20 | MOBLEY.FM Page i Wednesday, March 3, 1999 2:29 PM ROOT CAUSE FAILURE ANALYSIS 20 | MOBLEY.FM Page ii Wednesday, March 3, 1999 2:29 PM P LANT E NGINEERING M AINTENANCE S ERIES Vibration Fundamentals R. Keith Mobley Root Cause Failure Analysis R. Keith Mobley Maintenance Fundamentals R. Keith Mobley 20 | MOBLEY.FM Page iii Wednesday, March 3, 1999 2:29 PM ROOT CAUSE FAILURE ANALYSIS R. Keith Mobley Boston Oxford Auckland Johannesburg Melbourne New Delhi 20 | MOBLEY.FM Page iv Wednesday, March 3, 1999 2:29 PM Newnes is an imprint of Butterworth–Heinemann. Copyright © 1999 by Butterworth–Heinemann A member of the Reed Elsevier group All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, with- out the prior written permission of the publisher. Recognizing the importance of preserving what has been written, Butterworth–Heinemann prints its books on acid-free paper whenever possible. Library of Congress Cataloging-in-Publication Data Mobley, R. Keith, 1943- Root cause failure analysis / by R. Keith Mobley. p. cm. — (Plant engineering maintenance series) Includes index. ISBN 0-7506-7158-0 (alk. paper) 1. Plant maintenance. 2. System failures (Engineering) I. Title. II. Series. TS192.M625 1999 658.2’02—dc21 98-32097 CIP British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. The publisher offers special discounts on bulk orders of this book. For information, please contact: Manager of Special Sales Butterworth–Heinemann 225 Wildwood Avenue Woburn, MA 01801–2041 Tel: 781-904-2500 Fax: 781-904-2620 For information on all Newnes publications available, contact our World Wide Web home page at: http://www.newnespress.com 10 9 8 7 6 5 4 3 2 1 Printed in the United States of America Part I THEORY: INTRODUCTION TO VIBRATION ANALYSIS 1 Chapter 1 INTRODUCTION 2 Chapter 2 VIBRATION Chapter 3 VIBRATION Chapter 4 VIBRATION Chapter 5 VIBRATION Chapter 6 MACHINE Chapter 7 VIBRATION DATA Chapter 9 ANALYSIS ANALYSIS APPLICATIONS 3 ANALYSIS OVERVIEW 6 SOURCES 13 THEORY 17 DYNAMICS 26 TYPES AND FORMATS 42 Chapter 8 DATA ACQUISITION 49 TECHNIQUES 60 Part II FREQUENCY-DOMAIN VIBRATION ANALYSIS 65 Chapter 10 OVERVIEW 66 Chapter 11 MACHINE-TRAIN Chapter 12 DATABASE Chapter 13 VIBRATION DATA Chapter 14 TRENDING Chapter 15 FAILURE-MODE MONITORING PARAMETERS 71 DEVELOPMENT 97 ACQUISITION 112 ANALYSIS 125 ANALYSIS 138 Chapter 16 SIGNATURE ANALYSIS 181 Chapter 17 ROOT-CAUSE ANALYSIS 189 Part III RESONANCE AND CRITICAL SPEED ANALYSIS 200 Chapter 18 INTRODUCTION 201 Chapter 19 TYPES OF Chapter 20 EXAMPLES OF Chapter 21 TESTING FOR RESONANCE 202 RESONANCE 208 RESONANCE 213 Chapter 22 MODE SHAPE 222 Part IV REAL-TIME ANALYSIS 224 Chapter 23 OVERVIEW 225 Chapter 25 DATA Chapter 27 TRANSIENT Chapter 28 SYNCHRONOUS Chapter 30 TORSIONAL Chapter 24 APPLICATIONS 230 ACQUISITION 235 Chapter 26 ANALYSIS SETUP 246 (WATERFALL) ANALYSIS 255 TIME AVERAGING 259 Chapter 29 ZOOM ANALYSIS 265 ANALYSIS 267 GLOSSARY 286 LIST OF ABBREVIATIONS 291 INDEX 293 01.Mobley.1-6 Page 4 Friday, February 5, 1999 9:44 AM 4 Vibration Fundamentals Table 2.1 Equipment and Processes Typically Monitored by Vibration Analysis Centrifugal Machine-Trains Continuous Process Product rolls Boring machines Printing Mixers Hobbing machines Dyeing and finishing Gearboxes Machining centers Roofing manufacturing lines Centrifuges Temper mills Chemical production lines Transmissions Metal-working machines Petroleum production lines Turbines Rolling mills, and most Neoprene production lines Generators machining equipment Polyester production lines Rotary dryers Nylon production lines Electric motors Flooring production lines All rotating machinery Continuous process lines Source: Integrated Systems, Inc. P REDICTIVE M AINTENANCE The fact that vibration profiles can be obtained for all machinery that has rotating or moving elements allows vibration-based analysis techniques to be used for predic- tive maintenance. Vibration analysis is one of several predictive maintenance tech- niques used to monitor and analyze critical machines, equipment, and systems in a typical plant. However, as indicated before, the use of vibration analysis to monitor rotating machinery to