Introduction: In popular terms it has been said that machines 'talk' and through their sounds and vibrations, one can listen to their complaints and diagnose their ailments. Vibration based condition monitoring is the process in which the machine components are regularly checked and the condition i.e., whether it is healthy or faulty, is checked on the basis of vibration signals got from the machine components. Vibration monitoring can be broadly carried out at three levels 1: 1. Overall vibration level measurement, to detect that a problem exists.
Spectral or frequency analysis, to locate where the problem is in the machine. Special techniques, which can indicate what the problem is at a more detailed level The raw data from a vibration transducer mounted on a test structure is obtained in time domain. The vibration signal in time domain is useful to the extent of finding out the overall vibration level. The overall vibration level may not exactly indicate the impending defect that is growing in the system. The frequency that is responsible for a particular defect is to be identified rather than the overall vibratory level. For this the vibratory signal in time domain is to be converted to frequency domain using Fast Fourier Transforms and the vibration analyzers (FFT Analyzers) do this job 2. Special analysis techniques like envelope spectrum analysis, cepstrum analysis, spike energy method, shock pulse method, waterfall diagram etc., are used when the spectrum analysis does not give much information about the defect, or when the technique suits the system to be monitored than spectrum analysis.
This conventional vibration monitoring process is time consuming and it involves expensive instrumentation, accurate and repetitive measurements to be made and need an expert to interpret the measurements to the developing problem in the machine. Thus, if the absolute or overall vibration level is standardized then it makes vibration based condition monitoring program simple and effective than the conventional one. Hence the objective of the present study is to give an indication that, overall or absolute vibration level can be an alternative to frequency or spectrum analysis. Background and Scope of Work: Till today there are no standards available for determining the acceptable vibration level for machine tool spindles.
However there are some standards, which gives an indication of machinery health based on overall vibration level like ISO 2372, VDI 2056/1964, BS 4675, Indian Standards 4729, ISO 2373, ISO 3945, VDI 2059, IRD Mechanalysis Standard, Canadian Government specification and so on. The salient features of all these standards are as described in the Table 1. Salient Features of available Vibration Standards for Assessing the Machinery Health based on Overall Vibration Level All the above standards specify the acceptable vibration level for general purpose machineries like electric motors, pumps, generators, turbines, blowers, fans etc. Further these standards can only 5 be used as a guideline because they are based upon wide range of machines and cannot be expected to be accurate for every particular machine and its operating conditions. None of these gives acceptable vibration standards for machine tools. The present study is focused on specifying acceptable vibration levels for precision machine tools. Here an attempt has been made to establish vibration level under normal conditions and also to specify the vibration level due to problems like unbalance or bearing damage for machine tool spindles.
This can be used as direct tool for machinery health monitoring and will be an alternative to spectral analysis and other special analysis techniques. Data Segregation and Analysis Methodology: The study of vibration level involves segregation of the collected data and analysis of the segregated data.
Data segregation methodology: The noise and vibration laboratory of Central Manufacturing Technology Institute (CMTI) has a very large database of RMS vibration velocity values and frequency spectrum data, collected during their condition monitoring program of machine tools at various industries. The database of vibration of machine tools available includes vibration velocity in four frequency bands measured over a period of 3-4 years and they are chosen for the study. These bands are obtained from the spectral data of 10Hz to10 kHz bandwidth (as it is observed that, most of the machine tool vibrations are within this frequency range). These bands are: 10Hz- 1 kHz (RMS 1-1st Band), 1-3 kHz (RMS 2-2nd Band), 3-5 kHz (RMS 3-3rd Band), 5-10 kHz (RMS 4-4th Band) and overall vibration velocity (10Hz-10 kHz). The overall vibration level data to be studied is chosen from the database and segregated based on: 1. The machine type 2. The operating speed range of spindles 3.
Particular make or model of the machine Data analysis methodolog: The vibration data for defining the normal vibration level and damage factor due to unbalance and bearing damage is analyzed as explained below: 1. First, the vibration level data for healthy machines having no abnormalities for a particular type is taken from the database and the average vibration velocity value in all the four bands (i.e. Average RMS 1, RMS 2, RMS 3 and RMS 4) and the overall velocity value are calculated. Then the vibration level data of machines having problems like unbalance in spindle, bearing damage etc for each type of machine is studied by calculating the relative change in vibration level (Ratio of the increased vibration level caused due to a problem to the normal vibration level) i.e.
The factor by which the normal vibration value increases due to the bearing damage or unbalance in the spindle is determined. This factor is defined as the damage factor (DF). The machine tools considered for the study are precision machine tools. It is presumed that other problems, which influence the machine tool vibrations such as looseness, misalignment etc., are not there as these problems seldom occur in machine tools.
Hence the most common problems encountered in precision machine tools are either unbalance or bearing damage. So the study is focused on deriving the damage factors due to unbalance and bearing damage only for machine tool spindles. Machine tools with direct belt drive from the motor to spindle are considered for the study and hence applicable to such system only. The same procedure is used to establish the normal vibration level and the increased vibration level caused due to a problem, for all the machine tools considered for the study.
