Study on Methodology to Increase Fatigue Limit of Gears

Author:  Ando (1), Matsui (2) and Eto (2)
Source:  Conf Proc: ICSP-8 Sept. 16-20, 2002 Garmisch-Partenkirchen, Germany
Doc ID:  2002004
Year of Publication:  2002
Abstract:  
Authors: Kotoji Ando (1), Katsuyuki Matsui (2) and Hirohito Eto (2) (1) Yokohama National University, Yokohama-shi, Japan (2) Isuzu Motors Ltd., Kawasaki-shi, Japan Introduction At present, improvement of the fatigue limit of automotive components is a top priority. The demand is especially high for automobiles due to the environmental and fuel economy expectations. To improve the fatigue limit of automotive components, the following three methods are currently in common use: (i) Minimize surface roughness, (ii) Increase the hardness of the material, (iii) Introduce a large compressive residual stress at the surface. Item (iv) is an extremely adequate method; therefore, a close attention is being paid to achieve a minimized surface roughness at this time. Although item (ii) is a reasonable method, it is difficult to apply to the automobile gears and springs with hardness as high as 600-700 HV. Fatigue limit is proportional to hardness up to 400-500 HV. Above 500 HV, fatigue limit does not increase with hardness, but decreases as hardness increases.Therefore, it is not appropriate to increase hardness further for components such as gears and springs. Item (iii) is a general method and shot peening is used widely. However, it is extremely difficult to introduce a large compressive residual stress by shot peening since the hardness of automotive components reaches 600-700 HV. To solve above problems and improve the fatigue limit of automotive components, the authors conducted a study focusing on the following points: (a) What is the process of fatigue fracture and what is the resistance factor in each stage? (b) What stress ratio (R) can be applied to automotive components? (c) Why does the fatigue limit start decreasing when hardness reaches some level? Is there any way by which the decrease can be inhibited? (d) How can a large compressive residual stress be introduced to a material with 700HV or more? As a result of this study, it was found that the number of components subjected to cyclic loading with positive stress ratios (R>0) is unexpectedly high among automotive components such as gears and springs. Therefore, after working closely on the above four points concentrating on the R>0 components, the following results for improvement of the fatigue limit were proposed: (1) Increase the hardness of materials as high as possible. (2) Introduce compressive residual stress as high and deep as possible. (3) Decrease the grain size as much as possible. (4) Grind the surface region of components to remove early stage fatigue damage such as extrusion and intrusion, and stage I fatigue crack. (5) Heal the stage I fatigue crack during service if possible [1]. In this paper, the fatigue limit range of a gear was improved considerably, simply and economically, using above methods (1), (2) and (3) together.