T1-34
The values of F_{Bv} and F_{Bh }were calculated using floor vibration levels corresponding to line VC-B in Figure 15 (both for
vertical and horizontal directions). Since plots in Figure 15 are given for vibratory velocity V_{f}, vibration displacement ampli-
tudes X_{f} were calculated for each frequency of interest as X_{f} = V_{f}/23f.
Values of F_{A }calculated per Specification A are interesting only for comparison, since high precision microelectronic
production equipment is never used in conventional plant facilities, only in specially designed buildings complying with some
of VC criteria.
It can be seen from Table 4A that the lowest value of F_{Av} (case A) for vertical direction is 4.51 Hz. If vibration isolators
with medium damping 3
n
= 0.6 are used, then from Equation (12a) the required vertical natural frequency f_{v} = 4.51 0.6 =
3.04 Hz. However, if isolators made of rubber with high damping 3
n
= 1.2 are used, then f_{v} = 4.51 1.2 = 5.0 Hz, which can
be realized by passive isolators with soft rubber flexible elements.
Much stiffer isolators (f_{vz} > 14 Hz) can be used to comply with values of F_{Bv}, per Specification B, which represent
(according to not very stringent requirement VC-B) floor conditions at the microelectronics industry installations.
A similar situation is seen in Table 4B; however, realization of natural frequencies corresponding to F_{Bh }(4.7 Hz for 3
n
=
0.6, 6.63 Hz for 3
n
= 1.2) in horizontal directions with elastomeric isolators does not present any difficulty; even much lower
values can be easily realized.
References
[1] Rivin, E.I., Passive Vibration Isolation, ASME Press, N.Y., 2003
[2] Crede Ch. E., Vibration And Shock Isolation, John Wiley and Sons, Inc., New York, Chapter Three, 1951
[3] Mindlin, R.D., "Dynamics of Package Cushioning", Bell System Technical Journal, Vol. XXIV,
Nos. 3-4, July-October, 1945
[4] Hirschhorn, J., Kinematics and Dynamics of Plane Mechanisms, McGraw-Hill, 1962
[5] C.M.T. Wells Kelo Ltd., A Commercial Guide to Shock And Vibration Isolation, Sept 1982,
First Amendment, May 1983.