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Oscillation |
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Oscillations are the vibration with time of a quantity such as force, stress, pressure, amplitude, velocity, acceleration or jerk. |
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Vibration |
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The term vibration is defined as Mechanical oscillation or motion about a reference point of equilibrium. |
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Periodic Vibration |
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A Vibration whose instantaneous value of any future time can be predicted by an exact mathematical expression. Sinusoidal Vibration is the classic example of periodic or Deterministic Vibration. |
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Forced Vibration |
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The Vibratory motion of a system caused by some mechanical excitation is known as forced Vibration. |
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Free- Vibration |
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Free- Vibration occurs without forcing |
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Acceleration |
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Acceleration is a rate of change of velocity with time. Usually along a specified axis & Denoted by ‘g’ |
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‘g’ |
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The acceleration produce by Earth’s gravity. By international agreement, the value of gravitational unit is 9.80665 m/ sec2 |
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Amplitude |
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The magnitude of variation from its zero values known as Amplitude i.e. 0 to peak value. |
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Displacement |
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Displacement specified change of position and measured peak to peak. It is 2x amplitude of any variation. |
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Velocity |
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Rate of Change of displacement (x) with time, usually denoted by dx/ dt.
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Frequency |
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The reciprocal of Time period (in seconds). It is measured in Hz. |
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Frequency Response |
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It is defined as the portion of frequency spectrum, which a device can cover within specified limit of error. |
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Fundamental Frequency |
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It is defined as the no. of cycles per second of the lowest frequency component of a complex, cycle motion. |
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Frequency Spectrum |
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A description of the resolution into frequency of any signal. |
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Natural Frequency |
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It is denoted by Fn and known as the frequency of an undamped system’s free vibration. |
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Resonance Frequency |
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Resonance frequency is the same as natural frequency. It is also called as critical frequency. |
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Distortion |
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In machines, any unwanted motion. Any motion at harmonics or sub harmonics of fundamental frequency is known as distortion. |
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RMS or root mean square value |
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The square root of the time-averaged squares of a series of measurements. In case of sine wave, the RMS value is 0.707 x Peak value
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HOW TO SELECT A SYSTEM |
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The system rating are prepared as per direction of International Standards Organiasation document ISO5344 (BS6140). Following information assist in selecting a suitable configuration system for intended applications.
To get out rating of a vibration system based upon your test applications calls for extensive calculations and analysis as all parameters are interlinked to each other with specimen mass, dimension, frequency range, and test axis playing a vital role. The selection of a suitable Vibration Test System is in itself a most important aspect of application engineering, as system consist of various sub-systems which perform independent function but affect the overall performance of the system.
| Select a Vibration Shaker System |
The rating of the system is defined in terms of Kgf force and calculate using following formula.
Force (F) = Mass (m) ´ Acceleration (a)
Where :
Force (F) is in Kgf
Mass(m) is the total moving mass (Armature +
ILS + Payload support + Fixture + Object, all in Kg.
Note : Acceleration (g) due to natural Gravity = 9.8066 m/s”
Other then this some important factors as test profile, specimen dimensions, test axis, test type, guides for optimum shaker selection.
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| Sine Vibration Rating |
Sine vibration test is very common test having different profiles. Endurance by sweeping is the most appropriate method for simulating the effect of stress undergone by specimen in use. The displacement is associated with a corresponding value of acceleration in such a manner that the magnitude of vibration should be same at the cross over frequency. The frequency range may be thus programmed and swept continuously changing from constant displacement to constant acceleration and vice-versa at the cross-over frequency.
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| Random Vibration Rating |
Acceleration (g) in random testing is defined in terms of r.m.s. Value and calculated on the basis of Power Spectral Density (PSD). Random ratings have been calculated as per the directions of International Standards Organization document ISO 5344. The force rating are based on flat spectrum 100 to 200 Hz with 20 db/ decades roll-off from 100 Hz down to 20Hz, with loads equal to or greater than twice the armature mass.
ISO 5344 indicates that the ratio of peak to rms acceleration level must be at least 3:1 sigma, as measured on the vibration table.
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| Shock Vibration Rating |
The shock capability of the system is determined by the test profile such as pulse duration, pulse amplitude, pre and post pulse structure etc. Dynamic Associates & Services should be consulted to provide the most suitable system.
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