Basics of Vibration Analysis [PDF]

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Balancing and Diagnostic Systems



Welcome to the seminar: Basics of vibration technology – Measurement & Analysis



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Balancing and Diagnostic Systems



Lecturer :



Roland Kewitsch



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Balancing and Diagnostic Systems



Vibration analysis increases knowledge Provides necessary information for: Evaluation of machine condition Recognition of on-going machine damage symptoms Identification of the cause and the damaged components Prognosis of remaining service life



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Balancing and Diagnostic Systems



Machine damage in a power station Total destruction of a generator



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Balancing and Diagnostic Systems



Rolling-element bearing damage



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Diagnosis methods Vibration measurement and analysis Displacement, expansion and process value measurement Temperature, speed and phase measurement Lubricant analysis (e.g. spectroscope, ferroscope, radionuclide) Optical examination (e.g. endoscope, microscope) Non-destructive testing (e.g. ultra-sound, X-rays) 6



Balancing and Diagnostic Systems



Vibration Measurement in the past (& still today)



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Balancing and Diagnostic Systems



Diagnosis methods Diagnosis by vibration measurement



Overall methods



Machine assessment using Overall measurements



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Balancing and Diagnostic Systems



Diagnosis methods Diagnosis by vibration measurement Analytical methods Overall methods



Machine assessment using Overall measurements



Fault identification using frequency analysis measurements



Dynamic behaviour analysis



With self-excitation methods



With external excitation methods



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Measuring machine condition in the past



The first portable vibration measuring instrument from the Schenck company Demonstrated at an exhibition in Leipzig / Germany in 1925



Schenck was founded in 1881



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Modern machine diagnosis



Measuring machine condition with a modern measuring Instrument (VT-60)



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Measurement types for mechanical vibrations Vibration displacement „s“



in µm or mil



= deviation of measured point from rest position



Vibration velocity „v“



in mm/s or ips



= velocity with which measured point moves about rest position



Vibration acceleration „a“



in m/s2 or g



= acceleration with which measured point moves about rest position



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Balancing and Diagnostic Systems



Characteristics of composite vibrations • Narrow-band examination - Extraction into harmonic components (e.g. using a frequency analyser or tracking filters) • Broad-band examination - Through a summing formation in a defined frequency range (e.g. 10 …. 1,000 Hz)



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Balancing and Diagnostic Systems



Amplitude data for vibration measurement



s



so = speak = sm Saverage



srms = seff t



speak-peak = spp



su = speak = sm



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Balancing and Diagnostic Systems



Composite vibrations X



f t



X



2f



+



t



X



f + 2f



=



t



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Vibration in Time Domain vs. Frequency Domain x



x



t x



1



2



3



4



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9 10 11



1



2



3



4



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7



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9



f



x t



x



10 11



f



x t



f 1



2



3



4



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Balancing and Diagnostic Systems



Influence of integration - Practice Vibration velocity spectrum



Vibration acceleration spectrum



Vibration displacement spectrum



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Selecting the measurement type Vibration displacement: Machines with speeds under approx. 600 rpm (10 Hz) Structural vibrations or Relative motions (shaft vibrations) in journal bearing machines of any speed



Vibration velocity: Vibrations in machines with speeds above 600 rpm (10 … 1,000 Hz)



Vibration acceleration: Vibrations with frequencies of interest above 2,000 Hz 18



Balancing and Diagnostic Systems



Vibration types in machines



Rotor Relative shaft vibrations



Absolute bearing vibrations



Bearing casing



Foundation 19



Balancing and Diagnostic Systems



Measuring Absolute Bearing Vibration General rules: Measurement points should be exactly defined and clearly marked Measuring points should be flat, clean and free of grease Loose paint and rusted surfaces should be cleaned or avoided Sensor must sit securely and not wobble Sensor and cable should not move during measurement 20



Balancing and Diagnostic Systems



Acceleration sensors



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Balancing and Diagnostic Systems



Vibration velocity sensors



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Measuring Relative Shaft Vibration



A



45°



45°



B



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Eddy-current sensors Discrete type: Sensor with integral cable Calibrated extension cable Separate converter (oscillator)



Note: Cable lengths may not be altered! 24



Balancing and Diagnostic Systems



Eddy-current sensors Integrated type: Sensor with built-in oscillator and extension cable



Advice: Cable can be extended up to 1,000m in length Use in temperatures above 110°C is not possible



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Balancing and Diagnostic Systems



Machine assessment using the Trend



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Balancing and Diagnostic Systems



Machine assessment acc. to Standards and Guidelines A number of important Standards and Guidelines for rotating masses have been replaced during the last years by: DIN ISO 10816, parts 1 to 6 (absolute bearing vibrations) and DIN ISO 7919, parts 1 to 5 (relative shaft vibrations)



Reciprocating machines, including compressors, can be assessed according to DIN ISO 10816-6 (Reciprocating machines with > 100 kW) DIN ISO 8528-9 (Reciprocating internal combustion machines)



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Balancing and Diagnostic Systems



Assessment of an electric motor acc. to ISO 10816



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Assessment zones Assessment zones according to DIN ISO 10816: Zone A:



Vibration in newly-installed machines



Zone B:



Machines may be operated for an unlimited time without restriction



Zone C:



Machines may be operated for a limited time



Zone D:



Vibrations are at a dangerous level and may cause damage to the machines 29



Balancing and Diagnostic Systems



DIN ISO 10816 Part 3, Group 2 Medium-sized machines with nominal power from 15 kW to 300 kW; Electrical machines with shaft height 160 mm ≤ H >315 mm



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Vibrations created in damaged bearings



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Impulses from a damaged bearing



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Damage frequencies in a rolling-element bearing ß ß d n N



D



Contact angle Rolling-element diameter No. of rolling elements Speed of shaft



Outer race damage



fo =



n•N 2 60



( 1 - d cos ß ) D



Inner race damage



fi =



n•N 2 60



(1+



Rolling-element damage



fr =



D•N d 60



(1- d D



Cage damage



fc =



N 2 60



( 1 - d cos ß ) D



d cos ß ) D



[ ]² cos² ß ) 33



Balancing and Diagnostic Systems



Damage frequencies in a rolling-element bearing



Ball-bearing SKF 6211 Dimensions



Damage frequencies



D = 77.5 mm



Fo = N/60 4.1 = 205 Hz



D = 14.3 mm



Fi = N/60 5.9 = 295 Hz



n = 10



Fr = N/60 5.2 = 260 Hz



ß = 0°



Fc = N/60 0.4 = 20 Hz



N = 3,000 rpm



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Balancing and Diagnostic Systems



BCU signal process X



t



f



X



t



f BCU



X



t t 35



Balancing and Diagnostic Systems



Trend observation



Destruction



Example: Damage progress in a rolling-element bearing



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