Vibration Fundamentals Episode 7 pptx

10.Mobley.15 Page 174 Friday, February 5, 1999 10:38 AM 174 Vibration Fundamentals Figure 15.33 Bends that do not change shaft length generate radial forces only. Figures 15.32 and 15.33 illustrate the normal types of bent shafts and the force pro- files that result. V-Belts V-belt drives generate a series of dynamic forces, and vibrations result from these forces. Frequency components of such a drive can be attributed to sheaves and belts. The elastic nature of belts can either amplify or damp vibrations that are generated by the attached machine-train components. Sheaves Even new sheaves are not perfect and may be the source of abnormal forces and vibration. The primary sources of induced vibration due to sheaves are eccentricity, imbalance, misalignment, and wear. Eccentricity Vibration caused by sheave eccentricity manifests itself as changes in load and rota- tional speed. As an eccentric drive (Figure 15.34) sheave passes through its normal rotation, variations in the pitch diameter cause variations in the linear belt speed. An eccentric driven sheave causes variations in load to the drive. The rate at which such variations occur helps to determine which is eccentric. An eccentric sheave also may 10.Mobley.15 Page 175 Friday, February 5, 1999 10:38 AM 175 Failure-Mode Analysis Figure 15.34 Eccentric sheaves. Figure 15.35 Light and heavy spots on an unbalanced sheave. appear to be unbalanced. However, performing a balancing operation will not correct the eccentricity. Imbalance Sheave imbalance may be caused by several factors, one of which may be that it was never balanced to begin with. The easiest problem to detect is an actual imbalance of the sheave itself. A less obvious cause of imbalance is damage that has resulted in loss of sheave material. Imbalance due to material loss can be determined easily by visual inspection, either by removing the equipment from service or using a strobe light while the equipment is running. Figure 15.35 illustrates light and heavy spots that result in sheave imbalance. 10.Mobley.15 Page 176 Friday, February 5, 1999 10:38 AM 176 Vibration Fundamentals Figure 15.36 Angular sheave misalignment. Figure 15.37 Parallel sheave misalignment. Misalignment Sheave misalignment most often produces axial vibration at the shaft rotational fre- quency (1×) and radial vibration at one and two times the shaft rotational frequency (1× and 2×). This vibration profile is similar to coupling misalignment. Figure 15.36 illus- trates angular sheave misalignment and Figure 15.37 illustrates parallel misalignment. Wear Worn sheaves also may increase vibration at certain rotational frequencies. However, sheave wear is more often indicated by increased slippage and drive wear. Figure 15.38 illustrates both normal and worn sheave grooves. 10.Mobley.15 Page 177 Friday, February 5, 1999 10:38 AM 177 Failure-Mode Analysis Figure 15.38 Normal and worn sheave grooves. Belts V-belt drives typically consist of multiple belts mated with sheaves to form a means of transmitting motive power. Individual belts, or an entire set of belts, can generate abnormal dynamic forces and vibration. The dominant sources of belt-induced vibra- tions are defects, imbalance, resonance, tension, and wear. Defects Belt defects appear in the vibration signature as subsynchronous peaks, often with harmonics. Figure 15.39 shows a typical spectral plot (i.e., vibration profile) for a defective belt. Imbalance An imbalanced belt produces vibration at its rotational frequency. If a belt’s perfor- mance is initially acceptable and later develops an imbalance, the belt has most likely lost material and must be replaced. If imbalance occurs with a new belt, it is defective and also must be replaced. Figure 15.40 shows a spectral plot of shaft rotational and belt defect (i.e., imbalance) frequencies. Resonance Belt resonance occurs primarily when the natural frequency of some length of the belt is excited by a frequency generated by the drive. Occasionally, a sheave also may be excited by some drive frequency. Figure 15.41 shows a spectral plot of resonance excited by belt-defect frequency. Belt resonance can be controlled by adjusting the span length, belt thickness, and belt tension. Altering any of these parameters changes the resonance characteristics. In most applications, it is not practical to alter the shaft rotational speeds, which also are possible sources of the excitation frequency. Resonant belts are readily observable visually as excessive deflection, or belt whip. It can occur in any resonant mode so inflection points may or may not be observed 10.Mobley.15 Page 178 Friday, February 5, 1999 10:38 AM 178 Vibration Fundamentals Figure 15.39 Typical spectral plot (i.e., vibration profile) of a defective belt. Figure 15.40 Spectral plot of shaft rotational and belt defect (i.e., imbalance) frequencies. 10.Mobley.15 Page 179 Friday, February 5, 1999 10:38 AM 179 Failure-Mode Analysis Figure 15.41 Spectral plot of resonance excited by belt-defect frequency. Figure 15.42 Examples of mode resonance in a belt span. 10.Mobley.15 Page 180 Friday, February 5, 1999 10:38 AM 180 Vibration Fundamentals along the span. Figure 15.42 illustrates first-, second-, and third-mode resonance in a belt span. Tension Loose belts can increase the vibration of the drive, often in the axial plane. In the case of multiple V-belt drives, mismatched belts also aggravate this condition. Improper sheave alignment can also compromise tension in multiple-belt drives. Wear Worn belts slip and the primary indication is speed change. If the speed of the driver increases and the speed of the driven unit decreases, then slippage is probably occur- ring. This condition may be accompanied by noise and smoke, causing belts to over- heat and be glazed in appearance. It is important to replace worn belts. 