Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 3, Detuning and Proving Resonance
Section 8, "Reverse Bump" or Plucking the Suspect Part
Very large columns and beams, such as main steel support structures of buildings, do not as easily respond to the regular bump test. A structure such as a platform supported by columns will have individual resonant frequencies for each column and the platform. However, the total structure as a combination of columns and platform, etc., also has several flexibilities depending on the direction being considered. For example, a horizontal force can be applied in one direction at the top of the structure. Assume that the force is applied to the horizontal beam supporting the platform. The force will deflect the beam but will also cause the whole structure (including columns) to deflect as well. A regular impact bump test can be used for the horizontal beam, but will not necessarily work well for the total structure that includes all the vertical columns, beams and platform. To determine the resonant frequency of the total structure, a "reverse" bump or pluck test works very well.
Usually a clamp is attached to the top of the structure. A strong rope is tied to the clamp. Tension is applied to the rope using any practical means such as pulling with a turnbuckle, pulley, "come-along" or even a lift truck or tractor. If the structure is small, such as one supporting a fan or the total frame of a vertical pump, tension on the rope deflects the structure by several mils. For a very large structure, such as a tower of several stories, the structure can be deflected as much as a mm or two without harming the structure.
Position the pickup and set the vibration instrument for the expected frequency range. For an FFT or RTA, set for "peak hold." For a tuneable instrument, use the same procedure as for Filter Out/Off. Now cut the rope. The structure will spring back, giving its resonant frequency in a manner similar to regular bump tests. The rope will need to be retied, tension applied, cut, etc. Repeat the procedure several times to make sure repeat readings are obtained. It takes more time and patience than regular impact bump tests, but often reveals that the magnification due to resonance is not that of an individual part but instead that of the total structure.
The previous descriptions are meant to be used as a guide only. With the wide range of instrument types and capabilities available today, more specific instructions on settings, etc., cannot be given but can be obtained by consulting the instrument vendor.
This textbook contains only part of the information in our Practical Vibration Analysis seminar.