Cristian Dragomirescu

Theoretical and Experimental Studies on Magnetic Dampers

The proper operation and reliability of numerous mechanical devices depends on reducing the vibrations of embedded components, modeled as multi-degree-of-freedom dynamic systems. This can be achieved by using various types of absorbers. The design of such devices is conditioned by the understanding of the variation of their technical characteristics with the constructive parameters. The present paper is dedicated to a special type of dampers, based on the use of the magnetic phenomenon. However, available empirical formulae for the damping characteristics of magnetic dampers do not reveal all significant dependencies. Based on theoretical, numerical and experimental methods, the paper brings a number of contributions concerning the relation between the damping characteristics of a magnetic damper and some of its constructive parameters. Variation curves, useful both for theoretical studies and practical applications, are plotted.

Statistical Decision in the Automotive Material Selection

The objective of the research is the ranking of materials applied to an example from the automotive industry (material candidates for the car body construction). The variety of properties/attributes imposes different evaluations for metrical/ordinal scales and the necessary statistical calculus. Heterogeneity of the characteristics imposes the separation of the attributes into mainly two classes: functional (mechanical, physical, etc.), usually with metric scale and technological and environmental attributes with frequently ordinal scale. The paper analyses a practical case of the material for the car body by Multi Criteria Decision Making (MCDM) procedures like primary ranking and preference index value applied to mechanical, technological and ecological, respective all attributes, etc. All these methods are principally based on the variance, viewed as a risk measure. The final comparison has as result the most valuable materials: Titanium sheet, Glass Reinforced Plastics (GRP) and Carbon Fiber Composite.

Maintenance Costs Statistics for Urban Cars

A central economic problem in urban traffic is the total maintenance costs (failure replacements and preventive maintenance), as an important competitive level. The paper has, as main result, the statistical analysis of the dependence between the traffic urban mileage and the total maintenance costs. The variation of these costs depending on the mileage is very useful for any driver and employer. The paper uses some statistical models to evaluate the possibilities of calculation the maintenance costs and their schedule. The most important working tool in the paper is the regression analysis between the independent variable—mileage (measured in kilometres) and the dependent one—the maintenance costs. This statistical function can be used for prognoses and schedules of maintenance costs. Another statistical practical applications used for data modelling are power fit, modified geometric, cubic and normal models; the traffic urban mileage, and respective maintenance costs, has an univariate normal distribution (Shapiro-Wilk test); the derived distribution of both variables is a bivariate Gaussian model (Henze-Zirkler and Royston tests). Numerical adequate examples for automotive operation were processed with specialised software.

Considerations on the Vibratory Motion of a Medical Electro-Mechanical Equipment

The paper analyses the vibratory motion of an electro-mechanical device acting medical equipment. The study is performed using the specific methods of the theory of the dynamical systems. The induced vibrations, their variation in time and the evolution of the system, as a function of the initial conditions, are studied, examining the fulfilment of the constraints prescribed in the literature, in order to achieve a maximal therapeutic benefit. The suitable operation ranges are determined, which establishes the starting point for the design of such equipment.

Study on Vibration Transmission, with Application to the Calibration of a Measuring Stand

The paper studies the vibration transmitted by a mechanical oscillating system to its fixing device. The influence of the mechanical characteristics (inertial and elastic) of the device on the quantities specific to the free and forced vibration, respectively, of the principal system, is analyzed. In this context, the paper investigates the possibility to correlate the vibrations of the mechanical system with the vibrations measured on the fixing device. The conclusions of this study can be used for the calibration of vibration measuring stands. In the final part, a practical application is presented, aimed to compare the theoretical results with the data obtained by measurements performed on an experimental model built by the authors.

Virtual Laboratory for the Study of Kinematics in Engineering Faculties

In engineering faculties, kinematics is studied during the first year, providing the basic knowledge for understanding, modeling and designing mechanical systems. The traditional method of teaching kinematics uses drawings and mathematical equations to explain the motion of rigid bodies forming various mechanisms. From the experience of the authors, due to this approach students often tend to regard kinematics as excessively abstract, having difficulties in understanding the correspondence between analytical expressions and the actual behavior of mechanisms. The Virtual Laboratory of Kinematics VLK is an attempt to overcome these difficulties, by allowing students to visualize and interactively modify the motion and configuration of basic mechanisms. VLK consists of a package of software applications, which has been tested in theoretical mechanics classes during the past few years. The favorable feedback received from students encouraged the authors to improve it gradually and, recently, to completely re-write it, also implementing additional features and capabilities.