Vasile Cojocaru

Babeş-Bolyai University | UBB · Department of Engineering Science

In recent decades, enthusiasts and researchers from all over the world have invested a lot of energy and time in trying to develop linear propulsion systems using the inertial force generated by two masses revolving in opposite directions. The authors present an original concept for the propulsion system of a mobile platform by using the inertial f...

Among the FDM process variables, one of the less addressed in previous research is the filament color. Moreover, if not explicitly targeted, the filament color is usually not even mentioned. Aiming to point out if, and to what extent, the color of the PLA filaments influences the dimensional precision and the mechanical strength of FDM prints, the...

Additive manufacturing technologies enable the production of components with lightweight cores, by means of infills with various patterns and densities. Together with reduced mass and material consumption, infill geometries must ensure that strength and stiffness conditions are fulfilled. For the proper correlation of the infill type with the loadi...

In the context of global warming, Romania has assumed through European directives to increase the percentage of energy produced from renewable sources. In the last years, energy generated in hydropower plants had the largest contribution to the total renewable energy production. This is expected to continue in the forthcoming period, considering th...

The printing variable least addressed in previous research aiming to reveal the effect of the FFF process parameters on the printed PLA part’s quality and properties is the filament color. Moreover, the color of the PLA, as well as its manufacturer, are rarely mentioned when the experimental conditions for the printing of the samples are described,...

Polylactic acid (PLA) is produced from renewable materials, has a low melting temperature and has a low carbon footprint. These advantages have led to the extensive use of polylactic acid in additive manufacturing, particularly by fused filament fabrication (FFF). PLA parts that are 3D printed for industrial applications require stable mechanical p...

Initially developed as a rapid prototyping tool for project visualization and validation, the recent development of additive manufacturing (AM) technologies has led to the transition from rapid prototyping to rapid manufacturing. As a consequence, increased attention has to be paid to the mechanical, chemical and physical properties of the printed...

The characterization of flow through Pelton hydro turbines allows the optimization of their operation and maximization of energy performance. The flow in the injector of Pelton turbines and in the free jet area (the area from the injector outlet surface to the runner bucket inlet surface) is influenced by several parameters: the geometry of injecto...

The paper presents a way of estimation, by simulation, of friction forces that occur in the kinematical joints of mechanical systems. Friction forces between planar surfaces, in revolute joints, and in spherical joints are computed, and the results are compared with the values achieved from simulation. SolidWorks software is used for the simulation...

The paper shows a method of evaluation by simulation of reaction forces that occur in the spherical joints of a parallel topology robot. The geometrical model of the parallel topology robot, operated as a flight simulator, is presented. The structure of the model is of FP3 + 6·SPS + MP3 (or MSSM) type. Based on the input parameters, which are the r...

The paper proposes an inertial system capable of generating a propulsive force from the rotation on an eccentric circular path of 8 steel balls placed between two rotating discs constructed with a radial slot system. In addition to a detailed description of the mechanical components of this proposed device, particular attention is paid to the theor...

The paper presents the geometrical model of a parallel topology robot of MSSM type, operated as a flight simulator. Certain strokes of the driving kinematical joints lead to the simulation of real limit situations, which are dangerous for the human operator, such as a sudden pitch movement. The variations of different motion parameters are analyzed...

The paper presents the manner of determination of forces that act on a workpiece grasped by a pneumatically driven robotic gripper. The starting point consists of experimental determination of the pushing force necessary to move the workpiece held between the gripper's fingers. Then, by using theoretical relations, the friction force between the wo...

The gearbox housing is the assembly which surrounds the mechanical parts of a gear transmission. As its weight typically represents about 50% of the gearbox's, optimizations of shape and weight are of particular importance, leading to material and cost savings. This paper presents an example of how the design of a gearbox housing can be improved by...

Hydraulic turbine components that define the water flow, generally have a complex geometry. In the design of these components a major role is played by model tests. The complex geometry of runner�s blades and wicked gates determines high manufacturing times and high fabrication cost if they are made by classical manufacturing technologies (casting,...

The paper presents the comparative analysis of two types of welded structures used in railway bridges. The differences between the two types of structures are given by the configuration of welded joints (HEM profiles are used for both structures). Constructive and technological analysis made for each structure are presented, highlighting the differ...

