Krešimir Vučković

Effect of rotational speed on thin-rim gear bending fatigue crack initiation life

Thin-rim gears are often used in aircraft applications in order to reduce weight. The objective of this study is to investigate the effect of rotational speed (centrifugal force) on bending fatigue crack initiation life of thin-rim gear manufactured from case carburized 14NiCrMo13-4 steel. Stresses in gear are determined from two-dimensional finite element model, assuming plane stress conditions. The fact that, in actual thin-rim gear operation, a significant reversed stress occurs at the root of the tooth adjacent to the loaded tooth is considered. Material is assumed to be homogenous, isotropic and linear elastic. Elastic strains are calculated from obtained stresses and corrected using Neuber’s rule to account plasticity effects. The number of load cycles required to initiate bending fatigue crack is predicted using strain-life approach for variety of gear rotational speeds and rim thicknesses. Strain-controlled fatigue properties were approximated from material hardness, while the mean stress as well as residual stress effects are included through Morrow’s mean stress correction. The proposed approach is validated by comparison with available experimental data from literature and used for parametric studies. Predicted numbers of load cycles required to initiate potential bending fatigue crack are presented for the variety of cases studied.

Influence of the wall thickness on the allowable and failure pressures of two- and tree-way globe valve bodies

A globe valve is a linear motion valve used to shut off and regulate fluid flow in pipelines. Depending on the number of process connections, they are produced as two‑ or three-way valves. The main valve component carrying the internal pressure is the valve body. For safe exploitation, the valves are designed with the allowable internal pressure taken into consideration. The aim of this paper is to investigate the influence of the wall thickness on the allowable and failure pressures of two- and tree-way globe valve bodies, DN50 and DN100 respectively. Twice-elastic-slope (TES) and the tangent‑intersection (TI) methods are used to obtain the plastic collapse pressures at the critical location which was determined (Fig. 1a and 1b) at the location where maximum equivalent plastic strain throughout the valve body thickness reaches the outer surface. Obtained values are used afterwards to calculate corresponding allowable pressures according to the limit design method, while the failure pressure at the same location was determined as the highest point from the load-maximal principal strain curve. Calculated allowable pressure values, for both valve bodies, are compared with the corresponding ones obtained using the EN standard.