Design of Laminar Flow Restrictors for Damping Pneumatic Vibration Isolators

Pneumatic isolators provide excellent isolation of machine tools and measuring machines from seismic floor disturbances. Damping of the machine mass/air spring behavior becomes essential when automatic height adjustment is added. Air flow from the isolator to a tank can dissipate the oscillatory energy if it is passed through a restrictor. Commercially available isolators use a simple orifice as a restrictor, but flow through an orifice is non-linear with pressure. This non-linearity Insures that the damping is correct at only one amplitude of oscillation which can in turn lead to limit cycles as large as 1 mm. Conventional solutions to this instability problem include lowering the sensitivity of the self leveling valves, increasing the spacing between the isolators and lowering the center of mass of the machine with respect to the isolators. Laminar flow restrictors that provide optimum damping of an all amplitudes can be implemented with shim stock and spacers with no impact on the machine installation. Design formulas that accurately predict the experimental results will be described and some guidelines based on practical experience will be discussed.