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Tables & Breadboards

ScienceDesk™ As the transmissibility approaches zero, theposition of the ball is not affected by the vibration in the large mass. This is a classic example of seismic mounting.
BreadboardsBreadboard Supports Optical Tables Damping
Table Supports Damping refers to any process that causesan oscillation in a system to decay rapidly to zero amplitude. It is a very important phe- nomenon in vibration suppression or isola-tion. Damping causes the energy to be di-verted from vibration to other sinks. Damping in a system is usually defined as the ratio of actual damping (C) to critical damping (CC). Critical damping is theminimum amount of damping in a systemnecessary to prevent resonant oscillation fol- lowing application of an impulsive force. Damping is a resonant effect in that pri-marily, it affects the transmissibility func- tion at or near resonance. At resonance, the transmissibility becomes:Where
Non-Magnetic ℘ is the damping ratio, (C/CC)The height of the transmissibility peak atresonance is mainly determined by theamount of damping. In the absence of damping, the peak would be infinitely high. Also, a system with no dampingwould vibrate resonantly ad infinitum,even when the vibrational source (forcing function) is removed. Clearly, all real sys- tems contain damping to some degree. Using the suspended ball as an example,the simplest form of damping would be toimmerse the ball in a viscous oil. Viscous drag would convert the vibrational energy to heat by friction. Self Leveling Mechanism shown above. Seismic Mounting of Optical Tables The preceding discussion suggests in prin-ciple, a way in which an optical tablecould be vibrational isolated, i.e. seismical- ly mounted. If an optical table could be suspended from the ceiling by springs, then at frequencies much higher than thenatural resonance of the spring isolationsystem, ambient building vibrations would not be transmitted to the table. Obviously, this solution has no practical value, but itclarifies the general principles involved.Typically, ambient vibrations in a buildingtend to be in the range 10 to 50Hz for ver- tical components, and 1 to 20Hz for hori- zontal components.To seismically mount an optical table un-der these conditions requires a mountingsystem with a very low resonant frequency, i.e. a weak spring with a small spring con- stant. Unfortunately, even with the use of composites, optical tables are relatively massive, typically 500kg for a 1.2m x 2.4mtable. Such a system would involve highly extended springs and a very large travelrange of the table. However, it is interesting to note that if stiff springs were used in such a system, then the resonant frequency for pendulum motion could easily be less than 1Hz, i.e. the table would be well isolated from horizontal vibrationsof the building.The ideal mounting system shown below,has a rigid tabletop mounted to the floor in such a way that it can vibrate in vertical and horizontal directions, both with low resonant frequencies. Even though theseresonant frequencies can be made lowerthan most building vibrations, it is impor- tant that the height, and hence the width, of the resonant peak in the transmissibility curve be reduced. This is achieved byadding a damping mechanism, which en-sures minimum oscillation at resonance and short settling times. Summary of Key Points
â–  For best performance, an optical tableshould be seismically mounted.
â–  Transmissibility of the isolation system should be as low as possible.
â–  Below the first resonant frequency of the table and isolation system, trans-missibility is high.
â–  At resonance, damping is critical in re-ducing transmissibility
â–  Above resonance, transmissibility is de-termined by mass. Implications for Isolator Selection For optimum performance, the table andisolator system must have a resonant fre-quency well below the frequency of ambi- ent noise vibrations. Damping is critical to system performance.
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Tools of the trade - Vol 18 - 36188 RIGID TABLEsoft springisolatorsdamperdamper

Ideal seismic mounting of an optical table, consisting of weak spring supports with added damping Sales: 973-579-7227
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