Spall Size Research Articles

Spall size estimation for wind turbine pitch bearings: observation, signal processing method and experiments

It is essential to continuously monitor the spall size of wind turbine pitch bearings to prevent severe faults and catastrophic failure. In the field of spall size estimation for bearings, an essential step is to extract the entry and impact signals simultaneously. And this would become more difficult when it comes to the wind turbine pitch bearings due to the limited fault signals and heavy noise. In this paper, a coherent procedure is proposed to estimate the spall size for wind turbine pitch bearings. Firstly, the characteristics of entry and impact signals in actual wind turbine pitch bearings are observed to be low-frequency and high-frequency dominated, respectively. On the basis of characteristics analysis, a novel two-stage signal processing method called the wavelet augmented sparse dictionary, is proposed to extract the entry and impact signals, which combines the discrete wavelet transform and sparse representation technique. Finally, the spall size is calculated according to aforementioned extraction and geometric constraints. Results from real-world experiments demonstrate the effectiveness of the proposed method.

Adaptive time-domain sliding average method for spall size estimation of roller bearing with skidding

The size of the bearing spall fault can be estimated by the time interval between the roller's entry and exit points in the bearing vibration signal. However, the entry vibration response is typically low amplitude and masked in environmental noise. The time-domain average (TDA) method is effective for periodic impulse enhancement. However, bearing skidding is unavoidable during its running, and the cage's speed fluctuations caused by skidding will lead to non-periodicity of the impulses. The performance of TDA seriously deteriorated with such skidding because of the time-varying period. This paper presents the adaptive time-domain sliding average (ATDSA) method to address the bearing skidding problem. The similarity feature between each impulse segment is utilized, and a time delay matrix is constructed to estimate the skidding rate and mitigate the skidding effect. The frequency spectrum of the reconstructed signal shows clear sidebands with BCF interval. And the obtained averaged waveform has high resolution in both the low-frequency entry and high-frequency exit events. Simulation and experiment results show that it can effectively suppress the skidding effect and estimate the spall size correctly under different speeds.

Fault size estimation of rolling bearing based on weak magnetic detection

Spall is the main form of rolling bearing failure. To assess the bearing health status, this paper proposes a novel method based on the weak magnetic detection technology to estimate the spall size of the rolling bearing. The spall estimation model including the two key parameters of the roller motion speed and the entry-to-exit time is developed. They are both perception by the weak magnetic detection instead of the assumption. CEEMDAN combined with the de-trended fluctuation analysis (DFA) is introduced to identify the relevant mode including the key parameter information from the detected signal. Furtherly, the roller rotation speed and the entry-to-exit time are obtained by the feature spectrum based on the FFT and Hilbert transform, respectively. Comparison with the existing methods, the experimental result shows that the proposed method has a high performance and achieves less estimation bias.

A dynamic model for simulating vibration response of ball bearings with extended outer race defects for precise spall size estimation

A dynamic model for a ball bearing with an extended outer race defect has been proposed in this paper with an aim of precise fault size estimation. Both the varying stiffness and impact force excitation mechanisms have been incorporated. Besides, the continuously changing contact film damping and, the load zone and the resulting load distribution, are taken into account to accurately obtain the vibration response curve. The impulse force train due to impacts near the entry and the exit edges are also modelled and studied along with other important dynamic characteristics of the rotor bearing system under different operating conditions and defect parameters. The effects of different defect depth and its location, radial load, and shaft speed have been investigated. The theoretical findings were successfully validated using experimental results. The simulated results agree well with the actual vibration response of the defective bearing. Defects of five different sizes were diagnosed in light of the proposed method. A very good resemblance between the actual and the estimated values were achieved with a maximum error of 4.88%, corresponding to the smallest defect.

Experimental Investigation of the Spall Propagation Mechanism in Bearing Raceways

This article investigates the spall propagation mechanism for ball bearing raceways by focusing on an experimental investigation of cracks that evolve in the vicinity of the spall edge. Understanding the spall propagation mechanism is an important step towards developing a physics-based prognostic tool for ball bearings. This research reflects an investigation of different spall sizes that propagate naturally both in laboratory experiments and in the field. By using a combined model of a rigid body dynamic model and a finite element model that simulates the rolling element-spall edge interaction, our results shed light on the material behavior (displacements, strains, and stresses) that creates an environment for crack formation and propagation. With the support of the experimental results and the rolling element-spall edge interaction model results, three stages of the mechanism that control fragment release from the raceway were identified. In Stage one, sub-surface cracks appear underneath the spall trailing edge. In Stage two, cracks appear in front of the trailing edge of the spall and, in Stage three, the cracks propagate until a fragment is released from the raceway. These stages were observed in all the tested bearings. In addition, other phenomena that affect the propagation of the cracks and the geometry of the fragment were observed, such as blistering and plastic deformation. We include an explanation of what determines the shape of the fragments.

