The effects of material combination and surface roughness in lubricated silicon nitride/steel rolling contact fatigue

Four kinds of commercially finished 12.7 min HIPed silicon nitride bearing balls with surface roughness values R-a ranging from 0.002 to 0.016 mum were tested using a four-ball rolling configuration. They were rolling against two types of steel testing balls with different surface roughness and hardness, in fully lubricated condition at a maximum compressive stress of 6.58 GPa and at a speed of 10 000 rpm for over 135 million stress cycles. Rolling track surfaces were examined by microscope, SEM, 3-D surface analysis and interference profilometry. Experiment results show that the composite surface roughness are most influential. The shape of the surface topography of silicon nitride are not very sensitive. The slight difference. in steel hardness may lead to significant differences in steel fatigue life

Polyhedral Cones of Magic Cubes and Squares

Using computational algebraic geometry techniques and Hilbert bases of polyhedral cones we derive explicit formulas and generating functions for the number of magic squares and magic cubes.Comment: 14 page

A method for predicting IGBT junction temperature under transient condition

In this paper, a method to predict junction temperature of the solid-state switch under transient condition is presented. The method is based on the thermal model of the switch and instantaneous measurement of the energy loss in the device. The method for deriving thermal model parameters from the manufacturers data sheet is derived and verified. A simulation work has been carried out on a single IGBT under different conditions using MATLAB/SIMULINK. The results show that the proposed method is effective to predict the junction temperature of the solid-state device during transient conditions and is applicable to other devices such as diodes and thyristors

Characteristics of inhomogeneous jets in confined swirling air flows

An experimental program to study the characteristics of inhomogeneous jets in confined swirling flows to obtain detailed and accurate data for the evaluation and improvement of turbulent transport modeling for combustor flows is discussed. The work was also motivated by the need to investigate and quantify the influence of confinement and swirl on the characteristics of inhomogeneous jets. The flow facility was constructed in a simple way which allows easy interchange of different swirlers and the freedom to vary the jet Reynolds number. The velocity measurements were taken with a one color, one component DISA Model 55L laser-Doppler anemometer employing the forward scatter mode. Standard statistical methods are used to evaluate the various moments of the signals to give the flow characteristics. The present work was directed at the understanding of the velocity field. Therefore, only velocity and turbulence data of the axial and circumferential components are reported for inhomogeneous jets in confined swirling air flows

Measuring the energy handling capability of metal oxide varistors

Metal oxide varistors are widely used in many power electronics circuits to protect against transient over voltages. Certain applications are very demanding on the energy handling capability of the varistors. This paper gives an overview of the failure modes of ZnO varistors and investigates their characteristics when subjected to repetitive current pulses. It describes the puncture failure mode caused by melting of a region in the varistor of local current concentration. Experimental tests are performed to evaluate the puncture energy using an infrared imaging camera. A relationship between the energy absorption and the varistor maximum surface temperature is obtained. It is shown that the destructive energy depends strongly on the uniformity of the varistor; the more uniform, the higher the energy handling capability. The paper also presents the results of nondestructive tests using a scanning acoustic microscope to evaluate the uniformity of the varistor