Damping vibrations

Less noisy machinery may be on its way

A recent research project has demonstrated that thermal spraying can be used economically to dampen vibration on steel components such as wheels, which otherwise cause noise, premature fatigue and eventual failure.

It is possible to dampen vibration by exploiting a natural physical effect called magneto elasticity. Ferritic steels with a high chromium content have this damping effect, given heat treatment. However, chromium steels are costly and, therefore, are not often used.

However, recent research experiments by Sulzer Metco, the Ecole Polytechnique Federal de Lausanne and the Berne municipal transport services have revealed that a similar damping effect can be achieved by thermal spraying ferritic components with ferric-chromium aluminium alloys (Fe-Cr-Al) and heat treating them. This is more economic and opens the way towards much quieter trains and less noisy machinery, such as weaving machines. The City of Berne may use wheelsets damped in this way.

Magneto elasticity occurs because the structure of a ferric chrome alloy has countless different magnetic areas within it called Weiss domains. These are separated by domain boundaries called Bloch walls. Individual domains are magnetic in different directions.

If the alloy is vibrated, the domain walls shift and the spontaneous magnetism changes its direction, resulting in a change of volume. The vibratory movement is then absorbed by hysteresis effect or converted into heat, mainly by eddy currents, thus having a damping effect. This effect is increased by heat treatment as it releases the internal stresses which restrict the movement of the Bloch walls.

The thermal spraying method used was vacuum plasma spraying (VPS). The required coating (in this case Fe-Cr-Al) is fed in powered form into a plasma gas stream created by an electric arc ionising an inert gas, such as argon, in a spray gun. A second gas, normally hydrogen, is added to give heat control over the plasma, so that temperatures up to 16000 degreesC are achieved.

The coating particles impact against the surface of the component at a velocity of over 600m/s. The process takes place within a chamber in a pressure-controlled atmosphere to minimise oxidation. The result is a coating with a high density and cohesion.

When tested, the damping effect of the thermally sprayed component remained almost constant in the range from 10Hz up to 10kHz regardless of the vibration frequency.

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