A Study of Squeal Noise in Vehicle Brake System

Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition IMECE2011 November 11-17, 2011, Denver, Colorado, USA ASME 2011 International Mechanical Engineering Congress & Exposition ASME 2011 November 11-17, 2011, Denver, Colorado, USA

IMECE2011-62084 IMECE2011-62084 A STUDY OF SQUEAL NOISE IN VEHICLE BRAKE SYSTEM Xu Wang RMIT University Bundoora, Victoria, Australia

Sabu John RMIT University Bundoora, Victoria, Australia

ABSTRACT

He Ren RMIT University Bundoora, Victoria, Australia

thermal analysis are required. Many studies have recently been conducted to reduce the squeal noise using finite element numerical analysis [Hassan, et al, 2009].

Disc brake squeal can be classified as a form of frictioninduced vibration. Eliminating brake noise is a classic challenge in the automotive industry. This paper presents methods for analyzing the unstable vibration of a car disc brake. The numerical simulation has been conducted, and its results are compared with those from the experimental tests. The root causes of brake squeal noise will be identified. Potential solutions for elimination of the brake squeal noise will be proposed. Firstly, new materials and technologies for the disc brake application will be explored, secondly, it will be illustrated how to avoid the brake squeal noise problem from the brake system design. Brake disc design changes for improving cooling performance, and service solutions for brake squeal noise will be presented.

Thermal imaging camera was applied to analyze the heat distribution built up within the brake disc in an attempt to further understand what causes the disc to deform while the disc is in operation. Thermal imaging has shown that the vane pattern of the disc can cause a corresponding temperature profile on the surface of the brake disc. This relates to uneven heat transfer from the disc surface which should be avoided in order to minimize thermal distortion [Bryant, et al, 2008]. A polymer structure was considered for brake pads in which the partial stress does not increase in accordance with the dramatic changes in the real contact area when the polymer undergoes shear deformation, as the molecular structure of a polymer is capable of maintaining a high friction coefficient while reducing variation [Kenji, et al, 2009]. Friction experiments using parts formed from phenol resin and Polyamide-imide (PAI) were used to verify the improvements. The occurrence of brake squeal was halved by adopting a brake pad using PAI instead of phenol resin. A brake pad produced using the resin binder was found to be capable of improving brake squeal without reducing the effectiveness of the brakes. The squeal noise was reduced by approximately 20 dB with the brake friction coefficient at the same level [Kenji, et al, 2009]. Automotive brake squeal continues to challenge the industry engineers despite the efforts have been made to reduce its occurrence during the past years. The squeal noise has been a frequent source of complaint to many customers although it does not affect the performance of a vehicle. Brake squeal is high frequency noise (1 - 20 kHz) of a brake system, which is complex and influenced by many factors. Research into predicting and eliminating brake squeal has been conducted since 1930s. However, the root causes of the brake squeal noise are still not clear, effective design and service solutions need to be explored. In this paper, experimental tests and numerical modal analyses will be conducted to identify the root causes of the brake squeal noise. A study on materials and design of brake components will be carried out to define suitable design guidelines to reduce or to eliminate squeal problems.

INTRODUCTION Brake noise can be classified into brake judder occurring at 0 ~ 100 Hz, brake groan/moan at 100 ~ 500 Hz, brake howl at 500 ~ 1000 Hz, low frequency squeal at 1000 ~ 3000 Hz and high frequency squeal at 3000 ~ 16,000 Hz. All the brake noises are responses of self-excited vibration except for the brake judder which is response of forced vibration. Brake squeal occurs when the frequency falls in the range of 1,000-20,000 Hz and it results in a high pitched squeaky noise [Himanshu, et al, 2005]. When a vehicle is braked and decelerated, the kinematic energy of the rotating brake rotor is converted to thermal energy through friction at the interface of brake pad and rotor. The brake pads have high-friction surfaces and serve to slow the rotor down or even bring it to a complete halt. Frictional heat is generated on the rubbing surfaces due to the interaction between the pads and rotor disc. This action allows the rotor disc to absorb 90% of generated heat energy by means of conduction from the friction interface. The surface temperature of the brake rotor disc as well as the pads will rise. Disc brake squeal was generated due to both the thermal effects and the structural compliance of brake components. In addition to executing the instability study of a typical passenger car brake system using the complex eigenvalue analysis method, nonlinear contact pressure analysis and a fully coupled transient

