A Study Of Squeal Noise In Vehicle Brake System

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Proceedings of the ASME 2011 International Mechanical Engineering Congress & ExpositionIMECE2011November 11-17, 2011, Denver, Colorado, USAASME 2011 International Mechanical Engineering Congress & ExpositionASME 2011November 11-17, 2011, Denver, Colorado, USAIMECE2011-62084IMECE2011-62084A STUDY OF SQUEAL NOISE IN VEHICLE BRAKE SYSTEMXu WangRMIT UniversityBundoora, Victoria, AustraliaSabu JohnRMIT UniversityBundoora, Victoria, AustraliaABSTRACTHe RenRMIT UniversityBundoora, Victoria, Australiathermal analysis are required. Many studies have recently beenconducted to reduce the squeal noise using finite elementnumerical analysis [Hassan, et al, 2009].Disc brake squeal can be classified as a form of frictioninduced vibration. Eliminating brake noise is a classic challengein the automotive industry. This paper presents methods foranalyzing the unstable vibration of a car disc brake. Thenumerical simulation has been conducted, and its results arecompared with those from the experimental tests. The rootcauses of brake squeal noise will be identified. Potentialsolutions for elimination of the brake squeal noise will beproposed. Firstly, new materials and technologies for the discbrake application will be explored, secondly, it will beillustrated how to avoid the brake squeal noise problem fromthe brake system design. Brake disc design changes forimproving cooling performance, and service solutions for brakesqueal noise will be presented.Thermal imaging camera was applied to analyze the heatdistribution built up within the brake disc in an attempt tofurther understand what causes the disc to deform while the discis in operation. Thermal imaging has shown that the vanepattern of the disc can cause a corresponding temperatureprofile on the surface of the brake disc. This relates to unevenheat transfer from the disc surface which should be avoided inorder to minimize thermal distortion [Bryant, et al, 2008].A polymer structure was considered for brake pads inwhich the partial stress does not increase in accordance with thedramatic changes in the real contact area when the polymerundergoes shear deformation, as the molecular structure of apolymer is capable of maintaining a high friction coefficientwhile reducing variation [Kenji, et al, 2009]. Frictionexperiments using parts formed from phenol resin andPolyamide-imide (PAI) were used to verify the improvements.The occurrence of brake squeal was halved by adopting a brakepad using PAI instead of phenol resin. A brake pad producedusing the resin binder was found to be capable of improvingbrake squeal without reducing the effectiveness of the brakes.The squeal noise was reduced by approximately 20 dB with thebrake friction coefficient at the same level [Kenji, et al, 2009].Automotive brake squeal continues to challenge theindustry engineers despite the efforts have been made to reduceits occurrence during the past years. The squeal noise has beena frequent source of complaint to many customers although itdoes not affect the performance of a vehicle. Brake squeal ishigh frequency noise (1 - 20 kHz) of a brake system, which iscomplex and influenced by many factors. Research intopredicting and eliminating brake squeal has been conductedsince 1930s. However, the root causes of the brake squeal noiseare still not clear, effective design and service solutions need tobe explored. In this paper, experimental tests and numericalmodal analyses will be conducted to identify the root causes ofthe brake squeal noise. A study on materials and design of brakecomponents will be carried out to define suitable designguidelines to reduce or to eliminate squeal problems.INTRODUCTIONBrake noise can be classified into brake judder occurring at0 100 Hz, brake groan/moan at 100 500 Hz, brake howl at500 1000 Hz, low frequency squeal at 1000 3000 Hz andhigh frequency squeal at 3000 16,000 Hz. All the brake noisesare responses of self-excited vibration except for the brakejudder which is response of forced vibration. Brake squealoccurs when the frequency falls in the range of 1,000-20,000Hz and it results in a high pitched squeaky noise [Himanshu, etal, 2005].When a vehicle is braked and decelerated, the kinematicenergy of the rotating brake rotor is converted to thermal energythrough friction at the interface of brake pad and rotor. Thebrake pads have high-friction surfaces and serve to slow therotor down or even bring it to a complete halt. Frictional heat isgenerated on the rubbing surfaces due to the interactionbetween the pads and rotor disc. This action allows the rotordisc to absorb 90% of generated heat energy by means ofconduction from the friction interface. The surface temperatureof the brake rotor disc as well as the pads will rise. Disc brakesqueal was generated due to both the thermal effects and thestructural compliance of brake components. In addition toexecuting the instability study of a typical passenger car brakesystem using the complex eigenvalue analysis method, nonlinear contact pressure analysis and a fully coupled transient11Copyright 2011 by ASMECopyright 2011 by ASME

