What a Vibration Test Is For
5 sections · about 51 minutes of reading · 9 figures
After this chapter you can read a test requirement and say what it is trying to prove, which step of the hardware's life it represents, where its numbers came from, what "pass" means, and what the report will have to show.
About the Author. Kevin Cross spent nearly fifteen years at Sandia National Laboratories, where he ran vibration and shock testing at its remote hazardous test facilities, qualifying complete nuclear weapon systems as well as their components, with extensive experience bringing major vibration and shock test systems into service, from procurement and installation through commissioning and operator handoff. He now runs KC Service and Consulting, where he services, starts up and repairs electrodynamic shaker systems and consults with aerospace, defense and industrial labs on test planning, setups and new systems, and this series is drawn from both jobs.
The requirement was simple: show that the article could survive being moved around by forklift. It had been moved by forklift for fifty years. The handling gear that carried it in the field was older than most of the people in the room, and it had never failed.
The environments team did not have much real data for that step, so they built a mass mockup (a stand-in with the article's mass and center of gravity), bolted a field data recorder to it, and went looking for forklift shock. They recorded a great deal of it. They drove it across railroad ties, over four-by-fours laid on the concrete, into potholes deep enough to bottom the suspension. Every run went into the file. Then the file was enveloped, and the envelope got the margin that qualification requires, and the profile went into the test plan with a document number on it.
The article went on the shaker in its handling gear, and the gear failed. Not the article; the gear that had carried it through every depot and flight line for half a century without a crack. We had built a test that was harder than anything the sequence contained and then proved that the hardware could not survive a life it had never lived. The data were real; the step was not. Nobody had placed the test on the sequence before the profile was written. Nobody had asked what a good test was. Nobody had asked which step the data represented, or whether a forklift driver had ever, once, aimed for the railroad ties.
That is what this chapter is about.
It is also what the series is about. The people running a vibration test are most often mechanical and aerospace engineers in a structural dynamics role, using an electrical machine for a mechanical purpose. The shaker system as a whole, the amplifier behind it and the control loop that drives it, is the part of their work they were taught least, and that gap, more than the standards, is where tests go wrong. This series is written to close it, for them and for the technicians and report signers who work beside them, and Volume 1 starts with the machine, one piece at a time. This first chapter has no hardware in it: whether a machine is good enough depends on what the test has to prove, so the test comes first, and the shaker starts in Chapter 2.
This volume is Volume 1, the machine: the shaker system, the facility, and keeping both running; Volume 2 is the test, and the two together are what it takes to start testing. Volume 3, on multi-axis and multiple-input multiple-output (MIMO) testing, is for the reader who already runs tests, and each volume stands alone, giving what its reader needs now and naming the volume that carries the depth. Where this volume meets a resonance, a Q or a transmissibility it gives what the reader needs in the place it is needed; Volume 2 teaches them in full at the front of its sine chapter, and every test type there leans on them.
A lab with only small machines, permanent-magnet shakers, modal shakers or a small system with its amplifier and field supply in one cabinet, reads this volume with a shorter path. Chapter 2 and Chapter 3 apply in full, less the field supply of 3.3 where the field is a magnet, and the bench shaker of 2.6, where the operator is the interlock, is that lab's own paragraph. Chapter 4 applies to the amplifier whatever its size. In Chapter 5, 5.2 is cooling at the small end and stands in for most of the chapter. The fixtures and mounting of Chapter 7 apply unchanged. Chapter 8 applies with the field-coil items dropped on a magnet machine. Head expanders and slip tables (Chapter 6), the facility (Chapter 9) and the machine's side of the plan (Chapter 10) shrink: read them for the checks (the moving mass, the overturning moment, the electrical service, lifting, hearing and lockout) and pass over the plant, pits and isolated bases a small machine does not have.