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Copyright © 2011 by ASME


EXPERIMENTAL MEASUREMENT AND DATA ANALYSIS OF BRAKE SQUEAL NOISE The brake squeal noise was measured by microphones and analyzed using the software Head Acoustics Artemis. The experimental setting is shown in Figure 1. The experiment was carried out in a basement parking place in order to record clear braking sound without other background disturbance such as other cars or road noise. One microphone was installed inside the testing vehicle at the driver’s left ear. Two microphones were installed outside the vehicle at 7.5 meter away from either side of the testing vehicle and at a height of 1.2 meter from the ground. Both the two microphones (Model 330 Series) point toward the centre line of the vehicle. The testing vehicle had an odometer reading of 60000 km and was driven along the path with a length of 20 meter. As the testing vehicle was decelerated by braking with a deceleration of 0.5 g (4.9 m/s2) passing through the two external microphones, the sound pressure signals were recorded through sound level meter preamplifiers (Model 330 Series) and by laptop computers with sound card and the software Cooledit97. The sound pressure data was also recorded by the Head Measurement System (HMS) III in the front passenger seat and analyzed using the software Artemis. The noise spectra measured inside and outside the vehicle are shown in Figure 2 and Figure 3.

The high frequency brake squeal is identified at 6479 Hz. The main brake squeal noise problem for the test vehicle is seen to be a low frequency brake squeal. SPL amplitudes of the squeal noise at 1941 Hz inside and outside the vehicle are listed in Table 1. It is seen in Table 1 that the outside noise at the frequency had 10 dB(L) higher sound pressure amplitude level than the inside noise. Table 1 Sound pressure amplitude level comparison for the noise inside and outside the vehicle at 1941 Hz.

SPL (dB)

Inside 62

Outside 72

Figure 2: Sound pressure spectrum contours for squealing noise in different frequency ranges, recorded outside the vehicle (Max 95 dB(L)).

FINITE ELEMENT ANALYSIS

Figure 1 Microphone positions in the measurement.

It is clearly seen from Figures 2 and 3 that the noise peak frequency bands are around 375, 765, 1138, 1559, 1941 and 6479 Hz. The noise spectrum contour patterns measured inside and outside the vehicle are shown to be similar except that the spectrum amplitude level inside the vehicle is much lower than that outside the vehicle. This has established the correlation of the brake noise heard inside and outside. The low-frequency noise at 375 Hz is called “brake “groaning” while the low frequency noise at 765 Hz is called “brake howl”. Any noise having a frequency above 1000 Hz is considered a squeal [Hassan, et al, 2009]. The low frequency brake squeal noise frequencies are identified at 1138 Hz, 1559 Hz and 1941 Hz.

A numerical model of a disc brake system was formed using the software ANSYS. Meshing of the matching surfaces of the brake pad and the brake rotor, the pistons and the caliper, the locating stud of the caliper support and the locating hole of the brake caliper was conducted with special treatment. Meshing element of the matching surfaces is one-to-one corresponding. There are 20,000 elements in the model as shown in Figure 4. After the material properties of the components were imported into the model, an applied force, constraints/boundary conditions were defined for the model, normal modal analysis was conducted by the unsymmetrical ANSYS solver over the frequency range of 1-20 kHz, where all the natural frequencies and mode shapes of the brake system were solved. The modal shape results at the natural frequency of 6474 Hz are shown in Figure 5.

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It is seen from Figure 5 that the bending modes of the pads and disc have similar characteristics. These bending modes couple due to friction, which forms unstable modes and produce a squealing noise. Therefore, the geometry parameters and material properties of the braking system should be modified to eliminate the brake noise.

bending mode of the disc may couple to generate dynamic instability in the system.

