Effect of UV radiation aging on creepage discharge characteristics of high temperature vulcanized silicon rubber at high altitude

The physicochemical properties and creepage discharge characteristics of aged high temperature Vulca nized(HTV)silicone rubber materials were investigated by ultraviolet radiati on(UV)aging method in this study.The experimental results show that as the aging time increases,the creepage discharge flashover voltage increases first and then decreases.But the aging time has little effect on the creepage discharge inception voltage.With the aging time prolonged,the discharge endurance time of HTV silicone rubber is shortened,and the creepage discharge development velocity is accelerated.In the short time of applying voltage to aging material,the magnitude of discharge in creases rapidly.According to the partial discharge characteristic parameters of creepage discharge,the whole creepage discharge process is partitioned into four stages.Compared with unaged HTV silicone rubber,the aged HTV silicone rubber has less fluctuation in performance parameters and a clear trend.The study found that UV aging not only affects the physicochemical and hydrophobic properties of the HTV silicone rubber,but also accelerates the development of creepage discharge under AC voltage.

Analyses of Creepages and Their Sensitivities for a Single Wheelset Moving on a Tangent Track

Creep forces depend greatly on creepages in the contact area forming between wheel and rail.The creepages are completely determined by the state of a wheelset moving on a track.In this paper the contact state of a single rigid wheelset moving on a tangent rigid rail,creepages and their sensitivities to some parameters of contact geometry are analyzed by semi analytical method and numerical method,respectively.Some important ideas will be provided for the studies done on the interactions between wheels and rails at high speed.

Effects of temperature on creepage discharge characteristics in oil-impregnated pressboard insulation under combined AC–DC voltage

Due to the complexity of the valve side winding voltage of the converter transformer, the insulation characteristics of the oil-impregnated pressboard(OIP) of the converter transformer are different from those of the traditional AC transformer. The study on effect of temperature on the creeping discharge characteristics of OIP under combined AC–DC voltage is seriously inadequate. Therefore, this paper investigates the characteristics of OIP creepage discharge under combined AC–DC voltage and discusses the influence of temperature on creepage discharge characteristics under different temperatures from 70 °C to 110 °C. The experimental results show that the partial discharge inception voltage and flashover voltage decrease with increasing temperature. The times of low amplitude discharge(LAD) decrease and amplitude of LAD increases. Simultaneously, the times of high amplitude discharge(HAD) gradually increase at each stage of creepage discharge with higher temperature. The analysis indicates that the charge carriers easily accumulate and quickly migrate directional movement along the electric field ahead of discharging. The residual charge carriers are more easily dissipated after discharging.The ‘hump’ region of LAD moves to the direction of higher discharge magnitude. The interval time between two continuous discharges is shortened obviously. The concentration of HAD accelerates the development of OIP insulation creepage discharge. The temperature had an accelerating effect on the development of discharge in the OIP under applying voltage.

Correlation between surface charge and creepage discharge on polymeric dielectrics under high-frequency high-voltage stress

In the present work,creepage discharge characteristics,i.e.amplitudes,phases,and repetitiveness,and surface charge dynamic behaviors under a 20 kHz high-frequency sinusoidal waveform high-voltage electrical stress were captured in a discharge chamber with temperature and humidity control.The results showed that the creepage discharges mostly occurred in the positive half phase,whose maximum amplitude increased with the development of discharge.The inception voltage of the creepage discharge is independent of the frequency of the external electrical stress.Once the discharge occurred,there were a large number of positive and negative particles ionized by a high electric field.Because of the much higher velocity of electrons than positive ions,the energetic discharge-produced electrons are likely to disperse away along the surface and be accumulated through adsorption,collision,and reactions.Moreover,the positive ions join the high-conductive discharge channel and disappear though the ground electrode.Thus,after high-frequency creepage discharge,only negative charges remained on the dielectric surface,as measured.Particularly,the creepage discharges mostly occurred in the positive half phase,owing to the reverse electric field induced by the accumulated negative charges.With the development of creepage discharge,some large-amplitude discharges began to occur in the positive-peak-phase region.The research concluded that the synergistic effect of negative surface charges and large-amplitude discharges eroded the dielectrics and excited the streamer to creep toward the ground electrode until flashover along the surface.Therefore,the correlation between high-frequency creepage discharge and surface charge is preliminarily revealed.

Novel micro-capsule-doped insulating material with hydrophobicity transfer capability triggered by creepage discharge

The lack of hydrophobicity transfer capability has restricted the applications of insulating materials in environments with heavy contamination.In this study,an epoxy resin insulating composite was prepared using a micro-capsule system to achieve hydrophobicity transfer triggered by creepage discharge.The influence of the micro-capsules on the electrical properties of insulating materials and the hydrophobicity of the test samples before and after the creepage discharge test was analysed.The results show that the poly(urea-formaldehyde)micro-capsules,which contained hydrogen silicone oil,have good morphology and thermal stability characteristics at temperatures below 222℃.Microcapsules have minor effects on the electrical properties of epoxy insulating materials.Further,the hydrophobicity transfer capability of the test piece was improved considerably compared with that of the pure epoxy insulating material.This study expands the application of micro-capsule technology in the field of insulating materials and provides a reference for the development of next-generation insulating materials.

