Juan C. Jauregui

Efficient method for detecting tool failures in high-speed machining process

This research presents a method for detecting tool failures in high-speed face milling. This method detects tool failures from vibration signature maps. Tests were carried out at different tool failure levels, spindle speeds, feed rates, and workpiece mountings. Vibration signals were obtained with an accelerometer and processed using the continuous wavelet transform methodology. The vibration signature maps showed that healthy cutting tools produce a periodic insert passing frequency and its harmonics. In contrast, a damage tool generates additional nonlinear and transient frequencies at nonsynchronous frequencies. The experimental results agree with vibration signature maps obtained from a simulated cutting force model. The proposed method is effective as a tool failure detection method when transient and nonlinear behaviors are presented in face milling process. Moreover, the proposed method showed good results at different process parameters and for several types of tool failures. Finally, it is important to point out the use of accelerometers because they present several advantages against other types of sensors. Advantages such as low cost, wide bandwidth, and easy implementation are important characteristics for tool condition monitoring in high-speed machining.

Experimental Characterization of Synchronous Vibrations of Blades

Blades in a turbine rotor synchronize their individual vibrations after a certain period of time. Each blade has a slightly different natural frequency due to manufacturing errors, and it responds individually to external excitations. Nevertheless, after a period of time the blades synchronize and vibrate at the same frequency due to weak interactions between them. In this work, experiments are reported that identify blade synchronization. Individual blade vibrations were measured simultaneously under different conditions. Measurements were made with accelerometers attached to the tip of each blade which were very light in comparison with the mass of the blade. The blades were first excited by an impact force and the natural frequencies were identified. Then, the blades were excited by airflow and individual blade-tip vibrations were recorded at several subsequent time intervals. Synchronization is analyzed through the correlation between the time responses of every blade. The correlation was calculated at each of these intervals and the results were plotted in maps. It was found that the synchronization evolves as a function of time; it is high after a medium time period and reduced at longer time periods. Thus blade synchronization presents a long wave variation that could be a source of a very low frequency vibration. It was also possible to say that synchronization is dominated by the structure of the rotor.Copyright

Effect of road profile on heavy vehicles with air suspension

This study presents a seven degree of freedom, dynamic model for a two-axle vehicle which is used to analyse the influence of road profile on the vehicles rollover capabilities and ride characteristics. Non-linear elements are included due to the air suspension featured in the vehicle. Road roughness is represented as a Power Spectral Density (PSD) function acting as an exciting force on the unsprung masses of the seven degree of freedom model, and solved numerically for both rollover simulation and lane change manoeuvre. Results indicate that road surface does have a significant impact in a vehicles performance since it may prompt the vehicle to flip over below its rollover threshold.

Requirements and Constraints for a Robotized Roll Hemming Solution

The objective of this study is the design of a new roller hemming tool. For this purpose, main parameters affecting the final quality of the rolled hemmed part have been studied in order to identify and outline the key factors in roller design as well as in roller hemming process stages. A preliminary set of experiments has been carried out to study the pre-hemming force and springback relation in an aluminium sheet. Experiments showed the required forces in first pre-hemming stage and allowed to calculate a stiffness coefficient based on the springback deformation angle.

Housing Stiffness Influence on Gearbox Dynamic Loading for Wind Turbine Applications

The design of wind turbine gearboxes must consider particular conditions such as wind speed variations, load changes, and they must be as light as possible. Although there are enough field data, their life prediction lacks of good estimation. It is common to find gearboxes with bearing or gear premature failures. The housing can dissipate noise and vibrations in a positive manner; but in wind turbines it can increase the synchronization among nonlinear elements. This phenomenon adds an additional dynamic load on the gears and rolling bearings.The work presented here is a method for estimating the influence of the housing stiffness on the dynamic synchronization of wind turbine gearbox elements. The analysis estimates a dynamic load factor that affects both gears and rolling bearings. The analysis takes into account the gearbox nonlinear elements, as well as the turbine blades excitations.Copyright

Identification of Bearing Stiffness and Damping Coefficients Using Phase-Plane Diagrams

The reliable identification of dynamic parameters in mechanical systems remains a big challenge, in particular for nonlinear systems. There is not a single mathematical model encompassing the universe of most systems. From a practical point of view, the identification of system parameters depends on the measurement data as well as on the reference model.This paper presents a novel method for identifying the dynamic parameters of a gas bearing, whose force coefficients are strong functions of frequency. The method is based on the analysis of the phase diagram with the model assuming a mass-damper-spring system with time-dependent force coefficients. The phase diagram could be implemented electronically for on line monitoring and ready fault detection.Copyright

The Application of Time-Frequency and Phase Diagram Analyses for the Early Detection of Faulty Roller Bearing

