Browsing by Author "Tesfaye, Tamrat (phD)"

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    Analysis of Stresses in Helical Gears by Finite Element Method
    (Addis Ababa University, 2007-10) Alemu, Negash; Tesfaye, Tamrat (phD)
    Gears are one of the most critical components in mechanical power transmission systems. The bending and surface strength of the gear tooth are considered to be one of the main contributors for the failure of the gear in a gear set. Thus, analysis of stresses has become popular as an area of research on gears to minimize or to reduce the failures and for optimal design of gears. This thesis investigates the characteristics of an involute helical gear system mainly focused on bending and contact stresses using analytical and finite element analysis. To estimate the bending stress, three-dimensional solid models for different number of teeth are generated by Pro/Engineer that is a powerful and modern solid modeling software and the numerical solution is done by ANSYS, which is a finite element analysis package. The analytical investigation is based on Lewis stress formula. This thesis also considers the study of contact stresses induced between two gears. Present method of calculating gear contact stress uses Hertz's equation. To determine the contact stresses between two mating gears the analysis is carried out on the equivalent contacting cylinders. The results obtained from ANSYS are presented and compared with theoretical values. Face width and helix angle are important geometrical parameters in determining the state of stresses during the design of gears. Thus, in this work a parametric study is conducted by varying the face width and helix angle to study their effect on the bending stress of helical gear.
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    Computer-Aided Aerodynamic and Structural Design of Horizontal-Axis Wind Turbine Blades
    (Addis Ababa University, 2009-06) Biadgo, Mulugeta; Tesfaye, Tamrat (phD)
    Designing horizontal-axis wind turbine (HAWT) blades to achieve satisfactory levels of performance starts with knowledge of the aerodynamic forces acting on the blades. In this thesis, HAWT blade design is studied first from the aspect of aerodynamic view and the basic principles of the aerodynamic behaviors of HAWTs are investigated. Second the structural design which consists of the blade material selection and the determination of structural cross section, FEM analysis is done using CATIA V5 R16 and ANSYS 10. The displacement, Von Mises and Principal stress results are determined, these results shows that ANSYS is more conservative than CATIA in FEM analysis. Blade-element momentum theory (BEM) known as also strip theory, which is the current mainstay of aerodynamic design and analysis of HAWT blades, is used for HAWT blade design in this thesis. BLADE DESIGN PROGRAM which is a user-interface computer program for HAWT blade design is written. It gives blade geometry parameters (chord-length and twist distributions) and design conditions (design tip-speed ratio, design power coefficient and rotor diameter) for the following inputs; power required from a turbine, number of blades, design wind velocity and blade profile type (airfoil type). The program can be used by anyone who may not be intimately concerned with the concepts of blade design procedure and the results taken from the program can be used for further studies.
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    Design and Analysis of a Mechanical Damper in a Milling Cutter
    (Addis Ababa University, 2009-07) Girma, Tassew; Tesfaye, Tamrat (phD)
    When an endmill is used in high-speed machining, chatter vibration of the tool can cause undesirable results. This vibration increases tool wear and leaves chatter marks on the cutting surface. To reduce the chatter vibration, a layered-beam damper is inserted into the hole at the center of the tool. The friction work done by the relative motion between the tool and damper reduces chatter vibration. The purpose of this research is to design and optimize the configuration of the damper to obtain the maximum damping effect. The analytical method has been reviewed, which is based on the assumption of constant contact pressure and uniform deflection. For the numerical approach, nonlinear finite element analysis is employed to calculate the distribution of the contact pressure under the centrifugal and cutting forces. The analytical and numerical results are compared and discussed. In order to identify the effect of the damper’s configuration, two design variables are chosen: the inner radius of the damper and the number of slotted dampers. During the parameter study and optimization, the inner radius is varied from 1.5mm to 3.5mm and the number of slotted dampers is varied from 2 to 10. Results show that the damping effect is maximum when the inner radius is 1.5mm and the number of slotted dampers is 5. However, this result depends on the operating condition. Thus, it is suggested to prepare a set of dampers and to apply the appropriate one for the optimum damping effect for a given operating condition.
