MODARES TECHNICAL AND ENGINEERING
Year:2007 | Volume:- | Issue:28 (SPECIAL ISSUE ON MECHANICAL ENGINEERING)|Papers Count:11

Thermal fatigue (heat checking), mechanical fatigue, wear and plastic deformation of critical areas of hot forging dies at working temperatures are the main mechanisms that reduce their lifetime. During forging processes the surfaces of the dies reach temperatures of 700-800°C.Therefore, hardfacing of these areas with nonferrous elevated temperature hardfacing alloys such as Stellite 6 can improve the performance and lifetime of the dies, many times. Hot hardness, galling resistance, hot corrosion and oxidation resistance, adhesive wear resistance, low friction coefficient and absence of allotropic transformation up to 1100°C are the most important properties of Stellite 6. H11 tool steel is widely used as hot forging die material. This steel because of its high alloy contents and, therefore, its hardenability is very sensitive to high cooling rates involved during welding cycles and hydrogen induced cracking (HIC). For this reason, in this research hardfacing parameters of Hl1 tool steel with Stellite 6 in TIG welding method have been investigated. According to the results, hardfacing of this steel in annealed hardfaced condition isn't feasible and it is recommended that preheating and intermediate temperatures during the hardfacing cycle between 310-370°C. The suitable current for TIG hardfacing of this steel by jp3.2 mm filler rod was determined to be 80-85 amperes for the first layer and 90-100 amperes for upper layers. The minimum thickness for obtaining maximum hardness in the hard facing layer (41-42 HRC) under these conditions was determined to be 3mm. It is recommended that the effective heat input for hardfacing of this steel under three-body heat transfer conditions would be less than 455kJ/m. It is also recommended that the Dt8®5 of H11 tool steel hardfacing cycle would be in the range 6 to 15.3s. Finally it is recommended that H11 hardfaced tool steel would be stress relived in the range 425 -500°C for 1 hour per 30mm base metal thickness.

In this paper, design and implementation of a linear DC motor (LDM) is described. The LDM under consideration is a flat type moving magnet. It has a simple structure and so it is easy to control. In this motor, trust force is generated by interaction between magnetic field of permanent magnet and conductors carrying controlled current. To maximize the efficiency of the motor, NdFeB (rare earth) permanent magnet is used. The optimum dimensions and geometrical parameters of the motor, such as permanent magnet and coil size are obtained by mathematical calculations. Finite element analysis is also used for precise calculations and modeling of the motor. The results show good agreement between experimental and theoretical (FEA) results.

The analysis of two-dimensional potential flow involving a free surface h~s attracted the attention of many researchers for a very long time. Flow characteristics of interest may be pressure distribution and/or velocity distribution through the entire flow domain and also the discharge coefficient, contraction coefficient and the free surface location. The computational difficulties in the calculation are due to the non-linear boundary conditions.This paper presents a new approach for the solution of variable domain problems and provides, for potential flow problems involving a free surface, a numerical procedure for calculating the free surface coordinates using the element free Galerkin method. The method uses the element independence characteristics of the element free Galerkin method and the nodal grid and integration cells remain fixed in successive iterations.The proposed method is well suited for the analysis of the problems with boundaries of arbitrary shape and can be used for the analysis of free surface flow through nozzles and different types of gates. Results obtained from the present method show good agreement with previous solutions for the water surface profile and pressure distribution both through the flow domain and on the boundaries.Predicted results are also confirmed by conducting a hydraulic model test for the bottom outlet of the Karun-III dam.

The use of FEM (Finite Element Method) in the design process and the stiffness evaluation of machine tool structures is very popular. But, there is always some uncertainties about the FEM results because of 1) impossibility of defining the existing boundary conditions accurately; 2) impossibility of defining the exact force system acting on the model; 3) geometric simplifications of the model. Therefore, physical modeling of the designed structure by using a proper material is quite reasonable. In this research, the designed structure of a Vertical Turning Machine has been modeled physically to study the FEM results obtained by computer. The physical model was made of Polyethylene with the scale of 1:4 in comparison with the actual cast iron model. In order to assess the stiffness of the physical model, a scrwejack was used for loading and the deflection values were recorded by using a few dial gauges. Agreement between the experimental and FEM results has been investigated.

A one-dimensional model is used in this paper to study the changes of air flow temperature along a wet underground channel. Heat and mass transfer in an underground wet channel whose surface is coated with thin water film, is studied. Furthermore, the model is extended for dry channels by introducing the surface wet ability factor. This model estimates temperature and humidity ratio variations along the channel and the cooling load capacity. Parametric study is carried out to investigate the effects of hydraulic diameter and flow rate on cooling performance of the channel.

