Recent Advances in Girth Gear & Pinion Manufacture
Abstract
Recent manufacturing advances for both girth gears and pinions have significantly increased the power density of gearing used on grinding mills. These advances include new materials which increase the hardness of girth gears, precision machinery to cut large diameter girth gears, carburizing of large diameter pinions, and tooth modifications finish ground into coarse pitch pinions. The combination of these technologies allows girth gearing to now transmit up to 10 megawatts per mesh and wrap around 40 foot diameter mill shells.
Introduction
The last decade has seen significant advances in the manufacture of girth gears and pinions. This paper details these changes providing an explanation of how girth gearing is keeping pace with advances in grinding mill technology.
History
Throughout the history of mining the guiding principal has clearly been "bigger is better". All aspects of mining equipment have dramatically increased in size. One only has to look at the trucks, conveyors, and grinding mills to gain an appreciation of the magnitude of the change in scale of this equipment. In fact, cable television channels routinely air specials admiring the marvels of present day mining equipment. This change of size is of course production driven. Fewer pieces of large equipment have proven more efficient at processing ore than numerous pieces of small equipment. Using fewer, larger, pieces of equipment requires this equipment to be more reliable then ever before to minimize downtime.
The changes to girth gears and pinions have been dramatic as shown in Graph One. As recently as 15 years ago the state of the art was a 3/4 DP ("34 module), through hardened 365 HB, as hobbed, pinion meshing with, at most, a 12 meter diameter, 285 HB gear. This gear set could transmit approximately 5 megawatts per mesh, yielding a 10 megawatt total power dual drive mill.
Current Capabilities
The current state-of-the-art involves 42 module, carburized and ground pinions meshing with 325 HB gears up to 14 meters in diameter. This gear set can transmit up to 10 megawatts per mesh yielding a drive for a 20 megawatt mill. Currently the largest diameter gear driven mill is a 36-foot SAG mill in Chile. It is driven by two 6,711 kW motors yielding 13,422 kW total mill power. The highest power single pinion drive mills currently in operation are 7,084 kW per mesh ball mills.
This dramatic increase in transmitted power has occurred because of substantial improvements in material technology and manufacturing capability, not theoretical changes in paper rating. The same American Gear Manufacturers Association (AGMA) rating standards 6004-F88 and 321.05 are still being used to establish current designs. The AGMA Mill Gearing Committee is currently meeting in an effort to develop a new rating practice for girth gearing. The new code may include life factors and updated material information allowing for even greater increases in transmitted power.
Regardless of the rating code used, the three major influences on gear set rating are hardness, diameter and tooth size. All three of these influences have been improved significantly as described below.
Material Technology
One of the most influential of the factors a gear designer has to work with is the hardness of the gear and pinion. In addition to the primary affect of increased contact stress (durability) and bending stress (strength) allowable values, an increase in pinion hardness has a secondary work hardening affect on the gear. Therefore, an increase in pinion hardness has a dual impact on the gear sets' allowable transmitted power. This obviously means the harder the pinion the better. The hardest pinions are surface hardened and the surface hardening method of choice is a process called carburizing. This process produces a surface hardness significantly harder than through hardening. The carburizing process produces a uniform case depth which is verifiable without destructively testing the actual pinion.
A hardness of 365 HB is typically the maximum for a through hardened mill pinion and 55 HRC is the hardness typically used for carburized mill pinions. The affect on the allowable transmitted power when increasing pinion hardness from 365 HB to 55 HRC is dramatic. Using values from AGMA Standard 6004-F88, the allowable contact stress number increases from 148,890 psi at 365 HB to 190,000 psi at 55HRC. This increases the pinion durability rating by 62.9%. The allowable bending stress number increases from 43,580 psi at 365 HB to 55,000 psi at 55HRC. This increases the pinion strength rating by 29.4%.