The paper presents a constructive solution for the metallic structure of a city railway bridge, with pedestrian area. The structure made of HEM beams has a modular design in order to facilitate the transport and in-situ assembling. The results obtained from static, dynamic and fatigue analyses are presented. Static analysis highlights areas with ma...

The paper presents the geometrical model of a robotic gripper with 3 fingers, accomplished by using SolidWorks software, used for grasping cylindrical workpieces of the same length, but with different diameters. Applying an action force to the gripper’s pneumatic cylinder piston, for a certain cylindrical workpiece, the contact forces between the g...

The paper presents the manner of determination of forces that act on a workpiece grasped by a pneumatically driven robotic gripper. The starting point consists of experimental determination of the pushing force necessary to move the workpiece held between the gripper's fingers. Then, by using theoretical relations, the friction force between the wo...

In the process of geometric optimization of the gear reducers, an important role is played by the decrease of the housing weight. The optimization of housing should include stress state analysis, as well as modal analysis. This paper focuses on the modal analysis of the housing from a two-stage cylindrical reducer transformed into a one-stage reduc...

The paper presents the 3D model of a robotic gripper and a way to determine the value of prehension force by using the SolidWorks software. A set of prismatic workpieces is considered, the contact force finger-workpiece being determined in SolidWorks Motion module for the most disadvantageous case-the heaviest workpiece, as well as von Mises stress...

The paper presents a 6-DOF industrial robot and the driving system of its base translational joint. By imposing certain positions of the translational joint and determining the durations in which these positions are reached, average speed is computed. The paper shows how the clearances influence the average speed, depending on the displacement valu...

Noise is an important component of environmental pollution, becoming in the recent years a global problem, due to its presence in all the issues of modern life. From the perspective of gearboxes, the produced noise may have several causes, as for example: meshing of gears, splashing of the oil, rotation of bearings, interaction of the gearbox with...

Several approaches can be used in the FEM studies to apply the loads on a plain bearing. The paper presents a comparative analysis of maximum stress obtained for three loading scenarios: resultant force applied on the shaft – bearing assembly, variable pressure with sinusoidal distribution applied on the bearing surface, variable pressure with para...

CFD (Computational Fluid Dynamic) is today a standard procedure for analyzing and simulating the flow through several hydraulic machines. In this process, the fluid flow domain is divided into small volumes where the governing equations are converted into algebraic ones, which are numerically solved. Computational results strongly depend on the app...

The manufacturing of hydraulic turbine runners designed for experimental research on the model is carried out, usually, by precision casting or CNC machining. These processes involve technological difficulties, high manufacturing times and high production costs, which are generated by the complex geometry and the small dimensions of the runner. Rap...

The measurement of cavitation erosion resistance of materials on vibratory devices can be applied in two testing configurations: “direct method”- with the sample attached to the sonotrode and “indirect method” – with the sample fixed on a device with an offset distance between the sonotrode and the sample. Based on the testing experience of CCHAPT...

In this paper are presented a graphic - analytic method that can be used to estimate the fatigue lifetime duration for an operating mechanism lever pin to a Kaplan turbine. The presented calculus algorithm is adapted from the one used by Fuji Electric to made strength calculus in order to refurbish a Romanian hydropower plant, equipped with a Kapla...

During the testing of the materials’ cavitation erosion resistance on vibratory systems by direct method (ASTM G32), the thread from the ultrasonic horn – test specimen connection, is subjected to fatigue. Operational experience has shown the occurrence of cracks and fractures in this area after a number of stress cycles that was lower than anticip...

This paper proposes a study on the workspace of the guiding device mechanism of a parallel topology robot. The kinematical scheme and the geometrical model of the guiding device mechanism of the parallel topology robot are presented. The lengths of binary links between the platforms determine the shape and the volume of the parallel robot’s workspa...

In this paper, the geometrical model of a guiding device mechanism of a parallel topology robot proposed as a flight simulator is presented. For the operation simulation, the input parameters are the relative displacements of driving kinematical joints links versus time. The displacements of 2 characteristic points on the mobile platform are determ...

The current trend in the construction of gearboxes, regarding the speed increase, favors the increase of the dynamic loads which are accompanying the operation of these kinds of machines. The phenomena of dynamic contact like frictions, collisions and shocks which are taking place in joints, engines and mechanisms during their movement, are generat...

