Vibration Control

Contents

Introduction

Vibration Isolation Theory(VIT)

VIT for Harmonic Excitation

Practical aspects of vibration analysis

Dynamic vibration absorbers

Vibration dampers

Introduction

Force and motions undesired out of vibration.

Base\foundation undergoes stress.

Required for long and stable operation of the structure.

Vibration isolation theory

Source:-A mechanical or fluid disturbance, generated internally by the machine. Path:The structural or airborne path bywhich the disturbance is transmitted tothe receiver.Receiver:- The responding system,generallyhaving many natural frequencieswhich can potentially beexcited by vibration frequenciesgenerated by the source.

Contd..

VIT for harmonic excitation

Contd..

Contd..

Q. What is the minimum stiffness of an undamped isolator to provide 81 %isolation for a 200kg fanoperating at 1000 rpm?

Q. What is Minimum static deflectionof an Undamped isolator to provide75% isolation to pump that operates at speed between 1500 & 2000 rpm?

Q. A 100 kg turbine operates at 2000rpm.What % isolation is achieved if it is mounted on 4 identical springs inParallel, each of stiffness 3 x 105 N/m?

Practical aspects of vibration analysis

Q. A milling machine of mass 450 kg operates at 1800 rpm and has an unbalance which causes a harmonic repeated force of magnitude 20,000 N. Design an isolation system to limit the transmitted force to 4000N, the amplitude of vibration during operation to 1mm, and the amplitude of vibration during start-up to 10mm. Specify the required stiffness of the isolator and the minimum mass that should be added to the machine. Assume a damping ratio of 0.05.

Contd..

Isolator selection:- Depends on static deflection, vibration amplitude and resonant amplitude. Other factors are cost, weight and space limitations.

Helical springs:-For large static deflections, flexible foundation acceptable.

Steel springs:-Used when mounting on concrete block.

Elastomers:-For small static deflection,

Pads made of cork, thermocoals are also used

Dynamic vibration absorbers

A vibration absorber is an auxilliary mass spring system

which causes steady state vibration.It increases the degree of freedom and thus can decrease the amplitude of vibration.

Vibration Dampers

An auxilliary system composed of an inertia element and a viscous damper for vibration control.

End of Chapter