This paper serves as a summary of the vibration analysis techniques available at the time to detect and track the severity of rolling element bearing defects. Both low frequency, velocity spectra detection and high frequency detection techniques are discussed with examples given. The author relates the strengths and weaknesses of each technique. A helpful restatement of the four stages of bearing degradation first classified by J.C. Berggren in 1988 is provided with helpful comments by the author.
This paper provides examples of how windowing and the three primary windows used in vibration analysis (rectangular, hanning & flat-top) affect the real frequency resolution of signals. The author makes a strong case that the standard formula for frequency resolution should be modified by a factor of 2 to provide the true frequency resolution possible for all signal combinations. Reading this short paper provides a better feel for how each window type affects the frequency resolution of our machine vibration signals.
This short paper describes how the log scale can be used to identify bearing defect frequencies and other smaller peaks within the spectrum. A case history is provided with plots that illustrate the effectiveness of the log scale at identifying small bearing peaks in the presence of a resonant peak of high amplitude.
This short paper describes the phenomena of “soft foot” in electrical motors and the effect this problem has on vibration data. The author describes the difference between a simple soft foot from a tapered or angular foot and how a tapered soft foot can be difficult to impossible to detect by using a dial indicator. A case history is provided showing how the author identified and solved a soft foot using vibration analysis.
In this short case history the author stresses the benefits of gathering dual-channel .vs. single-channel vibration data from proximity probes. A vibration problem with a high speed centrifugal compressor could have been mis-diagnosed if only single-channel data were gathered. The orbits of vibration data from the compressors were particularly helpful in identifying the pre-loaded condition present.
In this short case history the author used trending, spectral analysis, time waveform analysis along with on-site inspections to identify internal looseness within a 400 HP motor. It provides a good example of the characteristics one looks for when looseness is present within machinery.
This short case history does a good job of illustrating the power of enveloping and high frequency techniques at identifying bearing defects. The Peakvue brand of enveloping technique was used successfully by the author in detecting a cracked inner race on a fan bearing. This bearing defect was not seen in the normal velocity data but was easily seen in the Peakvue data.
This short case history does a good job of illustrating the power of enveloping and high frequency techniques at identifying bearing defects even on very slow turning equipment. The Peakvue brand of enveloping technique was used successfully by the author in detecting an outer race defect on a slow-turning (47 rpm) trunion bearing.
This short case history is of a gas turbine generator where misalignment was properly diagnosed and corrected using vibration analysis. The alignment problem was between the turbine & gearbox and caused by thermal growth changes and differences in the turbine support legs. It provides a brief study of the vibration tools used to solve this problem and insight into a less common source of misalignment.
This paper provides an excellent introduction to VFD drives, and their pros and cons as it relates to vibration analysis. While its focus is on VFD drives applied to the operation of vertical pumps, the subject matter certainly applies to most if not all machinery operating on VFDâ€™s. A case history is provided at the end of the paper that illustrates how adding a VFD drive to an existing vertical pump created vibration problems that were not present before. The solution to the resonance problem introduced by the VFD drive is described in detail.
As the industrial arena grows more sophisticated, it seems as though operations are confronting fewer and fewer broken machine shafts. When shafts DO break, however, there are almost always as many theories regarding the suspected culprits as there are people involved.
*This copyrighted article was first published in the July 2012 issue of Maintenance Technology magazine (maintenancetechnology.com), which has granted permission to the Vibration Institute to post the material solely in the Member’s Only Selection of technical papers in the association’s library. This article may not be copied or distributed without the express permission of Maintenance Technology. All rights reserved.
This paper reads as a history of how our understanding of machine problems such as critical speeds, oil whirl, oil whip and other forms of instabilities were developed. Most of this knowledge was developed many years ago as a result of machine failures that at the time could not be fully explained. Root cause failure analysis investigations were essentially performed on each type of failure by companies, research organizations, and in some cases government agencies to better understand the causes. In most cases models of the machines were fabricated and tested under similar conditions to duplicate the instabilities. This approach allowed detailed observations by researchers, better understanding of the problems, and eventual recommendations on how to best avoid these problems thru improved design and operation. Reading this paper made me appreciate even more those in the past that increased our knowledge so much in the field of rotor dynamics. It was interesting to me that in most cases experimental observations and general recommendations from these observations came first; theoretical explanation of them was developed after the observations had been made – this is the essence of the scientific method.
Combining Vibration & Tribology Analysis To Detect Gas Turbine Main Bearing Failure – A Case History
This paper is a short case history where vibration analysis, oil analysis & tribology were used to detect a problem with a gas turbine generator. While both vibration & oil analysis results were stable, the author began noting a significant change in particle count from one of the magnetic chip detector (MCD) units installed along the gas turbine. Detection by the MCD of a problem prompted him to focus additional PDM efforts on the portion of the machine measured by the MCD in question. The decision was made to accelerate scheduled overhaul of the machine and a catastrophic failure of the machine was likely avoided. This paper provides an excellent example of how employing more than one predictive maintenance technology on a critical machine can aid in detecting problems earlier and more precisely.
