Keywords: rail vehicle dynamics, wheel flat, average vertical profile, welded joint defect, Sperling's ride index, ride comfort.

The dynamic performance of a railroad vehicle as related to safety is generally evaluated in terms of specific performance indices. The quantitative measure of ride quality is one such performance index. Ride quality is interpreted as the capability of the railroad vehicle suspension to maintain the motion of the car body within the range of human comfort. Sperling's ride index, which is a measure of ride quality and ride comfort, is used by the Indian railways. Indian railways has started using air springs as a secondary suspension for coaches running on the suburban mass rapid transit system (MRTS) track in Chennai, India, in an effort to improve ride comfort. In this paper a comparison of the dynamic behaviour of two different coaches is made:

• A coach equipped with air springs as a secondary suspension
• A coach equipped with a conventional coil spring as a secondary suspension.

A typical Indian railway vehicle of the alternating current/electrical multiple unit /trailer (AC/EMU/T) type running on a broad gauge track has been used for the analysis. Both numerical simulation and experimental studies of the vertical dynamic behaviour of the railway vehicle are presented. Car body motion is described by the vertical displacement or bounce and rotation about the transverse horizontal axis or pitch. Similarly, the motion of the two bogie units is described by two degrees of freedom (bounce and pitch). The motion of each axle set is described by one degree of freedom corresponding to bounce. Totally, ten degrees of freedom have been used to describe the motion of the vehicle model. Linear governing equations of the motion of the vehicle have been solved and the natural frequencies determined. Different wheel-rail irregularities have been considered for dynamic response analysis: average vertical profile irregularities as random excitation [1], wheel flatness as periodic excitation [2] and welded defects as impulse excitation [3]. Dynamic equations for the vehicle system have been solved in the time domain to calculate the vertical response of the vehicle by direct integration technique using Newmark's method. Responses in time domain have been converted into the frequency domain using fast Fourier transform (FFT). Analysis has been carried out for both the coil spring and air spring.

Experimental investigations were carried out at different operating speeds under normal running conditions. Readings were taken at different stretches of the track on MRTS routes, at different speeds. The comparison of predicted and measured response shows fairly good quantitative agreement, with most of the measured spectral components being present in the predicted response. Comparison with the standards set by ORE [4], a coach with an air spring as a secondary suspension will have a better ride than a coach with a coil spring as a secondary suspension.

1

O.R. Jaiswal, R.N. Iyengar, "Random field modelling of railway track irregularities", Journal of Transportation Engineering, 303-308, 1995. doi:10.1061/(ASCE)0733-947X(1995)121:4(303)

2

S.G. Newton, R.A. Clark, "An investigation into the dynamic effect on the track of wheel flats on railway vehicles", Journal of Mechanical Engineering Sciences, 21(4), 287-297, 1979.

3

Y.Q. Sun, M. Dhanasekar, "A dynamic model for the vertical interaction of the rail track and wagon system", International Journal of Solids and Structures, 39, 1337-1359, 2002. doi:10.1016/S0020-7683(01)00224-4

4

ORE Report C116/RP 1-9 /EC, "Interaction between Vehicle and Track", ORE, Utrecht, 1971-1978.

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