Bearing Fault Detection and Diagnosis with m+p SO Analyzer

Transcription

1 Application Note Bearing Fault Detection and Diagnosis with m+p SO Analyzer Early detection and diagnosis of bearing faults FFT analysis Envelope analysis m+p SO Analyzer dynamic data acquisition, analysis and reporting system National Instruments acquisition hardware This document illustrates the Envelope analysis feature in the m+p SO Analyzer to detect and diagnose bearing faults. Other scalar criteria (like broadband or in band RMS, crest factor, Kurtosis, FFT and/or 1/N octave analysis) are also available for use as additional criteria depending on machine complexity and user needs. Starting from a brand new bearing in good health, correctly selected, installed and oiled, the first fault will typically be from a single localized defect. This will generate shocks each time a ball passes or hits that local defect. Initially associated energy during the shocks will be quite weak but with time, the defect will get worse together with the generated energy. So it is recommended to use a suite of tools that can cover all these situations. Application Note Bearing Fault Detection and Diagnosis with m+p SO Analyzer 1

2 Courtesy of ESIX Cherbourg Example test system: PCB 352C04 accelerometer NI USB 9234 acquisition frontend m+p SO Analyzer DSA Pro V4.3 software including Envelope analysis feature Test bench with a motor driven at 1500 rpm, including two sleeve bearings and an open shaft for a quick/easy bearing mounting/exchange Bearings ref (9 balls; FAG, SNR ) Impact frequencies related to a local fault: Application Note Bearing Fault Detection and Diagnosis with m+p SO Analyzer 2

3 BPFO: Ball Pass Frequency for a local fault on Outer race BPFI: Ball Pass Frequency for a local fault on Inner race BFF: f r : BD: PD: Ball Fault Frequency; that value is two times the Ball Spin Frequency (2 hits per ball revolution) for a local fault on rolling element relative rotational speed between both races Ball Diameter nominal diameter β: contact angle For most of these situations, we can approximate impact frequencies by the following formulas: BPFO # 0.4 x Nb balls x f rot BPFI # 0.6 x Nb balls x f rot For more precise values the bearing frequency calculators supplied by the bearing manufacturers should be used. Comparison between two bearings ref named OK_F and HS_C (= NOK-C): These 6206 bearings have 9 balls. The motor speed is 24.8 Hz (1488 rpm). From approximate above formulas we have: BPFO # 0.4 x 9 x 24.8 = 89.3 Hz BPFI # 0.6 x 9 x 24.8 = Hz Before any measurement, we clearly hear bearing HS_C is noisier than bearing OK_F, but we cannot say anything else. (see Listen *.SOT files with and without filtering) 1st step: FFT analysis up to 10 khz We see increased levels at high frequencies for the bearing labelled as HS_C. By computing a 1/3 octave synthesis from FFT spectra, we see an approximate + 20 db increase (ratio of *10) for the vibration level in the 1-10 khz frequency band. Application Note Bearing Fault Detection and Diagnosis with m+p SO Analyzer 3

4 Looking at the FFT spectra content up to 500 Hz we don t see any significant difference in level and in shape. However the bottom part of the spectra is a little higher for the bearing named HS_C than for the one named OK_F. The frequency family shows rotational speed and harmonics but there is no identifiable family related to any of the expected bearing fault frequencies. Application Note Bearing Fault Detection and Diagnosis with m+p SO Analyzer 4

5 2nd step: Envelope analysis in the 4 khz - 8 khz frequency band We select the frequency band for the demodulation (Envelope analysis) from the broadband FFT spectra and get: On the above Envelope spectra for the HS_C bearing, we see a peak family for which the fundamental frequency is # 134 Hz (i.e. the estimated BPFI). Moreover with see side bands spaced by rpm around main frequency components. So we can confirm the defect is due to a local fault on the bearing HS_C rotating inner race. With experience it is possible to assess the importance of this developing fault from basic FFT and Envelope analysis. This Envelope feature, used in addition to basic FFT analysis, is a very powerful tool for early detection and diagnosis on complex machines. Application Note Bearing Fault Detection and Diagnosis with m+p SO Analyzer 5

6 We thank Mr. Jérôme Thiebot (ESIX - Engineer School in Cherbourg - France) for the use of his test bench and m+p SO Analyzer system used to prepare the above example data. At the end of 2012 and in mid-2014 ESIX Cherbourg has also upgraded three practical vibration exercises for students with three m+p SO Analyzer plus NI CompactDAQ systems: TP1: Structural identification through a shaker excitation TP2: Structural identification through an impact test TP3: Bearing fault detection and diagnosis m+p SO Analyzer and NI frontend: TP3 arrangement Germany m+p international Mess- und Rechnertechnik GmbH Phone: (+49) (0) Fax: (+49) (0) sales.de@mpihome.com USA m+p international, inc. Phone: (+1) Fax: (+1) sales.na@mpihome.com United Kingdom m+p international (UK) Ltd Phone: (+44) (0) Fax: (+44) (0) sales.uk@mpihome.com France m+p international Sarl Phone: (+33) (0) Fax: (+33) (0) sales.fr@mpihome.com China Beijing Representative Office of m+p international Phone: (+86) Fax: (+86) sales.cn@mpihome.com listens to customers /01-15 Application Note Bearing Fault Detection and Diagnosis with m+p SO Analyzer 6