1、_SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising theref
2、rom, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions.Copyright 2012 SAE International All rights reserved. No part of this pub
3、lication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-4970
4、(outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.orgSAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/J2598_201207SURFACEVEHICLERECOMMENDEDPRACTICEJ2598 JUL2012 Issued 2006-01Revised 201
5、2-07 Superseding J2598 JAN2006 Automotive Disc Brake Pad Natural Frequency and Damping Test RATIONALEThe SAE Brake NVH Standards Committee reviewed the SAE J2598 standard and agreed that: - No modification is required in the measurement requirements/test configuration sections.- Additional methods t
6、o calculate the damping factor should be added to the standard (Chapter 9). FORWARDNatural frequencies and damping are important characteristics of a brake pad assembly. There are currently no recognized standards for measuring these quantities.1. SCOPE This procedure is applicable to brake pad mode
7、s between 500 Hz. and 16 kHz. The parameters measured with this procedure are defined as the first three natural frequencies, fn(n=1, 2, 3), and the corresponding loss factors, .1.1 Purpose This recommended test practice is intended to establish a standardized and repeatable method for performing na
8、tural frequency and damping measurements on a disc brake pad. 2. REFERENCES 2.1 Related Publications The following publications are provided for information purposes only and are not a required part of this SAE Technical Report.D. J. Ewins, Modal Testing Theory, Practice and Application, Second Edit
9、ion, 2000, Research Studies Press LTD, EnglandB&K Technical Review n1, 1994: “Digital Filter Techniques vs. FFT Techniques for Damping Measurements“, Svend Gade & Henrik Herlufsen SAE J2598 Revised JUL2012 Page 2 of 13 3. GLOSSARY AND TERMINOLOGY 3.1 NATURAL FREQUENCY One of the frequencies at which
10、 the pad naturally vibrates at when excited in a free-free condition. 3.2 LOSS FACTOR The energy loss factor, , which is the percent critical damping divided by 50. 3.3 DAMPING This is generally expressed as the percentage of critical damping. 4. ORGANIZATION OF THIS DOCUMENT This document separates
11、 the measurement procedures to obtain the brake pad natural frequency and that required to obtain the loss factor. The reason for this separation is that in the development of this procedure significant differences were found in the repeatability of the results for these two measurements between dif
12、ferent testers. The result is that highly accurate and consistent natural frequency data can be obtained with less rigorous measurement system and procedure requirements. This measurement procedure is presented in Part A.The more rigorous loss factor measurement procedure is presented in Part B. It
13、is important to note that although the requirements for this measurement are more stringent there is still greater variability in the loss factor results. The magnitude of this variability will be discussed in some detail in Part B.The first step is to define the basic measurement configuration and
14、the requirements common to both parts A and B. 5. MEASUREMENT REQUIREMENTS Excitation: A metal tipped impact hammer with force transducer is the preferred method of excitation. Other methods of excitation such as a shaker may be used with proper care and documentation of equivalent results. Response
15、: Typically a microphone or an accelerometer will be used to measure response. A single point laser vibrometer can also be used. If a transducer other than a microphone or accelerometer is used, a comparison test shall be run to show equivalence. The recommended microphone is a 0.5 in free-field con
16、denser type. The recommended accelerometer is one with an integrated amplifier or a charge type. The recommended accelerometer is different for parts A and B. The specific requirements will be defined in these sections. Frequency response from 500 to 16 000 Hz 3 dB. Dual-channel FFT analyzer capable
17、 of calculating a frequency response function (FRF) and simple coherence. SAE J2598 Revised JUL2012 Page 3 of 13 6. TEST CONFIGURATION The preferred measurement setup is shown in Figure 1. This configuration shows the use of a microphone for the response measurement and an impact hammer for excitati
18、on. The same setup can be used with an accelerometer. One may use equivalent non contacting measurements such as Laser Doppler Vibrometer, if the results can be shown to be equivalent.To be sure that the lower three natural frequencies and their corresponding loss factor are measured, it is necessar
