Vibration Isolator-Induced Resonance

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Vibration isolators are mechanical elements that prevent/limit the transmission of vibrations generated by machinery to the ground and vice versa.
These elements, which generally contain viscoelastic materials, perform the vibration isolation function based on their mechanical properties: ‘Stiffness (K-spring coefficient)’, ‘Mass (M)’, and ‘Damping (D)’. There are also versions where the spring and damping elements are mounted separately.

Case Study:

In ‘Motion Amplification’ measurements conducted on a fan experiencing structural vibration issues, ‘Rigid Body Modes’ resulting from the application of isolators have been observed.
Rigid body modes are natural frequencies (deliberately and intentionally created) resulting from the stiffness of the isolator and the mass of the machine mounted on it.
The vibration isolation function begins at 1.414 times these natural frequencies, which are formed as a result of the mechanical properties of the isolator. Therefore, from 1.414 times the highest of these natural frequencies onwards, vibration problems such as imbalance and misalignment occurring in the machine do not pass to the ground, or are not transmitted from the ground to the machine (or are transmitted to a certain extent).

To identify the structural vibration problem experienced in this equipment, an acceleration test was first performed, and the spectrum waterfall graph seen on the left was obtained.
As can be seen, vibration increases occur in three different frequency regions. Upon separate examination of these frequencies, it was understood that they are solid body modes resulting from the application of isolators.
The natural frequency observed at 16Hz is an elastic structural natural frequency and has not been included in this case analysis.

The X, Y and Z coordinate directions and coordinate system are superimposed on the photograph shown on the right.
The natural frequencies detected in this fan example at 8Hz, 11Hz and 24Hz, along with their mode shapes (vibration shapes), are listed below.

In the motion amplification video shown above, the translation mode in the Y direction is clearly visible. This mode occurs at 8Hz and causes the fan to vibrate as seen above during the equipment acceleration phase. This mode is quite far from the fan’s nominal rotation speed of 1480RPM (24.8Hz) (except for 20%), therefore it does not pose a threat.

This motion amplification video also shows the rotation mode around the Z-axis. This mode occurs at 11Hz and causes the fan to vibrate as seen above during the equipment acceleration phase. This mode is quite far from the fan’s nominal rotation speed of 1480RPM (24.8Hz) (except for 20%), therefore it does not pose a threat.

This motion amplification video shows the rotation mode around the Y-axis. This mode occurs at 24Hz and causes the fan to vibrate as seen above during the equipment acceleration phase. This mode is very close to the fan’s nominal rotation speed of 1480 RPM (24.8 Hz) (within 20%), therefore high vibration problems occur in the equipment when it is unbalanced. Vibration can be reduced by on-site balancing, but high vibration values will be seen again if imbalance occurs again.
The reason this mode occurs at 24Hz is that the mount stiffness (and therefore their resistance) and/or the number of mounts is incorrect. Sometimes this occurs during the design phase (incorrect design/mount selection), but more often the mounts lose their properties over time and become stiff.
Therefore, the selection of mounts should be carried out through engineering work, and the mechanical properties of the mounts should be periodically checked.