14 Sep 2004
High speed infrared cameras may boost the stopping power of express trains traveling at speeds of up to 300 km/h.
State-of-the-art infrared imaging is optimizing the performance of braking systems on France's famous Train à Grande Vitesse (TGV) express trains. The French study, commissioned by the national rail operator SNCF, involves the infrared camera specialist CEDIP, the brake pad manufacturer Flertex and the National Center for Scientific Research (CNRS).
As CEDIP spokesman Pierre Bremond explains, for a TGV train traveling at 300 km/h (186 mph) to make an emergency stop within the mandatory 3500 m each brake disc has to dissipate a staggering 14 MJ of energy within just 80 s.
In order for the train to decelerate safely and effectively it is important that the brake disc manages this heat load efficiently. The generation of excessive 'hot spots' could cause parts to deform or fail with potentially disasterous consequences.
"The aim of the study is to better classify and to explain the thermal gradients on the surface of the [brake] disc," said Bremond. "Hot spots count among the most dangerous phenomena in frictional parts leading to damage and early failure. These high local temperatures may also lead to unacceptable braking performance such as brake fade or undesirable low frequency vibrations called hot judder."
To take a closer look at the performance of different brake pad materials and brake designs, the team is employing CEDIP's latest high-speed infrared camera (JADE). The camera takes an infrared image of a steel brake disc (640 mm diameter, 45 mm thick) in action, effectively capturing a thermal snap-shot of the disc showing its temperature distribution and ability to dissipate heat.
Operating in the 3 - 5 micron waveband at a 200 Hz frame rate, the camera was able to measure hotspots ranging from 300 up to 1200 degC on the disc's braking surface.
Using an experimental rig to simulate TGV rolling stock, temperature measurements were made while the brake disk was spinning and during its braking phase. The team was able to freeze-frame the disc's motion by reducing the camera's integration time down to 10 microseconds.
"Thanks to the high frequency and high resolution of JADE cameras, our observations give new information on the conditions of hot spot appearance," said Bremond. "Thermal low cycle fatigue may be occurring and our first results show a relation with the development of cracks on the disk surface."
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