A compatible computational fluid dynamics procedure for the calculation of incompressible three-dimensional time-dependent flow with complicated free surface deformation is presented. A computer software is developed and validated using a variety of academic test cases. Two fluids are modeled as a single continuum with a fluid property jump at the interface by solving a scalar transport equation for volume fraction. In conjunction, the conservation equations for mass and momentum are solved using fractional step method. Here, a Finite Volume discretisation and collocated arrangement are used. The developed code results in accurate simulation of interfacial flows, e.g. Rayleigh-Taylor instability and two and three dimensional dam breaking problems. All results are in good concordance with experimental data especially when there are two phases with high density ratio.

Biodiesel can be used easily as an alternative fuel in diesel engines and is environmentally friendly. With increased global fossil fuel price, biodiesel is economically competitive, because, triglycerides and its derivatives are obtained from plant oil (edible, nonedible) or fats. In this research work, the possible application of biodiesel obtained from sunflower oil in a low speed diesel engine (M8/l Lister) was investigated. Performance parameters and exhaust engine pollutants in diesel and biodiesel compound on a volume basis (B00, B10, B20, B30, B40, B100) was studied.The results of the engine short time tests on the basis of ECE-R49 standard showed that by increasing biodiesel percentage up to 20 volume, same properties such as pressure peak in combustion chamber, exhaust engine temperature, ignition delay, engine speed, brake and indicatory efficiency are increased in order to improve engine efficiency. Also, due to combustion quality improvement, CO and UHC emissions are decreased, but NOx increased through higher combustion temperature and consequently higher pressure. Brake specific fuel consumption is minimum in compound, which is economically very important.

Backlash between meshing gears in gear rotating systems leads to vibro-impact phenomena which mostly causes excessive vibration and noise. The gear rattle is a noise which is produced by the vibroi-mpact in gearboxes, and which might be audible in specific driving conditions in the interior of the vehicle. In this paper a non-linear mathematical model of a simple gearbox system is developed to understand and control the problem. The analytical and numerical activities were performed for minimizing gear rattle. It is shown that analytical prediction agrees well with the numerical results.

High tensile strength, good impact strength and toughness are the main mechanical properties of low alloy high strength steel. One of the most important parameters affecting these properties are inclusions and their characteristic. So, in this research the simultaneous effects of N and Al on the mechanical properties of medium carbon low alloy ultra high strength AISI9335 steel (DIN35NiCrMoV) were studied. Cast samples with different Al and N were forged and heat treated to achieve adequate strength. Hardness, tensile and Charpy impact tests as well as light and SEM metallography were done on the samples to characterize the common inclusions of the steel.The results show that tensile strength and hardness of steels are increased; impact strength is decreased with increasing the Al/N ratio and increasing the N content up to 165 ppm, while tensile strength and hardness are decreased, and impact strength is increased with increasing the same ratio and decreasing the N content of steels down to 60 ppm.

The cam and follower mechanism is one of the usual mechanisms in industrial machines which is used for creating versatile and complex motions. The pressure angle and Hertzian stress are two factors that have undesirable effects on the dynamic behavior of cams. In general, for a specified follower displacement curve, increase in cam dimensions results in decrease of pressure angle and hertzian stress. Since the spaces for cam operation in motors and machines are limited and confined, a method for finding optimum cam is needed. In this article, a method for optimization of pressure angle at disk cams with reciprocating knife-edge and roller followers and also similar method for optimization of hertzian stress at disk cams with reciprocating flat-face followers which operate at bounded geometric space (geometric constraints), are introduced. Thus, in addition to precise description of objective functions, complex constraints are simplified to suitable mathematical relations by cutting the geometric limitations and finally the optimization algorithm is fully described, too.

In this paper, a mathematical model has been used for the simulation of mechanical steady continuous bending process of thin plate by four-roll machine. The differential equations governing the large deflection bending of elastic-perfectly plastic thin plate are derived by modeling the loading and unloading deformation of the operational process. By using the system equations, the physical quantities of (i) the final deformation and bend plate stress, (ii) the shift of contact-angle between bend plate and rolls, (iii) the applied forces on rolls, (iv) the position of the operative side roll, associated with bending of a plate under a desired curvature, are predicted for single-pass bending operation. The analytical results are compared with experimental results and good agreement was observed.