EXPERIMENTAL MEASUREMENTAND DATA ANALYSIS OF BRAKE SQUEAL NOISEThe brake squeal noise was measured by microphones andanalyzed using the software Head Acoustics Artemis. Theexperimental setting is shown in Figure 1.The experiment was carried out in a basement parking place inorder to record clear braking sound without other backgrounddisturbance such as other cars or road noise. One microphonewas installed inside the testing vehicle at the driver’s left ear.Two microphones were installed outside the vehicle at 7.5 meteraway from either side of the testing vehicle and at a height of1.2 meter from the ground. Both the two microphones (Model330 Series) point toward the centre line of the vehicle. Thetesting vehicle had an odometer reading of 60000 km and wasdriven along the path with a length of 20 meter. As the testingvehicle was decelerated by braking with a deceleration of 0.5 g(4.9 m/s2) passing through the two external microphones, thesound pressure signals were recorded through sound level meterpreamplifiers (Model 330 Series) and by laptop computers withsound card and the software Cooledit97. The sound pressuredata was also recorded by the Head Measurement System(HMS) III in the front passenger seat and analyzed using thesoftware Artemis. The noise spectra measured inside andoutside the vehicle are shown in Figure 2 and Figure 3.The high frequency brake squeal is identified at 6479 Hz. Themain brake squeal noise problem for the test vehicle is seen tobe a low frequency brake squeal. SPL amplitudes of the squealnoise at 1941 Hz inside and outside the vehicle are listed inTable 1. It is seen in Table 1 that the outside noise at thefrequency had 10 dB(L) higher sound pressure amplitude levelthan the inside noise.Table 1 Sound pressure amplitude level comparison for the noiseinside and outside the vehicle at 1941 Hz.SPL (dB)Inside62Outside72Figure 2: Sound pressure spectrum contours for squealing noise indifferent frequency ranges, recorded outside the vehicle (Max 95dB(L)).FINITE ELEMENT ANALYSISFigure 1 Microphone positions in the measurement.It is clearly seen from Figures 2 and 3 that the noise peakfrequency bands are around 375, 765, 1138, 1559, 1941 and6479 Hz. The noise spectrum contour patterns measured insideand outside the vehicle are shown to be similar except that thespectrum amplitude level inside the vehicle is much lower thanthat outside the vehicle. This has established the correlation ofthe brake noise heard inside and outside. The low-frequencynoise at 375 Hz is called “brake “groaning” while the lowfrequency noise at 765 Hz is called “brake howl”. Any noisehaving a frequency above 1000 Hz is considered a squeal[Hassan, et al, 2009]. The low frequency brake squeal noisefrequencies are identified at 1138 Hz, 1559 Hz and 1941 Hz.A numerical model of a disc brake system was formedusing the software ANSYS. Meshing of the matching surfacesof the brake pad and the brake rotor, the pistons and the caliper,the locating stud of the caliper support and the locating hole ofthe brake caliper was conducted with special treatment.Meshing element of the matching surfaces is one-to-onecorresponding. There are 20,000 elements in the model asshown in Figure 4.After the material properties of the components wereimported into the model, an applied force, constraints/boundaryconditions were defined for the model, normal modal analysiswas conducted by the unsymmetrical ANSYS solver over thefrequency range of 1-20 kHz, where all the natural frequenciesand mode shapes of the brake system were solved. The modalshape results at the natural frequency of 6474 Hz are shown inFigure 5.2Copyright 2011 by ASME

