The vibration environment of facilities used for research, photolithography, microelectronics manufacture, and similar activities has a significant effect on the performance of the equipment used within. As the dimensional requirements of today’s equipment and processes become more demanding, their sensitivity to external vibration increases. When designing such facilities, the stiffness of the floors and supporting structure must be such that the vibration levels are kept below that which causes problems for the equipment.
Determining these appropriate levels can be difficult. Equipment specifications for vibration tolerance are often vague, misleading, or non-existent. Instead, a series of generic criteria have been developed based on available specifications, tests, and
experiences that can apply to broad classes of equipment. Facilities can be designed with these generic criteria for the classes of equipment that they will use.
The first generally accepted set of criteria was developed by Eric Ungar and Colin Gordon in the early 1980s and were known as the “BBN” criteria. The criteria have gone through a number of changes since then, and are now standardized by IEST.
Vibration Measurement
All of these criteria are defined as RMS velocity spectra in 1/3 octave bands. Rather than specifying the vibration levels in fixed bands, proportional bands are used because they provide a more meaningful measure of how a structure reacts to broadband excitation at its resonant frequencies. It can be shown that a resonance of a given damping ratio responds with the greatest sensitivity to a frequency band of a width proportional to its natural frequency.
Most modern facilities, with adequate isolation of ventilation equipment and other rotating machines, have a vibration environment dominated by broadband energy and little pure tone energy . This broadband energy assumption is the basis for using proportional octave bands.
Vibration measurements should be made in 3 directions (vertical and 2 horizontal), and averaged over an adequate time period. In a steady-state or quasi-steady-state environment, linear averaging should be used. In an environment that is impacted by
occasional disturbances or transient events, peak hold measurements should be made.
In large spaces, where a single location may not be representative of the whole, multiple measurements should be made throughout and then averaged together in log space. If an is the spatially averaged measurement and a1, a2, … are the individual measurements, then the log-space average is defined as
IEST Criteria
The IEST criteria originally came in 4 levels, called VC-A (or “Vibration Criteria-A”) through VC-E. VC-F and VC-G have also been developed, but these were defined to characterize extremely quiet lab spaces, and not intended as design targets. The
criteria definitions are shown numerically in the table below.
NIST-A Criterion
The NIST -A criterion is identical to the VC-E curve at frequencies above 20 Hz, but maintains constant displacement at frequencies below this.
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