Contents of the Journal of Mechanical Engineering 51, 4 (2000)

L. NEMEC: A variant of the gradient-based optimization method with 
application to technical problems                                         197

R. KASTOVSKY: Machining centres based on hybrid kinematic 
structure (in Czech)                                                      203

J. REMENEC, L. STAREK: Numerical and experimental verification 
of updating methods for the vibrating mechanical systems (in Slovak)      213

R. DINDORF: Structural and energetic efficiency of electro-hydraulic 
positioning system                                                        235

J. BENO, M. MANTIC: Chip cross-sections for hard machining                249

A variant of the gradient-based optimization method is presented in this paper. The gradient of a multivariable function is found numerically by sensitivity analysis of variables. This way is convenient both for algebraic and transcendental functions and constrained and unconstrained tasks. The method is plumb applicable to various application problems because the transformation of the objective function to special mathematical form is not needed, even the explicit form of an objective function needs not be known.

Leading producers of machine tools and also many technical universities have still been developing various types of positioning devices with a parallel kinematic structure applied to machine tools. There are designed new kinematic mechanisms in this paper, which eliminate main shortcomings of realized devices. These mechanisms, called mechanisms with a "hybrid" kinematic structure, are modifications of mechanisms with hexapodal structure where spherical kinematic pairs are replaced by lower kinematic pairs, i.e. revolute and prismatic. Therefore, it is necessary to change the arrangement of individual construction parts of the machine tool and to complete additional degrees of freedom which must be performed by the rotary table together with workpiece.

The most widespread approach for numerical modeling in engineering design is the finite element method. For various reasons the experimental results and numerical predictions often conspire to disagree. There are a lot of methods for updating analytical models that were developed in the recent years. But none of these methods is satisfactory enough for all cases. The main purpose of this article is verification of methods for updating a numerical model of vibrating mechanical systems.

The paper deals with structural and energetic efficiency of throttle systems with electro-hydraulic control. The structural and energetic efficiency of the research stand for positioning of the cylinder with servovalve and proportional valve has been considered. Two basic control systems, most frequently used in hydrostatic drive systems, that is open-centre throttle control and closed-centre throttle control have been used in analysis. The formulas for hydraulic, mechanical, structural, and total volumetric efficiency have been derived. The diagrams of the positioning system efficiency with regard to the effect of parameters and the control structure have been determined. The results have been analysed in order to determine the structure and parameters ensuring minimization of power losses in the system.

The contribution deals with formulae for chip cross-section applied to the hard machining technology by which hardened steel can be cut. Simulation procedures are used and graphical interpretation of the chip cross-sections modelled is presented. In order to distinguish between various tool types, a simulation approach was applied for the zero nose radius as well as for defined tool nose radii. A comparison of the simulation results for chip cross-sections using three ways of its computation (classical approach known in metal cutting technology, approximate formulae after Degner and analytical approach after Usui et al.) is given.