THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

604 20. Vibration and Vibration Controltemperature, humidity, corrosion, presence of solvents and they allow maximumdeflections. But springs have almost no damping and hence can lead to very hightransmissibility at resonance. But dampers can be included in parallel with springsto minimize resonance effects.Elastomeric mounts are generally constructed of either natural rubber or syntheticrubber materials such as neoprene. Synthetic rubber is generally far moreimpervious to environmental effects than natural rubber. These types of mounts aregenerally used to isolate relatively small electrical and mechanical devices fromrelatively high-forcing frequencies. Elastomeric compounds inherently containdamping. Rubber can be used in either tension, compression or shear modes, but itis generally used in compression or shear and hardly ever used in tension modes.Isolation or resilient pads include a variety of materials such as cork, felt, fiberglass,and special plastics. These items can be purchased in sheets and cut intogaskets to fit the particular application. Also, they can be stacked to provide differentdegrees of isolation. Cork is available in squares 1 to 2.5 cm thick. Corkcan withstand corrosion and solvents, but felts being made of organic materialscannot be utilized in an environment where solvents are present. Cork and felthave damping ratios that typically range from ξ = 0.05 to 0.06.Inertia BlocksCertain very large machines such as reciprocating compressors generate largeinertia forces that result in unacceptably large motion that can cause the machine tofunction improperly. One means of limiting this motion is to mount the equipmenton an inertia base, which consists of heavy steel or concrete mass. This mass limitsthe motion by the dint of its heaviness overcoming the inertia forces producedby the mounted equipment. Also low natural frequency isolation needs a largedeflection isolator such as a soft spring. But the use of soft springs can lead torocking motions that cannot be tolerated. Hence, an inertia block mounted on theappropriate isolators can serve to effectively limit the motion as well as providethe required isolation. Inertia blocks also help to lower the center of gravity andthus provide an additional degree of stability. The introduction of additional massthrough inertia blocks decreases vibrational amplitudes and minimizes rocking.Alignment errors can be minimized because of the greater stiffness of the base,and moreover, these blocks can serve as a noise barrier between the floor on whichthey are mounted and the mounted equipment itself.Vibration AbsorbersIn order to better understand its operation, let us consider the vibration absorbermodel of Figure 20.10. The components of the vibration absorber are m 2 and k 2 .The applicable equations of motion for the system arem 1 ẍ 1 + k 1 x 1 + k 2 (x 1 − x 2 ) = fm 2 ẍ 2 + k 2 (x 2 − x 1 ) = 0