Your question is similar to how many go to paradise and how many go to hell. Maybe AGMA has some qualitative information as it comes out from the chart that contains the limits of destruction of speed reducers from different causes like the ones you mentioned.

I have an AGMA report published in 1999 called "Gearbox Field Performance from a Rebuider's Perspective" written by Eng. Charles Schultz, former VP of Brad Foote Gear Works, which gives information about the repair service of over 2000 different gearboxes that had been repaired over the last 15 years before the date the report was published. From 1999 to today I haven't found any updated data.

The following paper of AGMA may be of help. If you need something from AGMA standards please let me know. Good luck with your research.

07FTM19. How to Determine the MTBF of Gearboxes Author: G. Antony Mean Time Between Failures (MTBF) became a frequently used value describing reliability of components, assemblies, and systems. While MTBF was originally introduced and used mainly in conjunction with electronic components and systems, the definition and application for mechanical components, such as gearboxes, is not broadly available, used, or recognized. In the field of gears it is difficult to obtain an MTBF from the manufacturer due to the lack of applicable, generally recognized definitions and standards. The paper will evaluate, compare and suggest ways in determining a gearbox MTBF based on the already established, proven, design calculation standards and test methods used in the gear design. ISBN: 978-1-55589-923-3  Pages: 9 2006 PAPE

Dr. Costopoulos,

I've never heard and I'm interested to know more about MTBF. Would you have the AGMA 07FTM19 available? 

And if you need anything please don't hesitate in contact me as well.

Thank you very much for your tips.

Dear Horatio, This is not a standard. It is a paper presented in AGMA proceedings back in 2006. I do not have these proceedings. Only standards for use by my colleagues and of course you.

Ok, thank you anyway.

FYI, attached a Technical article from Power Transmission Magazine regarding MTBF.

http://www.powertransmission.com/issues/0408/antony.pdf

For gear failures, the following is an industrial guide: in every 10 gear failures two come from pitting and spalling, two come from scuffing, two come from bending fatigue, two come from impact load, and the rest two from several other causes.