Gear Design’s Stepchild Root Fillet Configuration!

January 1, 1970

PRESENTER: Raymond J. Drago is Chief Engineer of Drive Systems Technology, Inc. (DST)

Drago is a mechanical power transmission consulting organization that he founded in 1976. Prior to this, Mr. Drago worked for the Boeing Company – Helicopters Division until his retirement after 37 years of service. Currently Mr. Drago is involved in the analysis, design, manufacture, assembly, and testing of many gear systems. In his role with DST, Mr. Drago is active in all areas of mechanical power transmission, including the design and analysis of drive systems in a very diverse field of application from heart pumps to very large mining & mill gears. He has also prepared and delivered more than 150 seminars dealing with various aspects of gear design and analysis.


This webinar will address the design details and pitfalls related with the gear tooth fillet configuration NOT just the fillet radius specification. Often this parameter is not specified at all or only minimally noted on the engineering drawing. In fact, it is one of the most important factors in determining the actual bending strength of the gear tooth. In this regard, we will discuss the various types of fillet forms including full trochoidal, corner radius trochoidal and full circular. Superimposed on these is the complicating detail of undercut which is developed either unintentionally due to tooth number and pressure angle conditions or intentionally to allow finishing the tooth flanks only (i.e. by flank grinding, lapping, honing, hard cutting, etc.) without creating a step in the critical section region.

The very powerful effect of the fillet configuration will be demonstrated by actual calculation using various fillet configurations for the same basic tooth configuration. We will see that the specifics of the fillet configuration can affect the tooth strength by 50% or more. In addition, we will also address the actual deleterious effects of a step at the root upon tooth bending fatigue strength. This effect is a function of the location, depth and radius at the corner of the step and can vary greatly. To implement this knowledge, we will close with a presentation of the proper drawing call out for complete fillet configuration control that will insure maximum and more importantly, consistent and repeatable tooth bending strength.


This seminar will benefit both users and designers. It will provide a better understanding of the importance of the fillet while providing the tools necessary to effectively control and define this critical parameter. Those involved in gear cutting will also find the insight gained from this session to be useful in making decisions regarding tooling where the customers specifications (drawings) are lacking adequate definition while also addressing possible liability associated with inattention to fillet condition.


Attendees will learn to:

  • Understand the importance of the fillet form with regard to tooth bending strength
  • Recognize the difference between the “Fillet Radius” and the “Fillet Form”
  • Understand the role that an undercut at the fillet to facilitate finishing has on the tooth strength
  • Be able to address the effect of a step at the root due to either a finishing issue or wear on the tooth surface with regard to tooth bending capability
  • Address the influence of rim thickness on the stress concentration effect of the fillet
  • Understand the proper drawing callout information to define the desired fillet on the finished tooth
  • Recognize the differences between a full fillet, full circular fillet, flat bottom/corner radius fillet and undercut fillet configuration

You will have 30 days to view the archived webinar.