As previously mentioned, there is a dual impact when the pinion hardness is
increased. When the pinion hardness is increased the Hardness Ratio
Factor also increases, providing an increase in gear durability
rating. The Hardness Ratio Factor increases from 1.0530 when a 365
HB pinion is meshed with a 285 HB gear to 1.0865 when a 55 HRC pinion
is meshed with the same 285 HB gear. This increases the gear durability
rating by an additional 6.5%. It should be noted that the gear ratio
and surface finish of the pinion can also affect the Hardness Ratio
Factor and the values being reported are typical. It is also important
to recognize that changes in pinion hardness have no impact on the
gear strength rating. The gear strength rating is typically lower
than the pinion strength rating and therefore is the limiting factor
when determining strength rating of the set.
In order to manufacture large carburized pinions, a pit carburizer is required due to the size of the typical mill pinion. The equipment shown in Photograph One is capable of carburizing parts up to 1.8 m in diameter by up to 5.5 m in length. Pinions can be carburized to finish case depths up to 6 mm and a surface hardness of up to 58-62
HRC.
The carburizing system is computer controlled and by monitoring and adjusting the time and temperature the amount of surface carbon can be precisely established. This is verified by destructively examining test coupons included with every heat.
Increases in pinion hardness alone however are not the only technological advances allowing for the tremendous increase in transmitted power. The hardness of the girth gears has increased from 285 HB to 325 HB or greater. A new high hardenability Cr-Ni-Mo steel alloy was developed at Falk to achieve girth gear hardnesses above 285 HB. Currently, girth gears with a hardness range of 325 - 365 HB are in operation and 335 - 375 HB is possible. During the development of this new steel alloy, called Falk Gear Alloy #6(, numerous full size test blocks were destructively tested to ensure hardness below the root of the tooth. The teeth would quickly fail if only the surface hardness had been increased.
The AGMA 6004-F88 allowable contact stress number increases from 122,010 psi
at 285 HB to 135,450 psi at 325 HB. This increases the gear durability
rating by 23.3%. The allowable bending stress number increases from
35,445 psi at 285 HB to 40,854 psi at 325 HB. This increases the
gear strength rating by 8.5%.
The increase in gear hardness would be meaningless if the integrity of the casting was in question. A critical aspect of casting design is to ensure directional solidification occurs in the gear rim. Sophisticated computer software such as MAGMASOFT( is used to thermally model casting solidification. This software allows the casting engineer to identify and correct any solidification problems prior to manufacture ensuring the integrity of the gear blank. To obtain correct directional solidification in a gear blank casting, the inside rim and outside diameter surfaces cannot be parallel. This necessitates the use of an inside wedge as shown in Figure One.
The benefits of using an inside wedge has been proven by the thousands of gears in operation and by extensive structural finite element analysis (FEA). Placing the wedge on the inside rim of the gear has the additional advantage of moving the centerline of the rim section below the root of the tooth. This guarantees the integrity of the tooth zone.
A final critical element of casting design is the use of a full contact ring riser versus individual risers. The use of individual risers can cause directional solidification problems that yield inferior castings. A full contact ring riser provides the maximum contact area and volume of riser material necessary to adequately feed the gear rim, guaranteeing a quality blank.
It is important not to confuse directional solidification with directionality of mechanical properties. Castings are inherently homogeneous and isotropic. Forgings used for gear rims (rolled plate and rings) are inherently non-homogeneous and anisotropic. Directionality of mechanical properties is caused by the working of the grain flow into a particular direction. This condition can be very beneficial when the directionality resists the applied loads (for example a crankshaft). This however is not the case with gear rims as the direction of the grain flow yields lower mechanical properties in the direction of the applied loads. A comparison of the mechanical properties of a casting and forging of the same chemistry (4340) and hardness ("290 HB) are shown in Graphs Two through Five.
Graph Two - Mechanical Properties Tensile
Graph Three - Mechanical Properties Yield
Graph Four - Mechanical Properties Reduction of Area
Graph Five - Mechanical Properties Percent Elongation
This information does not mean that forgings are unacceptable for gear rims
as they have been used successfully for many years. It does mean
that the gear designer should be aware of the anisotropic nature
of forgings and apply the appropriate allowable stress value for
the direction of the applied loads.
