result in gear failure.
The current standard also allows the load distribution to be calculated by both empirical and analytical methods. (See Table 4) Previously, the load distribution was a fixed value based on face width. Load distribution factor is one of the parameters that go directly into the horsepower calculation. It accounts for uneven load across the face as a result of misalignment, deflection and bending of the teeth, as well as torsional twist and bending of the pinion shaft.
The use of an infrared temperature alignment technique has been successful in improving alignment. Deflections, however, can be accounted for only by machining the pinion to compensate for the known deflection at the operating load, and analyzed based on an analytical iteration technique.
The horsepower rating of a girth gear can be increased dramatically using the most optimistic values. Therefore, it is important that the user, mill manufacturer and gear manufacturer work together to apply values that are justified both by the manufacturing specification and by the operating environment.
It is recommended that the AGMA 6004 standard is followed, and a Quality 10 is specified for manufacture. However, the allowable horsepower multiplied by the service factor to be transmitted by the gear should be equal to a horsepower calculated using Quality 8. This allows some change between the machine shop and actual mounting and usage on a mill. This same result can also be achieved by raising the service factors when calculating horsepower. It is important that an agreement be made at the time of quotation and purchase to cover the items listed in Table 5.
Once the gear has been purchased, proper installation and maintenance will ensure that it performs to the standard. The gear must be installed to the mill's and gear manufacturer's specifications. As the gear transmits more horsepower and is manufactured to tighter tolerances, the installation tolerances must also become smaller. A gear manufactured to a Quality 10 should be mounted to a compatible level.
Maintenance Recommendations
The following maintenance recommendations are based on the experience and reports of servicemen and engineers in the field.
Heading the list is the misuse of lubricants. The application of heavy grease or asphaltic lubricants several times per shift was acceptable when mill horsepowers were 500 hp (373 kW) and multiple mills were used. Just as today's automobiles require multi-grade lubricants with an array of additives filtered to 10 microns (1,250 mesh), today's girth gears need sophisticated lubrication systems as well. Users would never think of running their car engines without lubricant, yet many are doing so with their girth gears. Lubricant applications are so infrequent that for some periods, the gears operate without lubrication.
Table 6 shows lubricants that are now being used. All are acceptable if they are properly used and the application is controlled.
Asphaltic compounds have been used for over 40 years. They require a spray system that will provide warnings of clogged nozzles or lines that are not providing any lubricant. The lubricant must be tenacious to prevent removal from gear teeth and to avoid the surface temperature increasing between 15-minute spray intervals. It is possible that the last minute of a 15-minute lubrication cycle is the most damaging.
Grease-type lubricants use a lighter base oil stock, and must be sprayed every two to five minutes. The shorter interval is necessary because grease is not as tenacious as asphaltics, and the sliding motion of the gear teeth can remove the lubricant film. Lubricant suppliers recommend spraying the pinion rather than the gear.
Fluid lubricants require continuous spraying and recirculation of the oil using a pump. The oil must be cooled and filtered. Since this system represents a closed system, similar to a large enclosed gear drive, the gear set must have a guard with seals and a sump. High-viscosity oil may require heating to allow filtering to 10 microns (1,250 mesh) to remove cement dust. The pinion and gear guard seals should be designed to prevent cement dust from entering.
Lubrication film, or thickness, as compared to the surface asperities, is the key to proper lubrication. Lambda (oil film divided by composite surface finish) should be at least 1.5 and preferably 2.0 or higher. Figure 1 illustrates a gear set with a 250 AA finish on the gear and a 125 AA finish on the pinion, as compared to a gear set with a 125 AA finish on the gear and a 64 AA finish on the pinion. It can be seen that the higher lambda ratios will dramatically reduce wear and increase the gear set life.
Figure 1
It is also extremely important to maintain alignment, so that loads are equally distributed along the gear teeth and maintained in conjunction with the design loads of the gear. In a case where severe misalignment occurs, one side of the pinion will be overloaded causing tooth scuffing or scoring at the tooth end. The best preventive maintenance practice is a periodic check of alignment using infrared instrumentation.
An infrared temperature technique using a portable instrument or permanent infrared multi-station fixed device or a scanning infrared device can be used. A word of caution - it has been observed that infrared instruments provide false absolute temperature readings when the pinion surface goes from full-lubrication coverage (dark) to a bright, shining surface when the lubricant has been removed. Each of these surfaces represents a different emissivity value and will provide false readings.
As a result of poor lubrication, pinions have been scuffed or scored because of an excessive temperature and the welding of the asperitives on the surface of the gear teeth in the contact zone. Figure 2 shows a severe case of wear and scuffing.
Figure 3 shows misalignment causing scuffing and scoring on the end of the tooth.
Several factors must be considered to ensure the cost-effective, reliable and dependable operation of mill gear drives. Proper selection, using AGMA standards, and maintenance procedures, including maintaining proper alignment and lubrication, are essential to "long-life" gear sets.
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