Issue 3

JTAM, Sofia, vol. 21 Issue 3 (1990)

Design of Hhydraulic machines with multielement devices

JTAM, Sofia, vol. 21 Issue 3 pp. (1990)

Space Locomotion Robot Dynamics. Befrequency Periodical Decision

M. Peikova
High. Inst. of Mech. and Electr. Eng., Sofia

The dynamics of a spatial legged robot model with five degrees of freedom in the walk space is investigated in the paper. The first approximate motion laws for internal resonance are defined, accounting for the initial conditions and following Poincare's theorem for nonlinear oscillations. These conditions are necessary and sufficient and they are relations between initial values of chosen generalized coordinates and velocities. Moreover, they guarantee the existence of periodical approximate solutions of the differential motion equations. The initial conditions and formulas of the motion law for steady regimes are obtained when motion is realized under chosen drive control moments. The obtained expressions for the motion laws guarantee convergent computation when using a computer.

JTAM, Sofia, vol. 21 Issue 3 pp. 013-018 (1990), [Full Article]

On the Optimal Control Synthesis for a Class of Nonlinear Mechanical Systems

P. Kiriazov, P. Marinov
Inst. Mech. Biomech., Sofia

The subject of this work are nonlinear mechanical systems whose number of degrees of freedom is equal to the number of the actuators. A decentralized controllability condition is imposed in the sense that the sign of any control input at the extreme value is the same as the sign of the corresponding acceleration, irrespective of the other bounded control inputs. The performance index is a weighted minimum time-energy loss criterion. In the direct search optimization schemes adopted, we employ appropriate spline approximations of the real optimum control laws via the Pontryagin's maximum principle. A sufficiently large set of feasible solutions can be obtained, among which a satisfactory suboptimal solution can thus be found.

JTAM, Sofia, vol. 21 Issue 3 pp. 019-023 (1990), [Full Article]

Wrist Kinematical Structures and Dynamics of the Orientation Motion of an Electromechanical Manipulation Robot

L. Akulenko, G. Boyadjiev, L. Lilov
Inst. Mech. Biomech., Sofia

The present paper gives an estimation of the positioning deviation after attaining an orientation wanted. Equations of motion of a local structure in terms of controlling stresses of its electric drive are obtained. The use of a spherical Joint, which enables the orientation of a solid body with a single fixed point, is verified. The joint kinematic structure is with rotating couples of 5th class. A number of kinematic structures are analysed and such a structure is found as a result, which gives the best interpretation of the solid body orientation and which is to be chosen as a structure of local motion.

JTAM, Sofia, vol. 21 Issue 3 pp. 024-036 (1990)

Dynamic Resistance Against Brittle Fracture of Construction Steels

K. Minchev
Inst. of Metal. Res. and Techn., Sofia

Construction steels, exploited under various rate-temperature conditions, are to be estimated, regarding the effect of notches and cracks. Such a possibility is given by the characteristics, introduced by the linear fracture mechanics. The determination of parameters, characterizing resistance against brittle fracture, together with the conventional strength characteristics, enables a more complete estimation of construction steels. Construction steels, type SAF (alloyed with nitrogen) are analysed in the paper, and their resistance against brittle fracture under dynamic loading is determined. There types of construction steel, developed in the Institute of Metal Research and Technology of the Bulgarian Academy of Sciences, are studied. The critical stress at the crack tip, as well as the crack critical size, are determined. This enables to compare the steel behaviour under dynamic loading in the – 100°-20°C temperature range.

JTAM, Sofia, vol. 21 Issue 3 pp. 037-042 (1990)

Uniqueness in a Particular Model of Elasticity

P. Dzhondzhorov
Inst. Mech. Biomech., Sofia

A particular model, describing transient motion in the reactor core of PWR, has been developed by the author in a previous study — it is a thermo-visco-elastic model with a temperature-rate dependence. Here the energy balance equation is obtained. On its basis the uniquences of the solution is proved under some acceptable restrictions, imposed on the material constants.

JTAM, Sofia, vol. 21 Issue 3 pp. 043-047 (1990)

Creep Stability of a Sloping Orthotropic Rotary Shell with Random Initial Shape Imperfections

V. Kolomak

The author studies numerically a sloping rotary shell and the effect of its initial axisymmetrical shape imperfection on the critical time. The distribution function of the latter and its probability properties are determined by using a known law of random distribution of the imperfection amplitudes, as well as an established deterministic relation between the initial imperfection and the critical parameter. The intensive axisymmetric buckling is used as an instability criterion.

JTAM, Sofia, vol. 21 Issue 3 pp. 048-050 (1990)

Determining the Parameters of a Boundary Layer at the Walls of an Axisymmetric Cavity in the Case of Filling with a Viscous Liquid

K. Daskalov, S. Slavchev
Inst. Mech. Biomech., Sofia

A study of the laminar filling of a vertical axisymmetric cavity with a heavy incompressible viscous liquid is carried out by the authors, regarding the fluid-gas dynamic problem of filling of a casting machine die. The method of determining the hydrodynamic pattern consists in solving the equation of nonsteady fluid flow with a free surface in the cavity, together with the equations of a non-steady boundary layer which develops at the cavity internal surface. The main result is that the liquid friction at the cavity walls (due to viscosity) does not affect considerably the filling process. Hence, the effect of viscosity could be eliminated when the full liquid-gas dynamics problem is solved.