This paper introduces the reader to VFD drives with a focus on how their introduction affects the vibration of rotating equipment. It begins with an introduction to variable frequency drives, explains briefly their major components, how they work, and what electrical frequencies (harmonics) are usually introduced when a VFD is employed. The tendency for VFD drives to excite system natural frequencies and cause resonance problems is mentioned. The author finishes the paper with two case histories where VFD drives played a major role in the vibration problems experienced by the rotating machinery.
This brief paper gives an account of how the author dealt with a vibration problem at a belt-driven cooling tower fan. Balancing the fan corrected part but not all of the problem. The remaining final problem involved excitation of a natural frequency at the motor by what turned out to be a high order multiple of the belt speed (frequency). Once the excitation source was identified, a simple change of belt tension eliminated the problem.
This brief paper gives an account of how the authors identified and solved what turned out to be a base looseness problem on an air preheater drive that was first suspected to be a problem with the motor. Phase measurements collected along the entire drive and its base were the key to identifying the exact source of the looseness. Once the looseness was corrected, vibration levels dropped and normal operation was restored. This paper is also a great case history where looseness was only indicated in the vibration spectra by high vibration at 1x rpm with no multiples of running speed being seen of any significance. Without the phase measurements and a trained analyst being on-site, the problem could easily have been misdiagnosed.
This brief paper gives an account of how the author identified and solved a torsional vibration problem with a methanol compressor. The problem symptoms and diagnosis are described along with the final solution which involved a better set of gears and most importantly a different coupling. A new elastomeric coupling replaced the original gear coupling – this reduced the torsional stiffness as well as added torsional damping to the system. Both were positive changes to the machine which reduced its vibration levels and extended its life.
This brief paper gives an account of how vibration analysis was used to detect a bearing fault in a vertical pump. As the bearing condition worsened, the pattern in the vibration data changed from the presence of defect frequencies to signs of excessive clearance or looseness within the machine. Due to the persistence of the vibration analyst, a complete failure of the machine was avoided.
This brief paper gives an account of how the author and his other plant personnel dealt with a carbon seal rub on a steam turbine following rebuild of the unit. The paper goes thru how vibration analysis was used to detect and then help to resolve the problem. Corrective actions for this problem were discussed.
This is a well written and brief paper that introduces the reader to vibration analysis of paper machine press rolls and rolls in general. Four case histories are presented within this paper involving the following problems: roll unbalance, roll barring, bearing defects & felt seam impacting. The special techniques involved in collecting roll vibration data are presented including the use of low frequency vibration sensors and time synchronous averaging. For anyone wanting an introduction to vibration analysis of paper machine rolls, this paper is a good start.
This paper is a case history of problems experienced after upgrade of two turbine-driven feed water pumps. Upon OEM upgrade of the two units, a thrust bearing failure was experienced by one of the two turbines. After rebuild of the turbine, initial vibration levels were acceptable, but after operating the unit for about 5 minutes, the vibration levels quickly rose to unacceptable levels. The unit was shutdown. A later solo run of the turbine saw no problems and low vibration levels. Both pipe strain & thermal growth of the turbine were suspected. Adjustment of the exhaust piping to reduce pipe strain was made and the thermal growth of the turbine measured using benchmark gauges. Measured thermal growth was found in significant variation with that expected by the OEM and previously accounted for in the alignment. Once the measured thermal growth amounts were accounted for in the shaft alignment, acceptable vibration levels were consistently measured on the machine whether hot or cold.
This paper is a brief case history of how cracked blades on a cooling tower fan were detected and a fan failure likely avoided by the use of vibration analysis. Vibration sensors were properly installed on the gearbox driving the fan and the vibration levels routinely monitored. When a significant change in levels was noted by the PDM group, and dominant vibration at the fan speed determined, a thorough inspection by maintenance was requested. This inspection found significant cracks in two of the fan blades that undetected, would have certainly resulted in a catastrophic failure of the fan.
This paper is a good read for anyone wanting to learn more about isolation of rotating machinery. It begins with a theoretical discussion of isolation and later presents the different types of isolators available today (elastomeric, spring, air, etc). Three real examples of isolated machinery are given (fan & two feeders). Throughout the paper, the inaccuracies of a single degree of freedom isolation model are stressed with an awareness of the flexural & torsional modes of the supported machines & bases above the isolators being emphasized. The structural dynamics of the foundation or floor supporting the isolators is also stressed.
This paper provides five case histories where vibration problems with compressors were identified, successfully analyzed and solved. Examples of the problems involve surge in a centrifugal compressor, torsional vibration on a compressor driven by a synchronous motor, and a type of instability known as the Morton effect. For those wanting to learn more about compressors and the application of vibration analysis techniques on them, this paper is a good choice.
This paper is a case study of bearing damage caused by electrostatic discharge. Many photos of the actual pad damage are given. Many characteristic orbits with the descriptive random spikes caused by electrostatic discharge are shown. An economic evaluation of the potential losses averted is included. Before and after orbits are shown that prove the newly installed grounding brush stopped the progression of the problem. Several references are given in the bibliography of other similar situations.