Technical Notes by SpectraQuest, Inc.

Vibration Signature of Defected Gear Tooth Using Spectral Analysis
The vibration signature of gear tooth seeded fault has been studied in this tech note. Experiments were carried out on Spectra Quest's Gear box Dynamics Simulator (GDS). The test rig simulates a two-stage parallel gear transmission. The input gear in the gearbox was intentionally faulted with increasing severity level. For each gear configuration, tests were conducted with and without loading applied by a magnetic brake. The experimental results show that due to the impact caused by the faulted tooth, strong sidebands arise around the meshing frequency in the spectra. At high severity level, the amplitudes of the sidebands may be even higher than that of the meshing frequency components. Download PDF file.

Vibration Signatures of Reciprocating Compressors
This tech notes studies the vibration signatures of a reciprocating piston compressor. A brief introduction of the operation principle and noise and vibration sources is presented. Experiments were carried out on a Spectra Quest's machinery fault simulator MFS Magnum with a single-piston compressor driven by the belt drive. The compression cycle and the related behaviors of different parts, including piston, inlet and discharge valves, are analyzed in detail from the acceleration data. Both the FFT-based spectral analysis and joint time-frequency analysis were conducted to study the frequency contents. Download PDF file.

Effects of Clearance on Damping in Fluid Film Bearing
Clearance of a fluid film bearing is a critical parameter affecting the eccentric ratio as well as dynamic characteristics of the rotor system. Four shaft/rotor configurations have been tested on Spectra Quest's Rotor Dynamics Simulator. For each test configuration, three bearing sets with different clearances were used to investigate the effects of clearance on damping in the fluid film bearing. It is observed that an increase in clearance will increase the modal damping because of the higher shear deformation. The experimental results also show that the critical speed and damping in the coast-down process are lower than those when the machine starts up. Download PDF file.

Vibration Signal Analysis of Fan Rotors
Fan rotor system can generate large amplitude blade-passing frequency (BPF) component if the gap between rotating vans and stationary diffusers is not equal. A series of experiments were conducted in this study using SpectraQuest's Machinery Fault Simulator (MFS) to observe the behavioral changes of the BPF and its harmonics before and after installing an obstruction nearby an axial fan and a shroud around a paddle fan. The results show that the obstruction had an obvious and consistent effect on the 1BPF and 2BPF components for the axial fan rotor system. However, for the two paddle fans tested, the shroud does not have a large effect on the 1BPF, 2BPF behavioral changes. Download PDF file.

Simulation and Vibration Analysis of Shaft Cracks
A Shaft crack is one of the most common defects in a rotor system and detection of such shaft crack is a very serious matter. In this study, shaft cracks were simulated and analyzed using SpectraQuest's rotor Machinery Fault Simulator (MFS). A series of experiments were conducted to observe the behavioral changes of the cracked shaft in critical speed, 1X and 2X frequency responses. The experimental results were found to be consistent with the theoretical prediction of the shaft crack. Download PDF file.

Rotating Machinery Faulty Diagnosis Techniques - Envelope and Cepstrum Analyses
This is the second tech note in Spectra Quest’s signal processing series. This tech note focuses on two techniques associated with rotating machinery fault diagnosis: envelope and cepstrum analyses. Rotating machinery faults usually cause strong harmonics and sidebands. Both the envelope and cepstrum analyses are useful tools to identify the fault frequencies and distinguish them from other frequency contents. Examples of bearing faults, broken rotor bar, and gearbox faults have been presented to demonstrate how to use the software package and how to interpret results. All the experimental data were acquired from Spectra Quest ’s Machinery Faults Simulator. Download PDF file.

Some Observations of the Detection of Rolling Element Bearing Outer Race Fault
Rolling element bearing is used widely in industry. It is one of the most vulnerable components in a machine because it is most often under high load and high speed running conditions. Prompt diagnostics of rolling element bearing faults is critical not only for the safe operation of machines, but also for the reduction of maintenance cost. Among the three main components in a rolling element bearing, the outer race, the inner race and the ball, this work studies the diagnostics of faults on bearing outer race. There is an argument that a rolling element will excite the natural frequencies of bearing component when it passes the fault on the outer race. Therefore, the bearing outer race natural frequencies have identified by numerical simulation and hammer test. Vibration data were collected on a Machinery Fault Simulator (MFS) with good as well as faulted bearings. The data were analyzed using the VibraQuest software and efforts were made to identify the bearing fault characteristic frequency and its harmonics. Download PDF file.

