Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 1 The powerful tool for detection and analysis of amplitude modulation. To detect and analyze: rolling-element bearing defects P u l s a t i on s (shock excitations) modulated random noise The powerful tool for detection and analysis of amplitude modulation. To detect and analyze: rolling-element bearing defects P u l s a t i on s (shock excitations) modulated random noise Selective Envelope Detection (SED)

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 2 Single shock pulse Time signal (waveform)FFT- autospectrum t1t1 Short shock pulse time Long shock pulse time t 2 = 2 x t 1 frequency f 1, max = 1/ t 1 f max up to > 50kHz Continuous spectrum frequency f 2, max = 1/ t 2 = 0,5 x f 1,max f max often < 10kHz Continuous spectrum

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 3 Periodic series of shock pulses Time signal (waveform)FFT- autospectrum t1t1 Periodic series of shock pulses time T frequency f 1, max = 1/ t 1 f max up to > 50kHz Line spectrum 1/T Periodic series of shock pulses time t 2 = 2x t 1 T frequency f 2, max = 1/ t 2 = 0,5 x f 1,max Line spectrum f max often < 10kHz

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 4 Series of shock pulses with accelerometer FFT- autospectrum f 2, max = 1/ t 2 frequency 1/T frequency 1/T f 1, max = 1/ t 1 Line spectrum Time signal (waveform) t1t1 Periodic series of shock pulses time T Periodic series of shock pulses time t 2 = 2x t 1 T

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 5 Modulated random noise Time signal (waveform) FFT- autospectrum time Continuous spectrum f max = kHz frequency 1/T

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 6 Amplitude modulated time signal (waveform) AM radio Principle of envelope detection Like an AM radio the envelope detection uses the propagation of higher frequencies Higher frequencies are used as carrier frequencies The low.frequency information modulates the carrier frequency Consequently it is possible to detect and analyze low- frequency activities Like an AM radio the envelope detection uses the propagation of higher frequencies Higher frequencies are used as carrier frequencies The low.frequency information modulates the carrier frequency Consequently it is possible to detect and analyze low- frequency activities

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 7 2. Rectifying (amplitude demodulation) 3. FFT-analysis 1/3 octave or broader Envelope spectrum (modulating frequencies) Envelope (energy vs. time) Selective Envelope Detection (SED) 1. Filtering (with bandpass filter) Bandpass filter Amplitude-modulated time signal (waveform) < 10dB 10 kHz 500 Hz (typical for 50Hz machine) time

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 8 Periodic series of shock pulses creates spectral lines over the whole frequency axis A selective envelope filter extracts higher frequencies from the signal The signal-to-noise ratio against dominant low frequencies like unbalance, etc. is improved The narrower the filter bandwidth the better the signal-to-noise ratio The narrower the filter bandwidth the more important is an exact tuning of the filter with respect to the signal source (--> measure FFT-autospectrum first and use different setups for SED). Periodic series of shock pulses creates spectral lines over the whole frequency axis A selective envelope filter extracts higher frequencies from the signal The signal-to-noise ratio against dominant low frequencies like unbalance, etc. is improved The narrower the filter bandwidth the better the signal-to-noise ratio The narrower the filter bandwidth the more important is an exact tuning of the filter with respect to the signal source (--> measure FFT-autospectrum first and use different setups for SED). Selective Envelope Filter - Bandwidth 10 kHz 1/3 octave filter 1000Hz...10kHz filter

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 9 A selective envelope filter placed on a significant structural resonance uses the additional gain of the resonance to improve the sensivity A selective envelope filter outside structural resonances is less sensitive Too high sensivity will detect minor faults which very likely cannot be verified by visual inspection A selective envelope filter placed outside structural resonances is more suitable for detecting modulated random noise A selective envelope filter placed on a significant structural resonance uses the additional gain of the resonance to improve the sensivity A selective envelope filter outside structural resonances is less sensitive Too high sensivity will detect minor faults which very likely cannot be verified by visual inspection A selective envelope filter placed outside structural resonances is more suitable for detecting modulated random noise Selective Envelope Filter - Structural resonances < 10dB 10 kHz On a resonance Outside significant resonances

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 10 During normal operation a good rolling-element bearing creates random noise without discrete frequencies The („white“) random noise has a flat FFT-autospectrum with energy up to high frequencies During normal operation a good rolling-element bearing creates random noise without discrete frequencies The („white“) random noise has a flat FFT-autospectrum with energy up to high frequencies Rolling-element bearing in good condition 40 kHz FFT-autospectrum acceleration up to 40 kHz Time signal (waveform)

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 11 Crack Direction of load The highest structural load inside a bearing is in the load zone of the outer race, some millimeters below the contact surface Most rolling-element bearing defects start with a crack or spalling (rupture) Ruptures provoke shock pulses up to very high frequencies - the structural resonances of the bearing housing will be excited The highest structural load inside a bearing is in the load zone of the outer race, some millimeters below the contact surface Most rolling-element bearing defects start with a crack or spalling (rupture) Ruptures provoke shock pulses up to very high frequencies - the structural resonances of the bearing housing will be excited Start of wear inside the bearing time signal (waveform) 40 kHz FFT-autospectrum acceleration up to 40 kHz Rupture

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 12 The rupture edges will smooth out - therefore the shock pulses will get smaller and longer The signal energy will be reduced to frequencies below 10kHz...20kHz The motion of the rolling elements will be amplitude modulated Increasing defects will cause the rolling elements to bounce on the contact area - erosion inside the bearing will be the consequence The rupture edges will smooth out - therefore the shock pulses will get smaller and longer The signal energy will be reduced to frequencies below 10kHz...20kHz The motion of the rolling elements will be amplitude modulated Increasing defects will cause the rolling elements to bounce on the contact area - erosion inside the bearing will be the consequence Progressive wear inside the bearing Time signal (waveform) 10 kHz FFT-autospectrum acceleration up to 10kHz

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 13 Envelope Detection & typical defects in rolling-elem. bearings 1. Outer race defect Lead time: months; Ball-pass frequency at outer race (BPFO) and harmonics 2. Inner race defect Lead time: days - weeks; Ball-pass frequency at inner race (BPFI with sidebands (RPM distance) 3. Rolling element defect Requires immediate action; Double ball-spin frequency (2x BSF) with harmonics; often in combination with various inter- harmonics 1. Outer race defect Lead time: months; Ball-pass frequency at outer race (BPFO) and harmonics 2. Inner race defect Lead time: days - weeks; Ball-pass frequency at inner race (BPFI with sidebands (RPM distance) 3. Rolling element defect Requires immediate action; Double ball-spin frequency (2x BSF) with harmonics; often in combination with various inter- harmonics Envelope spectrum BPFO RPM BPFI 2x BSF

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 14 Lubrication problem Misalignment due to bad assembly or unbalanced shaft Radial tension on bearing outer race Misalignment of bearing outer race Slip from loose bearing outer race 2x RPM 1x RPM 2x BPFO Harmonics of RPM Increased noise floor RPM 2x BPFO 2*RPM Envelope spectrum Envelope Detection & typical defects in rolling-elem. bearings

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 15 Example : FFT-autospectrum - Signal energy overview Acceleration up to 10 kHz Broad-band acceleration (here up to 40kHz, VT-60 up to 20kHz)

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 16 Example: Selective Envelope Detection - Diagram

Sales Meeting February 2002 Brüel & Kjær Vibro GmbH VT-60 R.4 & xms R.1 Folie 17 Selective Envelope Detection - Random setup m/s Hz