11.Mobley.16 Page 181 Friday, February 5, 1999 11:25 AM Chapter 16 SIGNATURE ANALYSIS Most failures of rotating and reciprocating machinery exhibit characteristic vibration profiles that are associated with specific failure modes. This phenomenon is due to the forcing function, caused by a developing defect, having a unique characteristic signa- ture. None of the filtered bandwidth monitoring methods provides the means to detect and evaluate these unique profiles. Signature analysis provides this capability and its use is required in a comprehensive predictive maintenance program. C HARACTERISTIC V IBRATION S IGNATURES A vibration signature provides a clear, accurate snapshot of the unique frequency components generated by, or acting on, a machine-train. Such a signature is obtained by converting time-domain data into its unique frequency components using a fast Fourier transform (FFT). Such a vibration signature, referred to as frequency-domain data, is used in signature analysis to evaluate the dynamics of the machine. Frequency-domain vibration signatures form the basis for any predictive maintenance program designed to detect, isolate, and verify incipient problems within a machine- train. These signatures are the basic tools used for in-depth analysis methods such as fail- ure-mode, root-cause, and operating dynamics analyses. Operating dynamics analysis TM , which is beyond the scope of this module, uses vibration data and other process parameters, such as flow rate, pressure, and temperature, to determine the actual oper- ating condition of critical plant systems. T YPES OF S IGNATURE A NALYSIS In general, new or immature predictive maintenance programs are limited to compar- ative analysis or waterfall trending. Although these comparative techniques provide 181 11.Mobley.16 Page 182 Friday, February 5, 1999 11:25 AM 182 Vibration Fundamentals the ability to detect severe problems, they cannot be used to isolate and identify the forcing functions or failure modes. These methods also are limited in their ability to provide early detection of incipient problems. As the predictive maintenance program matures, root-cause analysis and operating dynamics analysis TM methods can be used. With the addition of these more advanced diagnostic tools, vibration signatures become an even more valuable process perfor- mance improvement tool. Automatic Trending Analysis A predictive maintenance program utilizing a microprocessor-based vibration ana- lyzer and a properly configured database automatically trends vibration data on each machine-train. In addition, it compares the data to established baselines and generates trend, time-to-failure, and alert/alarm status reports. The use of just these standard capabilities greatly reduces unscheduled failures. How- ever, these automated functions do not identify the root causes behind premature machine-train component failures. In most cases, more in-depth analysis allows the predictive analyst to identify the reason for pending failure and to recommend correc- tive actions to prevent a recurrence of the problem. Again, the specific microproces- sor-based system used determines how much manual effort is required for more in- depth analysis. More In-Depth Trending Analysis More in-depth analysis is called for when the automatic trending analysis described in the previous section indicates that a machine-train is exhibiting excessive vibration. Obviously, machine-trains that are operating within acceptable boundaries do not require further investigation. Care should be taken, however, to ensure that the auto- mated functions of the predictive maintenance system report abnormal growth trends as well as machine-trains that are actually in alarm. Comparative Analysis (Waterfall Trending) FFT signatures that are collected on a regular schedule provide a means of trending that can help the analyst identify changes in machine condition. Changes in the oper- ating parameters, such as load, will directly affect the signatures generated by a machine. Unlike trending analysis, which is based on broadband and narrowband data, compar- ative analysis is a visual comparison of the relative change of the machine-train’s full vibration signature and its discrete frequency components over a period of time. Because vibration signatures are acquired at regular intervals in a predictive mainte- nance program, this form of trending is very effective in identifying changes in machine condition. 