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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 3, Detuning and Proving Resonance

Section 6, Bump Test Vibration Spectrum

A typical spectrum obtained from a bump test will look similar to regular spectra obtained from vibrating running machines, except that the various peaks will not necessarily be related to running speed. Instead, the peaks will relate to the resonant frequencies of various machine parts that were displaced by the bumping action. Typically, there is one peak that has a much higher amplitude than the rest. The largest peak usually represents the resonant frequency of the part that receives the bump test hammer's impact, although not necessarily in all instances. When the hammer impacts one part, it causes a deflection and release. However, the impact also causes some deflection and release on parts that are connected to the part receiving the hammer blow. If the other part is more flexible or has much less damping, then that part may cause the largest amplitude peak. The bump test, using an FFT, may not always determine with certainty which peak represents a specific part.

Once the bump test indicates that some part is resonant to a source vibration, determining the exact part is possible while the machine is running. When the machine is running and the suspect part is resonating, the point-to-point method of obtaining and plotting amplitudes to obtain the part's mode shape should help identify which parts are resonant and which are not.



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