Abstract
KEYWORDS – Self-excited vibration, Squeal, Disc brake, Dry friction, Coulomb friction
ABSTRACT - Most newer automobiles are fitted with disc brakes because of their high braking performance, favorable heat-dissipation characteristics, and limited maintenance requirements. However, disc brakes often generate a squeal noise due to friction during braking. This squeal is caused by a type of frictional self-excited vibration and can be quite unpleasant for drivers, passengers, and even nearby pedestrians. In luxury vehicles, brake squeal can even lead to customer complaints.
The present paper deals with disc brake generated in-plane squeal. The vibration modes of a brake unit during squealing are investigated and the relationship between the rate of occurrence of squeal and the rotation speed of the disc, as well as the brake pad pressure, are investigated. The present paper also deals analytically with inplane squeal using a concentrated mass model formed by the connection of massless beams, and the characteristics of in-plane squeal are investigated. The results are summarized below.
Based on the experiments conducted in the present study, the frequency of in-plane squeal was determined to be approximately 10 kHz. In-plane squeal is caused primarily by in-plane vibration of the disc. The disc vibrates in a coupled mode with the (8, 0) mode in the out-of-plane direction and the second mode in the in-plane direction. The characteristics of in-plane squeal are completely different from those of out-of-plane squeal. The generation probability of in-plane squeal changes according to the rotation speed and the braking pressure. The coefficient of friction has a negative slope with respect to the relative velocity between the disc and pads in unit A. Analysis revealed that the out-of plane pad vibration and two-diametrical-node vibration of the disc coupled by the second mode in the in-plane direction becomes excited due to Coulomb friction and can become more unstable by exciting in-plane vibration of the second mode by dry friction.
The bench test rig used in the present paper consists of a disc, a caliper, pads, a knuckle (which connects the brake unit to the car body), a hydraulic pump, and a geared motor. The in-plane squeal generated in this test rig is comparable to that generated in an actual vehicle. The analytical model used in the analysis was not faithfully equal to the dimensions of an actual disc brake unit because the goal of the analysis is to qualitatively, rather than quantitatively, obtain characteristics of in-plane squeal and to investigate the stability of the resulting selfexcited vibration system.
In-plane squeal is classified as a high-frequency squeal, but its characteristics are different from those of general, high-frequency, out-of-plane squeals. While in-plane squeal has been examined in a number of studies, the vibration generation mechanisms for in-plane squeal have yet to be clarified.
In the present study, we demonstrate that out-of plane pad vibration and two diametrical node vibration of disc coupled by the second mode in the in-plane direction becomes excited due to Coulomb friction. In addition, this unstable vibration can become more unstable, exciting in-plane vibration of the second mode by dry friction. The height of the hat of the disc contributes significantly to the coupling of in-plane and out-of-plane vibrations of the disc. This is the generation mechanism of in-plane squeal.