Volume 9, number 2 (2015)
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Item Achievements, agreements and quarrels of forefathers of mechanics(University of West Bohemia, 2015) Okrouhlík, MiloslavThe presented paper is devoted to deeds of Galileo Galile i, Johannes Kepler, Robert H ook, Christiaan Huygens and Isaac Newton with an intention to show their achievements in mechanics, their intellectual and scientific heritage, and also their personal vanities that sometimes led to harmful mutual relations.
Item Negative stiffness in gear contact(University of West Bohemia, 2015) Půst, Ladislav; Pešek, Luděk; Radolfová, AlenaThe tooth contact stiffness is very often included in dynamic mathematical models of gear drives. It is an important value for calculation of torsion eigenfrequencies as w ell as the dynamic properties of the whole transmission systems. Planetary gear drives have several advantages over simple parallel axis gears, especially due to theirs compact design and great torque-to-weight ratio caused by multiple parallel paths. However, the dimensional or mounting errors can cause that some planets have the tendency to take more load than the others. One of the ways how to improve load sharing is the application of flexible planetary pins or by using a free central wheel. However in such cases, the wheels motion is defined in one rotation coordinate and two translation coordinates — tangential and radial. The reaction force at radial change of axis distance is usually neglected. The focus of this contribution is to derive the stiffness of this radial connection and to analyse the influence of radial stiffness on planetary gear dynamics.Item Dynamics of beam pair coupled by visco-elastic interlayer(University of West Bohemia, 2015) Náprstek, Jiří; Hračov, StanislavAn exact method is presented for solving the vibration of a double-beam system subjected to harmonic excitation. The system consists of a loaded main beam and an auxiliary beam joined together using massless visco-elastic layer. The Euler-Bernoulli model is used for the transvers e vibrations of beams, and the spring-dashpot represents a simplified model of viscoelastic material. The damping is assumed to be neither small nor proportional, and the forcing function can be either concentrated at any point or distributed continuously. The method involves a simple change of variables and modal analysis to dec ouple and to solve the governing differential equations respectively. A case study is solved in detail to demonstrate the methodology, and the frequency responses are shown in dimensionless parameters for low and high values of stiffness and damping of the interlayer. The analysis reveals two sets of eigen-modes: (i) the odd in-phase mode s whose eigen-values and resonant peaks are independent of stiffness and damping, and (ii) the even out-of-phase modes whose eigen-values increase with raising stiffness and resonant peaks decrease with increasin g damping. The closed-form solution and relevant plots (especially the three-dimensional ones) illustrate not only the principles of the vibration problem but also shed light on practical applications.Item Crack path modelling in railway wheel under rolling contact fatigue(University of West Bohemia, 2015) Kotoul, MichalA computational model of crack path for two-dimensional p rimary crack situated in a railway wheel rim is designed. The railway wheel rim is placed on the wheel disc of railway wheel with interference fit. Crack behaviour is analysed in the case of rectilinear ride of a train under rolling cont act fatigue. Plank and Kuhn criterion is used to decide whether crack will either kink and follow mode I controlled (tensile mode) path, or it will propagate coplanar mode II controlled (shear mode). If mode I controlled crack growth is more probable then a direction of crack propagation is predicted using the maximum tensile stress range criterion. In this way a relationship between stress intensity factors and crack geometry is obtained. For comparison, crack behaviour in a solid railway wheel which is not subjected to pre-stress loading is also analysed. In the latter case the contact forces in the wheel-rail contact are considered to have i) only normal part ii) both the normal part and tangential part.
