The Million Dollar Musical Instrument
I remember vividly the moment I learned that I would get the opportunity to design my very first NVH production test machine. We were a not-quite-yet-launched startup coming to the rescue of a machine builder that had been charged with a purpose-built axle NVH tester. Eager and optimistic, I brought NVH know-how and years of experience to this newly formed alliance. Of course, we worked hard, learned much and ultimately delivered a machine that is celebrating its 11th year and millions of completed NVH tests, but in the beginning, to say things did not go according to plan is an epic understatement.
It began with the past. A legacy of attempts to measure gear mesh energy where the machine’s own background noise is amplified by resonance and contaminated by ambient inputs from plant traffic drowned the data. As the NVH guy on the new project, my goal was to design a driveline that would load the gearset without any machine self-noise or plant ambient contamination at all.
I was proud of my solution. First, imagine a giant barbell equivalent to the weight of a car spinning with a gearset in the middle. Now put a brake on each side and apply one at a time. This arrangement created a smooth passive torque load across the gears without the need for drive motors. The driveline was big and beefy to eliminate the possibility of resonance and to support these giant inertia weights. The result was a pure torque loading and the most noise-free harmonic signature of gear mesh you could imagine, and it also destroyed every gearset we tested.
They still tease me about these giant inertia weights. Today they are rusting away out back of some building northeast of Detroit, but the stiff driveline is still there. A couple of well-isolated motors create the torque and there are isolation couplings in front of them to take up the tiny deformations that are typical in a gear and that were amplified by my giant barbell with nowhere to go except into the surface finish of the gear teeth. There was also the discovery of a 40 second long beat period caused by the inopportune utilization of a .998:1 pulley ratio (you do the math) that destroyed repeatability, a realization of huge side loading into the pinion bearing caused by loose machining tolerances in the pinion flange – there is that stiff driveline again. The list goes on and touches on lubrication, temperature, part clamping, the weather (OK, not the weather since we don’t actually measure the noise part of NVH on this one).
So, 11 years down the road and a dozen or so successful machine designs for all types of rotating machinery and the enduring lesson of that first one is that the only thing to safely assume is that your assumption is wrong. These machines are big, heavy, strong, stiff and delicate. NVH data is usually at frequencies beyond what rigid body intuition will tell you is enough, and they will behave in ways that only living with one for a few years can teach. I know it’s only motors, belts, bearings, isolators, couplings and clamps, but to me each one is much more like a finely tuned musical instrument than a big production test machine. Making one perform requires attention to details and an expectation that it is going to teach you something new – and all you need to do is be ready to listen.
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