Towards Bearings Prognostics Based on Oil Debris

Abstract:
 Rolling element bearings are widely used in many mechanical systems and affect their safe operation and reliability. The use of an integral Oil Debris Sensor in the lubrication system allows continuous monitoring of chips and particles originating from an evolving failure in the mechanical system oil wetted components.
 This paper examines the use of the mass loss from an Oil Debris Monitoring sensor as a Health indicator to assess damage severity of a rolling element bearing. A series of experiments were performed on a test rig using angular contact ball bearings subjected to high rotational speed and loads to study the propagation of spall in the bearing raceway. Using test results, a spall size estimation model was developed. It uses the mass loss from the Oil debris and known bearing characteristics. The difference between the measured spall size in the tests and the model results were found relatively small.

Two-dimensional quantitative analysis model with a shallow spall of rolling element bearing based on early weak multi-impact characteristics

The shallow spall faults will lead to multiple additional contact phenomenon of rolling element passing through the defects in the early stage of crack-to-spall evolution. The existing bearing defect analysis models ignore this stage. In this paper, the mechanism of the shallow spall fault is analyzed, and the width and depth of the spall are quantitatively studied. Based on Hertz contact theory, a multi-impact dynamic model of raceway spall damage on the rolling element bearing outer ring is constructed by time-varying contact displacement and force. The dynamic response of raceway spall damage on the rolling element bearing outer ring is calculated by the Runge–Kutta method under different spall sizes and compared with the existing dual-impact model. The results show that the model accurately describes the phenomenon of weak multi-impact in early stage of the shallow spall. The two-dimensional spall including spall width and depth are quantified in this model, compared with the limitation of the dual-impact model which only quantifies the spall width. This model fills in the blank of the mechanism model with early weak multi-impact characteristics of bearing defect evolution in the whole life. It is of great significance for rolling element bearing damage degree evaluation.

A new model for bearing spall size estimation based on oil debris

This paper presents a study on damage severity estimation of spalled bearings based on an Oil Debris Monitoring (ODM). Several failure progression tests on angular contact ball bearings were performed. The spalls formed at the end of the tests were expanded over more then 180° of the bearing race. Based on the ODM data analysis, it was possible to discriminate between the three stages of spall growth i.e., initiation, steady-state propagation, and accelerated propagation. A well-established dynamic model of bearings was used to simulate the dynamic behaviour of the angular contact ball bearing with a variety of spall sizes on the inner race. The analysis of the simulated signals from the model revealed that the transition from the steady-state to the accelerated propagation stages occurred at the arc length between two adjacent balls. This finding allowed the development of a new ODM model for spall size estimation in the accelerated propagation stage. The new ODM model relates the loss of mass from a bearing, as measured by an ODM sensor, to the size of the spall in the bearing. The ODM model considers the change in the spall geometry as the bearing deteriorates. The estimated spall size was in good agreement (maximum error of 13%) with the measured spall size at the end of the failure progression tests.

Identification of fault size of rolling element bearing on outer race from vibration signature analysis

Prognostics deals with the estimation of the remaining useful life (RUL) of machine, which is predicated based on their present condition and their future operating condition. Rolling element bearing failure is one of the commonly explained reason for machine breakdown. At the point when machine is in running condition and fault occur in bearing, it is extremely difficult to identify fault size in bearing. Thus, for safety of machine it is necessary to change bearing before fault size increases to a significant level. Further, the replacement of bearing might be extremely costly and on other hand chances can’t be taken with safety aspects. Therefore, it is necessary to find the RUL of bearing by identifying the spall size of fault from vibration signal. Fault size estimation can be done by decomposition of vibration signal by using discrete wavelet transform and also introducing the autoregressive method, minimum entropy deconvolution and Hilbert transform. The decomposed signal is divided into peak corresponding to the ball enter into the fault and exit from the fault. Experiment conducted for various size of faults present on the 25 mm inner diameter polyamide cage deep groove ball bearing. The minimum size of 0.3 mm is detected in the present work for outer race defect with only radial load. Further, fault size estimation is investigated for 1mm defect present at outer race subjected to both radial and axial load

