FATIGUE FAILURE OF SPUR GEARS - A REVIEW

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1 FATIGUE FAILURE OF SPUR GEARS - A REVIEW Anand D. Kalani 1, Dr. Jeetendrakumar. A. Vadher 2 and Rita K. Jani 3 1 Mechanical Engineering Department, Gujarat Technological University, Gujarat 2 Professor, Mechanical Engineering Department, GEC, Palanpur, Gujarat 3 Associate Professor, Mechanical Engineering Department, SSEC, Bhavnagar Abstract - Gear Tooth fatigue failure is one of the critical occurrences other than noise level and vibration of gears, this leading to one of the key area of interest for every researcher. More the life of gear, longer will be the life of machine and higher will be the output. In the present paper an attempt is made to summarize the failures occurring in spur gears, causes of fatigue failure and different techniques researched and adopted to inhibit the failures and to achieve the desired working life of gears. Keywords Spur gear, fatigue failure, bending stress, fem, contact stress, review I. INTRODUCTION Spur gears are one of the historical machine elements which play an important role when there is a need for transmitting power. Each and every machine small or big uses gears for completion of their required motions. Gears can also fail in service due to different types of reasons. The working life of gear is dependent on design, material and load exposure of the gear tooth. Normally the gears fail by fatigue, impact fracture, wear and stress rupture. The frequency of occurrence of failures by fatigue is highest, descending to least by stress rupture. Tooth bending failure is one of the most common type of failure in gear. II. GEAR FAILURES The gear teeth are normally subjected to cyclic loads with results in tooth breakage failure or tooth wear failure. The gears normally fail at two different regions of the gear tooth flank or tooth root. The different modes of gear failure are as shown in figure 1. [1] Bending fatigue load results in crack development and propagation at the root area of gear tooth. This crack ends up in failure of tooth giving a halt to the whole transmission system. If the gear tooth fails on exposure to more than load cycles than it is a high cycle fatigue failure and lower than cycle s exposure is low cycle fatigue failure. Gear tooth fractures on result of overloading or stress developed beyond its breakage limit, inaccuracy in design or inconsistency in material of gear. When the teeth mesh with each there is direct metal contact which gives rise to contact stresses resulting in development of surface wear namely scoring, pitting and surface cracking. This is mainly due to lack of lubricant between the surfaces or inclusion of corrosive material like dirt, scales, rust between the contact surfaces. Mustaq M., et.al. conducted a study on failure of full depth spur gear made of 20 general materials and pressure angle of 20 and 25 pressure angle. The study revealed that gears made of case hardened steel failed by bending and tooth breakage while the non heat treated material, structural steel gears mostly failed by scoring and pitting. The gears made of cast iron mostly failed by bending of tooth. [2] DOI: /IJMTER TUFZA 182

2 Wear Moderate Wear Abrasive Wear Corrosive Wear Tooth Fracture Fatigue Breakage Overload Breakage Bending Fatigue Tooth Bending High Cycle Fatigue Failure Low Cycle Fatigue Failure Figure 1 Gear Failure III. CAUSES OF FATIGUE TOOTH FAILURE Fatigue failure of the gear teeth is the most common failure in gearing system. The tooth fails in fatigue by two ways (i) tooth bending fatigue (ii) surface contact fatigue failure. Fatigue failure of gear teeth are result of different conditions prevailing during the operation or inaccuracy within the gears. These includes gear misalignment, overloads on gears, dynamic imbalance, stress raisers, inconsistency in materials and subsurface defects in critical areas [3] Factors Affecting Fatigue Failure are Material Composition, Geometrical discontinuity, Surface condition, Size factor, Residual stresses, Operating temperature, Corrosion, Fretting, Operating speed. [4] IV. METHODS TO IMPROVE SPUR GEAR WORKING LIFE Lots of research has been done on spur gears to increase resistance to failure and increase the efficiency of spur gear. The researches have been done from the design stage of gear to ready to use gears. Different methods and procedures are developed to control the parameters of gears upto desired level. The gears were subjected to alteration in design or were subjected to different metallic treatment. Metal Treatment Spur gears exposed to high operating load and corrosive environment resulted in development of metallurgical process which up-shot the fatigue life of gears. The different metal surface treatments used are laser shock peening, shot peening, coating of harder material, case hardening [carburizing-nitriding] and presetting/pre-stressing. Surface treatment of the gears resulted in increase of residual stressed which resulted in increase of resistance to contact stress developed during mating of tooth of pinion and gear. Contact fatigue is one of the main failure modes of the gear. The contact fatigue test of gear was carried out in order to obtain the contact fatigue characteristics of 20CrMnMo carburized and hardened gears with or without shot peening, the paper presents a method combined with up-down test method and the grouping test method for contact fatigue test of gears. The experimental results show that the shot peening treatment can significantly improve the contact fatigue strength of the gear, thus improving the service life. [5,6] The effect of laser shock peening (LSP) on bending fatigue performance of AISI 9310 steel spur gear has been investigated. The bending fatigue test is conducted using the single-tooth bending method to compare fatigue performance of laser peened teeth and non-peened teeth. The fatigue test revealed that bending fatigue lives of gear teeth has been significantly improved after LSP in comparison with those non-peened teeth, and the bending fatigue limit is enhanced correspondingly. All rights Reserved 183

