Development of a Rubber for a Tuned Mass Damper for Rail Vibration A H Muhr For RIEG Discussion Meeting on Vibration Control with Elastomer Products DTR VMS, Trowbridge, 6 th June 2014 Based on PhD thesis of Nazirah Ahmad, co-supervised by David Thompson, ISVR, and Alan Muhr, TARRC Acknowledgments due also to Chris Jones (ISVR), Hamid Ahmadi & Judith Picken (TARRC) 1
Damping Vibration isolation 2
Muhr 1992 3
Vibration absorber (Den Hartog, 1928) p a = k m 2 2 2 c2 μ = 2m p 0 and - measure of the damping of the absorber For a given mass ratio β = infinite resonance peak at ω = p 0 = k1 m 1, optimum values of to two resonances, one at a higher frequency and one at a lower, each of height - nominal natural angular frequencies of absorber and main system m m 2 1 p 0 p p a 0 x1 P/k 1 and µ reduce the 2 + β = β 4
Absorber splits the single natural resonance into two, and damps both. The bigger its mass, the lower the peaks, and the wider the split. Optimisation achieved by choice of absorber frequency to give equal ordinates at S & T, and choice of absorber damping to make one or other of S & T a maximum wrt ω [Timoshenko, Young & Weaver, 1974, pp277-278] 5
6 Typical noise spectrum showing wheel, rail and sleeper noise components calculated using TWINS (Thompson et al., 1996)
Original prototype current product is bigger top mass thickness 1 thickness 2 visco-elastic material bottom mass A 2DOF tuned mass damper for rail web and foot was designed and tested in a collaboration between ISVR and Corus, and based on a castable PU after assessing 47 7 different polymers
The tuned mass damper attenuates propagation of flexural waves NOT a resonant vibration mode 8
Flexural waves in untreated rail (a) decay weakly with distance travelled SDOF Mass Absorbers, shown in (b), enhance the amplitude decay rate of flexural waves 9
Onset of flexural wave propagation decay rate of untreated ( ) and treated ( ) track (SDOF 10 dampers)
Effects on decay rate rail pad stiffness, no absorber rail pad stiffness, with absorber absorber stiffness absorber loss factor 11
Tests showed that it worked Measurement of total rolling noise at 100 km/h with and without rail absorbers Influence of wheel noise minimized by using a vehicle fitted with noise-reducing wheels. Average from three microphones at 3 m from near rail (Thompson et al., 2007) 12
But will it work at all temperatures? The temperature of rails in sunlight may differ from air temperature Rail temperature data is rare but exists for a site in Leominster Map data 2014 Google 13
temperature of rails in sunlight may differ from air temperature Air versus rail temperature for a Leominster site, UK. Data obtained at 14 10 minutes intervals from 1 st Feb to 30 th Nov [Chapman et al; 2008]
Objective of Nazirah s project: could optimisation of properties of the viscoelastic material reduce the noise further? DESIGN BRIEF: For -20 T 40 C, 1000Hz E = 14 MPa, 0.35 tan δ Cost of installed product to be minimised Longevity in the environment to be maximised MODIFIED DESIGN BRIEF: For -20 T 40 C, 300 freq 3000Hz 1.7 G / 8.3 MPa, 0.25 tan δ 15
Here and Next 3 slides give literature data at 500 or 1000Hz, T= -20, 0, 20 & 40 C for 33 polymers 16
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Best materials at start of project Nitrile blend Nazirah Ahmad MSc thesis PU, prior art for rail absorber 19
First, try linear viscoelastic (ie obeying Boltzmann s superposition principle) and thermorheologically simple (ie conforming to temperature-rate equivalence, as described by WLF) materials: Assume tanδ is constant over the specified temperature range T 1 T T 2 storage shear modulus G / depends on T according to: log 10 G ( T G ( T 1 2 ) ) 2 tanδ π 8.86( T2 Ts ) 101.6 + T2 T s 8.86( T1 Ts ) 101.6 + T T 1 s 20
