Abstract
Most automobiles nowadays are fitted with disc brakes because of their high braking performance, favorable heat-dissipation characteristics, and limited maintenance requirements. However, it is well known that disc brakes often generate a squeal noise, which is caused by friction during braking. A squeal is a type of frictional self-excited vibration that can be quite unpleasant for drivers, passengers, and even nearby pedestrians because the squeal is radiated from the disc brakes into the surrounding environment. In luxury vehicles, brake squeal can even lead to customer compliant, at great expense to the automaker. This 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 direction of rotation of the disc as well as the brake pad pressure are investigated. The effect of rubber installation on the brake pads on squeal is also investigated, as are the frictional characteristics of disc brakes. This paper also deals analytically with in-plane squeal by 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. The frequency of in-plane squeal was determined to be about 10 kHz by experiments conducted in this study. In-plane squeal is caused mainly by in-plane vibration of the disc. The disc vibrates in coupled mode with (8, 0) mode in the out-of-plane direction and 2nd 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, braking pressure, directionality of friction, and rubber installation on the pads. 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 two diametral node coupled by the 2nd mode in the in-plane direction becomes excited due to Coulomb friction. This phenomenon can become more unstable by being excited in-plane vibration of the 2nd mode by dry friction. The bench test rig used in this 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 has reproducible comparable with the squeal generated in an actual vehicle. In analysis, this analytical model proved not to be faithfully equal to the dimensions of an actual disc brake unit because the aim of this analysis is to obtain not quantitatively but qualitatively the characteristics of in-plane squeal and to investigate the stability of the resulting self-excited vibration system. In-plane squeal is becoming a serious problem recently. The phenomenon is classified as a high-frequency squeal, but its characteristics are different from those of general high-frequency out-of-plane squeals. While some reports have been published regarding in-plane squeal, the mechanisms of vibration generation of in-plane squeal have yet to be clarified. To summarize: the out-of plane two diametral node coupled by the 2nd mode in the in-plane direction becomes excited due to Coulomb friction, and the ring which is considered as disc also has an (8, 0) mode in the out-of-plane direction because the natural frequency of that mode is close to the frequency of unstable vibration. In addition, this unstable vibration can become more unstable to be excited in-plane vibration of the 2nd mode by dry friction. This is the generation mechanism of in-plane squeal.
KEYWORDS – Self-excited vibration, Squeal, Disc brake, Dry friction, Coulomb friction