It is seen from Figure 5 that the bending modes of thepads and disc have similar characteristics. These bending modescouple due to friction, which forms unstable modes and producea squealing noise. Therefore, the geometry parameters andmaterial properties of the braking system should be modified toeliminate the brake noise.bending mode of the disc may couple to generate dynamicinstability in the system.Figure 5 Brake disc mode shape at 6474 Hz.Figure 3 Sound pressure spectrum contours of the squealing noisein different frequency ranges, recorded inside the vehicle (max 85dB(L)).Figure 6 Free brake disc mode shape at 6343 Hz.The bending modes of pads and disc are more significant thantwisting modes, they eventually couple to produce squeal noise.The second bending mode of the pad has a frequency of 6640Hz and the ninth bending mode of the disc has a frequency of6343 Hz. These pad and disc bending modes may couple toproduce an intermediate lock, resulting in a squeal noise at afrequency close to 6474 Hz, which is very close to the measuredsqueal frequency of 6479 Hz.Figure 4 A FEA model of brake system.It is important to determine the modal behavior ofindividual components (disc and pads) when predicting thebrake squeal noise. A modal analysis performed on the free padand free disc model will give insights into potential couplingmodes. The natural frequencies and mode shapes of brake padsand disc can also be used to define the type of squeal noise thatmay occur in a braking system. As shown in Figure 6 and Figure7, it is seen that the second bending mode of the pad and ninthThe finite element modal analyses for other naturalfrequencies near 1138, 1559, 1941 Hz also show similar resultswhich illustrate that the brake squeal was caused by modecoupling occurring between the out-of-plane bending modes ofthe rotor and the brake pad. For higher modal frequencies, thefinite element modal analyses shows in plane mode couplingoccurring between the rotor and brake pad, although the higherfrequency squeal noise was not noticed from the vehicle brakenoise measurement data.ROOT CAUSES OF BRAKE SQUEAL NOISE3Copyright 2011 by ASME

Coupled vibration of the brake rotor and pad generates anuncomfortable noise. Brake squeal noise was caused by largeamplitude nonlinear vibration [Himanshu, et al, 2005]. Thebrake squeal at 6474 Hz is associated with frictional excitationcouple occurring between the out-of-plane bending modes ofthe rotor and the brake pad, with a phenomenon known asmodal “locking”. Modal locking of two or more modes ofvarious structures provides appropriate conditions for brakesqueal as the brake disc rotor typically vibrates with 2 to 4nodal diameters. Higher frequency squeal is produced byfriction induced excitation imparted on coupled resonanceoccurring between the in plane modes of the brake disc rotorand brake pad.Correct selection of brake disc material is one of mostimportant factors in elimination of brake squeal noise where thebrake disc material and its surface treatment should have stablemechanical and frictional properties, high wear resistancethrough the range of expected service temperatures, high heatabsorption capability, high thermal conductivity, high vibrationdamping capacity, minimal thermal expansion and high degreeof corrosion resistance.Cast iron is a popular material for brake discs. Due to itsproperties and low cost in manufacturing, it has been widelyused in disc brake system. Furthermore, some new technologiessuch as coatings and surface treatments can be applied to castiron discs to eliminate the brake squeal noise to prevent orminimize brake disc surface corrosion. These include alloyingto improve thermal conductivity and/or wear resistance,alloying or heat treatments to modify the microstructure forimproved vibration damping, composites of gray iron and othermetals or ceramics.Figure 7 Free pad mode shape at 6640 Hz.There are several factors that influence the brake squealnoise, they are: rough finish on resurfaced rotor discsloose fitting brake pads inside the brake caliperslack of silicone compound on the back of brake padmissing springs or anti-rattle clips that should be onthe caliper or padimproper tightening sequence of lug nuts or caliperhardwarecontamination of the brake pad such as that caused byleaked brake fluidhumidity weathertemperature variations of brake disc and padsBrake disc surface conditions contribute to the brakesqueal noise generation. Main brake disc surface damagepatterns which would induce the brake squeal noise are given inFigure 8.SOLUTIONS FOR BRAKE SQUEAL NOISEFigure 8 Main brake disc surface damage patterns causing brakesqueal noise.Some new materials, such as aluminum metal matrixcomposites, are a class of metal matrix composites in which analuminum matrix is reinforced with ceramic particles, whiskers,or short fibers. These materials have the potential for redefiningthe property limits of aluminum materials because of theirunique combinations of properties. For example, they wouldprovide the stiffness of titanium, better wear resistance than4Copyright 2011 by ASME

