Dynamic Vibration Absorber.pdf
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Transcript of Dynamic Vibration Absorber.pdf
Dynamic vibration absorber
SOLVE the virtual lab@ NITK Surathkal Machine Dynamics and Vibration Lab
Dynamic vibration absorber
Theory
Learning objectives
After completing this simulation experiment one should be able to
Model a given real system to an equivalent simplified single degree of freedom
system and reducing the vibration of the main system adding an absorber system with
suitable assumptions / idealisations.
Determine the amplitude of vibration for both main and additional system.
Tune a vibration absorber by varying dynamic system parameters.
Study the response of the vibration absorber for different excitation frequency and
amplitude.
Dynamic vibration absorber
Introduction
When a structure is externally excited, it will have undesirable vibrations. The amplitude of
vibration will be maximal when the system gets excited close to its natural frequency and this
can cause rapid catastrophic failure. Hence it becomes necessary to neutralize these
vibrations. One of the methods for neutralizing these vibrations is by coupling a vibrating
system to it so that the amplitude can be brought down to zero. This kind of vibrating system
is known as vibration absorber or dynamic vibration absorber (DVA). DVA are used to
control structural vibrations where it concentrates on neutralizing the amplitude of vibration
at resonance. Some practical examples of dynamic vibration absorber showed in Fig 1 and 2.
Fig 1: Dynamic vibration absorber in steel stack
Dynamic vibration absorber
SOLVE the virtual lab@ NITK Surathkal Machine Dynamics and Vibration Lab
Fig 2: Dynamic vibration absorber PCB
To study the dynamic absorber system, a real system considered as main system is modelled
as an equivalent single degree of freedom system and it is excited by a harmonic excitation
force F= F0 sinωt. The steady state response of the system is given by x= X sin (ωt+ϕ).
Steady state amplitude of vibration of the proposed single degree of freedom system will be
maximum at the resonance. To neutralize the effect at resonance, the main system couples
with an absorber system. This coupling will affect (suppress) the amplitude of vibration of
the main system. By the addition of absorber system, single degree of freedom analysis
cannot hold. Hence whole system should be considered two degree of freedom system.
Fig 3: A single degree of freedom system and vibration absorber system
Dynamic vibration absorber
SOLVE the virtual lab@ NITK Surathkal Machine Dynamics and Vibration Lab
Fig 4: Free body diagram of vibration absorber system
The free-body diagrams of the masses and are shown in Fig. 4. By application of Newton’s
second law of motion to each of the masses gives the equations of motion as:
1 1 1 1 2 1 2 0( ) sin( )m x k x k x x F t ... (1)
2 2 2 2 1( ) 0m x k x x ... (2)
Steady state response of two degree of freedom is assumed as,
1 1 sin( )x X t And 2 2 sin( )x X t
By substituting x1 and x2 in equation 1 and 2 we get,
2
1 2 1 1 2 2 0( )Xk k m k X F ... (3)
2
2 1 2 2 2( )X 0k X k m ... (4)
By solving the equation 3 and 4, amplitude of vibration of main system and absorber system
given by,
2
2 2
1 4 2
1 2 1 2 2 1 2 1 2( )
k mX
m m m k m k k k k
... (5)
and
2 02 4 2
1 2 1 2 2 1 2 1 2( )
k FX
m m m k m k k k k
... (6)
From equation 5 it can be observed that the amplitude of vibration of main system X1 can be
zero if numerator becomes zero.
2
2 2i.e. 0k m
or 2 22
2
2
n
k
m ...(7)
From equation 7, it can be concluded that when the excitation frequency is equal to the
natural frequency of the absorber, then main system amplitude becomes zero even though it
is excited by harmonic force.
Dynamic vibration absorber
SOLVE the virtual lab@ NITK Surathkal Machine Dynamics and Vibration Lab
Dimensionless form of equation 5 and 6 can be written as,
2
2
21
4 2 2
2 2 2 2
1 2 1 2
1
(1 ) 1st
X
X
... (8)
2
4 2 2
2 2 2 2
1 2 1 2
1
(1 ) 1st
X
X
... (9)
Equations 8 and 9 give the amplitude response of main system and absorber system as a
function of exciting frequency.
Where,
Xst = F0/k1= Zero frequency deflection of the main system
ω1= Natural frequency of the main system
ω2= Natural frequency of the absorber system
ω= Frequency of external excitation
μ= ratio of absorber mass to the main mass
Fig 5 shows the variation of X1/Xst for different frequency ratios.
Fig 5: Frequency response curves of main system
Dynamic vibration absorber
SOLVE the virtual lab@ NITK Surathkal Machine Dynamics and Vibration Lab
Let’s try to understand these equations by doing a few simple simulations, go to next tab
procedure to find out how to run the simulation to EXPLORE (expR) and to EXPERIMENT
(expT). A talking tutorial or a self-running demo with narration can be seen at EXPLAIN
(expN)