RFI 18-02: AGMA 6123-C16
Request for Interpretation for AGMA 6123-C16, Design Manual for Enclosed Epicyclic Gear Drives
Issued September 10, 2018
Question:
For identical input, Equation 39 of AGMA 6123-C16, for shear capacity, yields different result compared to similar equation (Equation 4) in Darle Dudley’s paper “When Splines Need Stress Control”.
However, if we remove the π from Equation 39, the results are the same. See Annex 1.
Please review attached sample calculations and explain the difference.
Response:
It appears that the original Dudley article referenced by Vanyo has an error in it. Within the “Spline Symbols” block, the article states: “tc = chordal thickness at pitch line, in. (approximately equal to D/2N)”.
I believe that the original intent of the statement was really meant to say that chordal tooth thickness is approximately equal to circular tooth thickness. Since circular tooth thickness equals πD/2N, it seems clear that in the printing process, π was inadvertently left off of the equation.
This does not pose a problem to the subsequent article equations since they all just use the symbol tc.
AGMA 6123-B06 and C16 are correct because we chose to use circular tooth thickness for tc and it is calculated correctly with the inclusion of π.
I believe that in general, the gearing industry has recognized this “omission of π” error in the original Dudley article, but I do not have a published reference to this. Perhaps someone else in the committee does.
I looked back in the documentation used by my company (Cincinnati Gearing Systems) to create a spline stress computer program in 1994 and our copy of the Dudley article “When Splines Need Stress Control” referenced by the questioner has the D/2N equation changed by hand marking to πD/2N.