Vibration Isolation

A vibration isolation system prevents one object from affecting another. Such systems are used extensively to isolate machinery (industrial and marine), civil engineering structures (base isolation in building, bridges, etc.), and sensitive components from the foundation/base. Vibration isolation schemes are to 1) reduce the propagation of base vibration to the isolated object (machinery) and 2) abate the transmission of vibration energy of machinery to the base. Moreover, in vehicular/marine, some industrial machines (such as mechanical presses), as well as seismic applications, isolators are also expected to lower the impact of shock from base to isolated object or vice-versa.
Passive vibration isolation solutions, normally provided by elastomeric (rubber) or metal spring elements, do not address all of the above concerns, simultaneously; their effectiveness in one of the above areas is normally at the expense of either not being effective in or adversely affecting other areas; see the parametric study presented below.
The engineering of passive mounting arrangments is rather complex and involves optimizing the mounting variables, in terms of their stiffness, damping and mass attributes so that their performance is acceptable (but not necessarily ideal) satisfying all of the above objectives. Utilizing the adjustability of air springs, feedback controls, and the magic of embedded computers DEICONS has devised Computer Controlled Air Isolation System, an uncompromising solution satisfying all the requirements of an ideal mounting system.
Please contact DEICON to discuss your sound and vibration isolation problem. If we do not already have the solution for it, we can analyze the problem, customize DEICON's ideal mounting solution or design other vibration isolation treatment for it, and oversee its implementation.
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In a parametric study the impact of varying the attributes of an isolated system (damping, stiffness, and mass) on its vibration isolation effectiveness is analyzed. The results of this parametric study are summarized in Table 1. The table shows the positive (enhancement), negative (deterioration), and zero (no effect) impacts of change in damping, stiffness, and adding mass (to the isolated object) on the vibration isolation effectiveness.


Table 1 Impact of mount parameter variation on vibration isolation effectiveness

 

Effect on

Shock Isolation

Low-Frequency Vibration Isolation

High-Frequency Vibration Isolation

Parameter Variation

Damping

increase

+

-

-

decrease

-

+

+

Stiffness

increase

+

-

0

decrease

-

+

0

Added Mass

increase

-

+

+


Evident from Table 1, no single solution enhances all the attributes of a mounting mechanism isolating an object subject to simultanious vibration and shock loadings, e.g., a diesel-generator on-board a watercraft. For example, enhanced low-frequency vibration isolation performance of a system with low damping and low stiffness is normally achieved at the expense of excessive displacement of the mounted mass around the resonant frequency of the system diminishing its shock isolation effectiveness.
Only adjustable mounting schemes under the control of a computer such as DEICON’s Computer Controlled Air Isolation System, can address the conflicting requirements on the attributes of isolators, providing both shock and vibration isolation. DEICON’s adjustable vibration isolation system uses air as the isolation medium. Air provides 1) the highest degree of isolation of any type vibration isolator, 2) negligible overall damping enhancing high-frequency vibration isolation, and most importantly 3) adjustability.



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The impact of varying the parameters of a mounted system (damping, stiffness, and mass), one parameter at a time, on the isolation effectiveness in a one degree of freedom system is analyzed.

The spring-mass-dashpot system of Figure 1 is frequently used as a one degree of freedom approximation of an isolation system. The goal is to isolate the base from the vibration of the mass caused by the excitation force F, i.e., lowering the force transmitted to the base Ft , while avoiding excessive vibration of the mass (bouncing) due to shock excitation at the base. The spring dashpot pair, known as Voight model, is commonly used to approximate the dynamics of commonly used viscoelastic isolators such as rubber mounts.

Figure 1 A simple mounting system

Figures 2(a), 3(a), and 5(a) depict the magnitudes of the frequency response functions (FRF) mapping the vibration excitation force (F) and shock excitation (x_base) to the transmitted force Ft and mass displacement x, commonly known as transmissibility, for varying a) damping, b) stiffnesses, and c) mass (by adding a dead weight to the mass), respectively. Figures 2(b), 3(b), and 5(b) depict the magnitudes of the frequency response functions (FRF) mapping the vibration excitation force (F) to the mass displacement x, for the same parameter variations.

