WHAT IS VIBRATION TESTING?

I teach people who operate shaker systems – they conduct vibration tests. I’m writing this article in the first person because I personally had similar conversations ….. about sixty years ago. I’m updating it, however, based on what every generation of my students has told me. Why? So that the story matches today’s shaker technology and testing applications.

Casual conversation. Hello, I’m Ed. Hello, Ed, I’m Wayne. What do you do, Wayne? I use a shaker system to perform vibration tests. Huh? What’s that? What’s a shaker? What do you mean vibration?

“Slow down, Ed. Let me start with that last question of yours. How did you get here today, Ed?” That’s myself, getting ready to explain my activities…

“I drove my car. Why?” responds Ed.

“Well, Ed, what were you feeling as you drove along?” I ask.

A thoughtful expression, then a smile of understanding. “Oh. That vibration. Is it a problem?” Ed asks.

Automotive applications To which I reply “It can be a problem for all the instruments and apparatus, electronic and other, that are hard-mounted to your car. They ‘feel’ it more than you, sitting on an upholstered seat, feel. Your hands and feet rest rather lightly on various surfaces but they sense some vibration. Right? Let’s focus on the radio, firmly attached to the instrument panel (IP). Engineering prototypes of that radio, I expect, were attached to a shaker programmed to reproduce (and somewhat exceed) IP vibrations measured on a prototype automobile, while that prototype vehicle traversed various road surfaces at some proving ground.”

If I haven’t lost Ed, I continue, “That vibration environment is more severe for instruments in the engine compartment and even worse for instruments attached to the engine. And still worse aboard sports cars, racing cars, diesel-engined trucks, tractors, construction equipment, etc.” Most severe: treaded vehicles such as tanks.

“Part of the vibration affecting those instruments comes from the engine and drive train. That part is fairly regular and predictable, relating to engine speed and wheel speed. But there’s also unpredictable, ‘random vibration’ input from the road surface, affected by the driver’s maneuvers.”

“So various automotive instruments come to my lab for vibration testing. Of course, many of them also get thermal, altitude, humidity and other climatic tests in our lab. But I’m the vibration guy.”

Airborne applications. Ed nods, and seems willing to hear more, so I continue. “Everything I’ve said applies to all aircraft, most notably on board helicopters.”

Ed agrees and cites personal experience as a helicopter crew member and in small, propeller-driven personal aircraft. “Lots of highly-necessary electronics on those, for sure, where we can’t tolerate failure.”

His agreement allows me to continue. “Aboard a jetliner in flight, Ed, you feel little vibration because turbine vibrations are at extremely high frequency. Further, engines on some aircraft are way out on the wing.

“However, at high jet speeds, there’s another problem. Air flow over all the surfaces (but especially near jet engine inlets and exhausts) can be turbulent, causing the skin to vibrate. Those vibrations are coupled inward, mechanically and acoustically, to cause everything on board to vibrate. Especially thin sections, like electronic printed wiring boards (PWB). So electronic boxes (sometimes just the wiring cards) come to me for vibration testing, for simulating in-flight vibration conditions.”

“Electronic products have,” I continue, “in the last few years, become so rugged that designers are placing instruments in higher-severity locations. This necessitates ever more severe vibration testing. So much electrical power pours into some of today’s ED shakers that forced air cooling is not sufficient. Some of the newest shakers have fixed and moving coils wound with hollow-core copper wire. Distilled water flows through the windings to carry heat to external heat exchangers. With say 4X the electrical power, such a shaker can generate 2X the force and reach 200g (gravitational multiples) acceleration rather than be limited to perhaps 100g.”

Here Ed interjects, “I know a little about rocketry. Is that an application of liquid-cooled shakers?”

“Yes,” I respond. “Rocket ignition, liftoff, trans-sonic flight and reentry are about the most severe of flight vibrations and lead to the most intense vibration (and acoustic) testing.”

Acoustic testing “Acoustic testing?” asks Ed.

“Yes, though it’s outside my present job. Rather than attach hardware to a shaker, as I do, they place hardware in a hard-walled reverberant room into which very intense sound is poured. Test personnel, of course, stay outside and remotely monitor the hardware under test. Alternately, the test piece can be placed in a progressive wave chamber.”