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    Finite Element Stress Analysis of Artificial Femur Head on Hip Joint Prosthesis
    (Addis Ababa University, 2013-01) Bantirga, Yigezu; Tesfaye, Tamrat (phD)
    Finite Element Stress Analysis on Artificial Femur Head of Hip Joint Prosthesis Yigezu Bantirga Addis Ababa Institute of Technology, 2013 Cobalt-chromium-molybdenum ball heads for total hip replacement are highly loaded in finite element load step on the basis of ISO 5832-12 rupture test to meet the stress requirements concerning strength and safety. High stresses inside the ball head originate from the press fit between the conical stem neck (made of titanium alloy) and the borehole of the ball. The aim of this study was the development of an optimized contour at the fillet inside the ball head and the borehole contact length depth by means of numerical methods, in order to reduce local stress concentrations. The finite element optimization method was applied on the customary engineering fillet radius to reduce local stress peaks. Since the static load from experimental realistic hip implant test is governed by the maximum von-mises, shear, normal and principal stresses in the contact area of the taper. The optimal reduced local stress of the examined ball head design was obtained in the medium neck contact length at o.8mm fillet for the relevant realistic load case of stumbling. Keywords: Femoral head, Stem neck, Optimum geometry, Stress concentration, FEA
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    “Modeling Static and Dynamic of Milling Machine End Mill and Tool Holder”
    (Addis Ababa University, 2010-03) Eshete, Shimels; Tesfaye, Tamrat (phD)
    An end mill is a tool used on a milling machine. A milling machine is a machine found in a metal working shop that is used to remove material from a metal block to make it into a finished part. And it is one of the tools used on the milling machine to make a particular type of cut. Productivity and surface quality in milling processes have direct effects on cost, production lead time and quality of machined parts. Chatter vibrations in milling, which develop due to dynamic interactions between the cutting tool and the workpiece, result in reduced productivity and part quality. Machine tool chatter causes machining instability, surface roughness, and tool wear in metal cutting processes. The excessive vibrations of the cutter and workpiece result in poor surface finish, dimensional accuracy and may damage the workpiece and machine tool. In addition to this cutting force produce deformations of the tool and this cause dimensional and form errors on the workpiece. In this research, generalized equations are presented which can be used for predicting static and dynamic properties of the cutting tool. Milling forces are modeled for given cutter geometry, cutting conditions and work material. The static and dynamic characteristics of tool and tool holder can be obtained by using finite element analysis (FEA); simplified equations are created to predict deflections of tools for given geometric parameters and material properties and also for natural frequencies and mode shapes of cutter dynamics. Finite Element Analysis (FEA) results are compared with the results obtained from analytic equations, Comparison of natural frequencies between carbide and HSS end mills. And a simple method to analytically calculate the approximate optimum depths of cut and determine the corresponding spindle speeds by creating the stability lobe diagram. Chatter vibration free spindle speeds and axial depth of cuts can be selected from stability chats. The characteristics and limitations of the stability lobe diagram are discussed. Case studies are also provided to illustrate the method and verify the results.
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    Technical Assessment of Hydraulic Systems Used in Mi-24 And Mi-35 Helicopter
    (Addis Ababa University, 2008-03) Kebede, Endalkachew; Tesfaye, Tamrat (phD)
    Helicopter is known as a rotating wing aircraft that consists of different subsystems in it. These subsystems are very closely interlinked to each other that make the helicopters a typical example of mechatronic system. One of these subsystems is a hydraulic subsystem, which is usually used for actuating landing gears, flight control boosters, weapon system and other purposes. Thus, hydraulic system can be the main core of helicopter movement that needs a special consideration to assess. Currently, almost all Mi-24 helicopters in Ethiopian airforce are grounded or inoperative because of hydraulic related problems. The main problems include lack of reliable safety system for landing gear emergency extension, guidance device operation and directional control damper pressure loss. There are also other problems that led these helicopters to crash as shown from some safety records. Because of such problems, these helicopters couldn’t function their expected mission. Whereas Mi-35 helicopters are in a better performance of flight evenif some improvements and modifications are still required. Moreover this, difficulty of life-saving by parachute from the helicopter in flight lowers the confidence of pilots and flight technicians. This thesis work is mainly focused on how this problem or loss can be avoided by identifying the problems and proposing the possible solutions and remedies to be taken. Based on the two specified helicopters, a case study has been implemented from the available documents and past experience. And finally, different solutions are proposed and selected to upgrade the existing hydraulic system of both Mi-24 and Mi-35 helicopters.
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    Weld Design of Heavy Duty Vehicle Bodies
    (Addis Ababa University, 2007-11) Dinbassa, Samson; Tesfaye, Tamrat (phD)
    The welding process has been used extensively in the fabrication of many structures including ships, airplanes, buildings, bridges, vehicle bodies, pressure vessels, etc. It provides many advantages over other joining techniques. To mention only a few, it provides better mechanical properties, air and water tightness and good joining efficiency. However, there are various problems such as residual stresses and shape distortion associated with the construction of welded structures. When a material is being welded, it experiences local heat due to the welding heat source. The temperature field inside the weldment is not uniform and changes as the welding progresses. The welding heat cycle gives rise to a complex strain field in the weld metal and in the base metal regions near the weld. These strains, along with the plastic upsetting, create the residual stresses that remain after the welding is completed. In addition, shrinkage and distortion are also produced. Residual stresses and distortion are highly undesirable in welding technology. Thermal stresses during welding often cause cracking. Some of the above weld related problems can be solved by adhering to a preset weld design practice and employing appropriate weld process procedure in the welding tasks. In our country, Ethiopia, there are limited numbers of heavy duty vehicles body builders. Those body builders extensively use welding. In those local body builders there is lack of proper weld design practice in the fabrication of heavy duty vehicle bodies. Moreover, there is no well developed/structured welding process procedure they have adapted except in the cases of internationally undertaken projects. This fact enhances the weld related problem mentioned above on the vehicle bodies fabricated locally. In this paper case study is made on four local representative vehicle body builders focusing on their weld design and WELD DESIGN OF HEAVY DUTY VEHICLE BODIES 11 processes. From the case studies the general problems related to the weld design and process of the local heavy duty vehicle body builders is identified. Eventually, general recommendation is made on weld design and process of heavy duty vehicle bodies that the Ethiopian heavy duty vehicle body builders are to adapt or practice.