Vibrational signature

Collection and analysis of vibration signatures is a complex procedure. By looking at a vibration spectrum, one can identify which components of the pump system are responsible for a particular frequency component. Comparison of vibration signatures at periodic intervals reveals if a particular component is deteriorating. The following example illustrates evaluation of the frequency composition of an electric motor gear pump system. 

Predictive maintenance utilizing vibration signature analysis is based on the following facts, which form the basis for the methods used to identify and quantify the root causes of failure ... 

Figure43.5 Typical frequency-domain vibration signature...

A frequency-domain vibration signature is generally used for the analysis because it is comprised of discrete peaks, each representing a specific vibration source. 

There is a cause, referred to as a forcing function, for every frequency component in a machine-train s vibration signature. 

While an increase or a decrease in amplitude may indicate degradation of the machine-train, this is not always the case. Variations in load, operating practices, and a variety of other normal changes also generate a change in the amplitude of one or more frequency components within the vibration signature. In addition, it is important to note that a lower amplitude does not necessarily indicate an improvement in the mechanical condition of the machine-train. Therefore, it is important that the source of all amplitude variations be clearly understood. 

Vibration does not just happen. There is a physical cause, referred to as a forcing function, and each component of a vibration signature has its own forcing function. The components that make up a signature are reflected as discrete peaks in the TFT or frequency-domain plot. 

Almost every unique frequency contained in the vibration signature of a machine-train can be directly attributed to a corresponding mechanical motion within the machine. For example, the constant endplay or axial movement of the rotating element in a motor-generator set generates elevated amplitude at the fundamental (lx), second harmonic (2x), and third harmonic (3x) of the shaft s true running speed. In addition, this movement increases the axial amplitude of the fundamental (lx) frequency. 

Actual time-domain vibration signatures are commonly referred to as time traces or time plots (see Figure 43.17). Theoretical vibration data are generally referred to as waveforms (see Figure 43.18). 

In order to analyze this type of plot, the analyst must manually change the time scale to obtain discrete frequency curve data. The time interval between the recurrences of each frequency can then be measured. In this way, it is possible to isolate each of the frequencies that make up the time-domain vibration signature. 

Each of these sine functions represents a discrete component of the vibration signature discussed previously. The amplitudes of each discrete component and their phase angles can be determined by integral calculus when the function /(f) is known. Because the subject of integral calculus is beyond the scope of this chapter, the math required to determine these integrals are not presented. A vibration analyzer and its associated software perform this determination using FFT. 

Unfortunately, not all distortions of acquired data result in a low-level alert. Damaged or defective cables or transducers can result in a high level of low-frequency vibration. As a result, the low-level alert will not detect this form of bad data. However, the vibration signature will clearly display the abnormal profile that is associated with these problems. 

Broadband analysis techniques have been used for monitoring the overall mechanical condition of machinery for more than twenty years. The technique is based on the overall vibration or energy from a frequency range of zero to the user-selected maximum frequency, Fmax Broadband data are overall vibration measurements expressed in units such as velocity (PK), acceleration (RMS), etc. This type of data, however, does not provide any indication of the specific frequency components that make up the machine s vibration signature. As a result, specific machine-train problems cannot be isolated and identified. 

Typically, a machine-train s vibration signature is made up of vibration components with each component associated with one or more of the true running speeds within the machine-train. Because most machinery problems show up at or near one or more of the running speeds, the narrowband capability is very beneficial in that high-resolution windows can be preset to monitor the running speeds. However, many of the microprocessor-based predictive maintenance systems available do not have narrowband capability. Therefore, care should be taken to ensure that the system utilized does have this capability. 

Reciprocating compressors have unique operating dynamics that directly affect their vibration profiles. Unlike most centrifugal machinery, reciprocating machines combine rotating and linear motions that generate complex vibration signatures. 

