Bending Stresses in Spur Gear Teeth: Proposed New Design Factors Based on a Photo-elastic Investigation

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Abstract
The paper embraces three related subjects: Photo-elastic measurement of stresses in spur gear teeth; interpretation of stresses so found to give revised strength factors for B.S. specification on the design of spur gears; and a tentative attempt to correlate the stresses found with fatigue-test results which have been reported by various investigators. The object of this paper is to propose maximum safe bending loads which involute spur gears can be allowed to carry. The gears investigated are confined to those which can be generated by standard cutters conforming to the 20-deg. pressure-angle, B.S. basic rack, without either causing undercutting near the root or undue pointing of the teeth. The values proposed are based on a photo-elastic investigation carried out by the author under the guidance of Professor W. A. Tuplin, at the Post Graduate School of Applied Mechanics, Sheffield University. Strength factors for spur gears are given in the form of charts. These factors embrace the whole possible range of spur-gear combinations, both with and without addendum corrections. To enable a rational assessment of strength to be made, a list of service factors, as well as a graph correlating bending-fatigue data for three classes of surface finish (ground and polished, shaved, hobbed) are given.

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