1 vibration basics0

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1 vibration basics0

  1. 1. Reliability Management Consultant VIBRATION MONITORING FUNDAMENTAL REALITIES All machines vibrate – only dead machines don’t Vibration is a symptom – to and fro motion A body vibrates because it has mass and stiffness & they are related.
  2. 2. Reliability Management Consultant VIBRATION MONITORING HARMFUL EFFECTS OF EXCESS VIBRATION Increased Stresses On Risk of fatigue. Machine Components Increased Load On Bearings Reduced bearing life. Higher Forces On Mountings Loosening of foundation Damage to supporting structure.
  3. 3. Reliability Management Consultant VIBRATION MONITORING WHAT IS VIBRATION ? WHAT CAUSES VIBRATION ? HOW DO WE MEASURE VIBRATION ? HOW DO WE ANALYSE VIBRATION ?
  4. 4. Reliability Management Consultant VIBRATION MONITORING WHAT IS VIBRATION ? • IT IS MOTION OF MECHANICAL PARTS BACK AND FORTH FROM ITS POSITION OF REST.
  5. 5. Reliability Management Consultant Imagine Imagine a bird perched on a thin twig of a tree. And the bird goes up and down. In other words it oscillates.How would you describe its motion over a period of time? Now imagine that two birds sit on the same branch and the frequency of oscillation increases to 20 times/min. The vibration increases greatly and the birds fly off. What has happened?
  6. 6. Reliability Management Consultant VIBRATION MONITORING What causes Vibration ? INDUCED FORCE & FREEDOM FOR MOVEMENT
  7. 7. Reliability Management Consultant VIBRATION MONITORING Types of Forces ? Inertia Forces Acceleration & Deceleration Looseness & Impacts
  8. 8. Reliability Management Consultant VIBRATION MONITORING Degrees of Freedom? Degrees of Freedom determines the direction in which we measure vibration
  9. 9. Reliability Management Consultant Basic Theory Of Vibration Simple Spring Mass System Upper Limit Neutral Position Lower Position Displacement Max Acceleration, Min Velocity Max Acceleration, Min VelocityMax Velocity, Min Acceleration IT FOLLOWS SINE CURVE
  10. 10. Reliability Management Consultant VIBRATION MONITORING Modes of Amplitudes • DISPLACEMENT • VELOCITY • ACCELERATION • SPIKE ENERGY
  11. 11. Reliability Management Consultant The significance  Displacement gives the strain energy  Velocity gives the kinetic energy  Acceleration gives the force  Spike energy/HFD/gE are special ways to measure acceleration only.
  12. 12. Reliability Management Consultant Physical Significance Of Vibration Amplitude Displacement : Stress Indicator Velocity : Fatigue Indicator Acceleration : Force Indicator
  13. 13. Reliability Management Consultant Amplitude Measurement 1. Displacement : Total distance traveled by the mass. Unit : Microns, Mils 2. Velocity : Rate of change of displacement. It is the measure of the speed at which the mass is vibrating during its oscillation. Unit : MM/Sec, Inch/sec 3. Acceleration : It is the rate of change of velocity. The greater the rate of change of velocity the greater the forces (P=mf) on the Machines. Unit : M/Sec2, Inch/sec2
  14. 14. Reliability Management Consultant VIBRATION MONITORING UNITS OF MEASUREMENT DISPLACEMENT - MICRONS VELOCITY - MM / SEC. ACCELERATION - MM / SEC2 , G
  15. 15. Reliability Management Consultant Scales Of Amplitude Peak PeaktoPeak RMS Av. Peak - a Peak to Peak - 2a RMS - 0.707 a Average - 0.637 a
  16. 16. Reliability Management Consultant VIBRATION MONITORING AMPLITUDE DESCRIPTORS • RMS - Displacement Velocity • PEAK - 1.414 RMS Displacement Velocity Acceleration • PEAK-TO-PEAK - 2 * PEAK Displacement
  17. 17. Reliability Management Consultant Measurement direction - Horizontal, Vertical, Axial.