Gears are frequently used in mechanical systems for power transmission, speed variation and for changing their operating sense. Mathematical modeling of gear transmissions offers a better understanding of their dynamic behavior. A significant amount of literature and studies are available in this field. Because the gears are critical components of...

The design of the housings of the gear reducers is made, usually, using empirical equations based on the center distance (the distance between shafts). These equations can lead to inappropriate stresses distribution and inadequate material consumption at the final product. In the manufacturing of large series and in the manufacturing of the gear re...

The large diameter threaded connections are components with high importance in the heavy mechanical systems. At the threaded elements subjected to variable loads one of the main causes of failure is represented by fatigue. This phenomenon is accentuated by the occurrence of high local stresses on the thread root. In this context, the correct evalua...

In the last twenty years the cavitation erosion resistance of various welded materials was subject of extensive studies. Despite these, the research field is still opened. The multitude of materials used for the fabrication of hydraulic equipment and the variety of the operating conditions in hydropower units require adapted solutions. This paper p...

Cutting vibration is a phenomenon that often appears in the process of metal machining, being one of primary factors that affects the process-ing quality and limits the productivity enhances. The analysis of vibra-tion in the cutting process became very important, because the vibra-tions may cause the damaging of the machine spindle, cutting tool,...

In the thread root area of the threaded bolts submitted to axial loading occur local stresses, higher that nominal stresses calculated for the bolts. These local stresses can generate failure and can reduce the fa-tigue life of the parts. The paper is focused on the study of the influ-ence of the thread root radius on the maximum local stresses. A...

The influence of the mesh parameters on the results of FEM simulations depends on the type of the study and the geometry of the system submitted to analyze. In this paper a gear housing was submitted to static analysis. Six studies with distinct values for the maximum dimensions of mesh elements were applied. The results show significant difference...

Cutting forces are classified among the most important technological parameters in machining process. Cutting forces are the background for the evaluation of the necessary machining power, as well as for dimensioning of the tools. Cutting forces are also having a major influence on the deformation of the work piece machined, its dimensional accurac...

The paper presents the geometrical models of cam and of semi-finished piece, obtained with CATIA software. Two cam technological routes are shown and the optimal route is chosen. From the technological route, rough milling operation is simulated with CATIA software. By using different milling parameters, various machining time values are obtained.

The influence of the radius of circular ribs on the maximum stresses and deformations of a corner bracket was analyzed. The results were obtained using simulations based on finite element method. Two geometries were studied: with one and two ribs. For each of these two geometries the radius of the ribs was ranged between 3 mm and 9 mm. The parts we...

The paper presents the life time estimation of a low level link of a parallel topology guiding device mechanism, depending on the fillet radius between two different sections of link. For finite element analysis and fatigue calculus, SolidWorks software is used.

The use of inertial force to generate controllable linear motion was investigated using a finite element simulation. A system of two asymmetrical elements with equal mass was modeled. The elements were symmetrically disposed to the revolution axis of the assembly and rotated with high angular velocity. The reduction of the two centrifugal forces on...

The paper presents the results of cavitation erosion tests performed on two types of samples. The materials of the samples are frequently used for manufacturing and repairs of the hydro turbines components submitted to cavitation. The first sample was made by welding of an austenitic stainless steel on austenito-feritic base material. The second sa...

The paper presents the finite element method analysis of a mechanical system with two asymmetrical loads moved on Viviani’s trajectories. Using a CAD model a motion study was performed and the loads and restraints were exported to a static analysis. The variation of Von Mises stresses and maximum deformations were determined at four values of angul...

In order to increase the lifetime of runner blades of Kaplan turbines damaged by cavitation erosion, an anticavitation lip is attached to the periphery of the runner blades on the suction side. The anticavitation lip overtakes the cavitation pitting which appears between the runner blades and the runner chamber. A blade with the original anticavita...

Residual stresses occur in materials as a result of mechanical processes: welding, machining, grinding etc. If residual stresses reach high values they can accelerate the occurrence of cracks and erosion of material. An experimental research was made in order to study the occurrence of residual stresses in the repaired areas of hydraulic turbine co...

The paper presents the fatigue analysis of an outer bearing bush of a Kaplan turbine. This outer bush, together with an inner one, bear the pin lever -trunion -blade subassembly of the runner blade operating mechanism. For modeling and simulation, SolidWorks software is used.

