Post on 08-Oct-2015
description
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