Gear-tooth Stresses and Rating Formulae

Abstract

The state of knowledge about stress conditions in gear teeth is briefly reviewed, and directions in which further research is required are indicated. The assumptions underlying methods of stress-comparison, and the chosen load-points, are considered in relation to profile-modification, leading to revised conceptions of “precision” and “commercial” classes of gear. The present (British standard) basis for the design of “commercial” teeth is stated and a new basis for the design of precision, high-duty gears is offered.Criteria of surface stress are re-examined and current British standard rating formulae for wear are shown to be equivalent to the use of an unmodified criterion Sc= Fc/ Rrin conjunction with a speed factor which is a function of pitch circle velocity and gear ratio. New values for zone factors and strength factors for spur gears have been determined on the revised assumptions, and it is shown how the factors for helical teeth may be derived from those for the virtual spur gears. The assumptions underlying rating formulae for variable load-cycles are also examined, and a revised formula is suggested whereby present inconsistencies are removed.