Figure 5 Brake disc mode shape at 6474 Hz.

Figure 3 Sound pressure spectrum contours of the squealing noise in different frequency ranges, recorded inside the vehicle (max 85 dB(L)).

Figure 6 Free brake disc mode shape at 6343 Hz.

The bending modes of pads and disc are more significant than twisting modes, they eventually couple to produce squeal noise. The second bending mode of the pad has a frequency of 6640 Hz and the ninth bending mode of the disc has a frequency of 6343 Hz. These pad and disc bending modes may couple to produce an intermediate lock, resulting in a squeal noise at a frequency close to 6474 Hz, which is very close to the measured squeal frequency of 6479 Hz.

Figure 4 A FEA model of brake system.

It is important to determine the modal behavior of individual components (disc and pads) when predicting the brake squeal noise. A modal analysis performed on the free pad and free disc model will give insights into potential coupling modes. The natural frequencies and mode shapes of brake pads and disc can also be used to define the type of squeal noise that may occur in a braking system. As shown in Figure 6 and Figure 7, it is seen that the second bending mode of the pad and ninth

The finite element modal analyses for other natural frequencies near 1138, 1559, 1941 Hz also show similar results which illustrate that the brake squeal was caused by mode coupling occurring between the out-of-plane bending modes of the rotor and the brake pad. For higher modal frequencies, the finite element modal analyses shows in plane mode coupling occurring between the rotor and brake pad, although the higher frequency squeal noise was not noticed from the vehicle brake noise measurement data.

ROOT CAUSES OF BRAKE SQUEAL NOISE

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Coupled vibration of the brake rotor and pad generates an uncomfortable noise. Brake squeal noise was caused by large amplitude nonlinear vibration [Himanshu, et al, 2005]. The brake squeal at 6474 Hz is associated with frictional excitation couple occurring between the out-of-plane bending modes of the rotor and the brake pad, with a phenomenon known as modal “locking”. Modal locking of two or more modes of various structures provides appropriate conditions for brake squeal as the brake disc rotor typically vibrates with 2 to 4 nodal diameters. Higher frequency squeal is produced by friction induced excitation imparted on coupled resonance occurring between the in plane modes of the brake disc rotor and brake pad.

Correct selection of brake disc material is one of most important factors in elimination of brake squeal noise where the brake disc material and its surface treatment should have stable mechanical and frictional properties, high wear resistance through the range of expected service temperatures, high heat absorption capability, high thermal conductivity, high vibration damping capacity, minimal thermal expansion and high degree of corrosion resistance. Cast iron is a popular material for brake discs. Due to its properties and low cost in manufacturing, it has been widely used in disc brake system. Furthermore, some new technologies such as coatings and surface treatments can be applied to cast iron discs to eliminate the brake squeal noise to prevent or minimize brake disc surface corrosion. These include alloying to improve thermal conductivity and/or wear resistance, alloying or heat treatments to modify the microstructure for improved vibration damping, composites of gray iron and other metals or ceramics.

Figure 7 Free pad mode shape at 6640 Hz.

There are several factors that influence the brake squeal noise, they are: • • • • • • • •

rough finish on resurfaced rotor discs loose fitting brake pads inside the brake calipers lack of silicone compound on the back of brake pad missing springs or anti-rattle clips that should be on the caliper or pad improper tightening sequence of lug nuts or caliper hardware contamination of the brake pad such as that caused by leaked brake fluid humidity weather temperature variations of brake disc and pads

Brake disc surface conditions contribute to the brake squeal noise generation. Main brake disc surface damage patterns which would induce the brake squeal noise are given in Figure 8.

SOLUTIONS FOR BRAKE SQUEAL NOISE

Figure 8 Main brake disc surface damage patterns causing brake squeal noise.

