G.X. Chen

Study on transition between fretting and reciprocating sliding wear

An experimental investigation was conducted to find the associated changes in characteristics of wear before and after the transition between fretting and reciprocating sliding wear. A set of experiments were carried out using a AISI 52100 steel ball rubbing against a plate specimen made from the same steel under dry condition. Wear coefficient, wear volume, coefficient of friction, profile of the scars and wear debris were analyzed. The results displayed that there were significant differences in wear coefficient, wear volume, profile of the wear scars and wear debris before and after the transition. Wear coefficient and wear volume at a constant sliding distance were found to be the most appropriate for identifying the transition amplitude between fretting and reciprocating sliding wear.

Experimental investigation into squeal under reciprocating sliding

Abstract Experiments on squeal under reciprocating sliding were performed by means of a ball against a block. Vibration accelerations, sound pressure level of squeal and tangential force were measured simultaneously. Under certain test conditions, the reciprocating sliding can create a whole process from squeal generation to disappearance. Based on power spectral density (PSD) and short-time Fourier transform (STFT) analyses on the vibration accelerations in that process, it was found that the dominant frequencies of the friction-induced vibrations associated with squeal is not varied. Examination of the friction–velocity slope shows that there is no invariable correlation between the negative friction–velocity slope and occurrence of squeal. Squeal can occur in regions with both negative and positive friction–velocity slopes.

Tribological and electric-arc behaviors of carbon/copper pair during sliding friction process with electric current applied

The tribological behaviors of carbon block sliding against copper ring with and without electric current applied were investigated by using an advanced multifunctional friction and wear tester, and the electric-arc behaviors were analyzed in detail. The results show that the normal load is one of the main controlling factors for generation of electric arc during friction process with electric current applied. The strength of electric arc is enhanced with the decrease of normal loads and the increase of electric currents. The unstable friction process and the fluctuated dynamic friction coefficients are strongly dependent upon the electric arc. The wear volumes and the wear mechanism of carbon brush were affected by the electric arc obviously. As no electric arc occurs, no clear discrepancy of the wear volumes of the carbon samples with and without electric current applied could be detected. While the wear mechanisms are mainly mechanical wear. However, under the condition of the electric arc appearance, the wear volume of carbon with electric current applied increases much more rapidly than that without electric current applied and also increases obviously with the increase of electric current strengths and the decrease of normal loads. The wear mechanisms of carbon block are mainly electric arc ablation accompanying with adhesive wear and material transferring.

A transient dynamic study of the self-excited vibration of a railway wheel set-track system induced by saturated creep forces

Two dynamic models of a wheel set–track system on a tight curved track and on a straight track are established, in which the friction coupling between the wheel and rail is taken into account. Both the transient dynamic and complex eigenvalue analyses are performed to study the unstable transient dynamics and stability of the wheel set–track system. It is assumed that in the models creep forces between wheels and rails are saturated, that is, approximately equal to the normal forces multiplied by the dynamic coefficients of friction. The simulation results demonstrate that the saturated creep force can induce self-excited vibration of the wheel set–track system. The normal contact force between the wheel and rail fluctuates at the same frequency as the wheel and rail vibrate when the self-excited vibration occurs. And the fluctuation frequency of the normal contact force falls into the range of 60–500 Hz, which corresponds to the frequency range of rail corrugation. This phenomenon indicates that the self-excited vibration of the wheel set–track system may be a main cause of rail corrugation occurrence. Parameter sensitivity analysis shows that the stiffness and damping of the rail fastener have important influences on the oscillation amplitude of the normal contact force. Bringing the friction coefficient below a certain level and increasing the damping of the rail fastener can suppress rail corrugation.

Correlation between the Wear and Vibration of the Contact Strip in a Contact Wire Rubbing against a Contact Strip with Electrical Current

Vibration of the contact strip is an important phenomenon in the process of electrical sliding of a pantograph–catenary system. The correlation between the wear and vibration of a contact strip in a contact strip rubbing against a contact wire was studied. A series of experimental tests was conducted on a high-speed block-on-ring tester under electric current of 200 A, sliding speed of 160 km/h, and normal loads of 40–120 N. Scanning electron microscopy (SEM) and optical microscopy were used to observe the morphology of the worn surfaces. Experimental results indicate that the friction coefficient decreases but the wear rate of the contact strip increases with increasing vibration acceleration. Arc discharge energy increases with increasing vibration acceleration. Suppressing vibration is a practical way to decrease arc erosion wear of the contact strip. The conductivity of the contact strip becomes progressively worse with increasing vibration acceleration.