3 The comparison of the bending fatigue resistance of two case hardened and three nitrided gear steels was performed. Five batches of test gear specimens were produced according to the current industrial practice. Single Tooth Bending Fatigue (STBF) tests were conducted to estimate the fatigue limits of the five gear materials. The experimental results allowed for a comparison of the bending fatigue performances of case carburized and nitrided gears, showing the high competitiveness of the nitride gears. [8] The resistance to rolling contact fatigue of WC/C (tungsten carbide/carbon) -coated gears was investigated. The WC/C low- friction coating was deposited using the physical vapour deposition process by reactive sputtering. The investigation of rolling contact fatigue was realised by means of a back-to-back gear test rig. Four material combinations of gears were tested: uncoated wheel and pinion, coated wheel and pinion, coated wheel and uncoated pinion, as well as uncoated wheel and coated pinion. The tests were performed for mineral gear oil of API GL-5 performance level and 80W/90 viscosity grade. The test results indicate that the resistance to the pitting wear of coated gears depends not only on coating material but also on which gear is coated. To increase the fatigue life of gears, the coating should be deposited on the gear with the higher number of teeth. [9] The aim of this work is to investigate into the possibility of enhancing the fatigue resistance of CrN-PVD coated spur gears, which were tested and a numerical simulation of crack propagation was carried out. Three types of material 15NiCr13, Ti6Al4V TiN coated and carburized gears were investigated. Benefits induced by the presence of the coating were pointed out. The investigation revealed that titanium coated gears were having more fatigue resistance compared to other two, which were at the same level of fatigue resistance. [10] Hard coatings have potential for increasing gear surface fatigue lives. Experiments were conducted using gears both with and without a metal-containing, carbon- based coating. The gears were case-carburized AISI 9310 steel spur gears. Some gears were provided with the coating by magnetron sputtering. Lives were evaluated by accelerated life tests. For uncoated gears, all of fifteen tests resulted in fatigue failure before completing 275 million revolutions. For coated gears, eleven of the fourteen tests were suspended with no fatigue failure after 275 million revolutions. The improved life owing to the coating, approximately a six-fold increase, was a statistically significant result. [11] The fatigue life of machine elements may be increased if beneficial compressive residual stresses are introduced. One mechanical methods of introducing compressive residual stresses is presetting. AISI 1040 steel spur gear teeth were preset. Fatigue testing was carried out resulting in increment of average gear life of 670% under maximum unit load condition. [12] V. ALTERATION IN DESIGN OF SPUR GEARS Spur gear design has always been complicated procedure as number of parameters to be controlled is more. Researchers have always been working on modification of spur gear right from the design stage. Modification of involute profile of gear, optimizing the parameters of gear design, increasing contact ratio, addendum modification, changing rim thickness are some of the procedure to redesign spur gears. The sharing of load done by more than one teeth of the gear have high contact ratio. High contact ratio results in low noise of the gear and less dynamic load as the load exerted on gear teeth is divided between more than one teeth. Methods used for achieving high contact ratio are [1] increasing addendum [2] decreasing pressure angle [3] larger pitch. High contact ratio gears have tendency to slide resulting in less bending stress on tooth. This can be achieved by tooth profile modification. The significant advantage of high contact ratio spur gear is less bending and tooth root stress, thus increased load carrying capacity [13]. High contact ratio gears can decrease bending stress by twenty percent and contact stress by thirty percent [14]. Tirumurugan and Muthuveerappan [15] published a research paper in which they calculated maximum contact and fillet stress for normal and high contact ratio gear. The effect of various gear parameters such as pressure angle, teeth number, gear ratio, tooth size and addendum on the All rights Reserved 184