Results and Discussion: From the study of absolute vibration level, the normal vibration level and damage factors, in general for all machine tool spindles, is defined and are tabulated in the Table 2. From the study of overall vibration level data, the normal vibration level for different categories of machine tool spindles is also derived. The normal vibration levels in different frequency bandwidth and the damage factors due to unbalance and bearing damage are also defined for these machine tool spindles and the results are depicted in the Table 3. An illustration is shown below by a comparative study of vibration spectra and the damage factor graph to indicate that; the overall vibration level can be an alternative to spectral analysis.
1(a) Comparative vibration spectra when there is an unbalance in spindle Fig. 1 (b) Damage factor graph for unbalance A comparative vibration spectrum is given in the Fig. Form the spectrum it is clear that, the unbalance causes the increase in the vibrations in low frequency regions (10Hz to 1 kHz) and the same is represented in the damage factor graph as shown in Fig.
1(b) plotted using the overall vibration level. 2(a) Comparative vibration spectra when there is a bearing damage Fig. 2 (b) Damage factor graph for bearing damage A comparative vibration spectrum is given in the Fig. From the spectrum it is clear that, the bearing damage causes the increase in the vibrations in high frequency regions (above 1kHz) and the same is represented in the damage factor graph as shown in Fig. 2.(b) plotted using the overall vibration level. Therefore the overall standardized vibration level can be a valuable tool for machine health monitoring without having to go for spectrum evaluation.
Conclusions: The Vibration standards estimated from the study for different machine tool spindles are given in Table 2 and 3. These standards can be used to monitor the overall health of machine tool spindles. After fixing the vibration standards for different types of machine tool spindles can be used as benchmark for the vibration severity of other similar machines. The damage factors obtained for different categories of machines also helps in assessing the health of machines without going for detailed spectral analysis.
Future Scope: 1. The damage factors obtained from this study could be integrated in software associated with traditional data collectors for on-line monitoring. This probably eliminates the need of an expert and simplifies the vibration monitoring program, and thus helps in eliminating the costly instrumentation required for spectrum evaluation and the time required for the analysis.
The results can be used to fix Alert, Alarm and Trip vibration levels for a particular machine type, for developing, online-condition monitoring system, which continuously monitors the machinery health. The present study is the first step taken towards establishing acceptable vibration level for machine tool spindles, which can be used as an alternative to frequency or spectrum evaluation for assessing the machinery health. The study can be extended for a longer period of service to enhance the reliability of the standardized absolute vibration level and damage factors for different machine tool spindles.
References: 1. Woodley, Machine Condition Monitoring-Sources of Equipment and Services. Gupta, Introductory Course on Theory and Practice of Mechanical Vibrations, 2nd Edition, New Age International (P) Ltd, Publishers, pp. Keith Mobley, An Introduction to Predictive Maintenance, Plant Engineering Series, Van Nostrand Reinhold, New York, pp.148, 1990. Srivastava, IRD Mechanalysis, Vibration Monitoring for Predictive Maintenance, Purchase, August 1993. Yadava and L.
Thuestad, Vibration Measurement and Analysis, National Productivity Council, New Delhi, pp. Rao, Vibratory Condition Monitoring of Machines, Narosa Publishing House, pp. 354-356, 2000. Rao, Handbook of Condition Monitoring, 1st edition, Elsevier Advanced technology, UK, pp.76, 1996. Satyan and H. Nagarajan, Predictive Maintenance through Vibration Monitoring, Technical article, Noise and Vibration Laboratory, CMTI, 1988. Mechanical Vibration of Machines with Operating Speeds from 10 to 200 rev/sec.- Basis for Specifying Evaluation Standards, ISO 2372, 1974.
Iso Vibration Standards Pdf
Vibration Limits for Maintenance, Canadian Government Specification, CDA/MS/NVSH/ 107. Proceedings of VETOMAC-2, 16-18 December, 2002 Kumaraswamy. J and Amol Kumar Nalavade Final year B.E., Department of Mechanical Engineering Dr.Ambedkar Institute of Technology, Bangalore - 560 056 Email: [email protected] [email protected] [email protected] S. Rao and Prakash Vinod RTE, Noise and Vibration Laboratory Central Manufacturing Technology Institute Tumkur Road, Bangalore - 560 022 Email: [email protected] [email protected] S. Rao and Prakash Vinod RTE, Noise and Vibration Laboratory Central Manufacturing Technology Institute Tumkur Road, Bangalore - 560 022 Email: [email protected] [email protected].
Performance criteria is one of the most—if not the most—important aspects of a successful preventive maintenance (PM) and condition-based maintenance (CBM) program. The asset owner must refine and accept all performance criteria for it to be meaningful, and the criteria must be documented to ensure consistent application. Specifically, when referencing national and international consensus standards as a basis for a facility's performance criteria, personnel must evaluate the criteria to ensure it is representative of the normal operating characteristics of the asset. In all of the CBM (and predictive maintenance PdM) courses I have developed over the years, I have always shared the following: 'It is the facility owner's responsibility to establish performance standards (objective criteria).
