FEED FORWARD FLYER                 ADVANCING THE TECHNICAL KNOWLEDGE OF MAINTENANCE TRADESMEN AND PLANT OPERATORS         ...
FEED FORWARD FLYER                 ADVANCING THE TECHNICAL KNOWLEDGE OF MAINTENANCE TRADESMEN AND PLANT OPERATORS         ...
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Vibration and its control

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BIN95.com Newsletter, vol 154 - Vibration and its control. Vibration in equipment is the result of
unbalanced forces. Out-of-balance is corrected by adding or
removing material so that when the equipment is operating the
unbalance is controlled to an acceptable level.

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Transcript of "Vibration and its control"

  1. 1. FEED FORWARD FLYER ADVANCING THE TECHNICAL KNOWLEDGE OF MAINTENANCE TRADESMEN AND PLANT OPERATORS VIBRATION AND ITS CONTROLABSTRACT vibration because the center of mass of the rotor is eccentric (notVibration and its control. Vibration in equipment is the result of running true) to its center of rotation. The spinning off-centerunbalanced forces. Out-of-balance is corrected by adding or mass is continually being flung outward. The machine’s bearingsremoving material so that when the equipment is operating the hold the mass in place and react against the developed forces.unbalance is controlled to an acceptable level. Keywords: spring Vibration results as first the mass is on one side of the bearingsstiffness, damping, center of rotation, center of mass, natural and then it is on the other. Balancing aims to distribute the massfrequency, isolation mount, counter balance, out-of-phase. evenly about the running center. The drawing below shows eccentricity between the center of mass and the center of rotation.The transference of unbalanced forces through equipment intoneighbouring structures causing them to shake is vibration. Themotion of a body is limited by what connects it to a machine andthe walls in which it moves. Every time there is a change ofdirection unbalanced forces produce a shock. This shock travelsthroughout the machine and is transmitted to all connected items.Mostly a spring is used to isolate vibration movement or a damperis used to absorb the movement. A full explanation of vibration Materials such as rubber dampen shaking. The rubber flexes andcontrol requires calculus and is beyond the scope of this article. absorbs the movement within itself. The sketch below is of aThe spring-mass-damper system sketch below is a simplified simple rubber vibration damper. Because rubber cannot compressrepresentation of a vibration control system. much to accommodate movement, rubber dampers are normally used for low amplitude, high frequency vibration where noise transference is a problem. Shock absorbers are used for large amplitude, low frequency situations where springs alone would produce bouncing. An example is in car suspensions.Spring force and damper pressure control the mass’ movement.The damper piston moves and so absorbs the vibration. Where asthe spring flexes and isolates the movement from its attachment.The rate of vibration is called the frequency. It is measured incycles per second and has the units Hertz. A four-pole electricmotor rotates at about 1500 RPM. This is 25 cycles per second or The natural frequency can be moved away from the forcing25 Hertz. Vibration caused by an external applied force is known frequency by changing the weight of the system. Making theas a forced vibration because the mass oscillates at the frequency frame of a vibrating machine heavier will lower the assembly’sof the external force. An example is the shake produced by the natural frequency. Concrete added to the base frames ofmoving pistons and crankshaft in a car engine. lightweight fans reduce vibration by lowering the natural frequency and moving it away from the forcing frequencyThe equation for the natural frequency of an undamped spring- produced by the rotating blades.mass system moving in one direction is A vibrating mass can also be isolated from its surroundings byfn = ½ π √(K/M) where K is the spring stiffness and M the springs. The springs deflect under the shaking body. Installingmass. isolation springs make the spring’s natural frequency theThis equation lets us find the resonance frequency for a mass– governing frequency for vibration transfer. Altering the springspring system. Such a system can be represented in the drawing stiffness allows us to select the desired amount of isolation.above by removing the damper. Spring stiffness controls the amount of vibration transferred to theWild gyrations develop when the forced frequency nears the attachment. Too stiff will transmit vibration, while insufficientsystem natural frequency. Every system has a natural frequency stiffness will cause bounce. The correct spring stiffness can beand will shake to destruction if it is forced to move at that rate. found using charts available from specialist vibration controlThis phenomenon is known as resonance. An example would be companies. The isolation springs must not have a naturalthe shattered wineglass caused by an opera singer’s voice or frequency near the forcing frequency of the isolated equipment.vibrations in long, thin shafts that start and then stop as the shaft In such a case the system would start to resonate and jump about.speed goes through its natural frequency speed. The sketch below shows an air compressor supported on springs. Stiffer springs are at the heavier end of the machine to both keepThe four methods of vibration control are listed below. the machine level and to prevent resonance developing as the mass increases. The drawing also indicates that vibration isReduce or eliminate the exciting force by balance or removal. usually in more than one direction.Use sufficient damping to limit amplitude.Isolate the vibration source from the surrounds by using springmounts of appropriate stiffness.Introduce a counterbalancing force opposite in phase to theexciting force.Most importantly every moving mass must be balanced about itscenter of rotation. The topic of rotary machine balancing wasintroduces in the December 1999 edition of FEED FORWARDFLYER. The article indicated that rotating masses must bebalanced to an acceptable standard. Out–of-balance rotors cause Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, Western Australia, 6102. E-mail: feedforward@bigpond.com. Because the authors and publisher do not know the context in which the information presented in the flyer is to be used they accept no responsibility for the consequences of using the information contained or implied in any articles
  2. 2. FEED FORWARD FLYER ADVANCING THE TECHNICAL KNOWLEDGE OF MAINTENANCE TRADESMEN AND PLANT OPERATORS VIBRATION AND ITS CONTROLAn out-of-phase mass is a method not often used to controlvibration. It is possible to use a weight with an opposite vibrationpattern to negate the out-of-balance forces. This method has beenused in motor car engines where a shaft with an eccentric mass isspun in the opposite direction to the crankshaft.Mike Sondalini - Maintenance Engineer Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, Western Australia, 6102. E-mail: feedforward@bigpond.com. Because the authors and publisher do not know the context in which the information presented in the flyer is to be used they accept no responsibility for the consequences of using the information contained or implied in any articles

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