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The Case of the Ruinous Resonance
Myron J. Boyajian, President, Engineering Consultants -- Design News, August 12, 2007
Problem-solving requires a flash of inspiration, an open mind to try new things or to try old ideas in a new way. This story illustrates how technology in one field helped solve a problem in a seemingly unrelated field.
The Scene of the Crime
In the day, as staff engineer for a major forklift manufacturer, I learned one of the most popular products, a 6,000/8,000 lb capacity, engine-powered forklift was experiencing widespread field problems (this was in the days when we had problems, not “issues”).
This model had received an improved engine, more efficient hydraulics, some re-styling and a few structural changes. Once in the field, customers began to complain exhaust pipes were cracking near the mounting clamps and then breaking, giving a hot rod roar.
These complaints went up the line from customers to service and engineering management. Of course what goes up must come down, which it did, landing on the shoulders of the truck product design engineer. We soon met to work out a solution to this growing problem.
The Investigation
Gathering field data, we listed loads, length of runs, lift heights, gasoline vs LP fuel and truck capacity (6,000 or 8,000 lb capacity). We considered vibration and shock loading, but rejected that idea as most of the troubled trucks were used in long, fast runs between factory buildings. Only a few trucks with broken pipes operated on shorter runs, but those had tall masts with high lifts. All signs pointed to prolonged high, but governed, engine revs, but what was the connection?
Now, here is where the unrelated technology comes into play. Not long before this forklift exhaust problem, I had been building a miniature pulse jet engine and my mind was filled with thoughts of resonance, tube lengths and the like. In our engineering meeting, I blurted out I thought maybe the exhaust pipe was resonating and breaking at nodal points where the mounting clamps were located. The truck design engineer brightened in agreement and asked how we could prove resonance was the culprit.
We started to calculate the frequency and wavelength of the exhaust pulses with the engine at full chat and compare that with the distance between the mounting clamps. As we began calculations, we looked at each other saying in unison (resonance, again), why not just run the engine and try to measure pipe deflections in situ.
First, we had to confirm vibration as a cause of breakage, and second, to visualize how to suppress the vibration. We did have a high-intensity strobe light, ideal for this job. We chalked a wide line along the horizontal exhaust pipe, placed the truck in a dark corner of the shop and started and revved the engine. In the strobe's flash we soon saw the pipe resonate and vibrate like a string on a violin. We measured almost one-half inch in the center of the span between the clamps.
The Smoking Gun
On this model, production changes included a relocated exhaust manifold port, longer exhaust pipe and relocated pipe clamps. We confidently concluded the changed mounting points and pipe length resulted in pipe resonance and subsequent cracking. We prototyped and installed an additional clamp and mounting bracket located midway along the pipe and the vibration stopped. A mandatory production change and field kits were issued and broken exhaust pipes soon ceased to be a problem.
Oh yes, what about my pulse jet, the inspirational source of the solution to this conundrum? Well, I did get it operational for a few seconds. After a resounding Bwaaah! (if you have ever heard a pulse jet, you will know what Bwaaah! means) my home-brewed reed valves burned and were blown out the tailpipe and inlet nose cone in a shower of sparks. My pulse jet experiment had ended, but its useful lessons remained.