  18. 18. Reliability Management Consultant General rule of direction  We measure H direction for machines on the foundation.  We measure V direction for machines on structures  We measure A direction when we suspect misalignment  There are advantages, but there are exceptions to this rule.
  19. 19. Reliability Management Consultant Simplest Transducers  Finger – index finger – displacement to even 10 microns and acceleration or impacts FEEL  5 Rupee coin a go no-go test  Eye
  20. 20. Reliability Management Consultant Other Transducers  Velocity  Proximity Probes  Accelerometers
  21. 21. Reliability Management Consultant Accelerometer
  22. 22. Reliability Management Consultant 1. Produces signal proportional to acceleration of seismic mass, can get disp, and vel also. 2. Large Frequency range 3. Smaller in size 4. Can be used remote Accelerometer
  23. 23. Reliability Management Consultant Fixing Transducers  Come back to the same point  9 inch probe  Magnet  Stud
  24. 24. Reliability Management Consultant Simple rules of Limits (Alarm)  For Velocity – 5 to 7 times of baseline  For Displacement – 4 to 5 times of Base  For Acceleration – 20 to 40 times Base  For Spike Energy – 30 to 50 times Base
  25. 25. Reliability Management Consultant How do we set the limits?  Baseline = Y mm/s; G, D  Warning = 1.5 x Y mm/s; 10 G; 1.5 D  Alarm = 5 to 7 x Y mm/s;30-50G, 4/5D Warning and Alarm Limits
  26. 26. Reliability Management Consultant VIBRATION Displacement: Sensitive to low frequencies. Good guide for imbalance, & other low frequency faults. Velocity: Has average sensitivity throughout the frequency range. Reasonably sensitive to imbalance, misalignment, looseness and rolling element bearing damage. Acceleration: Sensitive to high frequencies. Good for rolling element bearing damage.
  27. 27. Reliability Management Consultant Advantage Of Using Velocity?  Flat frequency range compared to displacement & acceleration.  Almost all machines generate fault frequency between 600CPM to 60KCPM  Velocity indicates fatigue.  Velocity is the best indicator of vibration severity.
  28. 28. Reliability Management Consultant Amplitude & Frequency Amplitude: It is the magnitude of Vibration signal. Units: Micron, MM/Sec, M/Sec2 Frequency: How many times oscillation is occurring for a given time period? Units: CPS(Hz), CPM
  29. 29. Reliability Management Consultant Physical Significance Of Vibration Characteristics Amplitude - How much is it vibrating? (severity) of the problem. Frequency - What is vibrating? Source of the vibration. Phase Angle – Relation of vibration? Cause of the performance.
  30. 30. Reliability Management Consultant Analysis with overall? What faults can we measure and How?
  31. 31. Reliability Management Consultant Detection By Vibration Analysis Unbalance  If the ratios of H:V:A = 5:4:1  And it is true for both bearings on either sides, expect for overhung rotors
  32. 32. Reliability Management Consultant Detection By Vibration Analysis Misalignment  If the ratio of H:V:A = 4:2:3  Sometimes the axial may be more than the horizontal. It may be due to less stiff rotors.
  33. 33. Reliability Management Consultant Detection By Vibration Analysis Mechanical Looseness, Structural Weakness, Soft Foot  The ratio of H:V:A = 2:4:1 for foundation  The ratio of H:V:A = 2:1:4 for structures  The ratio of H:V:A = 2: 4: 3 for soft foot
  34. 34. Reliability Management Consultant Detection By Vibration Analysis Antifriction Bearing Defects  The ratio of H:V:A = 5:3:1  and the gSE or gE is also high
  35. 35. Reliability Management Consultant Detection By Vibration Analysis Electrical Problems  Vibrations are more to the motor side than on the driven side or the vibrations disappear suddenly on switching off.  In some cases there is a distinct noise. Or the needle starts to flicker when near an electrical cable.

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