University of Cambridge > > Engineering Department Dynamics and Vibration Tea Time Talks > Measuring microns on rails: can we really do it and if so, how?

Measuring microns on rails: can we really do it and if so, how?

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ABSTRACT Dynamic loads giving rise to vibration and noise are excited by irregularities, in particular so-called “corrugation” on the running surfaces of both wheels and rails. The amplitude of irregularity that gives rise to significant load and noise can be tiny. A typical contact stiffness between wheel and rail is 1.5GN/m, so an irregularity of 10m can give rise to 15kN dynamic load. A 50 m irregularity at a sufficiently high frequency causes loss of contact. Anyone who travels on the Piccadilly Line to Heathrow (or on many other LU lines) may wonder why it is so noisy: the roaring at a pretty consistent frequency of 500Hz is almost entirely a result of rail corrugation.

These irregularities have to be measured, not only for purposes of research, but also to maintain the rails and ensure that irregularities are removed satisfactorily. The irregularities are typically removed using a reprofiling train (Network Rail has recently bought 4 grinding trains, each with forty-eight 30kW grinding motors; milling is also used, particularly to remove significant depths of material). Much of the disruption on London Underground on weekends results from reprofiling: the whole of the Victoria Line is reprofiled annually. The standard to which reprofiling is undertaken essentially requires that irregularities with an amplitude of at most a few microns remain on the rail.

If the maintenance contractor is to do this job properly, rails have to be measured both before and after reprofiling. Those measurements have to be accurate to microns and, preferably, they should be made from the reprofiling train itself. RailMeasurement Ltd (RML) have developed equipment that is probably the “state of the art” in this area. The original equipment, the hand-held “Corrugation Analysis Trolley” (CAT) is based on an instrument made in CUED in the 1970s. Unfortunately, although the precision of the CAT is better than a micron, it is suitable neither for measuring freshly reprofiled rails nor for use on a train.

RML ’s RCA can be used on a train, even when the rail is being ground or milled. RML recently won a contract to supply one of these to Crossrail to measure longitudinal irregularities to what are essentially acoustic requirements. CATs and RCAs are made in St Ives and supplied worldwide: more than 90% of what we make is exported.

Some results from both types of equipment will be shown, and, with luck, both a CAT and a measuring wheel from an RCA will be available for inspection. References

1 SL Grassie, “Rail irregularities, corrugation and acoustic roughness: characteristics, significance and effects of reprofiling on different types of railway system”, Journal of Rail and Rapid Transit, Procs of I mech E, 2012, 226F, 542-557

2 SL Grassie, “Routine measurement of long wavelength irregularities from vehicle-based equipment”, in “Noise and vibration mitigation for rail transport systems”, David Anderson et al (eds), Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2018, vol 139, chapter 24

This talk is part of the Engineering Department Dynamics and Vibration Tea Time Talks series.

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