SAE TECHNICAL PAPER SERIES
2008-01-2531
A Systems Approach to Eliminating Squeal in a Drum Brake Snehasis Ganguly, Huanan Tong and Yuri Karpenko Ford Motor Company
26th Annual Brake Colloquium & Exhibition San Antonio, Texas October 12-15, 2008 400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-0790 Web: www.sae.org
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2008-01-2531
A Systems Approach to Eliminating Squeal in a Drum Brake Snehasis Ganguly, Huanan Tong and Yuri Karpenko Ford Motor Company Copyright © 2008 SAE International
ABSTRACT The traditional analysis of squeal noise for drum brakes is done in a separate approach, with CAE and laboratory/experimental techniques done independently or in a non-iterative sequential manner. In this paper, an innovative approach of directing the frequency response testing based on CAE is used and the overall process is embedded in a system approach. The drum brake design was changed to accomplish higher loads in a car. The initial results of the tests came out noise during the vehicle test. After retrieving the noisy parts from the vehicle, it was tested for frequency response, but in a directional manner suggested by the CAE model. This new approach hasn’t been done before in industry practice. The CAE identified that two modes (around the noise frequency) swapped their orders compared to the old design and suggested design changes. The new design was evaluated with a mocked up prototype. This was followed by getting cast parts and testing them for frequency response. Eventually rig tests and vehicle tests followed, which proved that the new proposal fixed squeal problems.
INTRODUCTION Brake design is a system based approach. In most of the cases, while designing a new system, the project starts from modifying or tweaking an existing system. In this particular case, the system design was changed due to a continuous durability failure of the bearings. The root cause behind the durability problems was that the durability test induced loading in excess of the capacity of the bearing. The capacity of the bearing was deemed adequate for a certain group of customers and passed the European durability cycle, but never survived the North American requirements. While the remaining part of the Brake system remained essentially the same, the drum and the bearing design were changed to accommodate the new loads. The initial design was thus based on upsizing the bearing and changing the drum design to accommodate for this. The vehicle testing started with the new drum and bearing set. Loud sustained squeals were heard during this test with major frequency of 2.2 kHz and 8 kHz (see Fig. 1). Though squeal is an expected byproduct of any braking function, it has a lot of customer dissatisfaction in recent years. A major cause of the squeal is the friction force introduced
dynamic instability. During braking, huge amount of kinetic energy is dissipated into heat. Only small amount of this kinetic energy is enough to trigger dynamic instabilities; it makes high pitched vibration of drum and integral parts. This squeal can occur in a variety of environmental conditions. The front brakes in a car do the most braking operation and consequently could cause squeal. The rear drum brakes in many cars and trucks are being replaced by disc brakes. But, the disc brakes are expensive than drum brakes. Thus the drum brake design is going to stay for years. There are various causes and theories behind brake squeal [1-7]. The broad categories of theoretical model are stick-slip where the static coefficient of friction is greater than the dynamic. The second category of friction model has a negative slope in the friction coefficient-velocity characteristics. The third type is where the coefficient of friction is assumed to be constant with non-conservative frictional forces [1]. The origination of squeal comes from interaction of the friction material and the drums. However, it spreads through the web and rim of the shoes, springs, wheel cylinder ultimately to the back plate of the brake assembly. In the vehicle under consideration the squeal was a particularly difficult problem to tackle because the vibration energy was transmitted through the drum brake back plate into the knuckle-tie blade assembly of the suspension. The various noise factors which complicate the analysis include park brake setting and variation in critical components like grind radius, lining compressibility, friction etc. However, the present system was stabilized over several years with incremental improvement. For the case under study the system behavior changed from a prior model year, and vehicle test confirmed no such noise found with prior model drum-bearing set. The only change in the new design was in the drum-bearing set brake design. It was evident that the drum operating modes constitute a major portion of the noise. The process of attacking the problem is best understood with a flow diagram. That diagram is presented in Fig.2. The drum in question is a GEN-I type of design where the bearing is part of the unicast hub design. These type of bearings are somewhat older but cost effective. The car in question has been often run under a durability route to simulate the worst possible loading in customer usage. It has been seen that the bearings have repeatedly failed under the road loads. The situation becomes even worse with road loads that were increasing for a future model