Manufacturing Capability
To manufacture a gear that can wrap a 40-foot mill, a machine large enough to cut the teeth is required. The minimum gear diameter needed to wrap a 40-foot mill is approximately 13.8 meters. To cut the teeth in a gear of this diameter a machine like the one shown in Photograph Two is required. The machine is a MAAG SH1200 that has been installed with 14 meter diameter capability. This capability was put to use during the manufacture of the gear installed on the largest diameter gear driven mill previously mentioned. That gear has a diameter of 13.1 m and was manufactured to AGMA quality ten tolerances.
After carburizing the pinion teeth they must be finished machined. The machined shown in Photograph Three is a Hofler CNC grinder that provides increased diameter and larger module capability for ground pinions. This machine is a generating-type gear grinder with the capability to grind parts up to 4 meters in diameter, up to 1.56 meters in face width, between 42 Module and 3 Module and weighing up to 35,000 kilograms. The machine is CNC controlled with a CNC wheel dresser and is capable of making precision tooth modifications. A Klingelnberg TPF40 lead and profile checker with independent 2-axis movement is integrated with the machine providing on-board inspection capability.
The precise manufacture and verification of tooth modifications is extremely important for high power density gearing. Software has been developed to determine the magnitude of tooth modifications. While not fully accounted for in current AGMA standards, tooth modifications can significantly reduce the actual operating stress the teeth experience. Using data from a Falk program called Elastrostress( a graph such as the one shown in Graph Six can be developed showing the change in Hertzian contact stress as alignment is varied.
The ability to finish grind larger teeth is a significant advancement. This directly increases the strength rating of gearing. For many years 3/4 DP ("34 module) was the largest tooth that could be finish ground. This is important as carburized pinions are necessary to achieve the required durability rating of the gear set. However, without finishing capability the maximum tooth size of 3/4 DP ("34 module) limited the strength rating of the gear and therefore the total transmitted power.
The development of larger grinding machines as discussed above also allowed coarser pitch (larger module) teeth to be ground. The size change is significant as can be see in Figure Three. A 42 module tooth is 24% taller and wider than a 3/4 DP ("34 module) tooth. The increase in tooth size yields a corresponding increase in strength rating.
Summary
This paper has detailed some of the recent manufacturing advances for both girth gears and pinions. These advances have significantly increased the power density of gearing used on grinding mills. These advances include new materials which increase the hardness of girth gears, state-of-the-art foundry technology to guarantee casting integrity, precision machinery to cut large diameter girth gears, carburizing of large diameter pinions, and tooth modifications finish ground into coarse pitch pinions. The combination of these technologies allows girth gearing to now transmit up to 10 megawatts per mesh and wrap around 40-foot diameter mills.
References
Blair, M. and Monroe, R., 2000, "Castings or Forgings? A Realistic Evaluation", Engineered Casting Solutions, Figures 1 and 2.
Brown, J., March 1995, "Ring Gear Drives Huge Grinding Mill", Power Transmission Design, pp. 29 - 31.
Danecki, C., 1997, "Workshop SAG 97: Girth Gears at 18000 HP - Operation and Performance, (Update April 1997)," Workshop SAG 97 Conference, Vina Del Mar, Chile, 1997.
Danecki, C., 1996," SAG Mill Drives Girth Gears at 18000 HP - Operation and Performance," SAG Conference 1996, Vancouver, B.C., pp. 425.
Danecki, C. and Kress, D., 1995, "Grinding Mill Gear Drives for the Future," IEEE Technical Conference, San Juan, Puerto Rico, June 1995, pp. 381.
Kress, D. and Hanson, D., 1989, "Girth Gear Design Concept Through Operating Criteria," SAG Conference 1989, Vancouver, B.C., pp. 609.
Zwirlein, T., January, 1999, "Falk - Leading the Technological Change in Mill Pinion Manufacture", Falk Print 981205.
By Bill Hankes, Engineering & Quality Manager - Mill Products, Falk, Milwaukee, Wisconsin
|