The cavitation phenomenon is an important cause of wear and damages of Kaplan and bulb turbines. In order to decrease the erosion rate on the blade and on the runner chamber an anticavitation lip is disposed on the suction side of the runner blade. The goal of the research presented in this paper was to observe the flow behavior for two types of ge...

The repairs of the components of Kaplan turbines affected by cavitation erosion are made usually by weld overlays. The investigations made in-situ on Kaplan turbine runner blades with intense cavitation erosion led to the conclusion that on thick welded layers may appears micro cracks. A laboratory research was made on three samples with 7mm, 10mm...

The paper presents the analysis of an outer bearing bush from a Kaplan turbine, using the finite element method. The 3D model of the runner blade operating mechanism is shown. The motion is transmitted from the fork head through connecting rod to the pin lever - trunion - blade subassembly. This subassembly is borne on the hub by an outer bush and...

Hydraulic turbine components are exposed to cavitation. This phenomenon consists in formation and collapse of vapor bubbles in a fluid due to decreasing of pressure under the equilibrium vapor pressure of water. The repair work of the cavitation affected areas is done by welding. Through the high heat input, this procedure generates important resid...

The cavitation erosion is a major problem for the hydro turbine components. This phenomenon which consists in the progressive loss of material from a solid surface, affects the runner blades and adjacent areas of the runner. Usually, the repairs of affected areas are made by welding. In order to increase the cavitation resistance, new repair method...

This paper presents a comparative study between different values of stress concentration factor K(t) and fatigue notch factor K(f) determined with different methods for a specific part. The study was specially created for the pin lever of runner blade mechanism from a Kaplan turbine. The paper tries to offer the most adequate choice, from the autho...

Fatigue, erosion and cavitation are the main factors which may lead to failure and therefore to expensive maintenance activities in hydropower plants. Cavitation erosion is influenced by various factors, including flow velocity, material and size of the component, corrosion, roughness, temperature, thermodynamic effects, fluid properties and gas co...

This paper shows the activities in Robotics domain at „Eftimie Murgu” University of Reşiţa. Below are presented: the Devices Design and Industrial Robots laboratory, the situation of disciplines connected to Robotics, the situation of doctoral studies regarding Robotics, published or on-going publishing Robotics books, participation in national and...

This paper presents 2 chained mechanical transmissions used to obtain the base translation of an industrial robot: worm - worm gear transmission and motion screw - nut transmission. The kinematical scheme, the 3D models of the main parts of the mechanical transmissions and the 3D assembly model are shown. For modeling, CATIA V5 software was used.

The paper presents the steps for computer aided manufacturing (CAM) of the bushings technological class parts: design of the part in CAD software; generation of the parametrical link with MSExcel files; extract of the useful information for the technological design (mass, volume, surfaces); setting of the CNC lathe and necessary tools; programming...

The paper presents the state of the art and the tendencies in the improvement of the propulsion system for mobile robots. The first chapter represents an introduction in the field of propulsion systems. In the next chapters are presented the main types of propulsion used in present in the field of mobile robotics. The area of application for each t...

This paper presents robotic grippers with translational drivers and kinematical schemes of their mechanisms. Depending on the gripper’s mechanism, the fingers can execute translational or rotational motions. The values of the prehension force are experimentally determined for variations f driver’s input parameters (voltage).

The paper presents the geometrical optimization of a triple branch pipe submitted to an internal pressure. The goal of the optimization was to determine the optimum thickness of piping and branch pipe ribs, in the condition of reaching admissible values of the stress and displacement. The resistance calculus was realized with Cosmos DesignStar soft...

This article presents the components of a mobile robot assembled by modules; its mechanical, driving and command systems are described. The dedicated software and the programming manner of different tasks for the mobile robot are shown.

The paper presents a methodology used for rapid design of part solid models for the same technological family. The parameterized models, which allow the rapid generation of a new part with other dimensions / shape, can be obtained by associating variable parameters to the dimensions or to the shape of a part. The methodology was exemplified on a fl...

The 3D models of links of a planar parallel topology robotic guiding device and the joints definitions are presented. Some particular positions-orientations of the mobile platform are shown, obtained by simulating the links relative displacements imposed by the actuated joints.

Mobile robotics field has in present a very important rate of growth caused by the stretch of the applications area. The development of the classical utilisations (laboratory research and internal transport) and the raising of the new ones (exploration, surveillance, services, military, agriculture, etc.) are associated with the development of the...