Abstract
M.Ing.
Sound pressure levels exceeding acceptable limits are being generated by trains travelling on the
1000 m radius curved railway line past the town of Elands Bay. Unacceptable sound levels are
attributed mainly to top of rail wheel squeal. Top of rail wheel squeal belongs to the family of selfinduced
vibrations and originates from frictional instability in curves between the wheel and the rail
under predominantly saturated lateral creep conditions. In small radius curves, saturated lateral creep
conditions occur due to the steering of railway wheelsets with large angles of attack. Given the large
curve radius and the utilisation of self-steering bogies on the Sishen-Saldanha Iron Ore railway line,
curve squeal is a highly unexpected result for the 1000 m radius curved railway line. This is because
curving of bogies in large radius curves are achieved without high wheelset angles of attack leading to
saturated creep conditions. An experimental and analytical investigation was carried out to identify
the mechanisms influencing the generation of wheel squeal in large radius curves.
Simultaneous measurement of sound pressure and lateral wheel-rail forces were made during regular
train service in one of the two 1000 m radius curves at Elands Bay to characterise the bogie curving
behaviour for tonal noise due to wheel squeal occurring in the large radius curve. The lateral force
curving signature not only reveals the levels of lateral wheel-rail forces required for bogie curving, but
also whether the bogie is curving by means of the creep forces generated at the wheel-rail interface
only or if contact is necessitated between the wheel flange and rail gauge corner to help steer the
bogie around the curve.
The test set-up consisted of two free field microphones radially aligned at equivalent distances
towards the in – and outside of the curve in line with a set a strain gauge bridges configured and
calibrated to measure the lateral and vertical forces on the inner and outer rail of the curve. This test
set-up allowed the squealing wheel to be identified from the magnitude difference of the sound
pressures recorded by the inner and outer microphones in combination with comparing the point of
frequency shift of the squeal event due to the Doppler Effect with the force signals of the radially
aligned strain gauge bridges.
From the experimental phase of the investigation, it was found that wheel squeal occurring in the
1000 m radius curve at Elands Bay is characteristic of empty wagons and is strongly related to the
squealing wheel’s flange/flange throat being in contact with the gauge corner of the rail. Here high
levels of spin creepage associated with high contact angles in the gauge corner lead to high levels of
associated lateral creepage necessary for squeal generation. This is in contrast to lateral creepage
due to high wheelset angles of attack being the key kinematic parameter influencing squeal
generation in small radius curves. Furthermore, the amplitude of wheel squeal originating from the
passing of empty wagons was found to be inversely proportional to the level of flange rubbing on the
squealing wheel i.e. increased flange contact on the squealing wheel brings about a positive effect on
squeal control. Contrary to the empty wagons which are characterised by tonal curve squeal, loaded
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wagons requiring contact between the wheel flange and rail gauge corner in the 1000 m curve was
characterised by broadband flanging noise. It was concluded from measurements that flange contact
occurring under high lateral forces for steady state curving of loaded wagons provides the complete
damping necessary for squeal control.
The curve squeal noise that originated from the passing of empty wagons in the Elands Bay curve
could further be classified according to the frequency at which the squeal event manifested itself in
the curve, i.e. low frequency audible (0 – 10 kHz), high frequency audible (10 – 20 kHz) and ultrasonic
squeal (> 20 kHz). The vast majority of low frequency audible squeal events recorded in the 1000 m
Elands Bay curve occurred at approximately 4 kHz and originated from the low rail/trailing inner wheel
interface, whilst the vast majority of high frequency audible squeal events occurred in the frequency
range between 15 and 20 kHz and originated from both the high rail/leading outer wheel and low
rail/trailing inner wheel interfaces.