JTAM, Sofia, vol. 21 Issue 3 pp. 051-058 (1990), [Full Article]

On the Theory of Turbulent Free-Convective Jets

G. Mahviladze, I. Selezneva, V. Filipov

The authors study an axisymmetric jet with a positive floatability which propagates in a non-stratified medium and in a direction, parallel to the vector of weight force.

JTAM, Sofia, vol. 21 Issue 3 pp. 059-065 (1990)

Design of Hydraulic Machines with Multielement Devices

K. Varsamov
High. Inst. of Mech. and Electr. Eng., Sofia

Impellers and guide vanes with multielement blades, as well as their multielement inlet/outlet parts, are applied in some known schemes of hydraulic machines. The principal aim of such devices is to get some control on the pressure distribution and thereby – on the boundary layer separation, cavitation inception etc. An approximate method for solving both the direct and inverse problems for a multielement blade cascade in a stream filament with variable thickness is developed on the basis of G. Yu. Stepanoff's quasi three-dimensional theoretical model. The inverse problem for a multielement splitter design in a given axisymmetrical pipe is studied, too. In general, the method of boundary integral equations is used. Some results of the application of programme packages for the desing of diffusers, fans and pumps are presented, which confirm the practical applicability of multielement devices. As a conclusion, a brief discussion on the necessity of a further development of the model is given, primarily concerned with viscous effects.

JTAM, Sofia, vol. 21 Issue 3 pp. 066-075 (1990)

On the Behaviour of High-Pressure Pipe Lines under Seismic Loading

S. Kislyakov
High. Inst. of Arch. and Civ. Eng., Sofia

The author treats the problem of the earthquake-induced vibration of an open horizontal pipeline with constant cress section characteristics. The liquid issupposed to be in equilibrium. The horizontal seismic input is taken into account and it is introduced in terms of the real accelercgram. The pipe is modelled as a continuous beam on equidistant supports. The interaction between the adjacent spans is taken into account and the solution is obtained in terms of a generalized Duhamel integral, containing several input functions shifted to each other in time. In the course of the study the beam influent theory is used to deal with the spectral values, and some recommendations, concerning the accuracy of the design, are given.

JTAM, Sofia, vol. 21 Issue 3 pp. 076-081 (1990), [Full Article]

Stability of Rotary Elasstic Orihotropic Shells under a Combined Loading – Static External Pressure and Dynamic Impact. Critical Dynamic Loading – Part I

Pl. Pirgov, Y. Ivanova
Inst. Mech. Biomech., Sofia

The authors study the shell dynamic stability under static external pressure that does not exceed the upper critical value. The shell undergoes simultaneously an external impulse which significantly exceeds the static upper critical value. A geometrical method of the non-linear theory of elastic shells is used. The critical impulse, at which the shell loses stability, is determined as a pont of inflecion of the load – deflection diagram. A theorem is formulated which gives the sufficient condition for the determination of the critical impulse. Numerical results for the critical value of the dynamic impulse are obtained.

JTAM, Sofia, vol. 21 Issue 3 pp. 082-087 (1990)

Material Damage during Hot Rolling of Metal Sheets

R. Kazandjiev1, L. Stanev2
1Inst. Mech. Biomech., Sofia
2Inst. of Metal Res. and Techn., Sofia

The authors study material damage during metal hot rolling by using the criterion, developed by V. L. Kolmogorov. The basic relations of hot rolling of a metal sheet are given. The sheet critical deformation, for which cracks occur in the material, is calculated. An example of rolling of a sheet of aluminium alloy is given. Comparison with experimental results of other authors shows a satisfactory agreement.

JTAM, Sofia, vol. 21 Issue 3 pp. 088-093 (1990)

Determination of the Crack Resistance of Materials by Using Wedge Indentors and the Methods of Acoustic Emission

S. Vodenicharov, V. Miladinova, S. Todorov
Inst. of Metal. Res. and Techn., Sofia

The authors present a considerably simple method, by which all operations of cutting a notch, forming a crack and following the crack development are performed on one and the same device. A relation of the acoustic emission parameters and the moment of the crack critical spread is found, in order to calculate the material crack resistance. An aluminium alloy, known as a piston alloy and used for the production of parts with high strength and low weight, is tested. The basic advantages of the method are that it can also be applied for the test of brittle and hard material by using appropriate indentors. Moreover, the testing time is significantly shortened, as compared to that characteristic for the existing methods, and the manufacture of a large number of test specimens with a special shape is not necessary.

JTAM, Sofia, vol. 21 Issue 3 pp. 094-100 (1990)