year. To compensate for increasing road loads, the bearing sizes were upgraded. However, this resulted in weakening of the drum sections. The new designs were tested by finite element analysis and the sections of the drums were reinforced to account for the extra weights. This was verified by rotary fatigue testing. This process was repeated a few times and finally a design was finalized. As shown in Fig.2a, the CAD files from this design were used for component CAE normal mode analysis. As shown in Fig.2a approach, the test points and directions were determined by the CAE analysis. Without these planned points and direction, subtle changes (such as mode switch in this case) may not be found or confirmed by a regular frequency response test. This is the most innovative approach, which has not been seen before. For the well established complex eigenvalue system of predicting noise [4,8], it is important to be able to distinguish the various mode shapes and frequencies. However, this is an iterative process which involves improving the model sufficiently to match the frequencies and mode shapes as determined experimentally. The next step in the system approach is the assembly and complex eigensolution of the system as shown in Fig 2b. For this purpose, when the system is put together, several measurement points are determined from both putting an accelerometer in the appropriate locations, as well as measuring the system frequencies and mode shapes. The further system model tuning up (assembly constraints optimization) was a correlation work by using all available vehicle test results. It includes Los Angeles City Traffic, Marquette city traffic, Cold room etc. Finally a correlated model which predicts the noise on a vehicle and dyno is used to suggest improvements in the design. The prototypes as dictated by CAE were ordered, quickly tested to make sure they had a predicted response as desired and were used in a dyno and vehicle level testing. Finally after obtaining a good solution to the noise problem, the next step was a robustness analysis. Within the context of time and resources, a few critical factors based on experience are identified and robustness checks are done. At this stage CAE is used to further suggest improvements in the design.
SYSTEMS APPROACH TO CAE AND TESTING The first step in the drum design was a load analysis based on CAE. The sections of the drums were changed and the rotary fatigue tests validated the design. Finally at a later stage of the program, when test vehicles are available, the baseline LACT tests were run. As can be seen from Fig.1, the baseline design was extremely noisy in terms of the noise performance. The major noise frequencies produced were 2.2, 7.5 and 8.3 Khz. The frequencies were collected over the entire spectrum of the LACT test, which means that the data covered 27 shifts (about 30,000 stops). The drums from LACT were returned. As a very first step, the drums were tested between prior year model and new model design by frequency response method. However, the key here was to do an eigenvalue analysis of the drums by finite element method. This is important to understand the
difference in the drum design. Also, it helps in specifying in which direction the drums need to be excited during testing to bring out the mode shape differences. As a result of finite element analysis, it was seen that the mode shapes of the drum design had switched between the old and the new design around 2.2 kHz. It can be seen that in Fig.3, the drum 3rd bending mode and mounting surface leaning mode of the drums have swapped and the frequencies have also shifted. Here a mention must be made that in the brake supplier industry, to control quality of brake part, a common practice is just to measure the first resonance frequency. That method often leads to incorrect assumptions especially in cases like this. For CAE correlation work, matching all frequencies in certain range is also not sufficient. Different hardware could have the same frequency with different mode shape or different order of modes. It can be clearly seen that the breathing mode shape remained the same both in term of frequency and mode shape between two designs. However, the bending mode and mounting surface leaning mode has different order. Based on mode shape of CAE analysis, it was predicted that increasing the bending stiffness of the drum could switch back the mode order for the new designed drum-bearing set. The next step was to predict the instabilities with finite element analysis. While the LACT showed that the 2.2 and 8 Khz were produced during the vehicle tests, the rear brake system had system modes at 2.2, 3.8, 5.8 and 8 Khz. Based on the system CAE, it was seen that the 2.2 and 8 Khz bending modes grows to squeal under excitation conditions. System CAE optimized the lip stiffening size to clean the instability of 2 and 8 kHz (Fig.5). The next step is to validate the finite element process with a prototype. Unfortunately the prototype process is time consuming due to getting the castings. As an interim measure, for rapid prototyping purposes, a drum assembly was modified by welding a steel ring to an existing drum as shown in Fig. 4. This drum was tested immediately for frequency response and the results were directionally correct. Even though the final frequencies were not achieved, the modes shapes were swapped back. However, the exercise was critical to have confidence in the process. In the very next step, a cast part was obtained from the supplier and it was measured for its frequency response characteristics. Then it was put through a parallel dyno and vehicle testing due to time constraint of the program. Three LACT's were run along with three cold room and Marquette vehicles. As can be seen from Fig.6 with the new improved drum, the noise results were substantially improved. There was no noise hits above the audible 50db and only a few below 50 db. Further as a final validation, the drivers concluded that there was no audible noise while driving the vehicles with the modified drums.
