Where Do Allowable Stresses Come From?


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.


About Charles D. Schultz 678 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.


  1. I refer to your comments about the strength rating of internal gears, e.g. internal gear rings of an epicyclic arrangement. Experience and history shows that the tooth contact and bending stresses of an internal gear are very rarely the cause of internal ring failure. The mechanism for failure is usually that the ring is too thin radially and fatigue cracks develop because of the “elastic band” wrapping around planet gears. Beware of trying to make annulus rings too lightweight!

    To rate internal gear rings for strength and durability, it is worth checking through the many epicyclic gear trains in automatic automotive transmissions, where you will find that the annulus is rarely a problem. More importantly, in all epicyclic arrangements it is invariably the planet bearings that are the limiting feature. Improve bearings and their lubrication, and you will have a durable epicyclic transmission!

    • Richard, those are the very arguments that have derailed getting an internal gear bending strength rating method adopted for 61 years. Everything you mentioned is generally found to be valid. However, just because something isn’t a wide spread problem doesn’t mean we shouldn’t have a value on it. Our wind turbine builders need to get third party endorsement of their designs and those agencies want to see a bending strength rating. Decisions need to be made on material and heat treat for ring gears; without a bending strength rating we only have half the information we need.
      As for other failure modes, the standard will include a comprehensive list of ways ring gears can break and remind the designer that further analysis is needed.
      Thanks for your comment.

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