Some new materials, such as aluminum metal matrix composites, are a class of metal matrix composites in which an aluminum matrix is reinforced with ceramic particles, whiskers, or short fibers. These materials have the potential for redefining the property limits of aluminum materials because of their unique combinations of properties. For example, they would provide the stiffness of titanium, better wear resistance than

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steel, and tailor-able coefficient of thermal expansion, all while maintaining the light weight characteristics of aluminum. Another brake disc material silicon carbide (SiC) is a compound of silicon and carbon with chemical formula SiC. Grains of silicon carbide can be bonded together by sintering to form very hard ceramics which are widely used in brake disc applications requiring high endurance, because its operational temperatures are not limited by brake disc rotor material, and its frictional properties are better at higher temperatures. It is light weight despite of a high cost.

brake squeal noise generation and control, which needs further studies in future.

Brake disc shape should also be fine-tuned for fin configurations and inlet outlet fin geometries to maximize airflow for effective heat removal during braking to attain high cooling performance. Cooling performance can be achieved through optimization using CFD.

1. Hassan, Muhammad Z., Brooks, Peter C. and Barton, David C. (2009). Thermo-Mechanical Contact Analysis of Car Disc Brake Squeal, SAE Int. J. of Passeng. Cars – Mech. Syst., Volume 1, No.1, pp 1230-1239. 2. Himanshu, M., Wayne, N., Tom, K., Louis, K., and Erwin, J. (2005), Brake Analysis and NVH Optimization Using MSC.NASTRAN, www.mscsoftware.com/support/library/conf/auto99/p01699. pdf 3. Bryant, D., Fieldhouse, J., Crampton, A. and Talbot, C. (2008). Thermal Brake Judder Investigations Using a High Speed Dynamometer, SAE Paper 2008-01-0818. 4. Kenji A., Masaaki N., Yukihiro S., Yasuo F., Hiromichi Y., and Igor S. (2009). A Study on Friction Materials for Reducing Brake Squeal By Nanotechnology, TOYOTA Technical Review, Volume 56, No.2, August, 2009, pp85 – 89.

Matching of brake pads and brake disc materials plays an important role in the brake squeal noise generation and control, which needs a lot of fundamental material property studies and tests. Anti-squeal shim is one of effective service solutions to reduce the brake squeal noise. The shim is installed on the backside of the pads, between pads and caliper pistons. The shim has a sandwich structure of constrained layer damping with two steel plates separated by a viscous-elastic core shown in Figure 9. This shim is a very thin and can be attached onto the back-plate of brake pad, which attenuates the vibration energy from the brake pad. The shims provide a permanent vibration damper and reduce the vibration transmission from the brake pad to vehicle chassis.

New brake disc materials, better surface treatments and cooling designs, will help to reduce the possibility of the brake squeal noise generation. Service solutions such as adding antisqueal shims and greases are recommended for elimination of the brake squeal noise. REFERENCES

Alternatively, anti-squeal grease can be applied onto the back of the pads when the brake pads are removed as shown in Figure 10. Anti-squeal grease is a kind of high-temperature silicon grease. This may be one of the low cost solutions for elimination of the brake squeal noise. CONCLUSIONS Vehicle brake squeal noise has been studied. Vehicle measurement and finite element analysis simulation have been conducted to identify the root causes of the brake squeal noise. Potential solutions for elimination of the brake noise have been recommended. It is concluded that the disc surface finish, installation quality, weather conditions, and contamination on the brake pads all contribute to the brake squeal noise. The brake squeal noise which was measured from the vehicle tests was caused by the frictional excitation couple occurring between the out-ofplane bending modes of the rotor and the brake pad. Higher frequency squeal is produced by friction induced excitation imparted on coupled resonance occurring between the in plane modes of the brake disc rotor and brake pad. Matching of brake pads and brake disc materials plays an important role in the

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Friction material -pad

Back plate

Shim Steel VE core

Pad Brake disc

Back plate

Shim

Steel Adhesive Pad

Brake disc

Figure 9 Anti-squeal Shim

Figure 10 Anti-squeal Grease

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