TMC Catalog

The condition for absolute stability is:Equations 14 and 15 give “rules of thumb” for calculatingthe stability of a system. As with all such rules, it is only anapproximation based on an “average” isolation system.It is always best to use as low a COM as possible.[12][14]Because MaxDamp ® isolators use a single air chamber,they are more stable, and the rule becomes:And the formula for absolute instability is:[15]with the volume between being “possibly” or “marginally”stable. The ratios A/V are not universal and should beconfirmed for different capacities and models of isolatorsbut are approximately 0.1 in –1 for (A/V) Top and 0.05 in. –1 for(A/V) Tot . Figure 10 illustrates what the marginally stableregion looks like for two chamber isolators. Unfortunately,the COM of many systems ends up in this indeterminateregion. These rules do not account for the actions of theheight control valves, which will always improve a system’sstability. If the payload has a mass which can shift (a liquidbath or a pendulum) these rules can also change.HUnstableBorderlineStableTwo-ChamberIsolatorsWFigure 10COM[13]Note that the effective support point for TMC’s GimbalPiston isolators is approximately 7 in. below the topof the isolator. For lightly loaded isolators, these rulesunderestimate system stability. If your system violates theseequations, or is borderline, the stability can be improvedusing counterweights, special volume isolators, differentisolator valving, etc. Contact a TMC sales engineer foradvice on the best approach.4.0 High-PerformanceTable TopsTable tops are the platform for conducting many typesof meas urements and processes. They can serve as amechanical reference between different components(such as lasers, lenses, film plates, etc.) as well as simplyproviding a quiet work surface. Tops typically use one ofthree constructions: a composite laminate, a solid material(granite) or a lightweight honeycomb. The choice of constructiondepends on the type and size of the application.Figure 11 shows a typical laminated construction.These are usually 2 to 4 in. thick and consist of layersof steel and/or composite materials epoxy-bondedtogether into a seamless stainless steel pan with roundededges and corners. A visco-elastic adhesive can be usedbetween the plates to enhance the damping provided bythe composite layers. All bonding materials are chosento prevent delamination of the assembly due to heat,humidity, or aging. The ferromagnetic stainless steel panprovides a corrosion-resistant, durable surface which workswell with magnetic fixtures. “Standard” sizes for thesetops range from 24 in. square to 6 x 12 ft, and can weighanywhere from 100 - 5,000 lbs. This type of constructionis not well suited to applications which require large numbersof mounting holes (tapped or otherwise). The ratioof steel to lightweight damping composite in the coredepends primarily on the desired mass for the top.10www.techmfg.com • 978-532-6330 • 800-542-9725 (Toll Free) • Fax: 978-531-8682 • sales@techmfg.com101