11.Mobley.16 Page 183 Friday, February 5, 1999 11:25 AM 183 Signature Analysis Figure 16.1 Illustration of a waterfall plot. Displaying the signatures in a waterfall or multiple-spectra display (sequentially by data-acquisition time) allows the analyst to easily see the relationship of each fre- quency component generated by the machine (see Figure 16.1). Any significant change in the amplitude of any discrete frequency is clearly evident in this type of dis- play, which is used in many of the figures in subsequent sections. Although comparative analysis can be used to help the analyst identify specific changes that are generated by process changes, each signature must be normalized for process variations. Therefore, as part of the acquired data set, the analyst must record the specific process conditions for each data set. With this information and the water- fall display of vibration signatures, the analyst can quantify the changes that result from variations of these parameters. Developing problems within a machine-train can be identified by comparing the FFT signature to the following: (1) a baseline or reference signature, (2) previous signa- tures, or (3) industrial standards. This method determines if a potential problem exists and can be used to isolate within the machine-train the probable source of developing problems. [...]... (see Figure 16.5) 11.Mobley.16 Page 1 87 Friday, February 5, 1999 11:25 AM Signature Analysis Figure 16.3 Multiple-signature display Figure 16.4 Ratio of two signatures 1 87 11.Mobley.16 Page 188 Friday, February 5, 1999 11:25 AM 188 Vibration Fundamentals Figure 16.5 Difference of two signatures 12.Mobley. 17 Page 189 Friday, February 5, 1999 11:31 AM Chapter 17 ROOT-CAUSE ANALYSIS Root-cause analysis... frequency components, variations in load change the ampli­ tude For example, the vibration amplitude of a centrifugal compressor taken at 100% 12.Mobley. 17 Page 198 Friday, February 5, 1999 11:31 AM 198 Vibration Fundamentals Figure 17. 4 Screen display cursor position does not provide true frequency load is substantially lower than the vibration amplitude in the same compressor oper­ ating at 50% load In addition,... from its true centerline By 12.Mobley. 17 Page 196 Friday, February 5, 1999 11:31 AM 196 Vibration Fundamentals Figure 17. 3 Machines in parallel should be analyzed as a group using common-shaft diagnostics, the analyst can detect deviations from normal oper­ ating condition and isolate the probable forcing function Data Normalization Data acquired as part of a regular vibration- monitoring program must be... roll and its drive-train’s vibration profile 13.Mobley.18 Page 200 Friday, February 5, 1999 11: 37 AM Part III RESONANCE AND CRITICAL SPEED ANALYSIS 200 13.Mobley.18 Page 201 Friday, February 5, 1999 11: 37 AM Chapter 18 INTRODUCTION Resonance is a large-amplitude vibration caused by a small periodic stimulus having the same, or nearly the same, period as the system’s natural vibration In other words,... knowledge of the machine-train or vibration analy­ sis techniques, but both data sets must be acquired under the same operating conditions Increases in relative strength indicate more vibration and a developing problem in the machine-train Cross-machine comparison is an extremely beneficial tool to the novice analyst Most vibration monitoring systems permit direct comparison of vibration data, both filtered... sometimes result For example, gear-mesh frequency locations are generally integer multiples (5×, 10×, etc.) and 12.Mobley. 17 Page 1 97 Friday, February 5, 1999 11:31 AM Root-Cause Analysis 1 97 bearing-frequency locations are generally noninteger multiples (0.5×, 1.5×, etc.) Plotting the vibration signature in multiples of running speed quickly differentiates the unique frequencies that are generated by bearings... dynamics affect the vibration spectrum However, the ana­ lyst may have to evaluate the entire process system to determine the reason behind a chronic machine-train failure If the chronic problem is system related rather than machine-train related, the knowledge of process dynamics required to perform the 189 12.Mobley. 17 Page 190 Friday, February 5, 1999 11:31 AM 190 Vibration Fundamentals analysis... TECHNIQUES The following diagnostic and data techniques are discussed in this section: commonshaft analysis, shaft deflection, and data normalization 12.Mobley. 17 Page 192 Friday, February 5, 1999 11:31 AM 192 Vibration Fundamentals Common-Shaft Analysis Vibration analysis requires the use of proper evaluation techniques that permit the analyst to understand fully the operating condition of each machine-train... of a specific shaft, monitoring all measurement points on a common shaft simplifies comparative analysis of the unique frequency components created by that shaft 12.Mobley. 17 Page 194 Friday, February 5, 1999 11:31 AM 194 Vibration Fundamentals Even though this technique does not identify the specific failure mode or problem (e.g., misalignment, imbalance), it does isolate the source or location of the... of 178 0 rpm during setup The analyst then assumes that all data sets were acquired at this speed In actual practice, however, the motor’s speed could vary the full range between locked rotor speed (i.e., maximum load) to synchronous (i.e., no-load) speed In this example, the range could be between 175 0 and 1800 rpm, a difference of 50 rpm This variation is enough to distort data normalized to 178 0 . sheave imbalance. 10.Mobley.15 Page 176 Friday, February 5, 1999 10:38 AM 176 Vibration Fundamentals Figure 15.36 Angular sheave misalignment. Figure 15. 37 Parallel sheave misalignment. Misalignment. or may not be observed 10.Mobley.15 Page 178 Friday, February 5, 1999 10:38 AM 178 Vibration Fundamentals Figure 15.39 Typical spectral plot (i.e., vibration profile) of a defective belt. Figure. 15.38 illustrates both normal and worn sheave grooves. 10.Mobley.15 Page 177 Friday, February 5, 1999 10:38 AM 177 Failure-Mode Analysis Figure 15.38 Normal and worn sheave grooves. Belts