I have a case where vibration is measured at DE of an alternator driven by diesel engine (10 cylinders). Increased level of 5x component (firing frequency) has been observed. What could be the possible fault condition ?
What is the cylinder arrangement ("V" or in-line), 2-stroke or 4-stroke, and the firing order? Something in the drive train that is intended for the reduction of torsional vibration, an elastomeric coupling for instance, may have degraded and caused the 5th order to increase.
As much of my resource material is quite old, I unfortunately I don't have the phase diagrams for any V-type engines. You should be able to get that information readily from the engine manufacturer. It will tell you if the "5x component (firing frequency)" is a major or minor order. You might try contacting them yourself, again you need the firing order which you didn't supply.
With regard to the phase diagrams see the paper at Torsional Vibration in The Diesel Engine. The content is very old but go to page 55 and you'll see a table of the anticipated minor and major orders for a 8 cylinder 4-stroke diesel.
I previously mentioned an elastomeric coupling. Is one being used, and if so have you inspected it? Typically, these couplings have some form of a rubber element. The rubber elements tend to age over time and get hard hence driving the coupling stiffness upward. This can change the torsional response of the system over time.
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