Known Unknowns

In my last posting I mentioned the need for humility and skepticism. Before anyone think this is heading off into political commentary, let me explain why I think these are important attitudes to bring into your gear work. One of my favorite “gear guys” of all time, the late Don McVittie, was fond of saying “In God we trust, all others bring data.” We hear many claims about “game changing breakthroughs” and “important new understandings” over a long career in any field. The magazine prides itself on publishing only peer reviewed articles but even with that proviso there is a risk of information coming through that will not hold up over time.

I discovered that there was no SWAT team of “gear police” waiting to swoop down on sloppy or deceitful practitioners when I started working in the gearbox repair business. Some of the things we had to fix were obvious to us but not to the original builder. The best “data” on what works and what doesn’t work comes from the field. Testing, especially accelerated life testing, is helpful but not conclusive.

Testing suffers from modeling “errors” that can, in some instances, change the failure mode. When you increase the load or the rotational speed to reduce the life cycles needed to “confirm” the design you can switch from a durability failure to a strength failure. You can also mask lubrication problems. Send that gizmo out into the field where the oil level never gets checked or the assembly gets covered in a foot of dirt and a completely different situation develops.

We have lots of confidence in our rating formulas; eighty years of experience and several generations of revisions makes it unlikely that a failure will occur that we cannot explain. But what about the “successes” that surprise us? I have seen parts that look to be identical deliver widely varying service lives. Back when we made race car parts, the same clutch dogs that failed within 100 laps for one driver survived 300 laps with a different guy at the controls.

Racing also shocked me with the stress levels routinely experienced. Watch a Top Fuel dragster crew completely rebuild their motor between runs yet never touch the rear axle gearing. Then run a typical ten or twelve inch diameter spiral bevel set through your rating software at 5,500 hp at 7,000 rpm. Count the decimal places in the predicted life. We have a lot to learn about short cycle gear strength.

You may say to yourself “those are perfectly made, high dollar components.” I won’t disagree. And there are failures we do not see on the television broadcast but the fact is our analysis does not explain why they survive at all. Wouldn’t it be fun to see some “honorable data” on highly stressed parts?

About Charles D. Schultz 639 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.

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