this this slideshare presentation we discussed about difference of vibration system for forced damping here this is having with simple definition and for dynamic of machinery without equation and simple method.
2. Damping
a process whereby energy is taken from the
vibrating system and is being absorbed by
the surroundings.
Examples of damping forces:
internal forces of a spring,
viscous force in a fluid,
electromagnetic damping in galvanometers,
shock absorber in a car.
3. Free Vibration
Vibrate in the absence of damping and
external force
Characteristics:
the system oscillates with constant frequency and
amplitude
the system oscillates with its natural frequency
the total energy of the oscillator remains constant
4. Damped Vibration
The oscillating system is opposed by
dissipative forces.
The system does positive work on the
surroundings.
Examples:
a mass oscillates under water
oscillation of a metal plate in the magnetic field
5. Damped Vibration
Total energy of the oscillator decreases with
time
The rate of loss of energy depends on the
instantaneous velocity
Resistive force ∝ instantaneous velocity
i.e. F = -bv where b = damping
coefficient
Frequency of damped vibration < Frequency
of undamped vibration
6. Types of Damped Oscillations
Slight damping (underdamping)
Characteristics:
- oscillations with reducing
amplitudes
- amplitude decays
exponentially with time
- period is slightly longer.
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2
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7. Critical damping
No real oscillation
Time taken for the displacement to become
effective zero is a minimum.
Types of Damped Oscillations
8. Heavy damping (Overdamping)
Resistive forces exceed those of critical damping
The system returns very slowly to the
equilibrium position.
Types of Damped Oscillations
9. the deflection of the pointer is critically damped
Example: moving coil galvanometer
Damping is due to
induced currents
flowing in the metal
frame
The opposing couple
setting up causes the
coil to come to rest
quickly
10. Forced Oscillation
The system is made to oscillate by periodic
impulses from an external driving agent
Experimental setup:
11. Characteristics of Forced Oscillation
(1)
Same frequency as the driver system
Constant amplitude
Transient oscillations at the beginning which
eventually settle down to vibrate with a
constant amplitude (steady state)
12. In steady state, the system vibrates at the
frequency of the driving force
Characteristics of Forced Oscillation
(2)
13. Energy
Amplitude of vibration is
fixed for a specific driving
frequency
Driving force does work on
the system at the same rate
as the system loses energy
by doing work against
dissipative forces
Power of the driver is
controlled by damping
14. Amplitude
Amplitude of vibration
depends on
the relative values of
the natural frequency
of free oscillation
the frequency of the
driving force
the extent to which
the system is damped
15. Effects of Damping
Driving frequency for maximum amplitude becomes
slightly less than the natural frequency
Reduces the response of the forced system
16. Forced Vibration (1)
Adjust the position of the load on the driving
pendulum so that it oscillates exactly at a
frequency of 1 Hz
Couple the oscillator to the driving pendulum
by the given elastic cord
Set the driving pendulum going and note the
response of the blade
17. In steady state, measure the amplitude of
forced vibration
Measure the time taken for the blade to
perform 10 free oscillations
Adjust the position of the tuning mass to
change the natural frequency of free vibration
and repeat the experiment
Forced Vibration (2)
18. Plot a graph of the amplitude of vibration at
different natural frequencies of the oscillator
Change the magnitude of damping by rotating
the card through different angles
Plot a series of resonance curves
Forced Vibration (3)
19. Resonance
Resonance occurs when an oscillator is acted upon by a
second driving oscillator whose frequency equals the
natural frequency of the system
The amplitude of reaches a maximum
The energy of the system becomes a maximum
The phase of the displacement of the driver leads that of
the oscillator by 90°
20. Resonant System
There is only one value of the driving
frequency for resonance, e.g. spring-mass
system
There are several driving frequencies which
give resonance, e.g. resonance tube