Review of the Paper on

Recent advances in nonlinear passive vibration isolators

Author: R.A. Ibrahim

ByDiala Uchenna

OutlineIntroductionBasic characteristic of nonlinear isolatorsUltra-low-frequency vibration isolationsBase isolation of structuresIsolation of liquid storage tanksNonlinear viscoelastic and composites

isolatorsViscoelastic isolator materialsIsolation of aerospace structuresIsolation of automotive systemsConclusion

IntroductionThe aim of vibration isolation is to curb

undesirable vibration to keep its harmful effects maintained within tolerable levels.

A linear isolator attenuates transmitted vibrations above an excitation frequency, which exceeds the value of where is the undamped natural frequency of the isolator.

The transmitted force, Ft reaches a value less than the excitation force, F0 at an excitation frequency >

The ratio Ft/F0 is known as the transmissibility, TR.

Basic characteristics of nonlinear isolatorsNeed for nonlinear isolation

Linear vibration isolators are efficient if their natural frequencies are less than the frequency of excitation.

This makes it’s application limited since it is unable to deal with severe environmental disturbances (e.g Shocks, random ground motion and Impact loads)with a wide spectrum (Low -> High).

Linear isolators, due to excess deflections experienced from severe environmental disturbances, can cause over-stress and damage to the system. Hence the need for nonlinear isolation systems which is more efficient here.

Basic characteristics of nonlinear isolators.. contd…

Shifting of resonance frequency The resonance frequency of an isolation system

can be shifted to the left or right of the linear resonance frequency on the transmissibility plot depending on the type of nonlinearity to be applied. This can help prevent unwanted vibration levels.

Chaotic responseA nonlinear isolator, subjected to a harmonic

disturbance, have tendencies of showing some chaotic behaviours over a certain range of system and excitation parameters.

Basic characteristics of nonlinear isolators.. contd…Influence of internal resonance

Nonlinear systems are subject to internal resonance which is due to interactions between vertical and horizontal vibrations of a rigid body and the rotational vibration about its centre of gravity.

Influence of random excitationSpecial isolation systems are required for

systems subjected to random excitations. Nonlinear isolators are able to operate within a wide range of frequencies hence are more appropriate here.

Ultra-low-frequency Vibration isolators

These type of isolators are required majorly in improving the sensitivity of terrestrial gravitational detectors which are limited at low frequencies due to seismic actions. Some isolation methods employed here include:Folded Pendulum isolatorThe X-pendulum isolation tableConical Pendulum

Ultra-low-frequency Vibration isolators

Folded Pendulum isolator

Active seismic isolators for interferometric gravitational wave detectors (IGWD) rely on low-noise low-frequency accelerometers designed for advanced mirror isolation systems of interferometric gravitational wave detectors. The accelerometer consists of a small monolithic folded pendulum with 2 s of natural period.

Ultra-low-frequency Vibration isolators

X Pendulum isolator

Conical Pendulum

This is a vibration isolation table using crossed wire suspensions (X mechanisms). A load table is suspended using a combination of normal and upside-down X mechanisms so that it moves freely with a very long period in both horizontal directions but is moderately stiff in all other degrees of freedom.

Pendulum r is under tension and carries the weight of the structure. The lower end of the rigid beam is constrained to move in a vertical line and a is typically made equal to r. The suspension point b, may be chosen to give straight line or large-radius curved motion.

Base Isolation of StructuresThe motive here is to design isolation

systems against seismic activities for ground structures like multi-storey buildings, bridges and liquid storage tanks etc to preserve structural integrity and avoid injury to occupants.

Two major isolation methods are involved:Laminated-rubber bearing isolators with/out

lead-coreFrictional-type sliding isolators

Schematics of some Base Isolation systems

Laminated-rubber bearing isolators with lead-core

Friction base isolators

Isolation of Liquid Storage tanksLiquid storage containers are usually mounted on the ground either

unanchored or anchored. Large unanchored tanks usually experience various forms of damages

due to seismic activities especially the elephant foot bulge.For the isolation of liquid storage tanks, the Friction Pendulum System

is well effective.

Friction Pendulum Isolation System

Nonlinear Viscoelastic and Composites isolatorsViscoelasticity is the property of materials that

exhibit both viscous and elastic characteristics when undergoing deformation. 

Nonlinear characteristics are reflected in mechanical properties such as stiffness and damping whose parameters depend on frequency and temperature.

Viscoelastic materials are widely used for vibration isolation in automotive and aerospace industries.

Viscoelastic Isolator MaterialsNonlinear natural rubber

When rubber material is used, loads producing small deflections are carried comfortably in shear.

During large deflection produced by overload, the rubber becomes loaded in compression thus acting as a compression spring to the motion.

Composite SpringsVarious composite materials due to their excellent

properties have been applied in some composite springs.

The sulcated spring was introduced by Scowen and Hughes in order to utilize the high tensile strength of fibre-reinforced plastic in spring systems.

Isolation of aerospace structuresThis is essential for space launch vehicles

including space telescopes and microgravity effects on liquids, combustion and material management.

Vibration isolations with constant stiffness elements and damping elements such as viscoelastic materials, viscous fluids etc have been extensively used in aerospace applications.

The Whole-space vibration isolation systems known as Single-axis SoftRide systems have been developed for the first and second Orbital/Suborbital Program (OSP).

Isolation of automotive systemsDesigns of rubber and hydraulic mounting

systems have emerged in the automotive industry to isolate the power-train system, engine and engine cooling system.

Hydraulic engine mounts exhibit nonlinearities due to Internal fluid flow characteristicsCompliances of elastomeric fluid chambersVacuum formation

Viscoelastic materials such as rubber are widely utilized in vibrating damping mounts here.

Isolation of automotive systems contd…Magnetorheological fluids was proposed for

use in hydraulic engine mount for damping vehicle noise and vibration.

An MR fluid is a type of smart fluid in a carrier fluid, usually a type of oil.

When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid.

The yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. 

MR Fluid

Schematic of a magnetorheological fluid solidifying and blocking a pipe in response to an external magnetic field

ConclusionRecent advances in base isolation systems have been

reviewed.Features of nonlinear isolators have been described.Ultra-low frequency vibration isolators have been

mentioned and Base isolation systems discussed.Protection of buildings, bridges and liquid storage tanks

against earth-quakes has been achieved by different types of base isolation systems such as rubber-lead bearings and friction-pendulum systems.

Viscoelastic nonlinear isolators exhibit different mechanical characteristics due to the dependence of their stiffness and damping parameters on frequency and temperatures.