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Damping Variation

Clear from the transmissibility plot of Figure 2(a), lower damping does not affect neither the transmission of force from the vibrating mass (e.g. diesel generator) to the base nor the transmission of shock inputs at the base to the mass at low frequencies, increase them around the resonant frequency, and lower these transmissions at high frequencies. This is why in most isolation applications the mount is selected to be highly underdamped with the resonant frequency of the isolated system less than 70% of the lowest vibration and shock excitation frequencies of the system. Of course, when either or both excitations (vibration and shock) are broadband (like discontinuous shock excitation common in marine applications), the above stated guideline will not be effective; broadband excitation will set off the resonance of the mounted system causing the mass to bounce. Figure 2(b) indicates that variation in damping only influences the transmission of vibration force to the motion of the mass at resonance; the lower the damping the more severe the resonance.

The reason practitioners do not embrace the obvious solution to the resonance problem, i.e., adding damping, is that damping deteriorates the high-frequency vibration isolation effectiveness of the mount resulting in transmission of noise.

Figure 2 Transmissibility and displacement Frequency response functions for different damping ratios.

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Stiffness Variation

Figure 3(a) indicates that the softer mounts (mounts with lower stiffness) decrease both the transmission of force from the vibrating mass to the base and the transmission of shock inputs at the base to the mass over a reasonably large range in frequencies (which normally coincide with the frequency range of interest in most mounting applications), including resonant frequency. Figure 3(b) indicates that soft mounting adversely affects the transmission of vibration force to the motion of the mass at lower frequencies, including at resonance; the lower the stiffness the larger the low-frequency motion of the mass caused by vibration forces. This adverse effect of soft-mounting would be tolerable if a) the excitation is harmonic (not impulsive such as shock inputs) and b) the resonant frequency of the isolated system is below the lowest excitation frequencies of the system.

Figure 3 Transmissibility and displacement Frequency response functions for different stiffnesses.

Frequently, e.g. in marine vibration isolation applications, the isolated object is excited by both periodic vibration, e.g., the force of firing in a diesel engine, and impulsive shock, e.g., the base excitation in a marine diesel generator or any other vehicular engine applications. The base excitation has normally a broadband spectrum, so there is always some energy at the resonant frequency(ies) of the isolated system. If the mounts are highly underdamped, the vibration amplitude at this (these) frequency(ies) becomes excessive. This problem can be addressed by adding damping to the mount. But in vibration isolators, as stated earlier, broadband damping which results in high frequency vibration (and noise) transmission is avoided.

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Adding Mass
Increasing the mass of the isolated object by adding an auxiliary mass to it, in order to vary the attributes of the isolation system, is a common high-frequency vibration isolation practice. One way of realizing this scheme is to install the vibrating machine on a massive concrete block and isolate the block from the foundation by rubber or neoprene isolators. A schematic of a simple mounted system with an auxiliary mass, m, added to the isolated body (machine/equipment), M, is shown in Figure 4.

Figure 4 A simple mounting system with added mass (m)

Figure 5(a) indicates that the addition of extra mass to the mounted object, enhances the high-frequency shock and vibration isolation attributes. It does not change the low-frequency dynamics of the system, except at resonant frequency. It also lowers the resonant frequency of the system while making the system more oscillatory (underdamped); note that damping ratio is inversely proportional to the square root of the mass. Again, as in soft-mounting the adverse effect on resonane would be tolerable if a) the excitation is harmonic (not impulsive such as shock inputs) and b) the resonant frequency of the isolated system is below the lowest excitation frequencies of the system.

Clear from Figure 5, the high-frequency isolation enhancement of added mass will be realized when a sizeable amount of it, e.g., at least half the mass of the machine being isolated, is used. To put this in perspective, for isolating a 170 KVA diesel generator weighting about 2 tons, one needs to attach a slab of steel weighting at least 1 ton to the machine to make the effort worthwhile. The inconvenience of implementing this solution, especially in retrofit applications, not to mention the potential problems associated with excessive static displacement of the mount due to the added mass is clear. In addition this solution has a considerable weight penalty which is highly objectionable in vehicular/marine applications.

Figure 5 Transmissibility and displacement Frequency response functions for different added masses (m).

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Double (Two Stage) Mounting

A somewhat common practice for enhancing the high frequency vibration isolation, is to place another mount between the added mass m, see Figure 4, and the isolated object M. This arrangement, called double mounting is discussed in detail here.

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Computer Controlled Air Isolation System
Advanced Air Isolation Systems for Precision Devices
Sound and Vibration Control