1,721,003 research outputs found
Gear Tooth Root Bending Strength Estimation under the Assumption of Fatigue Limit Existence
Full text linkBeing able to properly predict gear failure is a key aspect to achieve a reliable light-weight gearbox. Among the several gear failures, tooth root bending fatigue is considered as the most dangerous one because it implies the stoppage of the whole gearbox. In order to characterize a gear for this phenomena, Single Tooth Bending Fatigue (STBF) tests are the most performed ones. However, as in STBF test THERE IS no sliding/rolling contact and as the specimens are teeth rather than gears, some differences occur between the test conditions and those of the real case. This paper deals with the statistical ones that is the estimation of the gear SN curve starting from the teeth one. The teeth SN curve has been estimated by means of a statistical model developed considering Murakami's idea of nonpropagating crack. Then, a methodology based on statistic of extreme is adopted for the purpose of estimating the gear SN curve
Non-Newtonian cfd modelling of a valve for mud pumps
Full text linkMud pumps, like those used in the field of oil well drilling, are typically of the reciprocating type and are designed to circulate drilling fluid under high pressure down the drill string and back up the annulus. automatic valves must be applied to the fluid end in order to grant the desired pumping effect. from the engineering point of view, the design of the valve geometry must ensure that the phenomenon of cavitation does not occur and that, during the pumping action, the stiffness of the reaction coil spring is able to avoid reaching the condition of end stroke of the valve. Cavitation consists in the development of vapour cavities in the liquid phase. Inside the cavities, the pressure is relatively low. When subjected to higher pressure, the voids implode and generate an intense shock waves that promote the wear for the components (i.e. valve, valve seat, etc.). a deep understanding of the fluid behaviour is crucial for an effective design. Transient CfD simulations of the valve opening have been performed using a non-Newtonian fluid model able to describe the drilling muds. after a deep literature review, the Herschel-Bulkley model was selected as the most suitable for emulating the drilling mud. With the abovementioned approach, the reaction spring and design the valve seat to avoid premature wear phenomena were properly designed. The simulations have been also done considering a Newtonian fluid behaviour, in order to better understand the importance of considering the non-Newton behaviour for an effective design
Estimation of gear SN curve for tooth root bending fatigue by means of maximum likelihood method and statistic of extremes
Full text linkGear failure due to tooth root bending fatigue is one of the most dangerous gear failure modes. Therefore, the precise definition of gear bending fatigue strength is a key aspect in gear design. As a matter of fact, in order to assess a gear component, an accurate estimation of the component SN curve is required. This curve must properly take into account three main aspects: the slope of the fatigue strength region, the slope of the region ahead the fatigue knee and the position of the knee itself. In addition, with the aim of being able of considering different reliability levels, a proper estimation of the associated dispersion is required too. Single Tooth Bending Fatigue (STBF) tests are usually used to investigate the tooth load carrying capacity with respect to the bending failure mode. However, due to the test rig configuration, two main differences between test and real case are present. Firstly, the statistical behavior is different, since in the meshing gear the strength is determined by its weakest tooth, while in a STBF test the failing tooth is predetermined. Secondly, the load history is different.Therefore, additional effects have to be taken into account to obtain the gear SN curve starting from STBF tests. In this article, due to its capability of handling interrupted tests (e.g. runouts), Maximum Likelihood Estimation (MLE) has been used to estimate, in the most reliable way, the SN curve from experimental points. SoE (Statistic of Extremes) has been adopted to move from the STBF SN curve to the gear one, as, by means of a simple mathematical passage, SoE enables the estimation of the strength of the weakest tooth among the z gear teeth and, as a consequence, of the gear. The effect of the different load history is considered by adopting a literature-based approach (i.e. use of corrective coefficient). This paper describes in detail the proposed calculation method and shows its application to determine the SN curve in a practical case
A VIBRATION MONITORING SYSTEM FOR DETECTING PITTING DAMAGE IN SPUR GEAR TESTS
No full textThe paper presents a vibration monitoring system developed in order to detect macro-pitting damage in gear tests. This system has been installed on a back-to-back spur gear test rig and utilized during endurance pitting tests in parallel with a standard visual inspection method. The results, obtained by means of the analysis of the acquired vibration signals and expressed in terms of a synthetic vibration parameter, are correlated with the evolution of pitting damage on the tooth flanks of the test gears
Dynamic modeling of gears: An innovative hybrid FEM-analytical approach
