Identification of blocked efforts in the railway context
In the railway industry, blocked forces, i.e., residual forces that occur at the contact interfaces between mechanical components (such as bogie interfaces, components, and suspension elements), play a crucial role in interior acoustic quality. These forces can be identified using specific measurement methods.
The indirect method, standardized by ISO 20270:2019, is based on transfer function measurements using an impact hammer and accelerations for the different operating phases. It is particularly useful when force sensors cannot be used directly. In addition, the direct method, when feasible, measures forces using sensors installed at contact points, ensuring greater accuracy (under certain conditions of host structure rigidity).
Measurement methods based on specific processes for the railway industry
Measuring these forces requires the acquisition of precise parameters, such as frequency response functions (installation at standstill with impact hammer loading) and acceleration spectra for different operating conditions for the indirect method, and forces measured directly with force sensors for the direct method. Due to the different entry points and stresses on train bodies, these measurements are critical for analyzing the influence of blocked stresses on acoustic quality and certification. This also allows rolling stock manufacturers to specify blocked stresses for different components (HVAC, electrical equipment, etc.).
Consideration of railway equipment operating configurations
In the railway sector, operating configurations can vary considerably depending on operating conditions. This includes different rotating speeds on component parts (fan, compressor). Measurement tests must therefore take these different conditions into account to ensure that the forces measured are representative of operating situations.
Structural rigidity for accurate force measurements
Finally, for accurate direct measurements, when using direct methods to measure force, the test bench mounting structure must be sufficiently rigid within the frequency range relevant to railway equipment. A rigid mounting structure ensures that the force sensors only record forces that are not disturbed by support modes. Rigidity is therefore a key requirement in test benches for direct methods.
This is not the case for indirect measurement methods: measurements can be taken directly on the train or on a test bench without any particular constraints.