4 tanδ = 0.2 tanδ = 0.35 tanδ = 0.5 3.5 3 log 10 (G'(T 1 )/G'(T 2 )) 2.5 2 1.5 1 0.5-100 -80-60 -40-20 0 20 40 T s / C 0 NR, IIR Ratio of G / at specified temperature extremes T 1 = -20 C, T 2 = 40 C as a function of choice of T s and tan δ 21
Ratio of G / plotted against temperature for T s = 20 C, normalised at 10 C. The curves represent values of tan δ = 0.05, 0.1., 0.2, 0.4 & 0.8 22
ISVR technique for dynamic properties Reasonably free of artefacts for 300 < freq < 3000Hz large seismic mass on soft rubber mounts thin pads for soft rubber to avoid wave effects at high freq, thicker for stiff rubber [eg at low temp] to avoid artefacts at low frequencies Stiff steel yoke Excitation from electrodynamic shaker c d Samples of resilient material (2 each side) Bolt for applying compression Piezoelectric force transducer Accelerometer High impedance foundation (large steel block) 23
24 effect of adding filler to butyl rubber at 1 khz. ( ) unfilled ( ) 40 pphr carbon black
25 Results for EPDM + black + oil at 1 khz. Because tanδ is lower, the variation of modulus with temperature is also lower.
The 2DOF damper has been installed on 160km of trackwork distributed in 16 countries See www.southampton.ac.uk/engineering/research/impact/ sustainable_expansion_of_rail_networks.page based on the original PU, not Nazirah Ahmad s materials 26
Conclusions railway rolling noise dominated by sleepers up to 400Hz rail for 400 to 1500Hz wheels at higher frequency noise is radiated from both lateral and vertical motion of the rail vertical contribution is more important Vibration absorbers are effective against both rail contributions require high tanδ and minimal frequency and temperature sensitivity of stiffness polymer science + high tanδ high frequency and temperature sensitivity of G / We have sought a best compromise by choosing a low T g (and hence T s ), fine tuning with plasticiser incorporating filler optimising choice for actual rail temperature range by using average noise reduction, weighted by frequency of occurrence of 27 temperature bands
References Ahmad N, Thompson D J, Jones C F C, Muhr A H 2009 Predicting the effect of T on the performance of elastomer-based rail damping devices J Sound & Vibration Research, 322, 674 689 Chapman L T, Thornes J E, Huang Y, Cai X, Sanderson V L & White S P (2008) Modelling of rail surface temperatures Theoretical & Applied Climatology, 92, 121 131 Muhr A H 1992 A comparison of rubber and metal springs for vibration isolation Plastics, Rubber & Composites, 18, 3-7 Thompson D J, Hemsworth B & Vincent N, 1996 Experimental validation of the Track-Wheel Interaction Noise Software prediction program for rolling noise I: Description of the model and method, J Sound & Vibration, 193, 123-135 Thompson D J, Jones C J, Waters T P & Farrington D 2007, A tuned damping device for reducing noise from raliway track, Applied Acoustics 678, 3 57 Thompson D J 2008 Railway Noise and Vibration publ Elsevier Timoshenko S P, Young D H & Weaver W, 1974, Vibration Problems in Engineering,4 th Edition, publ Wiley, New York, pp277-278 (not included in 5 th Edition, 1990) 28
More Vibration Absorber refs: C.M. Harris, 1987, Shock and Vibration Handbook, Third Edition, McGraw Hill. R.G. White and J.G. Walker, 1982, Noise and Vibration, Ellis Horwood Publishers. J.P. Den Hartog, 1956. Mechanical Vibrations, Second Edition.Dover Publications S.S. Rao, 2003, Mechanical Vibrations, Fourth Edition, Prentice Hall. Snowden JC 1968 [Ch 4 ] Vibration & Shock in Damped Mechanical Systems, publ Wiley 29