Pumps and Systems, March 2009
Enveloping is a tool that can give more information about the life and health of important plant assets. It is primarily used for early detection of faults in rolling element bearings and gearboxes. Enveloped acceleration is an especially valuable parameter to trend, as the progression of machine condition can be evaluated. Armed with good information and assisted by service experts, plant engineers can be confident in the proper operation and management of the important assets in their care.
Enveloping can reveal faults in the earliest stages of development, before they are detectable by other machinery vibration measurements. Without an early fault detection technique like enveloping, personnel must wait until the latter stages of failure, when overall vibration increases, lubricants become contaminated and temperatures rise. By this time, the remaining usable life of the failing machine elements could be short and the damage more extensive than if the fault had been detected earlier.
The enveloping technique enables the detection and analysis of low-level, repetitive vibrations by extracting them from the overall machinery vibration signal. Enveloping thus facilitates earlier prediction of failure in machines with metal-to-metal contact. While the examples in this article are based on rolling element bearings, the techniques also can be applied to gearboxes and electric motors with commutators.
It is important to note that the successful application and interpretation of enveloping data require experience. Enveloping is just one tool in the analyst's toolbox, and it is best used as one of a number of techniques for complete monitoring of a machine.
Enveloping Isolates Signals of Interest
Enveloping is a multiple-step process that extracts signals of interest from an overall vibration signal (Figure 1). In a rolling element bearing, the interaction between bearing elements and defects excites a structural resonance in the bearing support structure. A seismic transducer measures the vibration and the signal is band-pass filtered to keep only signal components around the resonance frequency. The filtered signal is rectified and then enveloped, which removes the structural resonance frequency and preserves the defect impact frequency. A low-pass filter then eliminates some of the extraneous high-frequency components, and a spectrum is generated. Frequency components are correlated with physical bearing parameters, and a trend of the spectra can show progression of defects.
Figure 1. Typical steps in the implementation of enveloping
Analysis of the enveloping process begins with the source of the vibration signal. As the elements of the bearing interact with each other and with defects, forces are coupled to the machine casing and produce vibration. These defect interactions behave as impacts that excite a natural resonance frequency of the machine, causing it to ring (Figure 2). The amplitude of the ringing decays until the next impact, which re-excites the resonance. The defect amplitude modulates the natural resonance response at the impact frequency. The defect-related signal becomes part of the overall vibration of the machine.
Figure 2. Defect impact causes ringing of the machine structure at its natural frequencies
Because of its higher frequency response characteristics, an accelerometer is generally used to measure the vibration signal for enveloping, which is often called acceleration enveloping, or high frequency acceleration enveloping. High-frequency vibration signals, such as the resonance carrier of the defect signal, do not travel far in a homogeneous machine structure; metal imperfections, joints and gaskets cause further significant attenuation (Figure 3). It is critical that this low-level, high frequency signal be coupled efficiently into the accelerometer. The accelerometer should be mounted as close to the bearing as possible and near the load region of the bearing, where signals are coupled more effectively to the machine case.
Figure 3. Vibration signal transmission loss in homogeneous and nonhomogeneous materials
Figure 4. Accelerometer vibration waveform output with embedded defect signal
The output of the accelerometer (Figure 4) contains three important frequencies: a relatively low-frequency, high-amplitude rotor-related vibration; the modulated structural resonance frequency; and other high-frequency vibration components, including harmonics of the structural resonance frequency. Though the signal is complex, application of the enveloping technique allows the determination of an impact frequency associated with the defect, which provides valuable information about machine condition.