Applications of Time-Frequency Analysis Techniques on Machinery Fault Diagnosis
Spectra Quest, Inc. is planning to publish a series of tech notes on the applications of advanced signal processing techniques on machinery fault diagnosis. We start this series with timefrequency analysis techniques in this tech note. Generally speaking the sound and vibration signals obtained from a rotating machine are time-variant since they are strongly related to the rotational speed which is not constant even in the macro steady state. Since the mostly used signal processing method, the Fourier analysis, is only suitable for stationary signals, the development of the joint time-frequency analysis is demanded. Both the linear and quadratic time-frequency analysis approaches have been introduced. The performances of these algorithms were compared. Several tips were then summarized for you to pick an appropriate method for your application. Several successful applications of the time-frequency analysis methods on the machinery fault diagnosis were presented in detail. Download PDF file.

The Effects of Shaft Rub on Bearing Housing Vibration Signatures
In this study the vibration signature on bearing housing caused by shaft rub was studied by applying a rotor rub device on a machinery fault simulator. Full annular rub was simulated with three types of rub head materials: aluminum, brass, and plastic. Experiments were carried out at three rotatory speeds, i.e., 1000, 2000 and 3000 RPM. Acceleration signals on bearing housing were collected and analyzed. Several observations on the effect of rub on bearing housing vibration were presented. Download PDF file.

Vibration and Force Signatures of Bent Shaft
In this work, bearing vibration and reaction force signatures caused by bent shafts were studied experimentally. The vibration and force spectrum signatures between baseline and bent shafts (center bent and coupling end bent) under two shaft speeds, 1000 and 5000 RPMs were compared. The bearing reaction force associated with a center bent shaft under different shaft speeds were calculated by subtracting the baseline reaction force from the total reaction force. The experiment results indicate that for low shaft speed (1000 RPM), the effect of bent shaft on bearing vibration can be neglected. Under high speed (5000 RPM), a center bent shaft will increase the bearing vibration. However, a center bent shaft will increase bearing reaction force for both low and high speeds. The effect of a coupling end shaft on bearing vibration and reaction force is not conclusive. Finally, the bearing reaction force is not affected by the shaft speed significantly. Download PDF file.

Air Bubble and Cavitation Vibration Signatures of a Centrifugal Pump
In this work, a centrifugal pump was tested for its vibration signatures under different operational conditions. The two abnormal operating conditions studied are air bubble and cavitation. A transparent plastic cover was used in the experiments to observe the cavitation. It was found that the pump has higher vibration amplitude in the axial direction than in the radial direction. From the experiments, it was also determined that significant amount of air bubbles will increase vibration component associated with impeller vane pass frequency significantly. Cavitation might excite high frequency structural resonance. It may also reduce the impeller vane pass frequency vibration. Although cavitation is less likely to happen on a slow speed pump, it will develop very fast if it happens. Download PDF file.

In this study, the effects of load on oil whirl and whip were studied by using a rotor dynamics simulator with fluid film bearings. Rotor displacement during machine run up and coast down under different loading conditions were measured, analyzed and presented. Besides the oil whirl and whip introduced in traditional textbooks, harmonics of oil whip are observed. Some other vibration components associated with oil whip are also observed. Download PDF file.

In this work, vibration signature caused by gear tooth seeded fault is studied in the time and frequency domains. The experiment was conducted on a Machinery Fault Simulator ^TM (MFS). The pinion gear in the gearbox was intentionally faulted with increasing severities, and a vibration signal was collected for each case. The signal was analyzed in a time and frequency domain. The results indicated that the vibration signature of a faulted bevel gear tooth is a pulse in time domain. Because of this impulse signal, strong sidebands arise in the spectrum around the mesh frequency. Download PDF file.

Dynamic Behaviors of Rotor with Fluid Film Journal & Rolling Element Bearings
Using a MFS-Lite Extension machine, we studied the behavior of a rotor shaft supported on fluid film journal bearing as well as rolling element bearing from the perspectives of rotor resonance frequencies and rotor orbit shapes. The experiment results indicate the effect of bearing types on the rotor natural frequencies can not be neglected. Moreover, the rotor orbit displays totally different shape for journal fluid film bearing and rolling element bearing. Download PDF file.