steel, and tailor-able coefficient of thermal expansion, all whilemaintaining the light weight characteristics of aluminum.Another brake disc material silicon carbide (SiC) is acompound of silicon and carbon with chemical formula SiC.Grains of silicon carbide can be bonded together by sintering toform very hard ceramics which are widely used in brake discapplications requiring high endurance, because its operationaltemperatures are not limited by brake disc rotor material, and itsfrictional properties are better at higher temperatures. It is lightweight despite of a high cost.brake squeal noise generation and control, which needs furtherstudies in future.Brake disc shape should also be fine-tuned for finconfigurations and inlet outlet fin geometries to maximizeairflow for effective heat removal during braking to attain highcooling performance. Cooling performance can be achievedthrough optimization using CFD.1. Hassan, Muhammad Z., Brooks, Peter C. and Barton, DavidC. (2009). Thermo-Mechanical Contact Analysis of CarDisc 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 ry/conf/auto99/p01699.pdf3. Bryant, D., Fieldhouse, J., Crampton, A. and Talbot, C.(2008). Thermal Brake Judder Investigations Using a HighSpeed 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 forReducing Brake Squeal By Nanotechnology, TOYOTATechnical Review, Volume 56, No.2, August, 2009, pp85 –89.Matching of brake pads and brake disc materials plays animportant role in the brake squeal noise generation and control,which needs a lot of fundamental material property studies andtests. Anti-squeal shim is one of effective service solutions toreduce the brake squeal noise. The shim is installed on thebackside of the pads, between pads and caliper pistons. Theshim has a sandwich structure of constrained layer dampingwith two steel plates separated by a viscous-elastic core shownin Figure 9. This shim is a very thin and can be attached ontothe back-plate of brake pad, which attenuates the vibrationenergy from the brake pad. The shims provide a permanentvibration damper and reduce the vibration transmission from thebrake pad to vehicle chassis.New brake disc materials, better surface treatments andcooling designs, will help to reduce the possibility of the brakesqueal noise generation. Service solutions such as adding antisqueal shims and greases are recommended for elimination ofthe brake squeal noise.REFERENCESAlternatively, anti-squeal grease can be applied onto theback of the pads when the brake pads are removed as shown inFigure 10. Anti-squeal grease is a kind of high-temperaturesilicon grease. This may be one of the low cost solutions forelimination of the brake squeal noise.CONCLUSIONSVehicle brake squeal noise has been studied. Vehiclemeasurement and finite element analysis simulation have beenconducted to identify the root causes of the brake squeal noise.Potential solutions for elimination of the brake noise have beenrecommended.It is concluded that the disc surface finish, installationquality, weather conditions, and contamination on the brakepads all contribute to the brake squeal noise. The brake squealnoise which was measured from the vehicle tests was caused bythe frictional excitation couple occurring between the out-ofplane bending modes of the rotor and the brake pad. Higherfrequency squeal is produced by friction induced excitationimparted on coupled resonance occurring between the in planemodes of the brake disc rotor and brake pad. Matching of brakepads and brake disc materials plays an important role in the5Copyright 2011 by ASME

Friction material -padBack plateShimSteelVE corePadBrake discBack plateShimSteelAdhesivePadBrake discFigure 9 Anti-squeal ShimFigure 10 Anti-squeal Grease6Copyright 2011 by ASME

Disc brake squeal can be classified as a form of friction-induced vibration. Eliminating brake noise is a classic challenge in the automotive industry. This paper presents methods for analyzing .

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