Scientia Iranica
Volume:18 Issue: 5, 2011

The main purpose of this paper is to propose a frictional damper so as to enhance structural damping leading to the chatter suppression in the slender endmill tool. The proposed damper is composed of a core press fitted inside a multi-fingered hollow cylinder. Core and cylinder both are press fitted into an axial hole inside the tool. First, an analytical model is developed for the damper. Then, a parameter study is conducted to find optimum parameters of the damper for the increase of the stability limit as much as possible. In order to validate the analytical model, a nonlinear finite element model of the damped milling tool is simulated using ANSYS software package. Having finite element model, the stability lobe diagrams for simple tool and optimally damped tool are extracted from frequency response functions. Modal test is applied to a real specimen of damped tool in order to validate finite element results. Good agreement between experiments and modeling is obtained.

This paper is an attempt to develop a design methodology for a special deployable structure for potential use in micro-satellites. The basic form of this structure is a hexagonal prismatic tensegrity structure, which, after being rigidified, is used as the supporting structure of a mesh-like antenna. Here, the objectives of presenting the design methodology are to prevent structural elements from failure, while maintaining the structural natural frequency and mesh tension above an intended value and in addition, to minimize the overall mass. Here, the suggested design strategy combines the need for a behavioral study (i.e. fast and wide range evaluation) at the beginning of the design, with the goal of an exact optimum design in the final stages. It is shown that the final design chart possesses a multi-linear behavior, which could highlight the major effects of internal forces and construction constraints on the design.

Analytical solutions for the vibration of beams with variable cross-sections are, in general, complex and, in many cases, impossible. On the other hand, approximate methods, such as the weighted residual, Rayleigh–Ritz and finite difference methods, also have their own shortcomings, such as a limited number of natural frequencies and low accuracy. In this paper, using the wave propagation method, the beam is partitioned into several continuous segments, each with a uniform cross-section, for which there exists an exact analytical solution. Waves entering a segment in positive and negative directions are calculated from waves that entered the initial segment. Then, by satisfying the boundary conditions, the characteristic equation is obtained and all natural frequencies are calculated. Also, using the sum of waves at each point that are moving in positive and negative directions, the mode shapes are obtained. To verify this modified method, frequencies whose mode shapes are in a polynomial cross-sectioned beam having an exact analytical solution are compared and thereby proven to be highly accurate. Therefore, this method can also be used to calculate natural frequencies and their mode shapes in beams with variable cross-sections without any analytical solution.

In this paper, rapid and globally convergent predictive tool for dynamically loaded journal bearing design is developed. For accomplishment of such an aim, a neural network model of crankshaft and connecting rod bearings in an internal combustion engine is developed as an alternative for the complicated and time-consuming models. Six most important parameters are selected as inputs of neural network. These parameters are: oil viscosity, engine speed, bearing radial clearance, bearing diameter, slenderness ratio and maximum force applied on bearings. Also, some significant parameters are calculated as neural network outputs. These parameters include: all components of friction loss, all components of oil consumption, minimum oil film thickness, eccentricity, oil temperature rise and displacement relative to shell. In addition, an optimum analysis is performed. To achieve such a target, multi-objective optimization methodology is a good approach inasmuch as several types of objective are minimized or maximized simultaneously. The optimization goal is to minimize friction loss and lubricant flow as the two objectives and develop a Pareto optimal front.

The aim of this paper is developing a new hierarchical model to evaluate and rank the sawability (power consumption) of carbonate rock with the use of effective and major criteria, and simultaneously taking subjective judgments of decision makers into consideration. The proposed approach is based on the combination of Fuzzy Analytic Hierarchy Process (FAHP) method with TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) methods. FAHP is used for determining the weights of the criteria by decision makers, and then rankings of carbonate rocks are determined by TOPSIS. The proposed method is applied for Iranian ornamental stone to evaluate the power consumption in rock sawing process. A variety of two groups of carbonate rocks (7 types) were sawn, using a fully-instrumented laboratory cutting rig at different feed rate (200, 300, 400 cm min−1) at constant peripheral speed (1770 rpm) and depth of cut (35 mm). During the sawing trials, the ampere and power consumption were monitored and calculated as performance characteristics of the saws. The results of the sawing trials (power consumption) were used to verify the result of the applied approach for ranking the carbonate rock sawability. It was concluded that the sawability of carbonate rocks can reliably be ranked, using the developed approach.