19、y to excite both torsional and bending modes. The lowest three natural frequencies may then be selected from these results with high confidence.Figures 2 to 4 show three suggested measurement configurations. Using all three of these combinations of force input and response locations, one can be conf
20、ident of that the lowest three natural frequencies will be measured.FIGURE 1 - BLOCK DIAGRAM OF A BRAKE PAD LOSS FACTOR MEASUREMENT SETUP The Figure 2 setup will be prone to the excitation of the bending modes of the brake pad.To assure that torsional modes are excited and measured, the configuratio
21、ns shown in Figures 3 and 4 can be used. In this arrangement, it is important to excite and measure at the farthest edges of the pad (even if it means exciting on chamfers).AnalyzerBrake padMicrophoneImpactHammerFoamOvenSAE J2598 Revised JUL2012 Page 4 of 13 FIGURE 2 - BRAKE PAD NATURAL FREQUENCY AN
22、D LOSS FACTOR MEASUREMENT CONFIGURATION NUMBER 1 SAE J2598 Revised JUL2012 Page 5 of 13 FIGURE 3 - BRAKE PAD NATURAL FREQUENCY AND LOSS FACTOR MEASUREMENT CONFIGURATION NUMBER 2 SAE J2598 Revised JUL2012 Page 6 of 13 FIGURE 4 - BRAKE PAD NATURAL FREQUENCY AND LOSS FACTOR MEASUREMENT CONFIGURATION NU
23、MBER 3 SAE J2598 Revised JUL2012 Page 7 of 13 7. OPERATION In an effort to minimize the effect of environmental conditions, the measurement should be conducted between 18 C and 24 C and 30 to 80% relative humidity. Record the ambient temperature and humidity. Place the brake pad on a foam pad as ind
24、icated on Figure 1. For Part A any soft closed cell foam pad can be used. For Part B, a specific foam pad must be used. Other equivalent means of simulating free-free mounting conditions are also acceptable. Care should be taken to avoid restraining the motion of the pad at antinodes. Locate the mic
25、rophone or accelerometer on top of the pad, as indicated on Figure 1. The gap between the microphone and the pad should be around 5 mm. If an accelerometer is used, bond it to the pad using Loctite adhesiveThe measurement can be done with or without a brake insulator. Record the microphone/accelerom
26、eter and hammer positions used for this test.Connect the impact hammer to channel 1 and the microphone or accelerometer to channel 2 of the analyzer. Prepare the analyzer for data acquisition and analysis according to the following: Analyzer frequency range from 500 to 16 000 Hz. Pre-trigger delay p
27、roperly set to ensure that the entire force pulse is captured. Trigger level adjusted such that the data acquisition system properly captures the impact and acoustic response. The maximum value of frequency resolution should be 5 Hz. A force window shall be used for the excitation data acquisition.
28、Absolutely no windowing should be required on the response channel. Should the response not decay completely in the sampling window, the analyzer parameters must be adjusted to allow the complete decay to be captured. This adjustment may be done by increasing the number of sample points, adjusting t
29、he period, or the frequency range. Record the analyzer setup. It is important to adjust the analyzer so that the FRF between the microphone/accelerometer and the impact hammer and the coherence function are both displayed as shown in Figure 5. Hit the pad with the hammer a minimum of 5 times at the
30、same location. These hits must occur preferably on the backing plate (hitting the friction material is an option). The analyzer should be configured to compute a linear average the results of these impacts to obtain a high quality signal. The quality of the signal will be judged by computed coherenc
31、e. Reject the measurement if the coherence is below 0.9 for the resonant peaks. Avoid double hits. Data obtained with double hits shall be rejected. When using a microphone, reduce the gap between the microphone and the pad if the quality of the FRF needs to be improved.SAE J2598 Revised JUL2012 Pag
32、e 8 of 13 8. PART A: MEASUREMENT OF PAD NATURAL FREQUENCIES Response: Typically a microphone or an accelerometer will be used to measure response. A single point laser vibrometer can also be used. If a transducer other than a microphone or accelerometer is used, a comparison test shall be run to sho
33、w equivalence. The recommended microphone is a 0.5 in free-field condenser type. The recommended accelerometer is one with an integrated amplifier or a charge type with a mass of less than 1 g. Operation: Identify the resonance frequencies on the FRF (Figure 5). Repeat the measurements for configura
