Excellent Post Oscar and great work in determining the failure mechanics and evaluating bolt history. This post should be read by all incoming engineering freshmen to indicate the processes undertaken by working engineers in solving a problem. It's a great example of what to do when confronting a situation and retrieving data relative to that situation. Many thanks for your post.
A lot of us are not used to consideing that a bolt under load still has a reesonant frequency, and I would not have thought that a tooth-frequency vibration would be strong enough to cause any problems. But eveidently such a signal can contain quite a bit of energy, which is a good education for the rest of us.
I can explain about how the different bolt changed the resonant frequency, which is that the stiffness term affaect resonance at least as much as the size term does. That is an EE simplification, which comes from needing to design electronics that idi not resonate and break the leads of the components. THAT can be a real problem.
@Leigh: Actually, the second bending mode is more like an S shape, but you've got the right idea. And I think you answered my question about how the resonant frequency was changed. I was thinking of it as a free body, but obviously the bolt tension will affect the frequency, and a guitar string is a great analogy.
Being an EEng, some of the terminiology I'm not familar with.
When you say first and second modes of the bolt. Does that mean bending axialy like a V is the first mode and a W the second mode or does it refer to axial compression? If so doesn't the clamp point and tension change this frequency (Like a guiter string changes frequency depending on tension and bridge position). How do you actually measure this tension?
And how on earth do you attach sensors to bolts inside a complex spinning gearbox?
LOng ago, way before college, I did wonder about the relationship between bolt torque and bolt tension. That was because I had discovered that just a bit of grease or oil would make a bolt run down much easier. I never did get a good explantion about it, other than being told to clean the threads with thinner to avoid the bolts breaking. Much later I learned about bolt preloads and how the clamping load was the critical parameter.
One of the inherent problems with bolt failure is the tightening to a specific torque vs tightening to a required tension. A blob of grease on the threads of a bolt that is tightened to the same torque as a bolt with no applied lubricant can have a wildly different tension for the same bolt types.
Forensic engineering analysis can be one of the most challenging and rewarding tasks for an engineer. It's a wonderful way to use the applied science that we all learn in school, and one of the great fields for an engineer to be involved in. Hats off to this author.
Oscar, thanks for a timely article. Bolted joints, modal analysis, waterfall plots... this fits into some of the stuff I'm working on right now.
The only thing I don't understand is how switching from steel to a nickel-based superalloy gave you a huge shift in the resonant frequency.
As I understand it, for a given geometry, the resonant frequency depends on modulus and mass. As far as I know, the elastic moduli of the two materials are almost identical. Nickel-based superalloys typically weight 5-10% more than steel, but you said there was a weight reduction -- presumably as a result of going from a full shank to a reduced shank. But it seems to me that any mass reduction would have to be relatively small.
I can see how you got out of a fatigue problem by going to a higher-strength bolt with a reduced shank (and rolled threads!), but I don't understand how you were able to change the resonant frequency.
This is another great example of how experience can trump technology for solutions.
When I read the word "bolt" in the title, my mind immediately went to the new Bay Bridge here in San Francisco. There's a new problem with threaded rods (being called bolts in the news).
The Brooklyn and Golden Gate Bridges combined took less time to build and they are still standing. I don't doubt that they approached problem solving in the same way. "Sometimes less is better." - well said!
Oscar, when I first started reading your article I was reminded of working on a gearbox in an old sportscar my brother had. Actually, your situation was different. My brother ripped off the heads of a few case bolts becuase he did not pay attention to the high torque if the tool we were using. It was a real pain.
I am very impressed with your use of charting in solving this problem. Good work.
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