The loads induced by the belt increase the pressure on the loaded bearing and decrease the pressure on the unloaded bearing. An evaluation of process rolls should include a cross-comparison of the overall vibration levels and the vibration signature of each roll s inboard and outboard bearing. 

Selecting a low-frequency cut-off does not improve resolution and is strictly an arbitrary omission of visible frequency components within a vibration signature. The FFT is calculated on a bandwidth having a lower limit of zero and an upper limit equal to the maximum frequency, Fmax, which is selected by the user. The only reason for selecting a minimum frequency other than zero is to remove unneeded low-frequency components from the signature display. 

Resolution Resolution is the degree of spacing of visible frequency components in the vibration signature and is proportional to the bandwidth. The equation for resolution is given below ... 

Table 44.6 is a vibration troubleshooting chart that identifies some of the common failure modes. This table provides general guidelines for interpreting the most common abnormal vibration profiles. These guidelines, however, do not provide positive verification or identification of machine-train problems. Verification requires an understanding of the failure mode and how it appears in the vibration signature. 

Mechanical looseness Looseness, which can be present in both the vertical and horizontal planes, can create a variety of patterns in a vibration signature. In some cases, the fundamental (lx) frequency is excited. In others, a frequency component at one-half multiples of the shaft s running speed (0.5x, 1.5x, 2.5 x, etc.) is present. In almost all cases, there are multiple harmonics, both full and half. 

Other forms of mechanical looseness increase the noise floor across the entire bandwidth of the vibration signature. While the signature does not contain a distinct peak or series of peaks, the overall energy contained in the vibration signature is increased. Unfortunately, the increase in noise floor cannot always be used to detect mechanical looseness. Some vibration instmments lack sufficient dynamic range to detect changes in the signature s noise floor. 

How misalignment appears in the vibration signature depends on the type of misalignment. Figure 44.39 illustrates three types of misalignment (i.e., internal, offset, and angular). These three types excite the fundamental (lx) frequency component because they create an apparent imbalance condition in the machine. 

Ghosts are caused when two or more frequency components couple, or merge, to form another discrete frequency component in the vibration signature. This generally occurs with multiple-speed machines or a group of single-speed machines. 

This method provides the means to quickly monitor the mechanical condition of critical machine components, not just the overall machine condition. This technique provides the ability to monitor the condition of gear sets, bearings and other machine components without manual analysis of vibration signatures. 

Unlike the two trending techniques, signature analysis provides visual representation of each frequency component generated by a machine-train. With training, plant staff can use vibration signatures to determine the specific maintenance required by plant machinery. 

All vibration monitoring systems have finite limits on the resolution, or ability to graphically display the unique frequency components that make up a machine s vibration signature. The upper limit (Fmax) for signature analysis... 

To determine the impact of resolution, calculate the display capabilities of your system. For example, a vibration signature with a maximum frequency (Fmax) of 1000 Hertz taken with an instrument capable of 400 lines of resolution would result in a display in which each displayed line will be equal to 2.5 Hertz or 150 rpm. Any frequencies that fall between 2.5 and 5.0, i.e. the next displayed line, would be lost. 

C. Herrmann, J. Neugebauer, and M. Reiher, Finding a needle in a haystack direct determina tion of vibrational signatures in complex systems. New J. Chem. 31, 818 831 (2007). 

The results of this DFT study confirm that aluminium and PT oligomers interact preferentially through the formation of covalent bonds between Al-atoms and the a-carbons of thiophene rings. This type of reaction is expected to take place when A1 is deposited on the polymer layer as well as when PT is adsorbed on the metal surface. In all cases, the formation of the Al-C bonds and the geometric modifications induced in the organic molecules lead to important changes in the vibrational spectra which should allow the experimental detection of the vibrational signatures of the species formed at the Al-PT interface. 

To obtain molecular vibrational signatures using QWAIMD, we have recently introduced [149,150] a scheme using the Fourier transform of a unified veloc-... 

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