INTRO: Reflecting increased concern over the environmental impact of railway operations, noise limits for new vehicles are being introduced by the European Union. Recent research has focused on wheels and braking systems as well as the track
BYLINE: Dr David Thompson and Dr Chris Jones
Institute of Sound & Vibration Research, University of Southampton
THE European Union’s programme to develop Technical Specifications for Interoperability (TSIs) includes setting noise limits for new trains under both static and running conditions. The TSI for high speed trains has already been introduced1, and that for conventional rolling stock is nearing completion.
As explained in an earlier article (RG 7.02 p.363), rolling noise is usually the dominant source of noise at speeds below 300 km/h and can be attributed to components radiated by both the vehicles and track. Vibration of both the wheel and rail is caused by surface ’roughness’ at their interface (Fig 1), with irregularities in the wavelength range from 5mm to 500mm mainly responsible for this. The relative importance of the components of sound radiation from the wheel and track depends on their respective design as well as on train speed and the wavelength content of the surface roughness, although in most cases both sources (wheel and track) are significant.
As the noise radiation depends on the roughness of both the wheel and track, it is possible that a rough wheel causes a high noise level that is mainly radiated by the track vibration or vice versa. It is therefore difficult to assign noise contributions solely to the vehicle or infrastructure.
In order to establish a noise limit for railway vehicles, a standard measurement method is required that gives a reliable and representative result. The ISO 3095 standard2 dating from 1975 is inadequate in this respect, as it merely states that the rail surface should be in good condition. Variations of up to 10dB are possible for smooth wheels within this condition on the track. A revised version of ISO 3095 is therefore in preparation, including a definition of a rail roughness limit below which the roughness spectrum of the test site should lie. The idea is that, during the running tests, the roughness should be dominated by that of the wheels.
Variability is also possible due to the track design at the test site, but this is not completely eliminated by the standard. The TSI for high speed trains, for instance, defines a suitable track for use in the test in terms of a minimum rail pad stiffness. The recently-completed Stairrs project3 funded by the EU included the development of experimental methods to separate the wheel and track components of the noise generated by a moving train, but these are not without their own difficulties.
Braking systems
To reduce rolling noise, the options are to reduce the roughness, to design the wheels and track to produce less noise for a given roughness, or to use barriers either on the train or at the trackside. From the vehicle designer’s point of view, the main feature affecting the wheel roughness is the braking system.
Traditional tread brakes, in which cast iron brake blocks act on the wheel tread, lead to the development of high levels of roughness on running surfaces due to the formation of local hot spots. This high roughness in turn leads to higher levels of rolling noise. With the advent of disc brakes, such as on the MkIII coach design introduced by British Rail in the mid-1970s, it became apparent that tread brakes are noisier. The difference between a MkIII and its tread-braked predecessor, the MkII, is about 10dB. Modern passenger rolling stock is mostly disc-braked for reasons of braking performance, and this brings with it lower noise levels.
However, train noise is usually dominated by freight traffic. Freight vehicles are generally noisier and often run at night when environmental limits are tighter. For freight traffic in Europe a number of factors have meant that tread brakes with cast iron brake blocks have remained the standard. These include cost, the longevity of wagons (typically 50 years) and, most importantly, UIC standards for international operation.
Since 1999 the UIC has been pursuing an initiative to replace cast iron with composite material4, which does not produce hot spots and therefore leaves the wheel relatively smooth. It was hoped that cast iron blocks could be replaced without further modification of the braking system, but in practice the composite blocks have a number of side effects including potentially higher temperatures. Development is therefore on-going and widespread introduction still a number of years away.
Wheel damping
Wheels form the main source of rolling noise from the vehicle, dominating the spectrum at high frequencies above 1·5kHz where wheel resonant modes occur with a large radial component. Wheels are normally very lightly damped, resembling a bell in shape. Impressive reductions in the reverberation of wheels can be achieved by simple damping measures. However, a wheel in rolling contact with the rail is, in effect, already damped to a considerably higher degree since vibration energy flows from the wheel into the track. To improve their rolling noise performance, the added damping must exceed this effective level of damping, which is one to two orders of magnitude higher than that of the free wheel.