CONCLUSION Component CAE comparison provided design change direction and detail plane of testing. System CAE analyses provided further understanding about the root cause of drum brake and optimized size of the change.
High fidelity system model has been used as most efficient validation tool. CAE based proposal successfully passed dyno and vehicle test validation. Major squeal issues have been solved by CAE proposed design change. CAE driven brake squeal fix process cut the traditional long resolution process to a very short one, kept the project on track to a very tight timeline.
ACKNOWLEDGMENTS This work was supported by Dwayne Mattison, Ken Pastor, Lee Smith, Oliver Nwankwo and Marty Verhun. Greg Folta and Sajid Siddique of Ford also contributed to the paper. John Deiderich and Jack Groat of Hayes Lemmerz supported the testing of the drums for structural integrity. The vehicle tests were run by LDW.
REFERENCES 1.Johan Hulten, John Flint and Thomas Nellemose, ‘Mode shape of a squealing drum brake’, SAE Paper no. 972028. 2.P221 Brake Squeal CAE Support, VASE NVH CAE Huanan Tong & Shih-Emn Chen,Truck CAE Hari Ganesan & Churn Wang, NVH Review March 2003 3.CAE Analysis of Aviator Low Frequency Brake Squeal, LM CAE: Pandit Rao, Gabriela Dziubinski, Dale White; NAE: Huanan Tong, Shih-Emn Chen, Ford Global Noise and Vibration Conference, October 2002 4.Complex Eigenvalue Analysis Method for Brake Noise Prevention, Shih-Emn Chen, Huanan Tong, Ford Technical Report, May 2000 5.Budinski K. G, Engineering Materials- Properties and Selection, Second Edition, Reston Publishing Co., Inc, Reston, VA, 1983. 6.S.W. Kung, G. Stelzer, K. A. Smith, 'A study on low frequency drum brake squeal', SAE 2004-01-2787. 7.K. Krishnapur and Jim Luo, ' Brake Squeal rig and LACT Vehicle Test correlation improvements- Focus on thermal conditionings', SAE 2004-01-2791. 8. S. Ganguly, H. Tong, Greg Dudley, Frank Connolly and Stan Hoff, ' Eliminating drum brake squeal by a damped iron drum assembly', SAE 2207-01-0952.
G> = 0.20
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Fig.1 LACT results correlated with system modes of drum brakes
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Fig.2a Flowchart of the process
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Fig.2b Finite element model of the system
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The welded prototype hardware FRF has proved this concept.
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Fig. 4 A rapid prototype proposal
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Fig. 5 High fidelity system model to predict the noise frequencies
Los Angeles City Traffic Test Comparison (about 30,000 Stops) 70
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Fig. 6 Los Angeles City Traffic Test Comparison
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