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Control of Natural Frequency

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How we can control the natural frequency and avoid resonance phenomenon from happening in the real life.
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     Tafila technical university Faculty engineering   Mechanical Vibration Prepared by: Mohammad Nashat Abu-Sabbah   Control Of Natural Frequency : Project subjectSupervised by: Dr. Nabil Shabattat   Date of doing the project: THURSDAY (4/5/2014).  Date of submission: THURSDAY (6/5/2014).    1-Introduction: First of all we have to know the title meaning of this project in order to h ave full understanding about it…      Vibration : it could be simply defined as the rapid motion that it repeats itself after an interval of time. Types of vibration   1-Free vibration: Occurs when a mechanical system is set off with an initial input and then allowed to vibrate freely. Examples: pulling a spring back and then letting go or hitting a tuning fork and letting it ring. The mechanical system will then vibrate at one or more of its natural frequency and damp down to zero. 2-Forced vibration: Is when a time-varying disturbance (load, displacement) is applied to a mechanical system,   The disturbance can be a harmonic, non-harmonic but periodic, non-periodic, or a random. Examples: shaking washing machine due to an imbalance, transportation vibration (caused by truck engine, springs, road, etc.), or the vibration of a building during an earthquake.    Control Definition : is to adjust or the ability to manage a machine, vehicle, and other moving object, or limiting or regulating something.    Natural frequency  : is the number of complete cycles per second if a body is set to vibration and left free, it will vibrate with a frequency natural to that body called natural frequency,   but if it is not left free, and then it will vibrate in accordance with the frequency of the vibrating agent. It is called forced vibration. Note: that if the frequency of forced vibration happens to be the natural frequency, then Resonance will be set up. What it is resonance?? Mechanical resonance is the tendency of a mechanical system to respond at greater amplitude when the frequency of its oscillations matches the systems natural frequency of vibration (its resonance frequency or resonant frequency) than it does at other frequencies.  Why we worry about resonance occurring? Because it may cause violent swaying motions and even catastrophic failure in improperly constructed structures including bridges, buildings and airplanes — a phenomenon known as resonance disaster. So we can obviously see that avoiding resonance disasters is a major concern in every building, tower and bridge construction project. How we can avoid resonance occurrence? It is well known that whenever the frequency of excitation coincides with one of the natural frequencies of the system, resonance occurs. The most prominent feature of resonance is a large displacement. In most mechanical and structural systems, large displacements indicate undesirably large strains and stresses, which can lead to the failure of the system; hence in any system resonance conditions must be avoided. In most cases, the excitation frequency cannot be controlled, because it is imposed by the functional requirements of the system or machine. We must concentrate on controlling the natural frequencies of the system to avoid resonance.
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