vibration analysis in machines

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Vibration Analysis Welcome to the presentation on By ……….Sri Durga

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PREDECTIVE MAINTENANCE Maintenance depending upon the condition Costly trial and error approaches to solve a problem can be avoided since analysis identifies the nature of the problem .

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TYPICAL ROTATING MACHINE A. Structure B. Base frame & Bolts C. Casing D. Rotor E. Shaft F. Bearings G. Coupling A D G C C F F F B E B

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CONDITION MONITORING TECHNIQUES FOR ROTARY EQUIPMENTS 1. Physical Monitoring (LLF) 2. Performance Monitoring 3. Vibration Monitoring 4. Noise analysis 5. Bearing condition monitoring 6. Motor winding monitoring 7. Wear debris Monitoring(Tribology) 8. Thermography. Why use Vibration…… All machines vibrate. Every machine defect generates its own unique vibration frequency An increase in vibration indicates of a developing problem.

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Vibration is simply the motion of a machine or machine part back and forth from its position of rest.. The simplest way to understand vibration is to follow the motion of a weight suspended on a spring. Until a force is applied to the weight to cause it to move, there are no vibrations. Vibration is the response of a system to some internal or external excitation or force applied to the system . AXIAL RADIAL VIBRATION Force

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Fundamental synchronous rotor response equation Synchronous response (displacement) = disturbing force / Dynamic Stiffness Ie., This relationship indicates that an increase in vibration amplitude can be caused by an increase in disturbing force or decrease in synchronous dynamic stiffness.

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VIBRATION PARAMETERS AMPLITUDE ---- How much vibration ? FREQUENCY ---- How many times the machine or part vibrate per minute ? PHASE ---- How is the machine or part vibrating in relation with a reference point? Amplitude of vibration Time Frequency Phase A curve leads B curve by approx 90 0 Force A B

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AMPLITUDE Displacement Displacement is a measure of the total travel of the mass - back and forth. Displacement can be expressed in microns or mils. Velocity -- Rate of change of displacement The velocity of vibration is a measure of the speed at which the mass is moving or vibrating during its oscillation.The units are mm/sec or inches/sec. Acceleration -- Rate of change of velocity Acceleration is greatest at the instant at which velocity is at its minimum, ie., at this point the mass has decelerated to a stop and is about to begin its motion again in the opposite direction. The greater the rate of change of velocity , higher will be the forces on this mass ( F = ma). It is measured in ‘g’. Minimum Velocity & Maximum Acceleration Maximum Velocity & Minimum Acceleration

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FREQUENCY Frequency is expressed in either cycles per minute (CPM) or in cycles per second(Hz) . This is very important vibration parameter in identifying the machinery distresses. Time Amplitude Time-period The amount of time required to complete one full cycle of the vibration is called the period of vibration. Vibration frequency is the reciprocal of the time period. It can be defined as a measure of no. of complete cycles that occur in a specific period of time.

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PHASE Phase between A&B is ‘0’ B A Phase between A&B is ‘180’ Phase is a measure of how one part in moving in relation to another part, or to a fixed reference point. Phase measurement is very important in analyzing the following distresses in the machines. Bent Shaft Cocked bearing Looseness Unbalance Misalignment ……. A B

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Vibration measuring / monitoring instruments Vibration meters- gives only overall vibration levels. Vibration Analyzers- detailed analysis of vibration behavior of the machine. Here we are having one vibration meter and Analyzer. MEASUREMENT SENSOR POSITION Horizontal direction Vertical direction Axial direction

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Vibration Analyzers Vibration Meter

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Pick Ups or Transducers · Converts mechanical energy (vibration) into analog electrical energy. · Mainly classified into contact (Seismic) and non- contact (proximate) pickups. · Seismic pick up measure casing vibration whereas proximity pick up measure shaft vibration. The basic classifications are: PICK UPS MOVING COIL (VELOCITY) VELOCITY ACCELERATION PIEZOELECTRIC SEISMIC PICK UP (CONTACT TYPE) ON LINE MEASUREMENT (Directly measures shaft vibration) PROXIMITY PICK UP (NON-CONTACT TYPE)

