5/28/2013 May 28th, 2013UNIT V -MECHANISMS FOR CONTROL
5/28/2013 May 28th, 2013UNIT IV -FORCED VIBRATION
5/28/2013 May 28th, 2013UNIT III -FREE VIBRATION
5/28/2013 May 28th, 2013UNIT II- BALANCING The balancing of rotating bodies is important to avoid vibration. In heavy industrial machines such as gas turbines and electric generators, vibration can cause catastrophic failure, as well as noise and discomfort. In the case of a simple wheel, balancing simply involves moving the centre of gravity to the centre of rotation. For a system to be in complete balance both force and couple polygons should be closed. Static Balance Rotating shaft unbalanced by two identical attached weights, which causes a counterclockwise centrifugal couple Cd that must be resisted by a clockwise couple Fℓ =Cd exerted by the bearings. The figure is drawn from the viewpoint of a frame rotating with the shaft, hence the centrifugal forces. Static balance occurs when there is no resultant centrifugal force and the centre of gravity is on the axis of rotation .To avoid the stress upon the bearings caused by the centrifugal couple, counterbalancing weights must be added. The unbalancing weights are an idealization, of course. For example, in the case of an automobile tire the imbalance is due to imperfections of manufacture that make the tire composition in homogeneous. Dynamic Balance A system of rotating mass is in dynamic balance when there does not exist any resultant centrifugal force as well as resultant couple. Unbalanced Systems There are many disadvantages to unbalanced systems. When a system is rotating with unbalanced masses unnecessary vibration occurs, further it may generate unwanted noise, excessive stresses in machine elements and reduce the reliability for all the parts associated with the rotating system. If a rotating system is not in balance, due to the unbalanced couple high bearing thrusts are generated. This will cause failure in bearings in short time periods. Shafts with unbalanced masses can be easily bent and fatigue failure may occur in particular number of cycles. When a system is rotating two rotors, although the system is in static balance, always there exists a couple relative to a one rotor. This unbalanced couple will generate a dynamic force couple in the two bearings which is known as rocking couple. Under conditions where rotating speed is very high even though the mass is low, as in gas turbines or jet engines, or under conditions where rotating speed is low but the mass is high, as in ship propellers, balance of the rotating system should be highly considered, because it may generate large vibrations it will cause failure of the whole system.
5/28/2013 May 28th, 2013UNIT I- FORCE ANALYSIS AND FLYWHEELS
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