Now that the correct spring deflection and load capacity are selected, the applications engineer can go through the various options in selecting the proper vibration isolator. And by decreasing the natural frequency of the isolation system, we increase the ratio of fd/fn, which will create a more efficient system. Typically, good value for deflection of the isolation system is about five to six times that of the structure, assuming we are still in the range of isolation efficiency. As we decrease the natural frequency of the isolation system, we decouple the natural frequency of the system from that of the slab. Solving this problem requires additional displacement in the selected isolation system. Therefore, it is very likely that amplification will occur if this system applies its vibratory force at the center of the span. By comparison, this is roughly the same deflection required for a 95% efficient system running at 1000 RPM on a rigid foundation. For a 20 foot beam span, the allowable deflection is 0.67 inches. Per current design guidelines, ASD & LRFD, a typical mid-point deflection in a beam is L/360, where L is the length of the beam. We can quantify its stiffness for illustration. The concern is that placing a vibrating system on top of this slab at roughly the same frequency will create a two mass and spring system which may create a resonance between the two systems.Īs can be seen from the above sketch, the stiffness of the slab is a part of the total system. At the center of a mid-floor slab, natural frequencies may exist that are relatively close to theoretical natural frequency for an efficient isolation system based upon a rigid foundation. From the transmissibility equation, it is seen that the ratio between the disturbing frequency and the natural frequency of the isolation system is the driving factor. For installations above grade, the stiffness of the structure may play a significant role in selecting proper isolation.
For the sake of vibration isolation, this is true. It is generally assumed that a slab on grade is infinitely rigid and cannot deflect. Installations above grade within a structure must be analyzed differently than those on a structural slab on grade. For specialty applications that fall outside the realm of these general categories, consult VMC Group’s Engineering Services Division for additional support. Generally, this is most of the information required. Is the equipment installed on grade or above, within the building?.Installation location within the building.Number of isolation points and weight distribution at each point, if available.Additional information required includes: However, more information is required to make the proper isolator selection when leafing through this catalog. It’s important to know the operating speed of the equipment when selecting vibration isolation materials and isolators.
As the required isolation efficiency increases, so does the cost and complexity of the isolation system. As the application gets more critical, a computer chip manufacturing plant, for example, isolation efficiencies in the high 90% range may be required. Generally, in most non-critical applications, a minimum isolation efficiency of 80% is acceptable.
To calculate the natural frequency of the isolation system, when given the operating deflection in inches, we use this equation: To measure transmissibility, we need to know the deflection, or natural frequency, of the isolation system. A properly designed isolation system will attenuate the vibrations to a level necessary for the application. Though, for most systems where the energy is decreased, we use the term attenuation. For this increase, we use the term amplification. It is also possible to increase energy through the isolator system. This factor is called the transmissibility. The amount of energy that passes through an isolator is measured as a percentage of the vibration energy produced by the equipment. For example, a heavy, sudden shock requires a higher deflection than a constant running engine. Theory shows that as the operating speed of the equipment increases, the required deflection to absorb this energy decreases. By providing deflection, the energy of the vibrating equipment is released over a longer period. The purpose of a resilient isolator system is to provide flexibility in the form of an isolation system that converts energy into a more manageable form.
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