Suited to kiln and mill applications worldwide is a new American National Standards Institute (ANSI) and American Gear Manufacturers Association (AGMA) gear rating standard, ANSI/AGMA 6014-A06 (6114-A06 metric edition). Titled Gear Power Rating for Cylindrical Shell and Trunnion Supported Equipment, the new standard was approved October 18, 2006, and supersedes the now withdrawn standard ANSI/AGMA 6004-F88. Formulation of the new standard began with preparation of a first draft in November 2001 by a committee comprising OEMs from the cement/mining industries as well as gear manufacturers; subsequent additions and revisions were introduced during three to four meetings annually over the next five years, until membership approval was obtained in July 2006.

GEAR-RATING STANDARDS ROLE

Rating standards provide a means to quantify the load-carrying capacity of a gear set, enabling measurement of open gear capabilities in the design stage — a crucial function, given their vital role in all cement plants. Failures and difficulties with gear sets often lead to equipment stoppage and loss of production.

Included in the new standard are annex discussions addressing installation, alignment and lubrication. For cement plant personnel, such guidance is indispensable for proper operation and maintenance of existing gear sets to ensure their longevity — hardly insignificant, considering the substantial lead time and cost of large ring gears. Other annex topics include multiple pinion drives, gear rim deflection, material mechanical properties, and a discussion of ausferritic ductile iron.

Essential not only for the designing engineer, the new open gear rating standard cites service factor requirements that may be used by purchasers to compare and evaluate gear quotations. Requirements listed in the standard for gear and pinion materials, as well as tooth quality, may be referenced in customer purchase orders or quotation requests.

AGMA STANDARDS EVOLUTION

Predecessors of AGMA 6014-A06 include AGMA 321.05 and, most recently, AGMA 6004-F88. First issued in 1943, AGMA 321.05 was a ‘tried and true’ conservative standard yielding positive results that helped establish its excellent reputation in the industry. That initial standard was superseded by AGMA 6004-F88 for two reasons: (1) it was not applicable to spur gears; and, (2) it failed to address quality or accuracy of the teeth, i.e., gear sets with teeth of higher accuracy were rated equal to those with less accurate teeth.

While these two issues were resolved with AGMA 6004-F88, the later standard did meet some industry resistance, as it resulted in much smaller gears for the same power throughput when a high tooth quality was used. Arguably, therefore, recommended minimum service factors published in AGMA 6004-F88 were too low. Some OEMs solved the dilemma by using their own minimum service factor criteria, typically 10 percent higher than those recommended in the standard. AGMA 6004-F88 also did not cover materials newer to the application, such as ductile iron and induction hardened pinions.

All such issues are resolved with AGMA 6014-A06. Guidance is provided for ductile iron and induction hardened steel stress allowables. Although not as conservative as AGMA 321.05, it's foundation on decades of historical data ensures a gear equal to industry demands. Other additions to the standard include specification of gear-rim thickness and stress cycle factor, which adjusts allowable stress numbers for the actual expected life of the gear set.

CITING AGMA 6014-A06

Service factors are specific to each standard. Accordingly, customer specifications must include both a required service factor and the applicable gear rating standard. A 2.0 service factor in AGMA 6004-F88, for instance, is not the same as a 2.0 service factor in AGMA 6014-A06; and, a 1.5 service factor may be acceptable in one standard, while not in another. Besides application data, end users or purchasing officials citing AGMA 6014-A06 in their specifications might consider the following example:

Gear set to meet the requirements of AGMA 6014-A06 for a bearing supported ball mill:

• Minimum durability service factor 1.75
• Minimum strength service factor 2.50
• Gear material: steel
• Pinion material: case carburized steel
• Gear material grade M2
• Pinion material grade M2
• Gear tooth quality Av 7
• Pinion tooth quality Av 7
• Life expectancy 25 years per flank

APPLICATION VS. BASE STANDARDS

A true application standard, ANSI/AGMA 6014-A06 was developed specifically — like its predecessors — for mill and kiln applications. Thus, using a general base standard or modifying another standard to suit is not necessary. Issues specific to mill and kiln applications are addressed, including:

• Semi-enclosed or open gear guards;
• Dusty environment;
• Size, i.e., diameter, facewidth and weight;
• Alignment characteristics due to equipment dynamics; and,
• Load characteristics.

Not to be recommended is rating a mill or kiln gear in the same manner as other more common applications, such as an enclosed gear box. AGMA 2001-D04 serves as the “mother” standard, which “provides the basis upon which more detailed AGMA application standards are developed.” Consequently, the task of the AGMA Mill Gearing Committee is to apply the collective experience of the open gear industry to modify and tailor AGMA 2001-D04 so the result (allowable power throughput) agrees with historical data. Key to developing a representative and accurate design standard is AGMA member experience, reflecting centuries of design, manufacturing and operational practice within the application.

COMPARISON WITH ISO

ISO currently does not provide a rating standard dedicated specifically to mill and kiln applications. While it remains a valid technique for rating mill and kiln gear sets, nonetheless, using ISO requires an experienced person able to assign reasonable values to all factors within the rating formulas. Therefore, a specification with ISO rating requirements must assign values for the rating factors in addition to defining minimum safety values. Otherwise, lacking a comprehensive picture, results are dependent on how liberal or conservative the designer chooses to be with rating criteria.

SUREFIRE SOLUTION

A new open gear rating standard, AGMA 6014-A06 is a true application standard specific to mills and cement kilns. Its formulation reflects historical data encompassing the experience of OEMs, manufacturers and end users. In addition to a rating method, which is vital to ensure an acceptable design solution, the standard also includes discussions on operation, maintenance and lubrication.

This article was adapted from a report provided by Jason L. Daubert, P.E., Senior Design Engineer, FLSmidth Inc.