University of Incheon, 12-1 Songdo-dong, Yeonsu-gu, Incheon, Korea

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1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Optimization of Shot-peening Conditions and Wave Profiles To Improve Durability of Wave Cushion Spring Tae-Il Seo 1,a, Byeong-Uk Song 1,b and Youn-Seung Jeong 2,c 1 Major of Mechanical Engineering & Robotics, Division of Mechanical System Engineering, University of Incheon, 12-1 Songdo-dong, Yeonsu-gu, Incheon, Korea 2 Tool & Mold Design, Yuhan college, 590, Gyeonin-ro, Sosa-gu, Bucheon-si, Gyeonggi-do, Korea a tiseo@incheon.ac.kr, b mempisto@incheon.ac.kr, c chys0323@yuhan.ac.kr Keywords: Shot-peening; Optimization; Wave cushion spring; Durability; Residual Stress Abstract This study presents a determination procedure of optimal shot-peening process conditions, wave profiles and thickness in order to precisely control and improve durability and fatigue life of wave cushion springs assembled into automobile automatic transmission clutch. For this shot-peening process, residual stress analysis was applied under various conditions. Experimental works were conducted to evaluate proposed approach by using X-ray diffraction residual stress measurement tester. It could be expected that this study provided guideline for wave spring optimal design and improving fatigue life. Introduction Wave cushion springs have been widely used for automobile automatic transmission. These springs have to be rigid, long fatigue life and very controllable because they are operating during car running and very important parts. To improve fatigue life of mechanical parts and structures, various researches and technologies have been conducted and developed until now [1]. Among them shot-peening process is one of widely used processes for improving fatigue life, which can provide compressive residual stresses on the workpieces after main manufacturing processes [2]. Almost fractures can be avoided and fatigue life can be improved by deliberately providing compressive stresses on workpiece to offset tensile residual stresses [3]. Then shot-peening process was widely used for automobile automatic transmission clutches as well as many other automobile parts [4]. For wave spring maiden of 50CrV 4 (Spring steel class DIN EN 10277), this study handled with the effects of shot-peening, fatigue life analysis, process conditions, wave spring profiles to obtain optimal shot-peening conditions, wave profiles, long fatigue life by X-ray diffraction residual stress measurement tester. Properties of used material 50CrV 4 of spring steel class (DIN EN 10277) was used in this study, which was widely used to manufacture various wave spring for automobile automatic transmission clutch. Table 1 shows chemical composition of 50CrV 4 [5]. Table 1. Chemical composition of 50CrV 4 Chemical composition (%) DIV. Carbon Chromium Vanadium Silicon Manganese Phosphorus Sulfur 50CrV Under 0.04 Under 0.04 Determination of optimal conditions of shot-peening for wave springs Shot-peening conditions are material and size of shot ball, air jet pressure, incidence angle of air nozzle, rotational speed of workpiece and shot-peening process time. Fig.1 shows shot-peening system used and optimal conditions of shot-peening process were obtained by using experimental methodology in this study. For fundamental conditions shot ball diameter was 0.3mm, spindle All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/05/16,06:52:41)

2 2288 Advanced Engineering Materials III rotational speed was 2 RPM and nozzle incidence angle was 45. Air jet pressure was 2 bar and 4 bar, and shot-peening time was 30sec, 60sec and 90sec; totally 6 cases were chosen for experimental works. X-Ray residual stress analyzer (model name XSTRESS 3000) of Stresstech was used to measure residual stress of wave spring after applying shot-peening process as shown in Fig.2. Table 2 shows summarized experimental results of residual stress measured by X-ray diffraction residual stress measurement tester at 6 pints as shown in Fig.3 after shot-peening under various conditions. Fig. 1. Shot-peening machine Fig. 2. X-ray diffraction residual stress measurement tester Fig. 3. Residual stress measuring point. Table 2. Compressive residual stress measurement results according to shot-peening conditions Shot-peening Compressive residual stress (MPa) Pressure Time P1 P2 P3 P4 P5 P6 30 sec bar 60 sec sec sec bar 60 sec sec Table 3. Representative value of compressive residual stress according to shot-peening conditions Shot-peening Compressive residual stress (MPa) Pressure Time Max. Min. Mean Dev. 30 sec bar 60 sec sec sec bar 60 sec sec Table 3 shows summarized experimental data by maximum and minimum values based on Table MPa of average residual stress was measured in case of 2 bar and 30sec MPa of average residual stress was measured in case of 2 bar and 60sec MPa of average residual stress was measured in case of 2 bar and 90sec. In this case significant difference of residual stresses does not appear. Moreover in case of 4 bar, MPa for 30 sec, MPa for 60 sec and MPa for 90 sec; significant difference of residual stresses does not appear. However taking into account deviation of residual stresses at 6 points, significant trend appears. The deviation was 34.9 MPa for 30 sec, 14.4

