Papers by Author: Jože Flašker

Abstract: The classical methods of solid mechanics assume that the homeomorphism of deformation sometimes does not provide an adequate description of the theological behaviour and fracture of real materials, while irreversible macro deformation and the fracture of solids are predetermined by material behaviour in nanoscale. When we talk about crack propagation, we must talk at least about four phenomena, namely about crack incubation, initiation, and crack propagation must be sleeted in two parts, to growth of short crack and to growth of linear elastic or long crack.

Abstract: For designing machines and devices the dimensioning with respect to service life is increasingly taken into account. This applies also for gearing which are still today one of very important components of almost all machines. The problem of determination of the service life of gearing is directly related to geometry of gears, multiaxial loadings, materials and appropriate models for prediction of the crack propagation. Obviously gears and gearing belong to the real complex structure, by correctly selected and carefully planned experiments we obtained results with which we can confirm and justified the mathematical model for calculating different parameters, i.e. service life.

Abstract: For designing machines and devices the dimensioning with respect to service life is increasingly taken into account. This applies also for gearing which are still today one of very important components of almost all machines. The problem of determination of the service life of gearing is directly related to geometry of gears, multiaxial loadings, materials and appropriate models for prediction of the crack propagation.Obviously gears and gearing belong to the real complex structure, by correctly selected and carefully planned experiments we obtained results with which we can confirm and justified the mathematical model for calculating different parameters, i.e. service life.

Abstract: A crack in the tooth root is probably the least desirable problem in gear unit operation; it often leads to failure. Signals produced by a gear with a crack in the tooth root, produced through real operating conditions, and signals caused by a faultless gear are used for the analysis. By monitoring vibrations it is possible to detect the presence of a crack. A fatigue crack in the tooth root brings about significant changes in tooth stiffness. Other faults are usually linked with modifications of other dynamic parameters. Time Frequency Analysis tools, e.g. Wavelets Analyses, are used to analyse a non-stationary signal. The wavelet transform is chosen for the analysis. The wavelet function similar to the dynamic reaction of the crack in the tooth root is selected. By means of the methods and the analysis presented in this paper, the reliability of determining modifications in signal vibrations is improved.

Abstract: In this paper, the concepts of preventive maintenance are pres¬ented in first part, and second part of the paper present several mathematical models for prediction service life as a base for preventive maintenance forecasting. These models aid in determining the frequency of inspec¬tions that will allow the condition of a piece of equipment to be determined in order to plan an appropriate maintenance action. The problem of determination of the service life is directly related to geometry, multiaxial loadings, materials and appropriate models for prediction of the failure propagation. The more precise modelling of these input parameters the more precise and reliable the results are. Therefore in the paper we will present models and procedures for calculations service life, remain service life and/or service control cycle.

Abstract: In order to calculate the service life as precisely and reliably as possible we need good mathematical models for describing loading, geometry, properties of materials and fracture mechanics parameters. It can be established whether a mathematical model is precise and reliable only by comparison of results of the method such as analytical methods in case of simple problems and experiment when real complex structure are deal with. Since gears and gearing belong to the second group, by correctly selected and developed test pieces and carefully planned experiments we obtained results with which we confirmed and justified the mathematical model for calculating mentioned parameters. To this end we will show in this paper series of experimental methods and test pieces used on the gears.

Abstract: Problems concerning gear unit operation can result from various typical damages and faults. A crack in the tooth root, which often leads to failure in gear unit operation, is the most undesirable damage caused to gear units. This article deals with fault analyses of gear units with real damages. A laboratory test plant has been prepared; it has been possible to identify certain damages by monitoring vibrations. In concern to a fatigue crack in the tooth root significant changes in tooth stiffness are more expressed. When other faults are present, however, other dynamic parameters prevail. Signal analysis has been performed also in concern to a non-stationary signal, using the adaptive transformation for signal analysis.

Abstract: A two-dimensional numerical model is used to describe the crack path in the lubricated rolling-sliding contact problems. The model assumes that the crack is initiated in a pre-existing micro pit, which resulted from the crack growth on the surface of a gear tooth flank. The lubricated rolling-sliding contact problem is modelled using the Hertz theory of contact, the Coulomb's law of friction and hydraulic pressure mechanism with constant pressure which simulates the effect of lubricant trapped into the crack. Different load cases are used to simulate the moving of a contact load. The crack propagation path is evaluated by a maximum tangential stress criterion and modified maximum tangential stress criterion which considers the stress intensity factors KI and KII, the T-stress, the critical distance ahead the crack tip rc, and the stress on the crack surfaces. The computational results show that the consideration of the T-stress has a significant influence on the crack path in the lubricated rolling-sliding contact problems.

Abstract: A crack in the tooth root, which often leads to failure in gear unit operation, is the most undesirable damage caused to gear units. This article deals with fault analyses of gear units with real damages. Numerical simulations of real operating conditions have been used in relation to the formation of those damages. A laboratory test plant has been used and a possible damage can be identified by monitoring vibrations. The influences of defects of a single-stage gear unit upon the vibrations they produce are presented. Signal analysis has been performed also in concern to a non-stationary signal, using the Time Frequency Analysis tools. Typical spectrograms, which are the result of reactions to damages, are a very reliable indication of the presence of damages.

Abstract: For accurate determination of the service life we must take into account the loading, which are in most cases random loading of variable amplitude, the geometry and material properties of construction elements which are known not to be constants. The more precise these input parameters are modeled; the more precise and reliable are the results. In our paper we will deal in detail with the model of crack initiation and propagation in the complex structures as a basis of the algorithm for calculating the service life. For determination of the service life for the area of short cracks we used Bilby, Cottrell and Swinden model which is based on the theory of continuously distributed dislocations and we complemented it with random generation of structure of material before cracks. For the long crack we have developed a stochastic model for determination of service life.