Things We Don’t Know

Noted gear consultant and gear educator Ray Drago is fond of saying that a consultant’s best answers usually start with “it depends.” Those of you have had the opportunity to attend one of Ray’s seminars are smiling right now. If you haven’t availed yourself of Ray’s classes (or other AGMA educational products) I urge you to do so sooner, not later. I was blessed to meet and learn from some of the best gear engineers over my career; sadly many of them are no longer available for consultation.

Which is a real shame for our profession, because these giants of our trade were instrumental in getting the industry to where it is today. They also, to use the vernacular, knew “where the bodies were buried.” Engineering, being an applied science, requires us to move forward with new materials and techniques BEFORE we have full scientific understanding of the factors involved.

This costs our employers and customers a great deal. Because we don’t know how grind steps affect tooth strength, we throw out parts with steps or spend lots of machine hours reworking them. Because we don’t have a direct relationship between our allowable stress numbers and standard laboratory sample tests, we can’t use the full potential of our gears.

Progress is being made in some areas, but it takes a long time to be widely shared. Legal and commercial concerns are part of the delay but a larger issue is the lack of corporate support for basic gear research. There just isn’t the will to do “science” at the company level, and our university-based gear research centers seem too far removed from the needs of the industrial gear community.

These are some of the topics I wish we knew more about:

  • Grind steps in the root
  • The connection between traditional material properties (tensile, yield, impact, etc.) and allowable gear loads
  • The relationship between forging properties (reduction ratio, grain flow) and allowable stress
  • The validity of Reliability factors as presently used
  • A method of evaluating the size factor

Feel free to tack your topics on to my list. One of the things I love about the gear trade is that after several thousand years we still have interesting things to learn about.

About Charles D. Schultz 661 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.


  1. Things that you wish we knew more about:

    •Grind steps in the root
    •The connection between traditional material properties (tensile, yield, impact, etc.) and allowable gear loads
    •The relationship between forging properties (reduction ratio, grain flow) and allowable stress

    All these are inter-connected (as I am sure you know) but the main problem with the inter-connection is the stress raiser in the gear root, which varies considerably with method of manufacture. Grinding steps can be avoided by full form grinding, but this in turn raises other questions – grinding burn, depth of case remaining after grinding, etc. Hobbing and shaving can be achieved with protuberance hobbing and flank shaving, although some volume manufacturers prefer to hob with a full form fillet, and flank only shave, leaving a step between flank and root fillet – this is then affected by the post shaving heat treatment. Most other manufacturing methods have different effects on the fillet form, the achieved case depth and the final critical geometry of the gear (and micro-geometry corrections affect it all as well) so coming to a relationship between gear stresses and tensile material properties not only depends the geometry, but also on the method and quality of manufacture – hence the degree of difficulty which it poses for the research teams across the world.

    Richard A Dudley – RicardoUKLtd

  2. Your comment about the duty we “older” gear engineers have to pay it forward by passing on our accumulated knowledge is right on. This is the impetus for the development of all of my seminars. But I have found that we also need to get young people interested in careers in engineering in general which may lead to the development of tomorrow’s gear engineers. In this regard I have also developed and present several seminars (usually just a couple of hours long) for young kids starting as young as kindergarten and on up through high school If we don’t get ’em while they are young we may not get ’em at all. One of my favorite venues is Grandparents University at West Chester University (see this article for more details on this splendid program:
    As I look forward to the start of my sixth decade in the gear “trade” I find teaching to bring me great satisfaction, especially teaching those who may not become gear engineers until after I travel to the “gear shop” high in the sky (at least I hope that I am headed up rather than down to the “heat treatment” department in the netherworld!)

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