FALLSEM2019-20_MEE2004_ETH_VL2019201001536_Reference_Material_II_20-Sep-2019_MOM_Module6.pdf - Balancing 1 Syllabus Balancing Static and Dynamic


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Balancing 1
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Syllabus Balancing: Static and Dynamic Balancing of Rotating masses, Balancing of Reciprocating masses. 2
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Causes of Unbalance All rotors have some eccentricity. Eccentricity is present when geometrical center of the rotor and the mass center do not coincide along their length. Rotor becomes unbalance due to this eccentricity. Examples of rotors are turbines, generator, compressor or an electric motor. In the design of rotating parts of a machine every care is taken to eliminate any out of balance or couple, but there will be always some residual unbalance left in the finished part because of 1. slight variation in the density of the material or 2. inaccuracies in the casting or 3. inaccuracies in machining of the parts. 3
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Unbalanced force on the bearing rotor system Bearing 1 Bearing 2 Shaft with rotors 4
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Importance of balancing or Effects of unbalance A level of unbalance that is acceptable at a low speed is completely unacceptable at a higher speed. As machines get bigger and go faster, the effect of the unbalance is much more severe. The force caused by unbalance increases by the square of the speed. If the speed is doubled, the force quadruples; if the speed is tripled the force increases by a factor of nine. And so on. In rotor or reciprocating machines many a times unbalance of forces is produced due to inertia forces associated with the moving masses. If these parts are not properly balanced, the dynamic forces are set up and these forces not only increase loads on bearings and stresses in the various components, but also unpleasant and dangerous vibrations. It also causes excessive wear and tear, vibration (which causes fatigue failure and faulty operation i.e. in machining or in printing machine). 5
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Balancing Balancing is the technique of correcting or eliminating unwanted inertia forces or moments in rotating or reciprocating masses and is achieved by changing the location of the mass centers. It is a process of designing or modifying machinery so that the unbalance is reduced to an acceptable level and if possible eliminated entirely. The most common approach is by redistributing the mass which may be accomplished by addition or removal of mass from various machine members. There are two basic types of unbalance: rotating unbalance and reciprocating unbalance, which may occur separately or in combination. 6
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