detect budding problems and to head off catastrophic failure is the dominant predictive maintenance technique used with maintenance management programs. A CCEPTANCE T ESTING Vibration analysis is a proven means of verifying the actual performance versus design parameters of new mechanical, process, and manufacturing equipment. Preac- ceptance tests performed at the factory and immediately following installation can be used to ensure that new equipment performs at optimum efficiency and expected life- cycle cost. Design problems as well as possible damage during shipment or installa- tion can be corrected before long-term damage and/or unexpected costs occur. Q UALITY C ONTROL Production-line vibration checks are an effective method of ensuring product qual- ity where machine tools are involved. Such checks can provide advanced warning that the surface finish on parts is nearing the rejection level. On continuous pro- cess lines such as paper machines, steel-finishing lines, or rolling mills, vibration 01.Mobley.1-6 Page 5 Friday, February 5, 1999 9:44 AM 5 Vibration Analysis Applications analysis can prevent abnormal oscillation of components that result in loss of product quality. L OOSE OR F OREIGN P ARTS D ETECTION Vibration analysis is useful as a diagnostic tool for locating loose or foreign objects in process lines or vessels. This technique has been used with great success by the nuclear power industry and it offers the same benefits to non-nuclear industries. N OISE C ONTROL Federal, state, and local regulations require serious attention be paid to noise levels within the plant. Vibration analysis can be used to isolate the source of noise gener- ated by plant equipment as well as background noises such as those generated by fluorescent lights and other less obvious sources. The ability to isolate the source of abnormal noises permits cost-effective corrective action. L EAK D ETECTION Leaks in process vessels and devices such as valves are a serious problem in many industries. A variation of vibration monitoring and analysis can be used to detect leak- age and isolate its source. Leak-detection systems use an accelerometer attached to the exterior of a process pipe. This allows the vibration profile to be monitored in order to detect the unique frequencies generated by flow or leakage. AIRCRAFT ENGINE ANALYZERS Adaptations of vibration analysis techniques have been used for a variety of specialty instruments, in particular, portable and continuous aircraft engine analyzers. Vibration monitoring and analysis techniques are the basis of these analyzers, which are used for detecting excessive vibration in turboprop and jet engines. These instruments incorporate logic modules that use existing vibration data to evaluate the condition of the engine. Portable units have diagnostic capabilities that allow a mechanic to deter- mine the source of the problem while continuous sensors alert the pilot to any devia- tion from optimum operating condition. M ACHINE D ESIGN AND E NGINEERING Vibration data have become a critical part of the design and engineering of new machines and process systems. Data derived from similar or existing machinery can be extrapolated to form the basis of a preliminary design. Prototype testing of new machinery and systems allows these preliminary designs to be finalized, and the vibration data from the testing adds to the design database. 01.Mobley.1-6 Page 7 Friday, February 5, 1999 9:44 AM 7 Vibration Analysis Overview Figure 3.1 Periodic motion for bearing pedestal of a steam turbine. Figure 3.2 Small oscillations of a simple pendulum, harmonic function. [...]