Quasi-static Analysis of Thrust-loaded Angular Contact Ball Bearings Part Ⅰ:Theoretical Formulation

Ball bearings play an important role in various rotating machineries,but the complicated kinematic and tribological features of ball bearings make many aspects of their operating behaviors still inconclusive.Most theoretical analyses of ball bearings up to date are based on either the hypothesis of race control or other empirical models to determine the ball motion of ball bearings,but none of these strategies can reveal and consequently employ the intrinsic coupling mechanism between the spin and the tangential traction of contacting bodies rolling upon one another.To remedy the deficiency of current analytical models for ball bearing analysis,the rolling contact theory is employed to establish an explicit link between motions and interactions within ball bearings.A differential slip model is established to precisely define the slip component due to the significant curvature of the common contact patches between the ball and inner/outer raceways.The creepage and the spin ratio are formulated to accurately define the relative rigid motion between the ball and the inner/outer raceway.Then a quasi-static analytical model is established that can accurately determine the motions of the balls and races of the ball bearing.It can also give a vivid description of the slip and traction distributions within the contact area.The analytical model can be effectively used to analyze the operational conditions and tribological features of solid-lubricated ball bearings.It can also be used optimize the construction of ball bearings for specific applications.

Quasi-static Analysis of Thrust-loaded Angular Contact Ball Bearings Part Ⅱ:Results and Discussion

Ball bearings are widely employed mechanical components characterized by high precision and quality,and usually play important roles in various rotary machines and mechanisms.Many advanced applications require a deep understanding of their various kinematic and tribological characteristics that are essential to predict the fatigue endurance,relieve the vibration and minimize the power dissipation of ball bearings in particular applications.An angular contact ball bearing under a specified operating condition is simulated with the quasi-static/creepage analytical model proposed in the preceding article.The results demonstrate that the ball bearing is a statically determinate system.That the balls spin on both inner and outer races means the ball is controlled by neither the inner nor the outer raceway.The friction between the ball and raceway renders the inner and outer contact angles unequal.The larger the coefficient of friction is,the larger the angle deviation.The tangential traction perpendicular to the rolling direction due to the spin induces a gyro-like rotation of the ball with respect to the raceway even if no inertial effects are considered.The tangential elastic compliance of contacting surfaces gives rise to locked areas within the contact patch and transforms the sliding lines from circles into spirals.The differential slip due to the close conformity of the ball and raceway makes the sliding and traction distributions asymmetric,which will influence the location of the spinning center of the ball with respect to the raceway.The quasi-static/creepage model can be used to reveal the operating behaviors of ball bearings running under steady conditions and to optimize the design of ball bearings for specific applications.

Numerical Analysis of Rolling-sliding Contact with the Frictional Heat in Rail

Thermal damage caused by frictional heat of rolling-sliding contact is one of the most important failure forms of wheel and rail. Many studies of wheel-rail frictional heating have been devoted to the temperature field, but few literatures focus on wheel-rail thermal stress caused by frictional heating. However, the wheel-rail creepage is one of important influencing factors of the thermal stress In this paper, a thermo-mechanical coupling model of wheel-rail rolling-sliding contact is developed using thermo-elasto-plastic finite element method. The effect of the wheel-rail elastic creepage on the distribution of heat flux is investigated using the numerical model in which the temperature-dependent material properties are taken into consideration. The moving wheel-rail contact force and the frictional heating are used to simulate the wheel rolling on the rail. The effect of the creepage on the temperature rise, thermal strain, residual stress and residual strain under wheel-rail sliding-rolling contact are investigated. The investigation results show that the thermally affected zone exists mainly in a very thin layer of material near the rail contact surface during the rolling-sliding contact. Both the temperature and thermal strain of rail increase with increasing creepage. The residual stresses induced by the frictional heat in the surface layer of rail appear to be tensile. When the creepage is large, the frictional heat has a significant influence on the residual stresses and residual strains of rail. This paper develops a thermo-meehanical coupling model of wheel-rail rolling-sliding contact, and the obtained results can help to understand the mechanism of wheel/rail frictional thermal fatigue.

Effects of Lateral Motion on the Creep Forces in Wheel/Rail Rolling Contact

The influences of the lateral motion of a single wheelset running on a tangent railway on the creepages and creep forces between wheel and rail are investigated with numerical methods. The effect of the yaw motion of wheelset is neglected in the analysis, and Kalker’s theory of three dimensional elastic bodies in rolling contact is employed to analyze the creep forces in the wheel/rail rolling contact with Non Hertzian form.

ANALYSIS OF CREEP FORCES OF WHEEL/RAIL

The effects of structure elastic deformations of wheelset and track on creep forces of wheel and rail are investigated.Finite element method is used to calculate the influence coefficients expressing the relations between structure elastic deformations of wheelset and track and the corresponding loads in the rolling direction and the lateral direction of wheelset,respectively.The influence coefficients of wheelset and track are used to revise some of the influence coefficients obtained with the formula of Bossinesq and Cerruti in Kalker's theory of three dimensional elastic bodies in rolling contact with non Hertzian form.The theory of Kalker and the modified theory of Kalker are respectively employed to analyze creep forces of wheelset and track.The numerical results obtained show a great influence of structure elastic deformations of wheelset and track on the creep forces.Therfore it is not reasonable that wheel and rail are treated as elastic half space in the analysis of wheel and rail in rolling contact,and the present theories of rolling contact based on the assumption of elastic half space need to be further improved