Various devices have been developed to increase the damping of railway wheels by absorbing energy from their vibrations and thereby reduce the noise produced. These include multi-resonant absorbers (Fig 2a) which have been used in Germany for 20 years, and are fitted to many vehicles including ICE high speed trainsets. Noise reductions of 6dB to 8dB are claimed. Another commercial form of damper involves multiple layers of overlapping plates known as shark’s fin dampers (Fig 2b).
Constrained layer damping treatments (Fig 2c) consist of a thin layer of visco-elastic material applied to the wheel and backed by a thin, stiff constraining layer (usually metal). We were first involved in the use of such a treatment on the Class 150 DMU in the UK at BRResearch in the late 1980s. This was subsequently applied to the whole fleet to combat a particularly severe curve squeal problem excited by contact between the wheel flange and the check rail. Sufficient damping can also be achieved using constrained layer damping to make significant reductions in rolling noise5. Lucchini has developed the Syope wheel on this principle, and found that reductions in rolling noise of 4dB(A) to 5dB(A) were possible (RG 7.02 p571).
Wheel design
Reductions in the wheel component of radiated noise can also be achieved by careful attention to the cross-sectional shape of the wheel. In recent years manufacturers have used theoretical models such as TWINS6 to assist in designing wheels for low noise.
As an example of the difference that the cross-sectional shape can have, three wheels are shown in Fig 3. Wheel (a) is used on DB express passenger rolling stock, (b) is a UIC standard freight wheel and wheel (c) was designed several years ago by the Technical University of Berlin on the basis of scale model testing. Fig 4 shows the predicted noise components from each type of wheel in each case. The track component of noise (not shown) is not affected by these changes and remains the dominant source up to 1kHz.
The results show that a wheel with a straight web such as (c) is beneficial compared with a curved web. This is because the radial and axial motions are decoupled. However, it is not always possible to use a straight web with tread brakes, as the curve is included to allow thermal expansion. Wheel (a) is particularly noisy, the main difference between this and wheel (c) being the transition between the inside of the tyre and the web. If they have profiles similar to (a), wheels have shown appreciable noise reductions by the addition of absorbers. Increasing the web thickness and particularly the transition between the tyre and web are effective means of reducing noise but also lead to increased mass.
Another aspect of wheel design that can be used to reduce noise is the diameter. Smaller wheels have higher resonance frequencies, so it is possible by reducing the diameter to push most of the resonances out of the range of excitation, that is above 5kHz. The trend in recent years towards smaller wheels for other reasons is therefore advantageous for noise. This also negates the increase in unsprung mass caused by increases in thickness.
Under the EU Silent Freight project7 a shape-optimised wheel was designed that allowed for tread braking. It had a diameter of 860mm, compared with the reference wheel which measures 920mm, as well as a thicker web. The reduction in diameter was limited by the desire to allow retrofitting to bogies intended for 920mm diameter wheels. Reductions of 3dB in wheel component of noise were predicted from these changes.
Shrouds and perforations
A novel solution tested under the Silent Freight programme was the perforated wheel (photo p637), which aimed to introduce acoustic ’short-circuiting’ and so reduce sound radiation from wheel vibration. The benefit achieved was limited to frequencies below 1kHz.
Barriers can also be used to reduce sound radiation, and efficiency is improved by placing the barrier as close as possible to the source. Silent Freight found that for certain types of wheel, a shield mounted on the wheel to cover the web can reduce noise (Fig 5). A more general solution is to place an enclosure around the bogie. If used in combination with low trackside barriers placed very close to the rail, reductions of up to 10dB can be achieved. Bogie-mounted shields have also been used in the Low Noise Train demonstrator developed by railways in Austria, Italy and Switzerland (RG 4.03 p194).
Bogie shrouds and low trackside barriers were also tested by Silent Freight and its companion project Silent Track, but in this case the objective was to find a combination that satisfied international gauging constraints. Unfortunately, this meant that the overall reduction was limited to less than 3dB due to the inevitable gap between the top of the barrier and the bottom of the shroud8, which can be clearly seen in Fig 6.
