INTRO: German Railway’s ICE3 high-speed trains will have just two brake discs per axle, compared to four on earlier ICE generations. Use of advanced brake pads with an improved contact pattern offers lower noise and a 50% increase in energy absorption
BYLINE: Dr-Ing Xaver Wirth
Senior Manager, Design & DevelopmentKnorr-Bremse SfS GmbH
THE DEVELOPMENT OF friction brakes for high-speed trains suffers from one fundamental law of physics: the kinetic energy of a vehicle increases with the square of its velocity. With relatively heavy trains running in the 250 to 300 km/h speed band, a very large amount of kinetic energy must be absorbed every time the train slows or stops.
Modern practice is to use electro-dynamic braking for most, if not all, service brake applications, and the ability to do this has been boosted by the current trend towards distributed traction drives, as on German Railway’s ICE3, with a greater number of motored axles spread through the train. However, it remains a condition of high-speed operation that, in the event of any failure of the electric brakes, during emergency brake applications virtually all of the energy will have to be absorbed by the friction disc brakes. Friction brakes must also absorb the bulk of the energy on loco-hauled vehicles, where dynamic braking is not available.
Today’s state-of-the-art brake discs for high-energy applications are assembled from steel hubs mounted on the axle, and friction rings connected to the hub in a manner that allows them to expand as they heat up during brake applications. The friction rings are mainly made from steel or ductile cast iron, such as nodular cast iron.
The brake pads normally used with these discs are made from organically-bonded composition materials, which can be pressed into large-area pad halves at reasonable cost (Fig 1a). However, these pads only provide an approximately uniform surface pressure and good contact pattern as long as their hardness and modulus of elasticity remain below specific limits. The contact pattern needs to be kept as constant as possible, so that the frictional energy, in the form of heat, is fed evenly into the brake disc. Otherwise the friction area will become overloaded in certain areas with a resultant risk of cracking.
This is the crux of the problem: the higher the thermal loading capacity of an organic brake pad, the higher its modulus of elasticity, and therefore the greater the potential damage that can be done to the disc.
Sinter pads harder
Metal sinter pads are capable of withstanding much higher temperatures, being harder by a power of ten when compared with organic pads. Fig 1b shows a typical sinter brake pad with separate friction elements attached to an intermediate pad support which provides for a more or less flexible action. With a nominal size of 40000 mm2, this pad offers a contact pattern which is just about tolerable on brake discs of heat-treated steel at medium levels of braking energy.
At high braking power levels, a poor contact pattern will result in local overheating of the friction surfaces, restricting the performance of the brake disc. Fig 3a (p480) is a thermographic picture of a disc during a brake application from 330 km/h using the sinter brake pads which are common today. It is noticeable that the disc shows hot spots at certain angles, with relatively cold areas in between.
In the higher-temperature zones, thermal expansion of the disc results in the development of ’high spots’ which then receive a disproportionately large share of the energy because surface pressure is higher here than in the cooler ’valleys’ between. If the yield point of the disc material is exceeded, residual tensile stress will develop after the disc has cooled, leading to the formation of so-called heat cracks.
Depending on the application and on material behaviour, heat cracks are only tolerable up to a specific length. Otherwise the forces of motion could cause the disc to break up, with fragments projected in all directions! Thus the risk of crack formation, along with the normal wear criteria, is a determining factor for the maximum permissible loading of a brake disc.
For this reason, today’s TGV and ICE trainsets are equipped with four discs per axle (right). This results in a heavy unsprung weight penalty; the brake discs on an ICE1 trainset total 520 kg per axle, whilst the four actuating calipers in the suspended section of the bogie add another 260 kg.
Lower weight and higher speed
For future vehicle generations, vehicle manufacturers are looking for lighter brake equipment able to operate at higher speeds. Knorr-Bremse decided that these specifications could not be met adequately by simply upgrading the conventional technology. Innovative solutions were required.
The first development was the aluminium-ceramic brake disc (RG 6.95 p357) which is now in production for a wide range of applications. These discs have been in use on the prototype København S-bane EMUs since early 1996, and an ICE1 trainset fully equipped with aluminium discs has been in operation since January 1997. Disc brakes using fibre-composite materials will be ready for series production in a few years. But equally important has been the development of the innovative Isobar brake pad, which offers a considerable reduction in weight as well as enhanced performance.
The name Isobar reflects the aim of equalising pressure at all points on the disc. The main objectives of the development project were:
