Microsoft Word RP32 18 Title Method for Testing & Evaluating Trailer Steel Leaf Springs Fatigue Life Previous Editions Originally issued July 1970, Revised, October 1975, December 1981, April 1987, Ja[.]
TTMA Recommended Practice RP No 32-18 Aril 20, 2018 Truck Trailer Manufacturers Association 7001 Heritage Village Plaza / Suite 220 Gainesville, VA 20155-3094 (703) 549-3010 www.trucktrailer.org Title: Method for Testing & Evaluating Trailer Steel Leaf Springs Fatigue Life Previous Editions: Originally issued: July 1970, Revised, October 1975, December 1981, April 1987, January 1992, June 2000, Reissued May 2005, April 2011, April 2018 1.0 Preface: 1.1 No part of this document may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval system without written permission from the Truck Trailer Manufacturers Association 1.2 Recommended Practices and Technical Bulletins are furnished by the TTMA as a guide to general practices in the manufacture, use, and repair of truck trailers However, the scope of the TTMA's Recommended Practices and Technical Bulletins is not exhaustive of all general practices in the manufacture, use, and repair of truck trailers and there may exist such general practices which not appear in either the Recommended Practices or Technical Bulletins 1.3 Recommended Practices and Technical Bulletins represent the state-of-the-art that existed at the time of its preparation Users of Recommended Practices and Technical Bulletins should familiarize themselves with advancements in practices that have occurred subsequent to the Recommended Practice's or Technical Bulletin's publication date 1.4 The TTMA has not undertaken any evaluation of all the conceivable ways in which Recommended Practices or Technical Bulletins may be used by manufacturers, users, or repairers of truck trailers nor the consequences of such uses Everyone who uses Recommended Practices or Technical Bulletins must first satisfy himself or herself that his or her safety, the safety of others, or the safety of the truck trailer and any other equipment will not be jeopardized by their use of information contained within the Recommended Practices or Technical Bulletins 1.5 The Recommended Practices and Technical Bulletins may contain terms or words with specialized meanings Definitions for such terms or words may be found in TTMA RP No 36 - Tank Trailer and Tank Container Nomenclature or TTMA RP No 66 - Trailer Nomenclature 1.6 Within the Recommended Practices and Technical Bulletins, "shall" is used wherever conformance with the TTMA publication requires that there be no deviation from the specific recommendation "Should" is used wherever deviation from the specific recommendation is permissible in complying with the TTMA publication 1.7 Conformity with TTMA publications by manufacturers, users and repairers of truck trailers is voluntary and any non-conformity with such publications is not indicative of the non-conforming practice being deficient 1.8 Any inclusion of Recommended Practices or Technical Bulletins within any contract, document or standard is voluntary, and any such inclusion shall not imply any endorsement or approval by the TTMA due to the multitude of ways in which the Recommended Practices or Technical Bulletins may conceivably be used RP No 32-18 2.0 3.0 April 20, 2018 Scope and Objective: 2.1 To outline a procedure for fatigue life testing of trailer steel leaf springs 2.2 To evaluate the fatigue test life cycles in order to establish an acceptable minimum "life" standard Test Procedure: 3.1 A minimum of five (5) spring samples shall be used to determine the estimated fatigue life cycles 3.2 Springs are to be tested using a fatigue test fixture that has the dimensional characteristics of the corresponding manufacturer's suspension The fixtures shall include hangers, torque arms, and axle assemblies that are manufacturer's designated parts 3.3 Torque U-bolts to suspension manufacturer's recommendations U-bolt clamping torques should be checked at sufficient intervals to assure adequate clamp load A schedule of checking clamping torques at 5,000, 10,000 and 50,000 cycles and at 100,000 cycle intervals thereafter has been used successfully Regardless of the torque schedule adopted, the same schedule should be maintained throughout the test series New U-bolts and spring saddles are to be used for each spring 3.4 Set test equipment to stroke control mode 3.5 Run the spring through at least one, but preferably five, preload cycles consisting of loading from zero to at least 150% of the rated load and back to zero 3.6 Determine the spring's static load deflection characteristics (spring rate) prior to fatigue testing These load deflection characteristics may be determined by either of two methods With electronic measuring instrumentation, record the deflection of each spring while loading it from zero to at least 150% of the rated load and back to zero For manual recording, load each spring in 1/4 inch increments until 150% of its rated load is obtained (smaller increments may be necessary for very stiff springs) At each load increment, the top of the spring is struck with a rubber faced hammer on both sides of the spring saddle to minimize the effects of interleaf friction The load after striking is to be recorded at each increment Repeat this procedure for the rebound direction (from 150% of the rated load back to zero.) 3.7 The upper stroke limit necessary to obtain 150% rated load in Step 3.6 is to be maintained throughout the test, regardless of any changes in load The lower stroke limit should be set for a preload of 500 pounds and maintained between 100-500 pounds throughout the test It may be necessary to periodically adjust the lower stroke limit to maintain the preload RP No 32-18 4.0 April 20, 2018 3.8 Springs are to be cycled at a frequency of 0.6 to 1.0 Hz Measure and record load deflection curves every 100,000 cycles 3.9 The fatigue life of a leaf spring is defined as the number of cycles required to achieve the first visually observable crack The number of cycles is recorded with a description and the location of this initial crack Continue cycling the spring at 0.6 to 1.0 Hz and record all further damage until the main leaf separates 3.10 A high volume air fan may be used to blow air on the spring assembly to maintain an operating temperature less than 175°F 3.11 Spring leaves may be sprayed with white paint for ease of crack detection Evaluation of Fatigue Test Results: 4.1 General Observations: Considerable variation is present in fatigue life of springs loaded with the same stress cycle The average life of the tested springs is not sufficient by itself to establish a judgment on the design, the material or on the production method which they represent The relationship between the number of applied cycles and the percentage of springs that failed at these cycles can best be analyzed with the help of statistical techniques that will systematically describe the “dispersion,” “spread,” or “scatter” of the recorded test results 4.2 Analytic Methods: Various statistical techniques that can be used for the evaluation procedures are described and explained in the "SAE HS J788" report, titled "Manual on Design and Application of Leaf Springs - Chapter - Operating Stress and Fatigue Life." When tested according to the methods described in this Recommended Practice, the minimum acceptable fatigue life for a leaf spring is 40,000 B10 with a Weibull slope of 4.3 Applications: In order to establish the fatigue life cycles which are acceptable in any spring design, it is desirable to compare that spring fatigue life to the spring life determined under this same procedure for a spring design that has been operating in the field with satisfactory results