There are many systems that monitor and analyze machinery conditions, but there is a lack on monitoring techniques that are able to identify early faults. This is critical since faulty roller bearings cause most of the problems in rotating machinery. Two major difficulties limit the early prediction of roller bearing failures: The acquisition of high frequency data, and the low energy levels emitted by surfaces cracks on rollers or tracks. Early fault detection requires adequate instrumentation and signal analysis. Therefore, it is important to identify its nonlinear response, and to be able to detect low energy signals in order to detect early faults. The vibration characteristics of the bearing depend on the rotational speed, clearance, radial load, rolling element stiffness, and the surface waviness. Additionally, they generate transient vibrations due to stiffness nonlinearities and structural defects. We present in this work a technique for the detection of early faults. The technique is based on the derivation of the rolling bearing nonlinear stiffness as a function of the roller translation and roller deformation. For the deformation, we determine the relative displacement of the roller tracks and we apply Hertz’s contact stress function. With this formulation we identify the nonlinear response to the external excitation forces. Then, the results are analyzed with a time-frequency map and the phase diagram; the two analyses are compared in order to determine the best signal procedure to identify early failures.© 2009 ASME

A discrete simulation-based algorithm for the technological investigation of 2.5D milling operations

Many applications are available for the syntactic and semantic verification of NC milling tool paths in simulation environments. However, these solutions – similar to the conventional tool path generation methods – are generally based on geometric considerations, and for that reason they cannot address varying cutting conditions. This paper introduces a new application of a simulation algorithm that is capable of producing all the necessary geometric information about the machining process in question for the purpose of further technological analysis. For performing such an analysis, an image space-based NC simulation algorithm is recommended, since in the case of complex tool paths it is impossible to provide an analytical description of the process of material removal. The information obtained from the simulation can be used not only for simple analyses, but also for optimisation purposes with a view to increasing machining efficiency.

Estimation of the Rotordynamic Coefficients of a Compressor Mounted on Gas Bearings Using the Phase Diagram and the Unbalance Response

This paper presents a novel method for identifying the dynamic parameters of a gas bearing, whose force coefficients are strong functions of frequency. The method is based on the analysis of the phase diagram with the model assuming a mass-damper-spring system with time-dependent force coefficients. Usually, it is necessary a controlled mechanism to find the transfer function, this condition limits the application of the method. On the other hand, estimation of the damping and stiffness parameters under real loading is very cumbersome and requires a special care on identifying the excitation forces. One of the main difficulties is the isolation of noise and those vibration signals with an unidentified source. In this work, the excitation force was taken from the unbalance loading of a rotor test. Therefore, there is no need for a special test rig. The dynamic parameters can be estimated analyzing data from the actual rotor mounted on the gas bearings. Identifying the parameters that cause gas bearing instabilities is a big challenge. The gas properties are very sensitive to temperature and pressure changes, and, as a consequence the bearing rotor-dynamic coefficients change drastically and the rotor behaves chaotically, which means that the dynamic parameters are nonlinear. In this research a methodology based on the phase diagram construction to identify nonlinear instabilities of gas bearings is presented. The results show the method capability to estimate the dynamic coefficients by the analysis of the energy variation.Among nonparametric methods, the phase diagram or phase space is in use to identify nonlinearities in dynamic systems. The identification is conducted through the analysis of the energy variations. The energy variations can be represented as a three dimensional function E(x,v,t). In this way the phase diagram can be related to the frequency and the dynamic parameters of the system. According to Taken’s theorem, a dynamic system can be obtained by reconstructing the phase diagram. Then, using this method, the damping and stiffness coefficients are estimated.Copyright

Identification of Nonlinear Instabilities in Rotors Supported on Gas Bearing

Identifying the parameters that cause gas bearing instabilities is a big challenge. The gas properties are very sensitive to temperature and pressure changes, and, as a consequence the bearing rotor-dynamic coefficients change drastically and the rotor behaves chaotically, which means that the dynamic parameters are nonlinear. In this research a methodology to identify nonlinear instabilities of gas bearings is presented. Estimating variations of the coefficients can be done using wavelets and the phase diagram. Wavelets allow the development of time-frequency maps where the behavior of each of the system frequencies can be analyzed. The time-frequency map displays frequency variations as a function of time. In this way, any change in the rotor dynamic parameter is reflected as variations in its associated frequency. Analyzing these variations it is possible to estimate the nonlinear parameters. With the phase diagram analysis, it is possible to evaluate changes in the dynamic behavior. The phase diagram relates the potential energy and kinetic energy at any time instant. If the system is stable, the phase diagram has a well define pattern that repeats steadily; whereas, if the pattern shows sudden changes in dimensions and shape, it indicates that the system parameters have change. This change may be chaotic.© 2014 ASME