Abstract
M.Ing.
The aim of Spoornet is to provide a minimise predictable service. In order to provide
a predictable service, it is necessary to move trains safely and effectively from the place
of departure to their destination. The keywords here are safely and effectively.
Although support functions such as infrastructure and train control procedures are vital
in moving the train, the train or rolling stock as it is generally known, warrants some
attention.
Defects on the rolling stock are very costly to Spoornet. This is mainly due to the fact
that a defect on the rolling stock that goes undetected can cause damage to the rolling
stock and the infrastructure. This damage can eventually lead to derailments.
Considering that a derailment can cost Spoornet millions of rand and cause delays to
the services, it is only logical to spend time preventing derailments. It is for this reason
that a workgroup was formed to investigate and solve the problem of defects causing
derailments and delays by developing an early warning system.
The need for an integrated train condition monitoring system became apparent when
considering an early warning system. The objectives of the integrated train condition
monitoring system are to provide train condition information to different users, and
alarms on detection of emergency or dangerous conditions. Various train defects that
may cause damage or derailments were identified. One of them being a flat wheel on
a rail vehicle. A flat wheel is characterised by the flattening of the wheel on one or more positions on
its circumference, so that the wheel does not have an even and completely round
profile. Flat wheels are mostly caused by the wheels of a vehicle becoming locked
during braking, and sliding along the rail track. The friction created by this action
grinds a flat spot on the wheel.
The flat wheel leads to a decline in the riding quality of the rolling stock and a rise in
the levels of vibration and noise is evident. But more importantly, the flat spot causes
the wheel to roll unevenly, creating impacts on the rail on some points. It is these
impacts that can cause damage to the rail and the rolling stock. Depending on the length of the flat spot, the vehicle type and speed, the stresses may be sufficient to
cause final failure of the rail or initiate fatigue cracks in the rail. Severe flat wheels are
a safety hazard and can in some cases, cause derailments and consequent delays to
trains. Smaller flat spots contribute to track deterioration and so increase maintenance
costs by damaging the rails, sleepers and ballast. Flat wheels can thus be very costly
to Spoornet and its public image. In addition to safety and economic considerations,
wheel flats reduce the comfort levels in the passenger coaches and the noises they make
is annoying.
In an attempt to restrict the damage caused by flat wheels, most railway administrations
place a limit on the length of the flats that may stay in service. But to effectively find
a flat wheel on rolling stock is currently a very expensive exercise. Flat wheels can be
detected by an audible knocking sound when standing next to the rail. This sound is
impossible for the driver to hear and therefore goes undetected. Normally flat wheels
are detected by random inspection of the rolling stock or when they are brought in for
a routine service. The service cycle on rolling stock can be up to 24 months in
Spoornet. Considering that a flat wheel has an impact roughly every 3m, a serious flat
generates roughly 160 000 impacts on a single trip on the coal heavy haul export line.
It is therefore clear that a flat wheel can cause a considerable amount of damage
between service cycles. The severity of the problem is however not accurately defined in Spoornet, because up
to a few months ago there was no detection system in use to determine the distribution
of flat wheels. The research department of the Deutsche Bundesbahn however
considers rail fractures due to the flat wheels to be a serious problem with a significant
annual replacement cost.
There are thus sound safety and economic reasons for wishing to understand the
mechanisms of flat wheels and to develop an early warning system using an automatic
detector.