Shipboard vibrations. “Think about all the electronics aboard a modern ship, Ed, particularly a combatant ship. There’s propulsion, one or more engines driving one or more propellers. And lots of other machines: pumps, fans, etc. On naval ships there’s the possibility of gunfire and enemy action, explosives hitting the ship or exploding nearby underwater and causing the ship to ‘jump’”.

“So various shipboard instruments come to my lab for vibration and/or shock testing. Tests are based on shipboard vibration and shock surveys.”

Time for coffee. Ed’s eyes are beginning to droop, but he seems determined to learn more, so I suggest a coffee break. This perks him up, and he asks the inevitable question.

What’s a shaker? “It’s a controllable source of vibration, Ed. For test purposes, I can match or simulate just about any of the kinds of vibration we’ve been discussing. For many years, all shakers were mechanical, something like the paint shaker at your hardware store. Today, for shaking large loads like complete cars and trucks, electrohydraulic (EH) or servohydraulic shakers are popular. They are driven by high pressure oil. For shaking smaller loads, such as some of the instruments we’re been discussing, electrodynamic (ED) shakers are widely used. Much like the loudspeaker in your home music system, ED shakers are driven by power amplifiers.

Multiaxis shaking. I continue, “From the earliest days of vibration testing on mechanical shakers, we have one holdover: first we shake in the product’s X axis, then in its Y axis and finally in its Z axis. Three tests.”

“That doesn’t sound realistic,” comments Ed.

“Right,” I rejoin. “It’s not realistic at all. Automotive vehicle and seismic test labs shake in multiple directions simultaneously, but nearly all use EH shakers. Multiple-axis ED shaking of smaller automobile components is established in Japan, and in a very few US military labs, but is still relatively rare in the US. The 2008 “G” revision to the widely-recognized Military Standard 810 has a new Test Method 527 on Multi-Exciter Testing.”

Attaching hardware “How do you attach the hardware you’re testing to your shaker?” asks Ed. “By means of a fixture, usually aluminum or magnesium for lightness coupled with rigidity. They can be cast, or smaller fixtures machined from solid stock. Most fixtures are welded,” I answer.

Control? “How do you control shakers?” asks Ed. This guy is wonderful. He’s asking just the right, logical questions.

So I tell him, “Well, if we’re looking for resonances in the product we’re testing, we command the shaker to shake the product at one frequency at a time but to vary that test frequency, to sweep it over a range of frequencies. Here I purse my lips and emit a whistle whose frequency sweeps from low to high (and attracts a certain amount of attention from others nearby.) Meanwhile, we monitor the product for resonant responses.”

“But more realistically, we command the shaker to vibrate randomly and to excite all the resonances simultaneously,” I state, blowing air between my teeth with a “ssssssssshhhhhh” sound.

“Control commands go into the keyboard of a specially-programmed computer.”

Resonances “What are those resonances you keep mentioning?” asks Ed. “Are they bad?”

“Well,” I respond. “Let me remind you of an everyday example you’ve probably sen and perhaps have felt at some car engine RPMs. Have you ever noticed the steering wheel moving with rather large displacement amplitude, larger than the input to the column? That magnification is called resonance. Possibly it annoys you. There’s a slight chance that in a few years that whipping of the steering column might cause bending fatigue failure.”

“When I shake an automotive or ship or land vehicle instrument, I’m looking for, for example, portions of printed wiring boards (PWBs) responding with greater motion than I’m inputting. That flexing may damage PWB wiring, it may damage the attached components, and it will damage the soldered connections between components and the PWB.”

Sealed box. “But what do you do, Wayne, if the PWB is inside a box where you can’t see it?” asks Ed.

“Uh, Ed, I just noticed it’s getting late. I’m flattered by your interest, but I need to check in at home. Listen. Your well-chosen questions indicate a need to know this stuff. Here’s my card. Please look at our Web site www.equipment-reliability.com and join other interested engineers at an upcoming short course. I look forward to seeing you again.”

In coming months Wayne will include these ideas in a series of three-day courses near Portland, Oregon, at Santa Barbara, California, at Boxborough, Massachusetts, at Redford, Michigan and at Orlando, Florida. See www.equipment-reliability.com for Wayne’s schedule and http://www.vibrationandshock.com/book.htm for his text. Also
http://www.equipment-reliability.com/wayne.htm for Wayne’s biography