One of the most important aspects of a gear rating standard is the allowable stress charts. For spur and helical gears we want to calculate durability and strength ratings for a wide variety of materials and heat treatments, so the charts have gotten large and require many footnotes.
AGMA and other standards agencies work very diligently to keep commercialism out of technical matters. All proposed changes are scrutinized by a broadly based committee of engineers from AGMA members before being adopted as part of the standard. Completed standards are then submitted for membership comments and approval. The objective is to deliver a reliable methodology for making gears that will meet industry expectations for performance.
Unfortunately, the allowable stress values cannot be directly derived from the material properties you would test in a metallurgical laboratory. There is no formula for taking tensile, impact, or other physical test results and calculating an allowable contact or bending stress allowable.
The values shown in the charts were negotiated over the years based upon committee member input and their field experiences. The process has been compared to making sausage; you don’t always want to know what goes into the sausage but as long as it tastes good and no one gets sick we come back for more sausage.
This situation frustrates companies that would like to use new materials, but protects the general public from untested products. All AGMA standards include language that permits the use of alternate methods and procedures — provided the design is properly tested. The “standard” methods represent the consensus of the best engineers in the trade and hundreds of years of collected experience.
On the Helical Gear Rating Committee we jokingly refer to the area beyond what the standards endorse as a “Land of Dragons.” Brave engineers can go there if needed. We enjoy hearing the tales of the survivors of those journeys and use them to redefine the borderline to Dragonland when the standards are revised. If you are one of those survivors your input is welcome at the AGMA committee near you.