# Vibration

A vibration is a movement of oscillation around a position of steady balance or an average trajectory.

## Vibratory movements

### Degrees of freedom

• a degree of freedom
• two or several degrees of freedom
• infinite number of degrees of freedom

### Causes

• vibrations natural (or free)
• maintained vibrations
• parametric vibrations
• self-excited vibrations

### Form

• linear vibrations
• nonlinear vibrations

### Object

• machine (motionless)
• vehicle (in displacement, Guidonnage)
• driving building

### Nature

• translation
• rotation (vibration of torsion)

## Means of investigations

Techniques of investigations exist to characterize a mechanical structure (or system). These techniques make it possible in particular to know the Eigen frequencies of this structure {also, but improperly, called frequency of resonance of the system]. The amplitude of the response of the system to efforts of identical or multiple frequency or under multiple of the Eigen frequencies is much higher than the amplitude of the excitation and more or less quickly will cause the ruin of the structure. This phenomenon is called resonance.

There exist two methods to determine the Eigen frequencies of a system:

• use of a vibrating pot for a vibratory characterization of the part (determination of the frequencies of resonance), vibratory fatigue tests,…

• use of a hammer of shock for a modal analysis of structure, the part being successively excited in several points and vibratory reaction measured using a vibration pick-up (Accelerometer, laser Vibrometer).

The experimental modal analysis makes it possible to determine the structural deformations according to the frequency.

The shock with the hammer makes it possible to excite the structure on all the frequencies (until approximately 10  Khz) with same the energy.

These two tests then make it possible to dimension vibratile studs to insulate the part, the machine of the ground, to modify the structure of the machine to avoid a premature deterioration of this one.

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