AGMA 925-A03, Effect of Lubrication on Gear Surface Distress

Comment restated:

I’ve been working with AGMA 925-A03, Effects of Lubrication on Gear Surface Distress. I’ve followed the example calculation in Annex D starting on page 43. I’ve been able to duplicate all the results, with the exception of the last result for “Probability of wear” on page 48. The result listed is Pwear = 5% or lower.

I’ve been able to duplicate all the numbers that go into the result:

(y) = 0.425354
(muy) = 0.215956
(Sigmay) = 0.112623
and x = ((y-muy)/Sigmay) = 1.859273

These results are calculated per Annex B – Normal or Gaussian probability. On page 39, under “Evaluation of Q” it states that if the absolute value of x is greater than 1.6448, then Q = 0.05. Since our value for x, 1.859273 is greater than 1.6448, our value for Q is 0.05.

The section goes on to state that if x is greater than 0, then the probability of failure = 1 – Q. Since our value for x is positive, our probability of failure is 1 – .05 = .95 or 95%. This does not agree with the 5% given on page 48.

What am I doing wrong?

AGMA’s response:

After considerable discussion, the Helical Gear Rating Committee concluded that the calculated results of the example problem in annex D of AGMA 925-A03 are correct. However, there may have been confusion introduced into the document when the reader compares the discussions in annex B, “Normal or Gaussian Probability”, clause 8.2.2 of the information sheet, and annex D.

To clarify the intent of the example problem, on page 48, where the risk of scuffing and the risk of wear are calculated using the discussion in annex B, the conclusion statements would be clearer if worded as:

  • Probability of scuffing failure … = 5% or lower
  • Probability of wear failure … = 5% or lower

The committee intends to discuss the text related to this subject during their periodic review of the document.