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Vibration Theory and Applications

INTRODUCTION

Vibration testing is performed to demonstrate the ability of equipment to withstand the expected dynamic vibrational stresses
encountered in service. Vibration is an oscillatory phenomenon, which may be periodic or random, or both occurring
simultaneously.

TYPES OF VIBRATION

One type of vibration, known as free vibration, occurs when a system is set off with an initial input and is then allowed to vibrate
without restraint. Vibration will occur at one or more of its natural frequencies and will eventually return to equilibrium after
dissipating the input energy. The other type of vibration, known as forced vibration, occurs when a disturbance, which varies
with time, is applied to the system. The disturbance may be periodic, transient, random or combinations thereof.

VIBRATION TESTING

Vibration testing is performed by mounting the unit under test on a shaker table with the use of a mounting fixture. The fixture is
a key component, which if improperly designed can cause incorrect test results. The fixture design should consider the
following:

•Allow for ease of mounting of the test unit
•Allow for testing in each of the three orthogonal directions, if required
•Ensure the absence of fixture resonances within the test frequency range
•Weight and force limitations of the vibration machine
•Distribution of vibration energy uniformly throughout the test item
VIBRATION MODES

The basic vibration modes are as follows:

•SINUSOIDAL – vibration characterized by amplitudes which vary sinusoidally with time
•RANDOM - vibration characterized by irregular non-repeating patterns and amplitude versus time . Random vibration more
closely simulates the real world than sinusoidal vibration. A typical example are road inputs to a moving vehicle.


The figure above shows the distinct difference between sinusoidal and random vibration.

Mixed vibration modes are as follows:


•SINE ON RANDOM – Sine and random vibration are produced simultaneously to simulate sources where both sinusoidal
and random vibration are generated in service
•RANDOM ON RANDOM – Two different random vibration patterns are produced simultaneously to simulate sources where
two separate random inputs are generated in service
Mixed vibration modes more closely simulate the real world environment than either sine or random separately.

GUNFIRE VIBRATION

Test pulses are also available which simulate the vibration caused by overpressure pulses produced in the vicinity of
armaments.

 



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