34、tion 2 (Figure 3) and configuration 3 (Figure 4). The user must report the natural frequencies computed for each configuration. The reported natural frequencies are the average of the lowest three natural frequencies found (Table 1). The variation expected from this measurement is natural frequency:
35、 5% from the average. 9. PART B: MEASUREMENT OF PAD LOSS FACTOR It is extremely difficult to measure damping consistently. In order to achieve a reasonable level of consistency, the following steps have to be followed: Response: Select an accelerometer with a weight less than 1 g. Examples of accele
36、rometers that can be used are: PCB 357C10 (0.45 g), PCB 357A09 (0.6 g), PCB 357A08 (0.14 g), Endevco 2222C (0.5 g), Endevco 2250A/AM1-10 (0.4 g), Endevco 22 (0.14 g) or equivalent. Foam: The foam rubber used to simulate the free-free conditions is GM251M (Type I or Type IA) from Plastomer Corporatio
37、n, Livonia, MI 48150, Tel: 734-464-0700 or equivalent. The foam thickness is 50 mm. Operation: Identify the resonance frequencies on the FRF (Figure 5). Repeat the measurements for configuration 2 (Figure 3) and configuration 3 (Figure 4). The user must report the natural frequencies and loss factor
38、s computed for each configuration. The first three natural frequencies are the averages of the equivalent lowest natural frequencies detected. The loss factor for each natural frequency is the lowest computed value (Table 1). SAE J2598 Revised JUL2012 Page 9 of 13 The loss factor for each natural fr
39、equency shall be measured using the half-power or 3 dB method. The loss factor is defined by ff= where f is the frequency bandwidth at 3 dB below the resonant peak and f is the resonant peak frequency (Figure 6). If another bandwidth n is used, the following formula can be used:ffn=1101220Where:n is
40、 the n dB down point, and if is the frequency bandwidth for n dB down point, Hz. A single degree of freedom parameter identification method can also be employed. The method used to calculate the loss factor (or damping factor) should be documented. Many modern analyzers will have this calculation bu
41、ilt in as an automated function. Whether this is done automatically by the analyzer or is done manually by the user, this calculation must be done using a linear interpolation between discrete points in the frequency response function. If only the discrete points of the FRF spectrum are used, signif
42、icant errors in the loss factor estimate are possible.Some analyzers have a function which gives you the damping ratio in %, by placing the screen cursor on the peak. The loss factor is then defined by501002* = .The measurement variation is estimated at 10% or typically 0.5% of critical damping.SAE
43、J2598 Revised JUL2012 Page 10 of 13 FIGURE 5 - EXAMPLE OF AN FRF (TOP) AND COHERENCE FUNCTION (BOTTOM)OF A DISC BRAKE PAD SAE J2598 Revised JUL2012 Page 11 of 13 FIGURE 6 - ZOOM AROUND ONE OF THE FRF PEAKS Repeat those measurements at different temperatures if necessary (Appendix A). 10. SUMMARY OF
44、RESULTS The same report is used for Parts A and B. Plot the FRF curve from 500 to 16 000 Hz as shown in Figure 5. Tabulate and identify the 3 lowest modes. For each mode, take the average of the frequencies measured for the 3 microphone-accelerometer/hammer positions. When appropriate select the mea
45、surement showing the lowest loss factor. Tabulate the frequency and loss factor .11. NOTES 11.1 Marginal Indicia A change bar (l) located in the left margin is for the convenience of the user in locating areas where technical revisions, not editorial changes, have been made to the previous issue of
46、this document. An (R) symbol to the left of the document title indicates a complete revision of the document, including technical revisions. Change bars and (R) are not used in original publications, nor in documents that contain editorial changes only. SAE J2598 Revised JUL2012 Page 12 of 13 TABLE
47、1 - SUMMARY OF NATURAL FREQUENCIES AND LOSS FACTORS Test Date _ Test Conducted By _ Temperature/Humidity _ Window used for Hammer:_ Description of Test Sample_Window used for Microphone / Accelerometer: Loss Factor Calculation Method: Insulator Present/Type _ Vehicle Application_ Analyzer Setup _ Pa
48、d ID Mode 1 Mode 2 Mode 3 Notes Nat. Freq., Hz Loss factor Nat. Freq., Hz Loss factor Nat. Freq., Hz Loss factor Pad ID Configuration 1 Configuration 2 Configuration 3 Notes Nat. Freq., Hz Loss factor Nat. Freq., Hz Loss factor Nat. Freq., Hz Loss factor PREPARED BY THE SAE BRAKE NVH STANDARDS COMMITTEE SAE J2598 Revised JUL2012 Page 13 of 13 APPENDIX A - MEASUREMENTS AT TEMPERATURE EXTREMES In some instances it may be desirable to measure the damping and natural frequency for materials at other than room temperature. Some brake mater