Operating an Atomic Force Microscopy (AFM) with the cantilever and sample immersed in a liquid has many advantages, including the elimination of capillary forces and reduction of van der Waals forces in the study of liquid–solid interactions. Accurately identifying the maximum of the amplitude–frequency curves at which resonances occur is a challenging issue. The frequency response of a cantilever beam in a viscous liquid near a surface depends on the hydrodynamic loadings. First, in this paper, there is a comparison of predicted resonant frequencies from five different theoretical models, with measurements for the case of an ambient liquid of infinite extent. The precision of each method is indicated. Then, the motion of microcantilevers of variable widths close to a solid surface is simulated. When the cantilever tip approaches the sample surface gradually, the effect of squeezed film damping causes the resonance frequencies to shift toward lower values at lower amplitudes, and subsequently as the tip-sample separation becomes smaller, the resonance peaks seem to vanish completely. The results demonstrate that any changes in the geometrical dimensions of the cantilever and in the fluid properties may influence the accuracy of the model. Furthermore, due to the considerable effect of tip-sample separation on the resonance, some models are restricted to be applicable only in the circumstances of free liquid.

The objective of this research is geometrical optimization of half toroidal Continuously Variable Transmission (CVT) in order to achieve high power transmission efficiency. The dynamic analysis of CVT is implemented and contact between the disk and the roller is modeled viaelastohydrodynamic (EHL) lubrication principles. Computer model is created using geometrical, thermal and kinetic parameters to determine the efficiency of CVT. Results are compared by other models to confirm the model validity. Geometrical parameters are obtained by means of Particle Swarm Optimization (PSO) algorithm, while the optimization objective is to maximize the power transmission efficiency. Optimization was conducted over a wide range of selected input parameters that are EHL oil temperature, roller tilting angle and rotational velocity of input disk. Optimization results show that the power transmission efficiency alters with changes of input parameters, while the optimized geometrical parameters are approximately the same. Variations of power transmission efficiency over a wide range of input parameters were calculated for optimized geometry, while the highest value of power transmission efficiency occurs in low value of EHL oil temperatures and input disk rotational velocities. The optimized structure shows average power transmission efficiency equal to 93.4% over a wide range of input parameters.

There are different ways in order to achieve higher velocity in underwater vehicles. One of these methods is using a body with special form. This paper presents a towing tank based experimental study on drag forces for different Reynolds Numbers of a special underwater model. This paper investigates drag force and drag coefficient in a different flow direction over the model. Obtained experimental results in towing tank are explained. Computational Fluid Dynamic (CFD) simulation also is performed using commercial CFD software package FLUENT 6.3.26. There is a significant decrease in drag coefficient of model moving with small diameter at upstream.

This paper presents an advanced methodology for collision-free trajectory planning of wheeled mobile manipulators in obstructed environments by means of potential functions. In the presented method, all mobile manipulator parts and environmental obstacles are modeled as ellipsoids. Due to collision avoidance, the ellipsoid equations are expressed in a reference coordinate system and the corresponding dimensionless potential functions are defined. Then, the trajectory planning of a spatial mobile robot in cluttered environment is performed, employing optimal control theory. Beyond simplicity and novelty of the proposed method, depletion of prior methods is rectified, which lead to excessive computation and singularity during process. Also, a number of simulations and experiments for Scout mobile manipulator are carried out, which illustrate the power and efficiency of the proposed method.