Interface - The Journal of Wheel/Rail Interaction

MASS-SPRING SYSTEMS

Using High-Performance Mass-Spring-Systems to Reduce Noise and Vibration in Track
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Steel Coil Springs
Neither the excitation frequency spectra generated by the trains nor the resonances of the affected building slabs along a railroad are homogeneous. That’s why the choice of a low-tuned mass-spring system is generally the safer option. The system tuning frequency is determined by the choice of the mass as well as by the elasticity of the spring layer. Here, steel coil springs provide a real advantage. Their high elasticity makes a low-tuned, high-performing mass-spring system possible even if the available mass is comparatively small. Steel coil springs also provide:

• linear load-displacement curve.

• no reduction in elasticity in the dynamic range.

• high vertical and horizontal load capability.

• high attenuation levels also in the audible range.

• high durability.

High-performance mass-spring systems incorporating steel coil springs with a tuning frequency between 5 and 9 Hz have been successfully used by trams, metro, heavy rail, freight and high-speed railways over the past 15 years. Mass-spring system designs (using several types of steel springs and spring elements) have been used in tunnels, above ground, and on bridges and viaducts, where mitigation of structure-borne noise and vibration is important.

Construction and track-kinematical aspects are decisive in determining the mass and shape of the concrete slabs. Optimization can result in a cost-saving reduction of the tunnel diameter and may represent the only option in a tunnel in which space is limited.

Figure 6 shows the high transmission loss levels achieved by a 6.7-Hz system relative to the excitation frequencies. The curve measured on the test stand (left) confirms the theoretically produced curve (right), and illustrates the mitigating effect of a set of steel springs (outer and nested) when used in trackbed isolation. This approach is required to achieve high insertion loss levels, represented by mitigation of an initial structure-borne noise at a specified point in a propagation path by application of an elastic element (1). Insertion loss represents the actual dynamic efficiency of a mass-spring system by taking the contributions of the local parameters, such as soil conditions, lengths of transmission paths, structural masses and frequencies, etc. into account. Still, the opportunity to measure immission levels under same overall conditions with and without the use of a mass-spring system rarely occurs. However, the use of an elastic bearing system with a low natural frequency that provides high transmission loss levels ensures high attenuation over the entire relevant frequency range. Compared to a higher tuned system, there is a clear advantage in the lower frequency range of mechanical vibrations, and in the higher frequency acoustic range, as well (see Figure 6).

Costs
Mass-spring systems are designed and custom-built to meet the specific requirements that are dictated by construction and train operation issues. They are tuned to obtain a defined immission reduction, depending on the application. When determining the cost of a mass-spring system, overall system costs, which include the pure spring material as well as construction and installation costs, must be considered. Overall system costs typically range between $1,500 and $4,000 (USD) or more per meter of track. Longevity and maintenance issues should also be taken into consideration.

Hans-Georg Wagner is Head of Building and Trackbed Isolation, GERB Schwingungsisolierungen GmbH & Co.KG

References
(1) DIN 45672-2, Vibration measurements in the vicinity of rail roads, 07.1995, Appendix B

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