Any defined performance standards should provide alarm values and shutdown values.' If consensus standards or even original equipment manufacturer (OEM) recommendations are used as a basis for specific asset performance standards, those recommendations must be evaluated and, in most cases, adjusted to fit a specific set of asset operating conditions. The facility owner's engineer is typically responsible for that decision.
Documenting the criteria and providing training are also necessary to ensure that the defined performance criteria are globally understood and that consistent and coherent data is collected for analysis. Ask yourself: Has your facility made this level of investment in its PM and CBM programs? Low-Frequency Vibration Monitoring The following discussion focuses on broadband, filter-out, low-frequency vibration monitoring (less than 2 kilohertz KHz). The discussion also deals with International Organization for Standardization (ISO) consensus standards associated with general equipment vibration. The notions and concepts presented in this article are also applicable to those consensus standards that directly apply to in-situ pumps and fans, Hydraulic Institute and Air Movement and Control Association (AMCA) International.
In the days before ISO 10816, Mechanical Vibration – Evaluation of machine vibration by measurements on non-rotating parts, Parts 1 through 6, there was ISO 2372, Mechanical vibration of machines with operating speeds 10 to 200 rev/s – Basis for specifying evaluation standards. ISO 2372 was replaced by ISO 10816 in 1995. One example of performance criteria that must be evaluated and adjusted to fit a specific set of asset operating conditions is the old '97from 1974—ISO 2372 Vibration Severity Chart.
A version of this chart is packaged with just about every vibration meter, vibration analyzer and vibration data collector on the market today. It is still referenced 20 years after ISO 2372 was withdrawn. After 40 years of presentation, you would think that most would understand that this chart is not an absolute—it provides guidance only. It offers a place to start when developing a CBM process. This chart was also referenced in the original ISO 10816 (ISO 10816-3, to be specific) as an appendix. In more recent revisions of ISO 10816, the appendix has been removed and the severity chart has been rewritten.
However, no matter what color you paint it, the severity chart is still there. ISO 10816 (the original and the latest iteration) also contains the following statement: 'The ISO does not provide pass or fail criteria. It provides reasonable guidance ensuring that gross deficiencies (read as unsafe) or unrealistic requirements (read as too limiting) are avoided.'
For whatever reason, few people notice this statement. Many technicians and engineers see the charts and tables and jump to the conclusion that these are absolutes.
Figure 1 (page 72) shows a variation of the old 2372 severity chart. Several things are important to note:. The severity chart is no longer specifically referenced in a consensus standard. The chart is not used as pass or fail criteria but as general guidance.
It is the facility owner's responsibility—not a third-party committee's—to establish performance standards (objective criteria). Case Study As an example, a global manufacturer with a fleet of facilities in the U.S., Taiwan, China, Japan and Korea that has been in business for more than 100 years drank the ISO 2372 Kool-Aid. During initial visits to each facility when the technicians walked through their vibration monitoring program(s) for small-frame pumps, they showed their latest manual data collection and on-line monitoring schemes. A variation of the ISO 2372 severity chart (Courtesy of ARES Corporation) Not surprisingly, none of the programs was consistent or even similar in terms of data collection and analysis.
That was the first fix: to establish a consistent and coherent program. However, one thing was consistent: a devout following of the ISO 2372 severity chart. The alarm points in all of the monitoring schemes were always in line with the severity chart. A red line at 7 millimeters per second (mm/s) root mean squared (RMS) was drawn across all of the trending data for their small-frame pumps—a solid D according to the ISO severity chart.
ISO 2372 (10816) Standards provide guidance for evaluating vibration severity in machines operating in the 10 to 200Hz (600 to 12,000 RPM) frequency range. Examples of these types of machines are small, direct-coupled, electric motors and pumps, production motors, medium motors, generators, steam and gas turbines, turbo-compressors, turbo-pumps and fans. Some of these machines can be coupled rigidly or flexibly, or connected though gears. The axis of the rotating shaft may be horizontal, vertical or inclined at any angle. Use the chart below combined with additional factors described in this manual to judge the overall vibration severity of your equipment.
ISO Standard 2372 The ISO standard number 2372 provides vibration amplitude acceptance guidelines for rotating machinery operating from 600 to 12000 RPM. It specifies overall vibration velocity levels rather than spectral levels, and can therefore be quite misleading. ISO 2372 specifies the RMS vibration velocity limits on a basis of machine horsepower, and covers a frequency range from 10 Hz to 1000 Hz.
Because of the limited high frequency range, rolling element-bearing problems can be easily missed. This standard is considered obsolete, and is about to be rewritten.
Level, VdB Level, IPS Less Than 20 HP 20 to 100 HP More Than100 HP 125 1.00 Not Permissible Not Permissible Not Permissible 121 0.63 Not Permissible Not Permissible Just Tolerable 117 0.40 Not Permissible Just Tolerable Just Tolerable 113 0.25 Just Tolerable Just Tolerable Allowable 109 0.16 Just Tolerable Allowable Allowable 105 0.10 Allowable Allowable Good 101 0.06 Allowable Good Good 97 0.04 Good Good Good.
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