Full text linkGearboxes are widely used in several applications ranging from the automotive to the industrial and robotic sectors. A planetary gearbox is a special kinematic gear arrangement that, taking advantage of a planet carrier, ensures high reduction ratios together with a very small design. Therefore, they are widely employed for transmissions which require a high power density. There are several fields of applications including, but not limited to, mechatronic, automation and wind power generation. To improve the design of new solutions, for performing monitoring activities on actual gearboxes and for the definition of maintenance schedules, the availability of physical models able to accurately describe the behavior of the system, both in healthy and damaged conditions, would represent a great support. Experimental and numerical studies of the behavior of gearboxes are already available in the literature. Nevertheless, while the experimental approaches are valid only for the specific configuration tested, the numerical techniques show limitations related to the computational effort required. This paper presents an innovative approach for the characterization of the behavior of two different geared transmissions. It is based on a hybrid approach that combines finite elements (FE) with analytical formulations. In detail, the solver computes separately the macro deformation of the bodies (numerical solution based on a coarse grid) and the contacts (solved analytically avoiding the need of mesh refinements). The computational effort is reduced significantly without affecting the accuracy of the results significantly. This approach was used to investigate and understand the vibro-dynamical behavior of a back-to-back test rig (typically used for the characterization of the surface fatigue strength of gears) and of an industrial planetary gearbox. The results obtained for the healthy - not damaged - gearboxes were compared with experimental measurements for both configurations in order to validate the hybrid approach. Once the models were validated, the same methodology was eventually used to study the effects of typical gear failures and in specifically surface fatigue (pitting), on the vibrational response. The capability to reproduce the effect of damages with the model of a gearbox represents the first indispensable step of a Structural Health Monitoring strategy. State-of-art and challenges are analyzed and discussed in the paper
Structural modelling of multilayer skis with an open source FEM software
Full text linkThe design process of a ski is characterized by a short time of development due to continuous advancements in the material science and in the manufacturing processes as well as in customer’s requirements. Nowadays, the development process is very often still based on several physical prototypes and trials and Finite Elements Analysis (FEA) is a significant method to reduce times needed. The aim of this work is to develop a reliable numerical simulation of an existing mountaineering ski, able to predict the performance of the real element. For this purpose, an initial mechanical characterization of all the constituents used in the ski manufacturing was performed. Tensile tests in two directions were performed on flat bone-shaped samples laser cut from sheets. Combining the results of the tensile tests with Digital Image Correlation (DIC) data it was possible to approximate the four in-plane (XY) elastic properties, namely, the two elastic modules, the shear module and the Poisson ratio (Ex, Ey, Gxy, νxy). The DIC free software used is GOM Correlate. Results of the combined “tensile tests – DIC” approach were after verified with FEM simulations reproducing the testing configuration. The digital model of the ski was created starting from the nominal geometry. The whole procedure of modelling, meshing and FE analysis was performed in the open source software Code_Aster/Salome-Meca. Using this kind of software, which code is free to use and modify, permits to reduce costs due to its free license. The real component was tested in a three-point bending and torsion test. This kind of experiments were replicated on the FEM model and results were compared. The comparison highlighted discrepancies of 2.5%–10% with respect to the real component
Development of a computational fluid dynamics simulation tool for lubrication studies on cycloidal gear sets
No full textIn the last decades, the growing mechatronic sector has promoted the development of more and more compact and efficient gearboxes. The margins of improvement are still big even if, sometimes, finding the optimal solutions is a trial and error procedure. For this reason, the development of dedicated tools for the optimization of the geometry and configuration of gearboxes can significantly increase the development effectiveness and help in reducing design costs. Moreover, having a more efficient solution could also reduce thermal problems during operation and increase the system reliability. The so-called 'thermal limit', i.e. the maximum transmittable power without an overheating of the systems, is particularly critical for high power density and compact solutions. Those relies mainly on planetary, harmonic and cycloidal architectures. While many empirical or analytical prediction models can be found in literature for the prediction of the power losses associated with the gear meshing and the bearing, few reliable models are nowadays available for the losses associated with the interaction with the lubricant, i.e. hydraulic losses. Experimental and computational fluid dynamics studies on parallel axis as well as planetary gear sets have been presented in the