Help Calculating AGMA Pitting Geometry Factor 2

[Andrewk89]

I'm using AGMA 908-B89 and trying to calculate "I" for a pair of external spur gears. I can't seem to match the "I" calculation from Equation 4.1 with the values published in the tables in Paragraph 7.

For example, 20 deg pressure angle, 0 deg helix angle, equal addendum Np=21, Ng=135. I get I=0.139 using Equation 4.1, but the tables show I=0.118.

I tried to figure it out from Appendix A, but I'm stuck. The value of "I" I get from equation 4.1 is exactly equal to the value of Cc from equation A.13.

Can anyone help explain what I might be missing?

 

[Andrewk89]

Example calculation attached

 

[CDGear]

It would appear that you need to "normalize" the following
dimensions: (Cr, Ro1, Ro2) per the second paragraph of section 3. BASIC GEAR GEOMETRY

This would then give these as:

Cr = 78
Ro1 = 11.5
Ro2 = 68.5
Rb1 = 9.866
Rb2 = 63.429

using these would calculate
Rm1 = 10.5
p1 = 2.955
p2 = 23.722

Using these would yield a value of 0.118 for the I factor

Hope this helps

Chris

 

[Andrewk89]

Quite easy to miss a key point, but it actually doesn't matter. When I was updating the calculations per your comment, I realized that I had erroneously used the helical gear formula (Eq. 4.4) for the pinion radius of curvature instead of the spur gear formula (Eq. 4.6).

When I corrected the formula for rho(1), I got the correct pitting resistance factor without non-dimensionalizing the variables.

Attached is the corrected calculation.