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COMMON MACHINERY PROBLEMS 1 Unbalance 2 Misalignment 3 Looseness 4 Eccentricity 5 Defective bearings 6 Resonance 7 Electrical problems 8 Aerodynamic / Hydraulic problems

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SPECTRUM A m p l i t u d e Frequency 1 X 2 X High Frequency component Spectrum is the basic and very essential plot to detect the problem in the rotating equipment. Each problem will have predominant vibration levels at a particular frequency by which problem can be identified.

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Unbalance Symptoms Frequency : Phase (Radial) : Amplitude : 1 x Rpm 90 0 proportional to Rpm Imbalance occurs when the shaft’s mass centerline does not coincide with its geometric centerline. DIAGNOSES Common causes of Unbalance : Non uniform density of material Porosity in casting Gain of material during operation (dust accumulation) Keys and Key ways Wear of machine components Cavitation damage on pump impellers.

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Frequency : Phase (Axial) : Amplitude : Off Set 1 x Rpm 180 0 Shift across the coupling High radial vibration Angular Frequency : Phase (Axial) : Amplitude : 2 x Rpm 180 0 Shift across the coupling High axial vibration Combined Frequency : Phase (Axial) : Amplitude : 3 x Rpm 180 0 Shift across the coupling High axial vibration MISALIGNMENT SYMPTOMS

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Frequency : Phase (Radial) : Phase (Axial) : Amplitude : 1 x Rpm 0 0 180 0 Shift across the coupling High axial vibration · High axial vibration · Dominant vibration normally occur at 1x rpm · Axial phase change between two bearings. · When electric motor have problems such as shorted laminations, they will thermally induce a bend as the machine heats up, with the resultant vibration increasing higher and higher as the rotor heats up. Bent Shaft Symptoms

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Looseness Symptoms Frequency : Phase (Radial) : Amplitude : 1 x,2x,3x……Harmonics Unstable High Radial vibration This includes : Structural looseness / weakness of machine part, base plate & concrete base Deteriorated or crumbled grouting. Distortion of frame or base (Soft foot). Loose hold-down bolts.

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Anti friction bearing problems Anti-friction bearings fail prematurely primarily due to Misapplication of the wrong bearing for the job. · Lubrication inadequacies · use of wrong lubricant · contamination · Improper storage · introduction of moisture · False brinneling when standing idle. · · Improper installation. Cage Failure. · Outer race Inner race Rolling element Cage Lubricant Faulty bearings can generate vibrations at rotational frequencies (ball rpm, ball pass frequency for outer race ball pass frequency for inner race etc.,) and at natural frequencies of the component .

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RESONANCE Any initiation of crack in the shaft can lead to reduction in stiffness which in turn reduces the natural frequency of the rotor. If this frequency matches with any of the disturbing frequency, resonance can occur. When natural frequency of the system matches with disturbing force frequency In resonance condition the amplitudes will be 10x to 50x times the normal amplitudes which can easily lead to premature failure. Resonance occurs when an exciting frequency coincides with a natural frequency. Forcing frequencies include those from sources such as unbalance, misalignment, looseness, bearing defects, gear defect etc.

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How much vibration is too much ... · It is important to bear in mind that the objective is to use vibration checks to detect trouble in its early stages and schedule an appropriate correction procedure. · There are no realistic figures for selecting a vibration limit which if exceeded will result in immediate machinery failure. But, to have some general indication of machinery condition, guidelines have been developed and used by experience and research : 1) I R D Mechanalysis 2) I S O - 2372 3) API Guide lines The limits for each equipment will be fixed depending on the previous history , type of the machine, criticality etc after referring to the general guide lines by the following homographs.

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Thank You

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