BYLINE: Ulrik Danneskiold-Samsøe
ChairmanØdegaard & Danneskiold-Samsøe A/S
ROUGH wheels rolling on smooth rail or vehicles with reprofiled wheels or new brake blocks running on rough rail give a combined roughness level which has a significant impact on the level of noise produced (Fig 1).
Readings taken during the recent commissioning of EMU cars built by Rotem of South Korea for MTR Corp of Hong Kong provide a good illustration of the importance of balancing roughness contributions from both the track and the vehicle. Fig 2 shows that the roughness levels of both wheel and rail were of the same order, although the wheel roughness was greater than the rail roughness in the important wavelength frequency interval between 10mm and 100mm, which determines the noise level. The combined roughness level was, however, so small that the cars complied with MTR specifications on operational noise.
How much rail grinding should be undertaken for the purposes of noise reduction? The answer is straightforward. Only grind to a level where the roughness after grinding is of the same order as the approximate level of wheel roughness. If noise levels need to be reduced further, wheel roughness should also be reduced in order to obtain a lower combined roughness.
The same argument applies to wheels, which should be reprofiled to a roughness level corresponding to the ambient rail roughness. To reduce noise levels in sensitive locations such as residential areas, ensuring high wheel surface quality may not be enough. It may also be necessary to grind the rails in order to provide a lower combined roughness.
The technology of roughness reduction is progressing. The use of composite rather than cast iron brake blocks results in a noise level which is approximately 10dB lower, provided the rails are sufficiently smooth. Furthermore, wheels fitted with high quality composite blocks may even generate less noise than disc-braked vehicles, because with tread brakes the wheel is surface is ’polished’ by the block.
This also has commercial benefits, as tread braking systems are generally cheaper to install and maintain than disc brakes. By applying this new technology, wagon owners should have no difficulty in complying with the noise requirements of the future Technical Specifications for Interoperability for conventional rail vehicles.
CAPTION: Fig 2. Rail (pink), wheel (blue) and combined (green) roughness levels measured during commissioning of Rotem-built cars in Hong Kong
CAPTION: Fig 1. Either high rail roughness or high wheel roughness is enough to cause a high noise level