Christian Noël, Founder – CARE-TRACK
Pad influence on noise and ground vibrations
The rail is submitted to 2 kinds of forces: the static load of the train axles and the dynamic forces coming from the levelling and wheel/rail contact defects which generate dynamic forces, sources of rolling noise (high frequencies) and ground vibrations (low and medium frequencies).
Ground vibrations occur under the sleepers supporting the wheel load, rolling noise comes from the fastened rail vibrations generated by these defects and propagating along the rail on both sides of the contact. When the static load is stress dependent, the dynamic forces are strain dependent. An ideal pad should be: +soft at high load to reduce the impact of the dynamic loads on the sleepers. +stiff at low load to prevent long propagation of vibrations along the rail.
The axle is resiliently supported under the primary suspension, the rail lays on the rail pad and the sleeper lays resiliently on the ballast. Between these 3 actors, the rail is the lightest component and vibrates to «fellow» the wheel/rail contact imperfections, the rail reaction is governed by the bending moment of the rail and by the elasticity under the rail foot. Therefore the characteristics of the dynamic deflection under the rail, when the wheel is loading the sleeper, are very important. This deflection is the sum of the ballast and the pad deflections. The ballast deflection is non linear and its dynamic loop shows an important damping effect when the pad deflection can be linear or not with a smaller damping effect.
The secant dynamic stiffness leads to the static load share on the main sleeper when the the tangent dynamic stiffness drives the amplitude of the dynamic overloads. Most domestic pads (grooved, studded, foamed pads) exhibit non linear deflections which concentrate the dynamic overloads on the main sleeper (the secant stiffness increases with the load) when a constant stiffness pad fixes the share of the total load, secant and tangent stiffnesses are equal: a linear deflection leads to smaller dynamic overloads and reduced ground vibrations.
The decoupling frequency of the rail depends on the pad dynamic stiffness under the constant loading of the fastening clips. The decay rate of vibrations along the rail is related to the pad dynamic stiffness under small strain amplitude. For the same rail deflection under the wheel load, a linear pad exhibits a higher dynamic stiffness than a non linear one. The result is smaller vibration amplitude damped more quickly= less rolling noise.