Impacts of Mode Mixity on Controlled Spalling of (100)-Oriented Germanium

Controlled spalling is a technology to prepare single-crystal thin films of semiconductors by fracture with a subsurface crack propagating nearly parallel to the substrate surface. Practical applications require uniform thickness and a smooth surface across the whole film. Both wafer-scale and patterned-stressor-defined small-area spalling of germanium substrates are conducted experimentally and numerically. River line features are observed on spalled surfaces close to lateral edges of the spall, regardless of the spall direction and the size of the spalled area. Three-dimensional finite element method modeling shows the river lines are caused by mixed mode I + III loading near the lateral edges of spall and predicts a spall depth variation near the lateral edges of spall due to mixed mode I + II loading. The absolute range of river lines increases with lateral size of spall, while the relative range of river lines decreases, consistent with variations in mode mixity.

Dynamic modeling and quantitative diagnosis for dual-impulse behavior of rolling element bearing with a spall on inner race

The dual-impulse behavior, which is produced by the spall located on the surface of the bearing raceway, is closely related to the size of the spall zone. This allows to establish an accurate mechanism of quantitative fault diagnosis and spall size estimation. However, dynamic model of dual-impulse behavior, which couples the nonlinear excitation force and the impact force induced by the interaction between the rolling element and the spall zone, have not been proposed before, especially under the elastohydrodynamic lubrication (EHL) condition. To address this issue, the corresponding dual-impulse excitation mechanism is investigated and explained, and the force produced by the collision of the rolling element with the trailing edge of the spall zone is derived. A coupled nonlinear dynamic model of the rolling element bearing (REB) with a spall on the inner race is proposed in this paper, in which the displacement and excitation force originating from the passing process of the rolling element over the spall zone are modeled as a piecewise function. The transient collision force excited by the strike of the rolling element on the trailing edge of the spall area is introduced, and the contact between the rolling elements and both the bearing races under EHL condition is modeled. To validate the proposed model, the dual-impulse time spacing (DITS) is analysed, and the vibration responses and its corresponding envelope spectra under simulation and experimental conditions are studied theoretically and experimentally. The simulated results agree well with the experimental results.

Tracking the natural evolution of bearing spall size using cyclic natural frequency perturbations in vibration signals

Spalling caused by fatigue is the most common reason for rolling element bearing failure, and spall size can be a good indicator to predict the remaining useful life of the bearing. Previous studies reported that, in the acceleration signal, a low-frequency step response is caused by de-stressing or re-stressing of the rolling element during its entry into and exit from the spall zone, and a high-frequency impact response is often induced when the roller hits the trailing edge of the spall. The conventional spall size estimation methods thus attempt to capture these events (entry, impact, and exit) and use their time difference to estimate the spall size. But the limitation of these methods is that they were mostly built on artificial spalls and have proved scarcely effective in dealing with natural ones. A recent study investigated the effect of these events on bearing stiffness and reported that the stiffness of the bearing structure decreases when the rolling element traverses the spall zone. Inspired by the study, a novel approach is proposed in this paper, which utilises the duration of natural frequency perturbations (induced by stiffness variation) of the bearing structure to perform spall-size estimation. Compared to the existing methods, this approach investigates the instantaneous resonance frequency variation during the spall span instead of the signal events at the entry and exit points. This novel approach along with three other representative existing methods are tested on three experimental datasets: one obtained by an artificially induced spall and the other two by naturally extended spalls. The performance analysis results reveal that all the methods are effective for artificial spalls, but only the proposed approach is effective for naturally extended spalls.

Physical Properties of Basalt Ejecta Boulders at Lonar Crater, India: Insights Into the Target Heterogeneity and Impact Spallation Processes in Basalt With Application to Mars

AbstractThe 1.88‐km‐diameter Lonar crater emplaced in the Deccan basalt is an excellent terrestrial analog of bowl‐shaped impact craters on Mars. It contains a pristine ejecta blanket with abundant basalt boulders. We measured the density, porosity, P and S wave velocities and attenuation, magnetic susceptibility, uniaxial compressive strength, and tensile strength of 313 core specimens from 65 basalt ejecta boulders. Using these values, we quantified the gross physical properties of preimpact target and applied them in the cratering equations and estimated the spall plate dimensions and ejection distances. The impact of 35‐m‐radius projectile with 20 km/s velocity on the basaltic target with 77.2‐MPa dynamic tensile strength, 4,163 m/s P wave velocity, and 2.56 g/cm3 density gives a spall thickness range (0.2 to 9.7 m) similar to the observed boulder sizes at Lonar crater. Because aqueous weathering and vesicularity are present in the target, the Lonar impact produced smaller spall sizes than those expected from a massive basalt target. We also measured the ejecta boulder sizes and shapes around a fresh impact crater (of similar size and target to Lonar) in Arcadia Planitia on Mars and calculated the spall thicknesses. The low‐velocity impact (5.5 km/s) of 50‐m‐radius projectile on the target similar to Lonar basalt gives a spall thickness range (1 to 25 m) similar to the observed boulder sizes at the Martian impact crater. The impact craters of similar sizes and targets on Earth and Mars produced different spall boulder size distributions reflecting the distinct impact conditions on these planets.

