This is especially the case in braking systems, where friction is both the principal performance factor and a potential cause of detrimental vibrations, noise, and excessive wear. In aircraft braking systems friction induced oscillations can lead to excessively high loads in the landing gear and the brake structure, with consequences ranging from noticeable human discomfort to structural failures of the brake components.
Squeal vibration is characterized by torsional motion of stators, torque tube, and piston housing.
Typical frequency range is between 100 Hz and 20 kHz. Squeal usually occurs during landing stops, but it can also be observed during taxis. Higher contact pressures and higher energies increase the severity of this type of vibration. Squeal can be excited by the characteristics of the friction material and by modal coupling between axial and tangential degrees of freedom of the brake system. 100 Hz to 300 Hz, roughly the same as the first squeal mode.