There are many other practical difficulties in enclosing the bogies, such as ventilation for the brakes and access for maintenance. Nevertheless, such vehicle-mounted screens are common on light rail vehicles and bogie fairings have also been tested on TGV trainsets, but with the aim of reducing aerodynamic noise.
Curve squeal and other noise
As well as rolling noise, wheels may produce curve squeal, an intense tonal noise generated when the vehicle negotiates a curve. This is caused when a wheel resonance is excited by unsteady lateral frictional (creep) forces at the wheel-rail contact. This type of excitation arises because the coefficient of friction usually reduces as the wheel begins to slide on the rail.
Known solutions include lubrication using either grease or water or the application of friction modifiers that reduce the difference between static and sliding friction coefficients. A small increase in the level of wheel damping can also be effective in eliminating squeal. Solutions should also be sought in vehicle design in order to reduce creep, but this is often in conflict with the design of bogies for stability at high speed.
A research project is underway to develop models for this at the University of Southampton and Manchester Metropolitan University, as part of the national Rail Research UK programme. This aims to couple vehicle dynamics and acoustics modelling, and to use laboratory test rigs to assist in developing and validating models.
As well as the wheels and track, it may be expected that the bogie and vehicle superstructure may vibrate and radiate noise. However, tests under the Silent Freight programme indicated that the contribution from a Type Y25 bogie was about 15dB to 20dB less than that from the wheels, while the component from the wagon superstructure was lower still. These are therefore insignificant compared with the wheels, and attempts to reduce noise by treating the bogie frame are unlikely to be effective. Bogies without primary suspension between the wheelset and the bogie frame may not perform so well, however.
Other sources on the vehicle are important, particularly engines and exhaust systems on diesel units, and cooling fans such as traction motor blowers. The limit set for a stationary vehicle in the TSIs will impact particularly on these sources.
References
1. Commission Decision 2002/735/EC concerning the Technical Specification for Interoperability (TSI) relating to the rolling stock subsystem of the Trans-European high-speed rail system. Official Journal of the European Communities 12.9.2002 L245/402-506.
2. ISO 3095-1975 Railway applications - acoustics - measurement of noise by railbound vehicles.
3. Dings P. Characterisation and classification of railway noise sources. 7th International Workshop on Railway Noise, Portland, Maine, October 2001.
4. Hübner P. The Action Programme of UIC, UIP and CER ’Abatement of Railway Noise Emissions on Goods Trains’. Proceedings of Internoise 2001, Den Haag, Netherlands, pp107-112.
5. Jones C J C and Thompson D J. Rolling noise generated by wheels with visco-elastic layers. Journal of Sound & Vibration, 231, pp779-790, 2000.
6. Thompson D J, Hemsworth B, and Vincent N. Experimental validation of the TWINS prediction program for rolling noise, part 1: description of the model and method. Journal of Sound & Vibration, 193, pp123-135, 1996.
7. Hemsworth B, Gautier P E and Jones R. Silent Freight and Silent Track projects. Proceedings of Internoise 2000, Nice, France.
8. Jones R et al. Vehicle-mounted shields and low trackside barriers for railway noise control in a European context. Proceedings of Internoise 2000, Nice, France.
CAPTION: This perforated wheel was developed and tested under the Silent Freight research programme Photo: AEA Technology Rail
CAPTION: Fig 1. Simplified block diagram of rolling noise generation
CAPTION: Fig 2. Wheel damping devices: (a) tuned resonance devices; (b) shark’s fin dampers; (c) constrained-layer damping
CAPTION: Fig 3. Three wheel cross-sections: (a) as used on DB express passenger rolling stock; (b) a UIC standard freight wheel; (c) a wheel designed several years ago by the Technical University of Berlin on the basis of scale model testing
CAPTION: Fig 4. Effect of the wheel shapes in Fig 3 on the wheel component of railway noise
CAPTION: Fig 5. Wheel-mounted web shields were also tested as part of the Silent Freight programme Photo: AEA Technology Rail
CAPTION: Fig 6. This combined shield and barrier system achieved a limited reduction in noise because of the inevitable gap between the top of the lineside barrier and the bottom of the bogie shroud Photo: AEA Technology Rail
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