A mechanics and signal processing based approach for estimating the size of spall in rolling element bearing

Rolling element bearing (REB) is a critical component in any rotating machinery. It is subjected to surface damage during operation due to localised fatigue. Any prediction about damage severity helps in effective maintenance planning. Defect size estimation is one of the methods of assessing damage severity in REB and is the subject of this study. Several researchers have formulated analytical as well as signal processing based methods to estimate the defect size in REB. The study proposes a method that is a blend of a mechanics-based approach and signal processing approach for estimation of the size of the defect. The mechanics-based approach is based on the Hertzian contact theory and considers bearing operational parameters along with the effect of the load zone. The analogy of correlating peak acceleration at impact with force in the load zone is proposed and validated experimentally. The signal processing approach for locating entry and exit events encompasses concepts of cross-correlation, Variational mode decomposition (VMD), curve fitting, and root location. Parameter optimization of VMD is done using non dominated sorting particle swarm optimization. The proposed method is used to estimate the size of the defect, which is in the form of an artificially generated spall. Experiments are performed at various speeds and sizes of spall on outer as well as inner raceways. The results advocate the improved efficacy of the proposed method in estimating the size of artificially generated spalls.

New approach for gear mesh stiffness evaluation of spur gears with surface defects

Gear tooth surface fatigue is one of the most common failure modes of gears. Surface fatigue manifests on two principle phenomena, namely spalling and pitting. Spalls are localized cavities on the teeth surface with relatively important depths, whereas pits are distributed cavities on all the teeth surface with small depths. Generally, spalls and pits appear with nonuniform shapes, making their modelization difficult for researchers. Therefore, common methods assume specific geometries (rectangles, triangles, circles, etc.) to consider surface defects on the gear mesh stiffness evaluation process. In this paper, a new method has been proposed to accurately calculate the gear mesh stiffness of spur gears with surface defects based on the potential energy method. A double discretization of the tooth surface was performed to consider defect depth variation in both width and length directions of the gear tooth. A contact detection algorithm is developed to detect actual contact points during meshing process. Simulation results showed that the deformation of the gear mesh stiffness due to irregular shaped spalls and pits was completely different to those of regular shape and depends on the defect severity. In addition, results showed that the spall size and position have an impact on the final deformation of the gear mesh stiffness. The proposed method allowed remedying to multiple problems exposed by researchers in modeling gear mesh stiffness of gears with tooth surface defects.

Experimental investigation of basalt fiber-reinforced polymer (BFRP) bar reinforced concrete slabs under contact explosions

This study evaluates the effectiveness of basalt fiber-reinforced polymer (BFRP) bar reinforcement on the blast-resistant performance of concrete slabs. Contact explosion tests of 30-cm-thick concrete slabs with three layers of reinforcement were conducted. Eight slabs were tested and four were reinforced with steel and four were reinforced with BFRP bars. Four charges of TNT, 0.8, 1.0, 1.6, and 1.9 kg, were set for each type of slab. Damage modes of the crater, medium spall, and severe spall of the normal reinforced concrete slab (NRCS) and BFRP bar reinforced concrete slab (BRCS) were reached under contact explosions of TNT charges of 0.8, 1.0, and 1.6 kg, respectively. The BRCS exhibited severe spall while the NRCS experienced breaching under explosions induced by 1.9 kg TNT. The threshold ranges of spall and breaching of the NRCS and BRCS were obtained. Compared with the NRCS, the BRCS exhibited better blast-resistance based on the smaller crater and spall sizes, the absence of a cross-shaped crack, and the presence of fewer cracks. Existing formulae were applied to predict the damage modes obtained in the tests. McVay's formula caused an overestimation in which the crater modes of all eight slabs were predicted. Although the formulae proposed by Morishita et al. provided better predictions, the thresholds of spall and breaching were inaccurate.

An Approximate Estimation Approach of Fault Size for Spalled Ball Bearing in Induction Motor by Tracking Multiple Vibration Frequencies in Current.

