Jerzy Piotrowski

Wheel-rail contact models for vehicle system dynamics including multi-point contact

Advanced modelling of rail vehicle dynamics requires realistic solutions of contact problems for wheels and rails that are able to describe contact singularities, encountered for wheels and rails. The basic singularities demonstrate themselves as double and multiple contact patches. The solutions of the contact problems have to be known practically in each step of the numerical integration of the differential equations of the model. The existing fast, approximate methods of solution to achieve this goal have been outlined. One way to do this is to replace a multi-point contact by a set of ellipses. The other methods are based on so-called virtual penetration. They allow calculating the non-elliptical, multiple contact patches and creep forces online, during integration of the model. This allows nearly real-time simulations. The methods are valid and applicable for so-called quasi-Hertzian cases, when the contact conditions do not deviate much from the assumptions of the Hertz theory. It is believed that it is worthwhile to use them in other cases too.

A simplified model of wheel/rail contact mechanics for non-Hertzian problems and its application in rail vehicle dynamic simulations

Abstract The presented model assumes semi-elliptical normal pressure distribution in the direction of rolling. The contact area is found by virtual penetration of wheel and rail. The normal pressure is calculated by satisfying contact conditions at the geometrical point of contact. The calculation is non-iterative, fast and completely reliable. It may be carried out on-line in MultiBody Systems (MBS) computer codes. The tests using the programme CONTACT by Kalker and experience from application in MBS codes show that the model is suitable for technical applications. The creep forces have been calculated with the FASTSIM algorithm, adapted for a non-elliptical contact area. Some applications in rail vehicle dynamics and wear simulation have been outlined.

SOME NEW RESULTS IN ROLLING CONTACT

SUMMARY In the present paper three problems in the simplified theory of rolling contact are investigated. As to the first problem, three benchmark loadings, derived by Kalker in 1973 for Hertzian rollingcontact, are in existence. Each of these loadings gives rise to a value of the flexibility parameter of the simplified theory. These values are combined to a single, creepage dependent value of the flexibility, which appears to have an error of at most 10 to 15%. Secondly, the law of Coulomb is generalised by introducing two values of the coefficient of friction. The FASTSIM algorithm is adapted to that, and it is found that the traction, and hence the displacement, show a discontinuity inside the contact area. The discontinuity in the displacement is removed by introducing damping in the constitutive relations of the simplified theory. The damping constant is determined experimentally. When the damping coefficient decreases, the damped solution tends to the solution obtained directly without damping. This...

Kalker's algorithm Fastsim solves tangential contact problems with slip-dependent friction and friction anisotropy

This paper presents two extensions of Kalkers algorithm Fastsim of the simplified theory of rolling contact. The first extension is for solving tangential contact problems with the coefficient of friction depending on slip velocity. Two friction laws have been considered: with and without recuperation of the static friction. According to the tribological hypothesis for metallic bodies shear failure, the friction law without recuperation of static friction is more suitable for wheel and rail than the other one. Sample results present local quantities inside the contact area (division to slip and adhesion, traction) as well as global ones (creep forces as functions of creepages and rolling velocity). For the coefficient of friction diminishing with slip, the creep forces decay after reaching the maximum and they depend on the rolling velocity. The second extension is for solving tangential contact problems with friction anisotropy characterised by a convex set of the permissible tangential tractions. The effect of the anisotropy has been shown on examples of rolling without spin and in the presence of pure spin for the elliptical set. The friction anisotropy influences tangential tractions and creep forces. Sample results present local and global quantities. Both extensions have been described with the same language of formulation and they may be merged into one, joint algorithm.

Smoothing dry friction damping by dither generated in rolling contact of wheel and rail and its influence on ride dynamics of freight wagons

Friction dampers of mechanical systems are frequently exposed to medium-frequency (M-F) dither generated in the surrounding environment. A dithered system of technical importance is the railway freight wagon with friction dampers in the primary suspension developing two-dimensional friction, where dither is generated by the rolling contact of wheel and rail. This paper presents some results of the investigation of the influence of dither on dry friction damping. This influence has been studied experimentally and theoretically, and the parameters of dither influencing dry friction damping have been indicated. An experimental set-up has been designed that allows investigating friction damping in the presence of dither. The experiments have shown that friction damping in the presence of the M-F dither behaves like viscous damping. This means that dither smoothes dry friction as far as damping is concerned. To investigate this phenomenon theoretically, a rheological model of dry friction has been proposed that is applicable to one- and two-dimensional friction. In the latter case, the model takes into account friction anisotropy. According to performed numerical simulations of freight wagon motion, with dither supplied to the model through measured vertical accelerations of axle boxes, smoothing dry friction by dither strongly influences ride dynamics of the wagon with friction dampers in the primary suspension. Smoothing dry friction by dither should be accounted for in numerical simulations of motion of vehicles with friction dampers in the primary suspension by employing a proper model of the two-dimensional friction and application of realistic dither generated by rolling contact.

