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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 8, Vibration in Bearings

Section 3, Considerations for Evaluating Amplitudes Due to Bearing Defects

Whether plotted by hand or by computer, trends of IBF units or vibration units (usually velocity) vs calendar dates are certainly valuable for monitoring how fast a bearing may be deteriorating. However, such trends do not always accurately predict the approximate time for failure. Usually trend plots of IBF units show increases as the bearing deteriorates, but as actual failure occurs (such as cracking), the IBF units may show a sharp decrease. When IBF units are still increasing, most careful analysts monitor trends of velocity amplitudes as well. But again, amplitude increase is not always present as a bearing deteriorates.

The difficulty is due to the entirely different nature of vibration amplitude increases due to bearing defects, as compared to amplitude increases due to other vibration sources such as unbalance, misalignment, bent shaft and so on. For example, as fly ash builds up on a fan, its unbalance may progressively increase. Increased unbalance shows increased amplitude at 1 x rpm. Or, for example, greater shaft-to-shaft misalignment will give higher amplitudes for its frequencies. Therefore, the analyst is tempted to think that greater defects at the source always produce larger amplitudes. True for almost all vibration sources. However, not necessarily true for bearing defects.

Sometimes bearing defect amplitudes increase as the defect gets worse -- but not always. Amplitudes at bearing defect frequencies may not increase much at all and, in some situations, may actually decrease as the bearing gets worse. Instead of the bearing defect amplitudes increasing, they may instead increase the number of sidebands. What may have started out as a relatively sharp peak may appear to be spreading out to cover a wider frequency range. As with trends of IBF units, just before or right at bearing failure, the vibration amplitude suddenly decreases. Therefore, trend plots based on amplitude, may or may not give an accurate picture of what is happening in the bearing.

Vibration amplitudes measured on the machine's case that are considered as acceptable or even smooth if originated from, for example, unbalance, are not acceptable for amplitudes due to bearing defects. For example, a pump's case reading of 0.1 in/sec or 3 mm/sec is considered acceptable, but if that amplitude originates from a bearing defect, that bearing is usually so bad that the defect is visible to the eye (conversions between English and metric units are approximate). Vibration amplitudes at bearing defect frequencies are usually very low. For a "regular speed" machine, such as 1800 rpm or 1500 rpm, a bearing is considered mildly bad at only 0.04 in/sec or 1 mm/sec. Although this amplitude appears mild, the bearing can still immediately fail. For very slow speed machines, such as papermachine dryer rolls (under 100 to 200 rpm), a bearing defect velocity amplitude of only 0.02 in/sec or 0.5 mm/sec requires a bearing change.

(In Update's course Practical Solutions to Machinery and Maintenance Vibration Problems Part II this subject is covered in considerably more detail and actually takes a full day. However, this textbook is for Practical Solutions to Machinery and Maintenance Vibration Problems Part I and the information provided is sufficient for analysis of bearing defects in about 90 percent of all situations.)

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