My newly designed footers are based on the concept of variable stiffness isolators as described by Eugene I. Rivin in his paper titled Vibration Isolation of Precision Objects, that was published in the Journal of SOUND AND VIBRATION, JULY 2006. In his paper Riven describes how constant natural frequency isolators (CNF) can be assembled to allow for adjustment of the load when compressing the isolator along the z-axis. In order to do this a bolt or screw is passed through the center of the isolator. It is important not to bond the isolator to any of the supporting surfaces. The screw can then be used to adjust the load on the isolator independent of the weight load. Adjusting the preload force applied to the isolator can vary effective stiffness. Riven states that “Such variable stiffness isolators are useful for experimental optimization of vibration isolation systems with complex excitations, with compliant supporting surfaces, as well as for adaptive and other actively controlled isolation systems and dynamic vibration absorbers. ” The purpose of the screw in my design allows the user to torque down the isolator thus “preloading” the spring isolator.
In his paper Riven has also established that “cylinders are naturally nonlinear in compression, and their nonlinear load-deflection characteristics result in quasi- CNF load-natural frequency characteristics. Stresses under load are very uniform and are two to three times lower for the same compression loads/deformations than for rubber elements bonded to metal plates, thus allowing larger compression de- formations. This results in smaller sizes, lower creep rates, and better fatigue resistance.” The shape of my silicone spring insert was designed to be cylindrical for this reason. The cylindrical isolator is compressed between the footer shell and floor of the component being isolated. In addition the silicone compresses against the threads of the screw. Since the silicone isolator is molded in the stainless steel footer shell as it compresses it is pushed against the lateral wall of the shell and this restrains deformation in the z-axis. “Stiffness ratios … for ideal shape rubber elements are in the range of (5-9Hz). These values can be reduced if the lateral deformation of the elements is restrained” … in the z-axis.
The conclusions of Rivin’s paper are as follows:
4. CNF isolators can be used as variable stiffness isolators.
5. CNF isolators are natural ‘snubbers’ due to progressively in-creasing stiffness. (A snubber is a device used to suppress (“snub”) some phenomenon, such as: Voltage transients in electrical systems. – WIKI)