Non-contacting determination of moisture content in bulk materials using sub-nanosecond UWB-pulses

This paper presents a new approach of noncontacting moisture determination in bulk materials utilizing time-domain transmissometry. The described method is independent of the mass per area of the material under test and does not suffer from measurement ambiguities, which are well known from frequency-domain methods. Ultra-wideband quasi-Gaussian monocycle pulses are applied with a cycle width of approximately 400 ps. The measurement signal is transmitted through the material over free space using double-ridged horn antennas. The received signal is acquired in the time domain and analyzed with different methods of data processing. The results obtained with multiple linear regression, principal component analysis, and artificial neural networks are compared with each other. Furthermore, a possible hardware implementation for a dedicated time-domain transmissometer is proposed based on a earlier developed time-domain reflectometer for the determination of food quality.This paper presents a new approach of noncontacting moisture determination in hulk materials, which is independent of the mass per area of the material. It also dues not suffer from measurement ambiguities, which are well known from frequency domain methods. UWB quasi ganssian monocycle pulses are applied with a cycle width of approximately 400ps. The measurement signal is transmitted through the material over free space. The received signal is acquired in the time-domain and analysed, using multivariate calibration techniques. Principal component analysis and principal component regression yield the best results. Two different types of antennas and their influence on the accuracy of the measurements are investigated.

Dielectric measurements using a coaxial resonator opened to a waveguide below cut-off

A model of a quarter-wavelength coaxial resonator opened to a semi-infinite circular waveguide below cut-off is presented in the paper. A full-wave analysis of the transition between coaxial-line and circular waveguide was used. The analysis was based on a mode-matching formulation. The circular waveguide is filled with a dielectric material. As a consequence, the parameters of the resonator, resonant frequency, quality factor and resonant transmission, change in relation to the complex permittivity of the material. A fully automated L-band microwave system with resonator sensor was fabricated and used for measurements of the permittivity of low-loss and high-loss powdered materials. Results obtained using the system are compared with those of other method.

Novel approach to the characterization of coaxial-to-microstrip transitions

Resonance measurement method has been employed in the procedure leading to coaxial-to-microstrip equivalent circuit determination. In comparison with previous approaches which used reflection or transmission measurements, the proposed method has the advantage of superior accuracy of measurements and may be implemented using scalar network analyzers. Discontinuities in connectors coaxial part have been accounted for. In addition, transition losses have been modelled by a frequency-dependent resistance. Equivalent circuits of two SMA-microstrip connectors have been presented. In one case the characterization has been extended to 30 GHz, covering thus the whole SMA connector operating frequency range. The accuracy of characterization has been confirmed by measurements on a pair of connectors separated by a section of microstrip. Potential applications of equivalent circuits obtained in such a way have been also discussed.

CAD Models of Connectors and Transitions used in Hybrid Microwave Integrated Circuits

Recently proposed resonance measurement based method of microwave transition characterisation has been employed when developing CAD models of transitions from coaxial connectors to different planar transmission lines. The considered cases include millimetric-wave connector - microstrip transition, SMA connector - symmetrical stripline transition and in-house developed SAM connectors for direct feeding of coplanar waveguide circuits. Equivalent circuits of transitions have been obtained and experimentally validated Examples of application include the improvement of millimetric-wave transition performance and an approach used in the design and calibration of a fixture for characterisation of devices mounted in coplanar waveguide.

The Elastic Cross-Influence between Two Quasi-Hertzian Contact Zones

SUMMARY In this paper a type of contact between two bodies is considered, which leads to the formation of two separate contact zones. The contact zones considered are Hertzian ellipses if the distance between them is large. When the distance between the zones is finite it is necessary to take into account the elastic cross-influence of the two zones. In this paper an approximate method is proposed which allows the determination of the actual contact zones as Hertzs ellipses but without disregarding the cross-influence. Numerical results are presented for two, two-humped bodies pressed against each other and for steady state rolling of a flanged wheel along a steel rail. It is shown that neglecting the cross influence for a flanged wheel leads to a negligible error in the contact forces but the forces are transmitted between wheel and rail through contact patches which are generally more slender than with the cross-influence neglected.