Contents of Journal of Mechanical Engineering 59, 3 (2008)

TONDL, A.: Analysis of a self-excited two-mass system                   115

CHRIAŠTEĽ, L., DZIANIK, F.: Hydrodynamic peculiarities of 
two-phase vertical bubbly flow                                          125

ABID, S., TAKTAK, M., DAMMAK, F., HADDAR, M.: An accurate 
two-node finite element for the pre-twisted beam modelling              135

ČERVEŇAN, A., PALČÁK, F.: Parametric modelling methodology 
of open mechanism for a robot                                           151

JÁNOŠÍK, Ľ., BUKOVECZKY, J., GULAN, L.: Use of load 
spectra in the design process of simplified tests of 
building machines                                                       163

A simple two-mass self-excited system is analysed. The stability of the equilibrium position is investigated. Two alternatives of self-excitation are considered – to the upper mass or to the lower mass. The passive and active suppression means are investigated. The active suppression means are due to the periodic variation of the upper mass.

In the paper, the two-phase vertical bubbly flow in loop bioreactor of airlift type is analysed. Authors start with drift flux model, which adequately characterizes such flow case. Using this theory and a new knowledge about two-phase friction factor, relationships enabling to calculate liquid flux and slip velocity in the reactor riser section are presented. Very good concordance with results of the authors' own measurements and with relationships of another authors was shown also for gas hold-up measurement results. Unknown parameters for this reactor type, which need further detailed research, are summarized.

In this paper the development of a two-node finite element for the pre-twisted beam modelling is presented. This element is based on the mixed-hybrid formulation where the equilibrium equations are satisfied. Shear effects are taken into account in this formulation. Only one element can simulate the total behaviour of the beam. This element gives a good solution at nodes for the twisted beam problem. To illustrate the good performance of the proposed element, a comparison with other results from the literature is presented. Distribution of resulting forces along the beam is also presented for different load cases.

Parametrization of virtual model is an effective way how to shorten its development time. In this paper the robotic arm is represented by open kinematic chain consisting of rigid or flexible parts connected by geometrical joints. The parametric modelling methodology for the robotic arm is based on local reference frame variable coordinates considering initial and desired configuration of a model. A set of rules was prepared for development of initial model. Required configuration of the robotic arm is achieved by the change of variable parameters representing the local reference frame coordinates of each part. The calculation of suitable values for coordinates is realized using special algorithm for inverse transformation.

In the system of product quality assessments, reliability assessment belongs to the most important ones. One of the reliability criteria is a structural life. In dynamically loaded structures, where it comes to accumulation of fatigue damage, life tests are an important part of new products development. It is inevitable to set effective procedures to verify the machine design even from the point of view of life evaluation. For this purpose, it is necessary to know a load of every structural element in its every kinematic point, as well as service loads in particular working regimes of a building machine. One of the ways to better characterize service load is the determination of load spectrum, i.e. to express cumulative counts and sizes of particular loads.

For speeding up a product development stages, it is necessary to develop methods of their testing in such a way that they shorten the time of product‘s testing as much as possible.