Misalignment Detection Using ODS
Misaligned rotating machinery is causing high cost to industry due to prematurely damaged machinery, lost production, and excessive energy consumption. Therefore, a robust, accurate, and convenient misalignment detection technique is needed. This work investigated the perspective of using operational deflection shape (ODS) to check the machinery alignment condition. Two types of misalignment are studied: angular and parallel misalignments. The results suggest that ODS is a potential method to detect misalignment. Download PDF file.

Numerical Simulation of the Propagation of a Crack in Gearbox
In this technical note, fracture propagation is simulated using a finite element model for a gearbox which is designed for a rotor dynamic test rig. The circumferential plates of the gearbox are welded along the side as well as the bottom edges. Micro-cracks are usually generated at weld toes and the welding process will introduce residual stresses. The effect of the locations of initial fracture on fracture propagation path is studied. Download PDF file.

Vibration Analysis of Belt Drives in Rotating Machinery
Belt drives are commonly used in a variety of power transmission applications. Improper installation and maintenance of belt can have an enormous impact on the operation and degradation of a machine. Vibration signatures are widely promoted for studying belt drive malfunctions. This note presents the results of an experiment performed on a two belt drive system. The effect of belt tension, speed, sheave eccentricity, and misalignment conditions were investigated. The results indicate that eccentric sheave show amplitude modulation and produce high vibration. Sheave eccentricity also excited belt natural frequencies easily. Meanwhile, the effect of misalignment on the vibration signature is not significant. Download PDF file.

Correlation between Belt Natural Frequencies and Vibration Spectra
SpectraQuest plans to publish a series of articles addressing these issues. This article, second in the series, will cover: how the natural frequencies get excited for belts in differing lengths, correlation between machine operating speed and natural frequency excitement, whether natural frequencies show up in the vibration spectra, belt dynamic stiffness versus static stiffness, and belt length effect on excitation and RPM. Download PDF file.

A Study of Rolling Element Bearing Defect Analysis
SpectraQuest plans to publish a series of articles addressing these issues. This article, first in the series, will cover the effect of rotor weight, speed, and defect severity on outer race fault spectra. Download PDF file.

A Study of Belt Natural Frequency
In the vibration analysis of machinery with V-belt drives, the belt behavior is an essential component to understand. Because of its special physical characteristics, there are many unclear issues regarding belts in the system. The SpectraQuest Machinery Fault Simulator provides a way to perform studies on a belt drive. This test is the first one in a series of tests on power transmission V-belts. Download PDF file.

Observations Concerning Misalignment Vibration Signatures
Misalignment is a common cause of machinery malfunction. Considering the importance of alignment, the vibration spectrum of misalignment is not well documented. This is the first report on the ongoing research to determine the unique vibration signature for misalignment at varying operating and design conditions such as speed, type and level of misalignment, coupling types and machinery dynamic stiffness. The SpectraQuest Machinery Fault Simulator? was used in the study to create the varying mechanical conditions. Triaxial vibration measurements were taken at each end of the coupling on the motor and rotor bearing housings. Data was collected at several other locations of the Simulator. The results indicate that the speed and the coupling type/stiffness have a strong effect on the vibration spectra. The level and type of misalignment had a significant effect on the vibration signature. No unique signature was observed, suggesting that care is needed to correctly diagnose misalignment. Download PDF file

A Constitutive Equation for Thermoviscoelastic Behavior of Polymeric Materials
Dr. Suri Ganeriwala's paper on thermoviscoelastic behavior is available for download. Download PDF file

Useful Accelerometer Articles from IMI
(each link opens a PDF file; right-click to save)

Download the Microphone Handbook by Mark Valentino from PCB

White Paper on Motor Current Signature Analysis
The main objective of this review is to provide industy with state of the are knowledge on condition monitoring strategies for assessing the operational health of induction motor drive systems. An overview of typical failure mechanisms and their causes is presented. The essential criteria for a reliable condition monitoring system are discussed and the theoretical principles behind the generation of unique signal characteristics, which are indicative of failure mechanisms, are presented. Online monitoring techniques which can be used to diagnose mechanical problems, stator and rotor winding failure mechanisms, and airgap eccentricity are decribed. The focus is on vibration, current, flux and partial discharge monitoring strategies. The application of monitoring systems in industry is reviewed and reference is made to on-site case histories. Recent developments, including the use of monitoring and analysis of electric current to diagnose mechanical problem is discussed. Future requirements for monitoring and diagnosing faults and fundamental causes in modern induction motor drives are presented. Download PDF file