Conclusions
All of these design, selection, and installation problems undermine
optimal isolator performance. The impact of the unintended
stiffness and damping can be quantified via testing. Data illustrating
the importance of these parameters are presented in Figure 5 for
two identical, large (100-HP) pump installations supported on steel
springs. Spectral vibration data were collected on the pump bases
by heel strike to illustrate the spring natural frequencies.
In Figure 5a, the natural frequency on the motor end of the
base appears to be about 4.4 Hz, implying a spring static deflection
of about 13-mm. On the pump side of the base, the damping
and stiffness provided by the attached piping nearly doubles the
natural frequency to 8.4 Hz. For a pump speed of approximately
1440 RPM (24 Hz), the 4.4 Hz and 8.4 Hz natural frequencies lead
to an isolator efficiency of 97% and 86%, respectively.