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1.Sources of Vibration

Vibration, which is commonly referred to as noise, can be segregated into three main categories:seismic (ground) vibrations, acoustic vibrations, and forces applied directly to the load on the

working surface. Seismic vibrations include all sources that make the floor under theexperimental setup vibrate. Common seismic vibration sources are foot traffic, vehicular traffic,

wind blowing the building, and building ventilation fans, to name a few. Many of the sourcesthat generate seismic vibrations also generate acoustic vibrations. The difference is that acoustic

vibrations are a measure of the effects of air pressure variations on the experiment. The finalcontributor to vibration is forces applied directly to the load on the working surface; these are

vibration sources that are directly coupled mechanically to the experimental setup but nottransmitted through the table supports. Examples include vibrations resulting from a moving

positioning stage with a sample on top of it or the vibrations transmitted to the working surfacevia vacuum system tubing.

Vibration Characteristics

Vibrations can be classified as either random or periodic. Periodic noise obviously includes theconstant vibrations caused by a continuously running vacuum system, but it also includes the

vibrations caused by the fans of an air handling system that turn on and off based on thetemperature of the room. Random vibrations are classified as vibrations from unpredictable

sources like wind blowing a building or a jack hammer crew digging up a water main in thestreet. In addition, it is important to know the frequency and amplitude of the vibrations.

Typically, the frequency of the vibrations will range from 4-100 Hz.

Many sources of noise contribute via more than one mechanism to the overall vibration of theexperimental setup. For example, a vacuum pump located on the floor beside the experimental

setup creates seismic vibrations in the floor as well as acoustic vibrations. Both of these channelsof vibration should be considered when analyzing noise sources. However, since mechanical

coupling efficiency is typically higher than coupling from acoustic sources, the largestcontributions to overall noise are generally due to seismic vibrations and forces directly applied

to the load. Hence, placing the vacuum pump on a vibration absorbing pad may provide thenecessary reduction in vibration to make its contribution to the overall noise insignificant when

compared to other sources.

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2. The main source of rotor vibration are;

The dynamic loads such as the centrifugal force of the rotating main rotor

blade, aerodynamic of the wing and tail unit of the helicopter. Generally such

loads bring about bending, tension, compression, torsion and shear strains inthe components of the helicopter structure. The dynamic loads which are

repeated periodically at greater frequency are called variable loads. The main

and tail rotors are the main source of variable loads. Other source of variable

loads such as the engine, transmission, main rotor pulsating air flow over the

fuselage are of less important.The static loads are the loads the value and direction of which remain

unchanged during the prolonged period of time.The major significant to thehelicopter are aerodynamic loads which cause oscillation (vibration) of the

helicopter components and leads to the existence of alternating stress then to

total stress finally to the failure of helicopter structure due to the material

fatigue. The main forces developed by the flapping and lag hinge dampers or by

varying the weight and stiffness of the blades e.t.c.