Fault size estimation is of great importance to bearing performance degradation assessment and life prediction. Until now, fault size estimation has generally been based on acoustic emission signals or vibration signals; an approach based on current signals has not yet been mentioned. In the present research, an approximate estimation approach based on current is introduced. The proposed approach is easy to implement for existing inverter-driven induction motors without complicated calculations and additional sensors, immune to external disturbances, and suitable for harsh conditions. Firstly, a feature transmission route from spall, to Hertzian forces, and then to friction torque is simulated based on a spall model and dynamic model of the bearing. Based on simulated results, the relation between spall size and the multiple characteristic vibration frequencies in friction torque is revealed. Secondly, the multiple characteristic vibration frequencies modulated in the current is investigated. Analysis results show that those frequencies modulated in the current are independent of each other, without spectrum overlap. Thirdly, to address the issue of which fault features modulated in the current are very weak, a fault-feature-highlighting approach based on reduced voltage frequency ratio is proposed. Finally, experimental tests were conducted. The obtained results validate that the proposed approach is feasible and effective for spall size estimation.

Probability-Based Algorithm for Bearing Diagnosis with Untrained Spall Sizes.

Bearing spall detection and predicting its size are great challenges. Model-based simulation is a well-known traditional approach to physically model the influence of the spall on the bearing. Building a physical model is challenging due to the bearing complexity and the expert knowledge required to build such a model. Obviously, building a partial physical model for some of the spall sizes is easier. In this paper, we propose a machine-learning algorithm, called Probability-Based Forest, that uses a partial physical model. First, the behavior of some of the spall sizes is physically modeled and a simulator based on this model generates scenarios for these spall sizes in different conditions. Then, the machine-learning algorithm trains these scenarios to generate a prediction model of spall sizes even for those that have not been modeled by the physical model. Feature extraction is a key factor in the success of this approach. We extract features using two traditional approaches: statistical and physical, and an additional new approach: Time Series FeatuRe Extraction based on Scalable Hypothesis tests (TSFRESH). Experimental evaluation with well-known physical model shows that our approach achieves high accuracy, even in cases that have not been modeled by the physical model. Also, we show that the TSFRESH feature-extraction approach achieves the highest accuracy.

Biofilm rupture by laser-induced stress waves increases with loading amplitude, independent of location.

Integral to the production of safe and biocompatible medical devices is to determine the interfacial properties that affect or control strong biofilm adhesion. The laser spallation technique has recently emerged as an advantageous method to quantify biofilm adhesion across candidate biomedical surfaces. However, there is a possibility that membrane tension is a factor that contributes to the stress required to separate biofilm and substrate. In that case, the stress amplitude, controlled by laser fluence, that initiates biofilm rupture would vary systematically with location on the biofilm. Film rupture, also known as spallation, occurs when film material is ejected during stress wave loading. In order to determine effects of membrane tension on the laser spallation process, we present a protocol that measures spall size with increasing laser fluence (variable fluence) and with respect to distance from the biofilm centroid (iso-fluence). Streptococcus mutans biofilms on titanium substrates serve as our model system. A total of 185 biofilm loading locations are analyzed in this study. We demonstrate that biofilm spall size increases monotonically with laser fluence and apply our procedure to failure of non-biological films. In iso-fluence experiments, no correlation is found between biofilm spall size and loading location, thus providing evidence that membrane tension does not play a dominant role in biofilm adhesion measurements. We recommend our procedure as a straightforward method to determine membrane effects in the measurement of adhesion of biological films on substrate surfaces via the laser spallation technique.

Estimation of the Defect Width on the Outer Race of a Rolling Element Bearing under Time‐Varying Speed Conditions

Fault diagnosis and failure prognostics for rolling element bearing are helpful for preventing equipment failure and predicting the remaining useful life (RUL) to avoid catastrophic failure. Spall size is an important fault feature for RUL prediction, and most research work has focused on estimating the fault size under constant speed conditions. However, estimation of the defect width under time‐varying speed conditions is still a challenge. In this paper, a method is proposed to solve this problem. To enhance the entry and exit events, the edited cepstrum is used to remove the determined components. The preprocessed signal is resampled from the time domain to the angular domain to eliminate the effect of speed variation and measure the defect size of a rolling element bearing on outer race. Next, the transient impulse components are extracted by local mean decomposition. The entry and exit points when the roller passes over the defect width on the outer race were identified by further processing the extracted signal with time‐frequency analysis based on the continuous wavelet transform. The defect size can be calculated with the angle duration, which is measured from the identified entry and exit points. The proposed method was validated experimentally.