4 sharing ratio and corresponding stress was investigated. Calculation of maximum fillet and contact stress in the case of normal contact ratio gear and high contact ratio gear using the load sharing ratio was performed. The addendum of the gear modification means changing the addendum diameter to avoid under cutting and interference of gear tooth. This method results in weakening of the tooth resulting in less use of this method. Addendum modification is called as profile shifting method. Analysis of effect of addendum modification on bending fatigue strength of cast iron and cast steel was performed by Oda et al. [16]. Effect of profile shift on the true stress at the fillet section along with most critical point and bending fatigue strength was analyzed in this study. The influence of profile shift on the contact ratio was also studied and contact ratio factor was obtained. Following conclusions were drawn from this study: (i) As the addendum modification coefficient is increased, the root stress factor decreases for both highest points of single tooth loading and tooth tip loading. (ii) If the proper amount of addendum modification is selected, the bending fatigue limit load of can be raised more than forty percent over standard gear. (iii) If the addendum modification factor is applied to cast iron and cast steel, the bending fatigue strength of profile shifted gear can be estimated with very high accuracy. (iv) If the addendum modification of gears is increased, the contact ratio of profile shifted gear decreases. In a technical paper, authored by Oda and Koida [17], the surface durability and pitting failure of spur gear with a smaller number of teeth with different amount of addendum modification were investigated with the help of running tests. Ferritic nitro-carburized gear with a small number of teeth was used by Oda et al. Various tests were performed for different addendum modification and compared with the standard one. Calculated values of hertz stress and specific sliding of spur gear with different number of teeth and addendum modification co- efficient were compared with the experiment results. Li [18] observed that increasing addendum definitely reduces contact stress in gear tooth. In the case of bending stress, this is not surely true. This is due to the fact that the increase in addendum increases the tooth length also. This results in an increment of bending stress instead of decrement. So, in order to implement longer addendum with high contact ratio, calculation of the scoring strength of the root and tip of the gear is also necessary. Chong et al. [19] presented a paper in which results for various loading conditions, rim thickness and support condition were obtained. It was found that the maximum stress exists on the tooth root surface and it increases as the rim thickness is decreased for the looser fitting hub. For tight fitting hub, tooth root stress decreases with a decrease in the rim thickness. It was also found that as the fillet radius decreases, correspondingly the tooth root stress also increases. This variation was found valid only for tooth surface stress. The internal tooth root stress was unaffected by the change in the parameters. Researchers have located the stress concentration area in the gear and tried to reduce the stress by introducing stress relieving features like holes. Fredette and Brown researched on spur gear having hole drilled along the axis of gears in order to decrease the stress concentration. Considerable reduction in the tensile root stress was achieved with this method. Maximum reduction of 8.8 % was achieved. Reduction of root stress increased the fatigue life of gear as capacity to withstand more load cycles increased upto 217 times of original.[20] VI. CONCLUSION Based on the research and findings done by the researchers, two dominating stress i.e contact stress and bending stress generated in spur gears has remained the base of their research. It can be concluded that mainly three methods (i) analytical method (ii) experimental method (iii) FEM method using computational ability are evenly important. Any method out of the three can support the other one for validating the results. AGMA and ISO design procedures are used by the researchers to design spur gears. Gear tooth is considered as cantilever beam for calculating All rights Reserved 185

5 stress and for contact stress Hertzian contact theory is used. Different techniques were adopted by the researchers to increase the working ability, efficiency and working life of gear. REFERENCES [1] E. E. Shipley, "Gear Failure," in Machine Design, ed Cleveland, Ohio: The Penton Publishing co., [2] M. M. a. Kader, S. P. Nigam, and G. K. Grover, A study on mode of failures in spur gears under optimized conditions, Mech. Mach. Theory, vol. 33, no. 6, pp , [3] K.I.Parashivamurthy Material Science and Metallurgy, [4] A. S. Kale, Bending Fatigue Failure In Gear Tooth, Int. J. Eng. Res. Technol., vol. 2, no. 6, pp , [5] W. Li and B. Liu, Experimental investigation on the effect of shot peening on contact fatigue strength for carburized and quenched gears, Int. J. Fatigue, vol. 106, no. 2017, pp , [6] Pariente IF, Guagliano M. Contact fatigue damage analysis of shot peened gears by means of X-ray measurements. Engineering Failure Analysis 2009; 16 (3): [7] C. Peng, Y. Xiao, Y. Wang, and W. Guo, Effect of laser shock peening on bending fatigue performance of AISI 9310 steel spur gear, Opt. Laser Technol., vol. 94, pp , [8] E. Conrado, C. Gorla, P. Davoli, and M. Boniardi, A comparison of bending fatigue strength of carburized and nitrided gears for industrial applications, Eng. Fail. Anal., vol. 78, pp , [9] R. Michalczewski, M. Kalbarczyk, W. Piekoszewski, M. Szczerek, and W. Tuszyński, The rolling contact fatigue of WC/C-coated spur gears, Proc. Inst. Mech. Eng. Part J J. Eng. Tribol., vol. 227, no. 8, pp , [10] S. Baragetti, Fatigue resistance of steel and titanium PVD coated spur gears, Int. J. Fatigue, vol. 29, no. 9 11, pp , [11] T. Krantz, C. Cooper, D. Townsend, and B. Hansen, Increased Surface Fatigue Lives of Spur Gears by Application of a Coating, J. Mech. Des., vol. 126, no. 6, p. 1047, [12] S. R. Daniewicz and D. H. Moore, Increasing the bending fatigue resistance of spur gear teeth using a presetting process, Int. J. Fatigue, vol. 20, no. 7, pp , [13] F. Karpat, S. Ekwaro-Osire, Dynamic analysis of high contact ratio spur gears with asymmetric teeth, Mechanical Systems and Control 11(2008) [14] J. Wang, I. Howard, Finite Element Analysis of High Contact Ratio Spur Gears in Mesh, J. Tribol. 127(3)(2005) [15] R. Thirumurugan, G. Muthuveerappan, Critical Loading Points for Maximum Fillet and Contact Stresses in Normal and High Contact Ratio Spur Gears Based on Load Sharing Ratio, Mech. Based Des. Struct. Mach. 39(1)(2011) [16] S. Oda, K. Tsubokura, C. Namba, Effect of addendum modification on bending fatigue strength of spur gear with high pressure angle, Bull. Jap.Soc.Mech.E. 259(209)(1982) [17] S. Oda, T. Koida, Study on load bearing capacity of gear with smaller number of teeth, Bull. Jap. Soc.Mech.E. 28(236)(1985) [18] S. Li, Effect of addendum on contact strength, bending strength and basic performance parameters of a pair of spur gears, Mech. Mach. Theory 43(12)(2008) [19] T. H. Chong, T. Suzuki, T. Aida, A. Kubo,H. Fujio, Tooth fillet Stresses of Gear with Thin Rim,Bull. Jap. Soc. Mech. E. 26(220)(1983) [20] L. Fredette and M. Brown, Gear Stress Reduction Using Internal Stress Relief Features, J. Mech. Des., vol. 119, no. 4, p. 518, All rights Reserved 186

INCREASE IN FATIGUE LIFE OF SPUR GEAR BY INTRODUCING CIRCULAR STRESS RELIEVING FEATURE

INCREASE IN FATIGUE LIFE OF SPUR GEAR BY INTRODUCING CIRCULAR STRESS RELIEVING FEATURE INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 6340(Print), ISSN 0976 6359(Online), Volume TECHNOLOGY 6, Issue 5,