Study on Rail Corrugation of a Metro Tangential Track with Cologne-Egg Type Fasteners

ABSTRACT In Chinese metro lines, rail corrugation on both tangential and tight curved tracks with Cologne-egg type fasteners is very severe. Based on the viewpoint of friction-induced vibration causing rail corrugation, the rail corrugation on a tangential track with Cologne-egg type fasteners is studied in this paper. A vibration model of an elastic multiple-wheelset-track system with Cologne-egg type fasteners is established. Both the complex eigenvalue analysis and the transient dynamic analysis are performed to study the stability and the dynamic performance of the wheelset-track system. The simulation results show that a low rail support stiffness value is responsible for rail corrugation on the tangential track. When the Cologne-egg fasteners characterised by a lower stiffness value are replaced with the DTVI2 fasteners characterised by a higher stiffness value, rail corrugation disappears. However, rail corrugation on tight curved tracks cannot be suppressed using the same replacement. The above conclusions are consistent with the corrugation occurrences in actual metro tracks.

Experimental Study on Corrugation of A Sliding Surface Caused By Frictional Self-Excited Vibration

abstract A series of tests is conducted on a pin-on-disc tester to study the effect of frictional self-excited vibration on evolution of the scar profiles. A laser displacement sensor is used to measure the profile size of the worn scars. An accelerometer is used to measure vibration of the pin specimen. The test results show that a sustained frictional self-excited vibration easily occurs under dry friction. When the vibration lasts for a long enough time, corrugation is generated on the sliding surface of the disc specimen. The wavelength of corrugation is approximately equal to the sliding speed multiplied by the period time of the friction-induced vibration.

Study on material transfer in the process of contact strips rubbing against a contact wire with electric current

A series of experimental tests on a block-on-ring tester were carried out to obtain a new understanding of severe material transfer in the process of contact strips rubbing against a contact wire with electric current. Three types of contact strip materials including an aluminum-based strip, a copper-based strip, and a pure carbon strip are tested in electric sliding against two contact wire materials including a pure copper contact wire and a copper–silver alloy contact wire. Test results show that there are serious material transfers in these three different friction couples in electric sliding. The aluminum-based strip has the severest material transfer, followed by the copper-based strip. The pure carbon strip has the minimum material transfer. It is found that the material transfer increases with the increase of the sliding speed, the arc discharge intensity, and the contact pressure. In the presence of electric current, contact strip materials are always transferred to the contact wire.

Friction-Induced, Self-Excited Vibration of a Pantograph-Catenary System

the pantograph is coupled with that of the catenary by friction. Stability of the pantograph-catenary system is studied using the finite element complex eigenvalue method. Numerical results show that there is a strong propensity of self-excited vibration of the pantograph-catenary system when the friction coefficient is greater than 0.1. The dynamic transient analysis results show that the self-excited vibration of the pantographcatenary system can affect the contact condition between the pantograph and catenary. If the amplitude of the self-excited vibration is strong enough, the contact may even get lost. Parameter sensitivity analysis shows that the coefficient of friction, static lift force, pan-head suspension spring stiffness, tension of contact wire, and the spatial location of pantograph have important influences on the friction-induced, self-excited vibration of the pantograph-catenary system. Bringing the friction coefficient below a certain level and choosing a suitable static lift force can suppress or eliminate the contact loss between the pantograph and catenary. [DOI: 10.1115/1.4023999]

Complex Eigenvalue Analysis of Railway Curve Squeal

A finite element complex eigenvalue analysis of curve squeal is carried out. Two models for the wheel tread/rail top contact and the wheel flange root/rail gauge corner contact are established. In the both models, the contact between the wheel and rail is simulated with a spring. The friction force is considered as the contact spring force multiplied by a coefficient of friction. The rail is supported by vertical and lateral springs at each sleeper. The simulation result shows that the coefficient of friction, contact point positions and stiffness of the rail support spring all have distinct influences on the curve squeal occurrence. Appropriate stiffness of the rail support spring can suppress or eliminate curve squeal.