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What the roughness of train wheels says about noise and vibration
The tread of train wheels has a significant impact on the amount of noise and vibrations generated by a train. The irregular shape of the wheels causes additional vibration disturbance. The roughness of the wheels, together with that of the rail, is responsible for generating rolling noise. Fortunately, the roughness of the wheel can be kept under control through the choice of braking system and maintenance. By measuring the roughness, it can be determined when maintenance is required.
Why is wheel roughness important?
When a wheel rolls on the rails, vibrations occur in both the wheel and the rail. These vibrations occur because the surface of the rail and wheel contains bumps. This unevenness is called roughness. We can hear and feel these vibrations. On the one hand, these vibrations cause the track and vehicle to emit noise. In addition, some of the vibrations travel through the ground to the receiver. Thereby, the more rough the wheel and rail, the more noise is produced.
To reduce roughness, wheels are made smooth through turning. However, the roughness is not constant. After the wheels are smoothed off, it will increase again. This increase in wheel roughness is partly determined by the train's braking system. If brake pads are used, the wheel roughness increases after turning due to the action of the brake pads which are pressed onto the tread of the wheels during braking. The type of brake pads is important here: (traditional) cast-iron brake pads cause more wheel roughness than more modern composite pads. If the train uses disc brakes, then the roughness of the tread is not affected by the brake pads and tread roughness increases much less. Trains with cast iron block brakes therefore produce more noise than trains with composite brake blocks or disc brakes.
When do we measure wheel roughness?
We measure wheel roughness during approval testing of a train, where the passing noise is measured. Then the tread of the wheels should be in good condition and free from damage. To check this, we measure wheel roughness.
Wheel roughness also plays an important role in mapping rail roughness levels with ARRoW. The wheel roughness of the train being measured should be low enough to properly detect differences in rail roughness. If we know the wheel roughness of the train being measured in advance, we can determine the rail roughness much more accurately. Wheel roughness measurement offers a solution here too.
How do we measure wheel roughness?
We measure wheel roughness with Müller-BBM's m|wheel. This involves placing a mechanical probe against the tread of the wheel, and by rotating the wheel, we measure the roughness around the entire wheel. So the wheels should be able to rotate freely, while the bogie must still be supported.
