Hanging lead weights do not simply lower the center of gravity. To do
this the lead weights must be firmly attached to the table. Hanging
lead weights will only increase the effective inertia in the direction
away from the weight. I think the main problem with increasing mass
away from the table is that the tables cannot attenuate low
frequencies (i.e. have relatively small damping for slow motions). As
you increase the mass of the system you lower the resonant frequency
and thereby reduce the effectiveness of the table and this is
especially bad for rotational oscillations around the center of the
table. As you raise the center of mass there is a massive
(sorry for the pun) increase in the moment of inertia of the table
which worsens the low frequency performance. In addition, the poorer
low frequency performance of the table will lead to excitation of
large masses supported away from the table top. This is perceived as
poor table performance but is is not really due to the table per se.
The real trick is to make sure that the resonant frequency of the bits
you want stable (with regard to each other) is far from the resonant
frequency of the support system to minimise their excitation. Since
better design is preferable to a fix, you should make sure the
supports have good damping at their resonant frequency. For example,
try filling the pillars with sand. This should help attenuate
vibrations after they are excited. If you hit the supports do they
'ring' or are they dead? Vibration control is extremely tricky, and
simply adding mass is not generally the best answer. What you need to
do is alter mass and damping together to control resonances in the
table/support system and/or decouple vibrating objects from each
other.
 
I hope this 2 cents worth helps...
 
Regards Mark Cannell
 
On Wed, 17 Jul 1996 14:21:52 -0500 W.E. Brownell wrote:
 
 
>
> The solution is fairly simple but not necessarily obvious, the
center of
> gravity may be lowered by hanging lead weights under the table.
>
> >Dear list,
> >   We have a BioRad MRC1000 with a motorized pillar to
> >permit its use with standard or inverted microscopes. It is
> >currently attached to a Zeiss Axiovert 100, and is also
> >surrounded by an aluminum bridge to support manipulators
> >and other equipment weighing about 30 pounds. The 3'x4'
> >isolation table on which it is now mounted is in bad
> >condition and we plan to replace it. We would like to
> >attain sufficient stability for patch clamping on the
> >microscope stage, as well as imaging at high magnification,
> >and would appreciate hearing from anyone with experience
> >on this matter. We understand that the high center of
> >gravity of the equipment stresses the capabilities of
> >standard vibration isolation equipment.
> >Thanks,
> >Len ZablowLeonard Zablow
> >Howard Hughes Medical Institute
> >722 West 168 St.
> >New York, N.Y. 10032
> >
> >Tel:(212)960-2246
> >Fax:(212)795-7997
> >
> >