Up Time Magazine- August/September 2008

Stage Three – Now the damage is more significant. If you removed the bearing you would see the damage. The bearing has less than 5% of its L10 life at this point. It is now time to replace the bearing; unless the risk of failure can be offset by the need to continue running the machine. The high frequency techniques will still indicate the presence of a fault. The Shock Pulse and Spike Energy (gSE) readings will still trend upwards. The peaks in the envelope spectrum will continue to grow in amplitude. In stage three you will definitely see peaks in the velocity spectrum that correspond to the bearing forcing frequencies (BPFO, BPFI, BSF and FT) depending on the fault condition. • If there is damage on the outer race of the bearing (horizontally oriented machine), there will be harmonics of the BPFO frequency. Initially the harmonics may have a lower amplitude than the peak at BPFO, but as the fault develops the harmonics will grow to be greater in amplitude than the peak at BPFO. • If there is damage on the inner race, there will be a peak at the BPFI frequency. There will be harmonics of this frequency, and sidebands of the running speed will surround the fundamental and harmonics. The side bands appear due to a phenomenon known as amplitude modulation. As the damaged area on the inner race moves in and out of the load-zone, the impacts will rise and fall (once per revolution). • If the rolling elements are damaged, there may be a peak at the BSF frequency, but more 7. 6. 5. 4. 3. 2. 1. 0. -1. -2. -3. -4. -5. likely at twice that frequency (because damage on the ball or roller will impact the inner race and outer race per revolution). Again, we will see harmonics and sidebands, however this time the side bands spacing will be equal to the FT (cage) frequency – because each ball/roller moves in and out of the load-zone with the cage. Time waveform analysis will also indicate the presence of the modulation, and impacts will be visible in the waveform. The time waveform shows you exactly what is happening inside the bearing – each impact is visible. Care must be taken to select the correct record length and resolution so that it is possible to see the detail required. Stage Four – Now the bearing has substantial damage. The bearing should be replaced; you are taking a significant risk of catastrophic failure to leave it in service. Vertical Figure 7 – A rolling element bearing highlighting the load-zone mm/sec 0 0.05 0.1 0.15 0.2 0.25 10. 8. 6. 4. 2. 0. -2. -4. -6. -8. -10. 0 0.05 0.1 0.15 0.2 0.25 g While the high frequency techniques can still be used, the bearing will go through some important changes which can cause the high frequency vibration to be reduced. Shock Pulse and Spike Energy vibration levels may drop. As the damage continues, a number of things can happen: 1. There is so much wear that there are no longer sharp edges, thus there is a reduction in the high frequency vibration. Damage Life Action Figure 9 – The Damage, Remaining Life and Appropriate Action for the Four Stages of Bearing Failure 2. There is now so much damage that the vibration loses its periodicity. Figure 8 - Time Waveform Analysis becomes a more valuable tool You may not see distinct peaks when bearings are in failure stage 2 or higher. On low speed in the spectrum at the key bearing machines, it is valuable at all stages. forcing frequencies. Those peaks will drop down, and seemingly Stage 1 Stage 2 Stage 3 Stage 4 random peaks Very Difficult to See Damage Difficult to See Damage Easy to See Damage Very Easy to See Damage will appear Sub-surface Damage Sub-surface Damage A Range of Severities A Range of Severities and the noise floor will rise 10 - 20% of L10 Life 5 - 10% of L10 Life < 5% of L10 Life < 1% of L10 Life up. This is true for the velocity Lubricate Correctly Lubricate Correctly Replace ASAP Replace NOW Continue Monitoring Monitor More Frequently Monitor More Frequently spectrum and the envelope Axial Stage 1 HFD Envelope Spectrum Time Waveform Yes: Effective Methods Trend Will Continue Upward Earliest Signs of Faults Noise Floor May Rise Maybe: Acceleration Spectrum No: Velocity Sprectrum No: Vibration in Noise Floor Effective for Low Speed Machines Stage 2 Yes: Effective Methods Trend Will Continue Upward Yes: Defect Frequencies Present Noise Floor Should Rise Yes: Acceleration Spectrum No: Velocity Spectrum Maybe: Vibration in Noise Floor Effective for Low Speed Machines Stage 3 Yes: Effective Methods Trend Will Continue Upward Yes: Defect Frequencies Present Noise Floor Should Rise Yes: Acceleration Spectrum Yes: Velocity Spectrum Yes: Impacts Visible Modulation Visible Stage 4 No: Less High Freq. Vibration Trend Will Drop Yes: Defect Frequencies Present Noise Floor Should Rise Yes: Acceleration Spectrum Yes: Velocity Spectrum Yes: Impacts Visible Modulation Visible Figure 10 – Appropriate Signal to Monitor During the Four Stages of Bearing Failure 66 august/september 2008

Table of Contents Feed for the Digital Edition of Up Time Magazine- August/September 2008

Up Time Magazine- August/September 2008 Contents Upfront Upclose Information Technology Lubrication Infrared Maintenance Management Motor Testing Precision Maintenance Reliability Ultrasound Vibration Upgrade

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