3 Advanced Materials Research Vols MPa for 60 sec and 10.1 MPa for 90 sec when 2 bar. The deviation was 29.3 MPa for 30 sec, 16.2MPa for 60 sec and 11.2 MPa for 90 sec when 4 bar. Increasing process time, shot-peening effects uniformly were applied and residual stress was uniformly distributed on whole wave spring. Determination of wave spring optimal shape by endurance tests To determine wave spring optimal shapes, 4 wave spring profiles were considered in this study, which were applied into 6-speed automatic transmission of domestic new models. Fig.6 shows these 4 different wave spring shapes; Fig.6-(a) shows sine wave profile, Fig.6-(b) shows 100% R triangular wave profile, Fig.6-(c) shows 50% R triangular wave profile and Fig.6-(d) shows 20% R triangular wave profile. (a) Sine wave (b) 100% R triangular wave (c) 50% R triangular wave (d) 20% R triangular wave Fig. 6. Wave profile for the evaluation of load characteristics The highest value of compressive residual stress was obtained when air jet pressure was 4 bar and shot-peening time was 90 sec through shot-peening process tests. Then these conditions were chosen as durability test with 4 different wave profiles as shown in Fig. 6 and 4 different thicknesses. 2Hz of repeated loads, between without loads (under 10N) and complete compression until thickness, was applied on the workpiece. Fig. 7 shows durability test system used in this study. It was assumed that the wave spring could be defined as infinite life and the test would not continue if times of repeated loads would be over 2,000,000. Table 4 and Table 5 show summarized fatigue life data of wave springs without and with shot-peening according to spring thickness and wave profile type. Theses results show that spring thickness can affect fatigue life and it was reduced according to thickness increasing. Then it could be the most suitable that thin thickness was chosen to improve wave spring durability. Fig. 7. Spring durability tester

4 2290 Advanced Engineering Materials III Table 4. Fatigue life minimum value from durability test results according to wave profile for each wave spring thickness under non shot-peening conditions Thickness (mm) Fatigue life (Cycles) Wave Profile Sine 100% R 50% R 20% R 1.2 Over 2.0 E6 Over 2.0 E E E E5 Over 2.0 E E E E E E E E E E E3 Table 5. Fatigue life minimum value from durability test results according to wave profile for each wave spring thickness under shot-peening conditions Thickness (mm) Conclusions Fatigue life (Cycles) Wave Profile Sine 100% R 50% R 20% R 1.2 Over 2.0 E6 Over 2.0 E6 Over 2.0 E E5 1.4 Over 2.0 E6 Over 2.0 E6 Over 2.0 E E E6 Over 2.0 E E E E5 Over 2.0 E E E3 This study presents determination of optimal shot-peening process conditions, wave profiles and thickness in order to precisely control and improve durability and fatigue life of wave cushion springs assembled into automobile automatic transmission clutch. It could be known that spring thickness can affect fatigue life and the thinner the longer fatigue life. X-ray diffraction residual stress measurement tester was used to quantitatively evaluate shot-peening effects for improving fatigue life by measuring residual stresses after applying shot-peening process. Residual stresses increased when air jet times increased. However increasing amount was not significant and deviation of residual stresses at different points reduced when air jet times increased. This means that sufficient process time can provide uniform residual stress on wave spring. To choose optimal wave spring profile and evaluate fatigue life, actual prototype springs were produced and tested by durability tester. It could be known that the thinner thickness the longer fatigue life and the bigger wave curvature the longer fatigue life, and shot-peening process could increase fatigue life as respect to without shot-peening. From this study it could be known that (1) sine wave profile was suitable to precisely control spring under hydraulic complete compressive load, (2) the thinner spring the longer fatigue life, (3) 100% R triangular wave profile was suitable to guarantee longer fatigue life and (4) shot-peening could obviously improve fatigue life. Finally it could be expected that this study provided guideline for wave spring optimal design and improving fatigue life. Acknowledgement This work was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy. References [1] S.H. Lee and D.S. Shim: J. of the Korean Soc. for Heat Treatment Vol.17, No.6 (2004) p.336. [2] N.S. Lee, W.J. Park and S.H. Park: The Korean Soc. of Mech. Eng. Autumn Conf. (2006) p.57. [3] K.D. Park, J.Y. Lee, Y.J. Shin and Ki: The Korean Soc. of Manuf. Proc. (2006) Eng. P.184. [4] D.S. Lee and S.K Cheong: The Korean Soc. of Auto. Eng. Vol.16, No.4 (2008) p.63. [5] K.D. Park and J.P. An: The Korean Soc. of Auto. Eng. Spring Conf. (2005) p.741.

5 Advanced Engineering Materials III / Optimization of Shot-Peening Conditions and Wave Profiles to Improve Durability of Wave Cushion Spring /