past. The goal of this research is the extension of the applicability range of those numerical approached to cycloidal kinematics for which no studies at all are available with respect to the hydraulic losses. The main challenge in numerically simulate the lubricant splashing in a cycloidal reduced is related to the topological modification of the computational domain during operation. For this purpose, a specific mesh handling technique, based on a 2.5D mesh, capable to handle the variations of the geometry of the domain was developed in the OpenFOAM® environment. The capability to analytically control the mesh generation at each time step ensures a very high numerical stability and a very high computational efficiency of the solution. Eventually, the approach was systematically applied to a real geometry and the results compared with those obtained for other gear architectures with comparable performances in terms of dimensions and reduction ratios
Aerodynamic study of moto rcycle racing wheels: A performance evaluation based on numerical CFD simulations
Full text linkIn any racing competitions, the aerodynamic performances of the equipment are determinant. This is true, for example, for cars, where the geometry of the bodywork and of the wings can ensure a lower Cx coefficient and/or a higher down-force and a higher handling. In other competitions, like rowing, the aerodynamics of the hull can reduce the effort done by the athletes. In the cycle and motorcycle racing competitions, other aspects related to aerodynamics become important, such as the manoeuvrability and stability. In the present research, a numerical approach was used in order to compare different front-wheel geometries (of a racing motor-bike) in terms of drag, lift and axial forces. Three different wheel designs have been compared. The first one consists in a traditional seven spokes aluminium design, the second wheel is a 6 spokes magnesium solution and the third a solid-disk wheel. Steady state as well as transient simulations was performed with OpenFOAM®, a free open-source software. This was selected because it allows a higher flexibility with respect to any close-source commercial software. The possibility to customize the solver as well as the boundary conditions allows the analysis of the physical problem of interest. The free license allows a high parallelization of the computations. The steady-state simulations were performed by freezing the wheel position and introducing a rotating reference frame. In this way, the computational time was significantly reduced. For the transient simulations, the computational domain was split into two subdomains. The internal one is cylindrical and contains the wheel. The rotational velocity of the wheel was imposed by applying a rigid rotation to the mesh of the internal subdomain. Mesh interfaces ensures the continuity of the solution across the domains
Numerical modeling of the churning power losses of gears: an innovative 3D computational tool suitable for planetary gearbox simulation
No full textThanks to the recent developments in the computer science, simulations are becoming an increasingly widespread approach that can help the designers in the development of new products. In the specific field of gearboxes, simulations up to now have been used mainly for structural and dynamic analysis. In these fields, the simulation tools have proved to be able to provide reliable information. Moreover, in the field of structural design and strength analysis, with respect to the various failure mechanisms involved, many analytical methods and international standard are available. On the other hand, at present for the prediction of the power losses and the efficiency of gears, neither accurate analytical methods nor automated simulation tools are available, in particular where the interest is focused on load independent power losses. The authors have been working on this topic for several years and have developed new methodologies based on computational fluid dynamics. With respect to general-purpose commercial software, the techniques that have been developed by the authors for the specific application of gears and gearboxes, allow a significant reduction of the computational effort and have the capability to take into account particular physical phenomena that occurs in gears, such as cavitation for instance, and for which no information available in literature, concerning their influence on gears efficiency. The purpose of this paper is to introduce an improved automated strategy, implemented in order to extend the applicability of the previously developed computational method to real complex gearboxes. With this additional improvement, some geometrical limitations adopted in the past can be removed and the tool is now suitable for the application to real complex gearboxes. In order to show the capabilities of this new strategy, a planetary gearbox, which represents one of the most complicated kinematic arrangements of gears has been selected and simulated. At the same time, the planetary gearboxes represents a configuration for which the numerical fluid dynamics simulation can give the major contribution tb the calculation of load independent power losses, due to the peculiar interaction between the lubricant and the planets which are supported by a rotating planet carrier. The simulations have been performed both with planar simplified models and with complete 3D models and compared with experimental data showing the goodness of the approach
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