... normalization, although the amplitude may be different 01. Mobley .1- 6 Page 11 Friday, February 5, 19 99 9:44 AM Vibration Analysis Overview 11 Interpretation of Vibration Data The key to using vibration signature analysis for predictive maintenance, diagnostic, and other applications is the ability to differentiate between normal and abnormal vibration profiles Many vibrations are normal for a piece of rotating... generate the spike of energy associated with direction reversal 01. Mobley .1- 6 Page 18 Friday, February 5, 19 99 9:44 AM 18 Vibration Fundamentals Figure 5 .1 Illustration of vibration cycles 1 f = T Another measure of frequency is the circular frequency, ω, measured in radians per sec ond From Figure 5 .1, it is clear that a full cycle of vibration (ωt) occurs after 360 degrees or 2π radians (i.e., one... equations: X 1 = a sin ( ω 1 t ) X 2 = b sin ( ω 2 t ) The total vibration represented by the solid line is the sum of the dashed lines The following equation represents the total vibration: X = X 1 + X 2 = a sin ( ω 1 t ) + b sin ( ω 2 t ) Any periodic function can be represented as a series of sine functions having frequen cies of ω, 2ω, 3ω, etc.: 01. Mobley .1- 6 Page 21 Friday, February 5, 19 99 9:44 AM Vibration. .. 01. Mobley .1- 6 Page 8 Friday, February 5, 19 99 9:44 AM 8 Vibration Fundamentals ACTUAL VIBRATION PROFILES The process of vibration analysis requires the gathering of complex machine data, which must then be deciphered As opposed to the simple theoretical vibration curves shown in Figures 3 .1 and 3.2 above, the profile for a piece of equipment is... personal or a more powerful computer to perform more sophisticated analyses, data storage and retrieval, and report generation 01. Mobley .1- 6 Page 14 Friday, February 5, 19 99 9:44 AM 14 Vibration Fundamentals Rotor Imbalance While mechanical imbalance generates a unique vibration profile, it is not the only form of imbalance that affects rotating elements Mechanical imbalance is the condi tion where... machine 01. Mobley .1- 6 Page 15 Friday, February 5, 19 99 9:44 AM Vibration Sources 15 Mechanical Motion and Forces A clear understanding of the mechanical movement of machines and their compo nents is an essential part of vibration analysis This understanding, coupled with the forces applied by the process, are the foundation for diagnostic accuracy Almost every unique frequency contained in the vibration. .. standpoint, simple harmonic vibration functions are related to the circular frequencies of the rotating or moving components Therefore, these frequen cies are some multiple of the basic running speed of the machine-train, which is expressed in revolutions per minute (rpm) or cycles per minute (cpm) Determining 01. Mobley .1- 6 Page 10 Friday, February 5, 19 99 9:44 AM 10 Vibration Fundamentals Figure 3.5... readings Transducer The transducer most commonly used to obtain vibration measurements is an acceler ometer It incorporates piezoelectric (i.e., pressure-sensitive) films to convert mechan ical energy into electrical signals The device generally incorporates a weight 01. Mobley .1- 6 Page 12 Friday, February 5, 19 99 9:44 AM 12 Vibration Fundamentals suspended between two piezoelectric films The weight... determine the contribution of any particular vibration source The French physicist and mathematician Jean Fourier determined that nonharmonic data functions such as the time-domain vibration profile are the mathematical sum of Figure 3.3 Example of a typical time-domain vibration profile for a piece of machinery 01. Mobley .1- 6 Page 9 Friday, February 5, 19 99 9:44 AM Vibration Analysis Overview 9 Figure 3.4... radians (i.e., one full revolution) At this point, the function begins a new cycle ω = 2πf For rotating machinery, the frequency is often expressed in vibrations per minute (vpm) or 01. Mobley .1- 6 Page 19 Friday, February 5, 19 99 9:44 AM Vibration Theory 19 Figure 5.2 Two harmonic motions with a phase angle between them ω VPM = π By definition, velocity is the first derivative of displacement with respect . ACQUISITION 11 2 ANALYSIS 12 5 ANALYSIS 13 8 Chapter 16 SIGNATURE ANALYSIS 18 1 Chapter 17 ROOT-CAUSE ANALYSIS 18 9 Part III RESONANCE AND CRITICAL SPEED ANALYSIS 200 Chapter 18 INTRODUCTION 2 01 Chapter. VIBRATION ANALYSIS 65 Chapter 10 OVERVIEW 66 Chapter 11 MACHINE-TRAIN Chapter 12 DATABASE Chapter 13 VIBRATION DATA Chapter 14 TRENDING Chapter 15 FAILURE-MODE MONITORING PARAMETERS 71. amplitude may be different. 01. Mobley .1- 6 Page 11 Friday, February 5, 19 99 9:44 AM 11 Vibration Analysis Overview Interpretation of Vibration Data The key to using vibration signature analysis