AGMA 13FTM09-2013 pdf free.AGMA Technical Paper Investigations on Tooth Root Bending Strength of Case Hardened Gears in the Range of High Cycle Fatigue By Dr. N. Bretl, S. Schurer,Dr. T. Tobie, Dr. K. Stahl and Dr. B.-R. Hohn, Gear Research Centre (FZG).
Both variants show tooth root breakages in the range of high cycle fatigue, but only the variant with low material cleanness shows breakages with crack initiation under the surface and the connected decrease of high cycle fatigue (see also model representation in Figure 10) compared to the variant with high material cleanness. The test results conclude that the variant with high cleanness does not show inclusions of critical size in the critical surface layer, which results in crack initiation under the surface because of the local stress situation. The result is that the load-carrying potential of shot peening can be used even in the range of high load cycles and subsurface fatigue seems to be avoided because of a high material cleanness (according to the model representation shown in Figure 11).
The test results show that the residual stresses and the material cleanness or rather the presence of inclusions of critical size in the material associated with the local load stresses, have a dominant and direct influence on internal fracture mode and resultant high cycle fatigue. In view of the experimental results, it becomes obvious that the initiation of internal cracks not only depends on the residual stress condition or the presence of inclusions. It seems rather to be an interaction of state of load stress, state of residual compressive stress and critical inclusions in the material.
In general, the experimental test results do not show an appreciable decrease of the tooth root loadcarrying capacity with a fatigue cycle limit of 100 x 106 compared to standard methods on gears with residual compressive stresses < 900 N/mm2 (typical clean blasted condition). According to this, the values of the tooth root load-carrying capacity for material quality MO published in the standard ISO 6336-5 [7] remain valid for the examined tooth sizes and common material cleanness even for a fatigue cycle limit of 100 x 106. A further increase of the load-carrying capacity due to shot-peening can be confirmed by the achieved test results. Nevertheless, especially for case hardened, shot-peened gears, additional influences on the high cycle fatigue should be taken into account.
Summary
The tooth root load-carrying capacity is one of the determining factors in gear design. Besides the strength of the material itself, the existing state of stress (load induced stresses and residual stresses) significantly influences the tooth root load-carrying capacity, also in particular the high cycle fatigue and the related fracture mode. In order to verify the high cycle fatigue behavior of the tooth root of case hardened gears. substantial theoretical studies as well as an extensive program of tooth bending tests have been carried out on case hardened gears of various sizes, materials and residual stress conditions.
Based on the test results and the knowledge of further investigations taken from the literature, a model representation was established to interpret the influence of different local stress situations on the tooth root load-carrying capacity and different fracture modes in high cycle ranges. Therefore, the idea of a stepwise S-N curve, already known from a variety of investigations in the literature for simpler specimens [8], [9], can also be transferred to gears. The different level locations of these two S-N curves for the different fracture modes mainly depend on the current residual stress condition, as well as on the material cleanness. They describe the occurrence of tooth root breakages with crack initiation on and below the surface and the related high cycle fatigue of case hardened gears.
The experimental test results have shown the high potential regarding tooth root load-carrying capacity due to peening operations in high cycle ranges up to 100 106 cycles. Additionally, the expenmental test results confirm the theoretical statements of the presence of a stepwise S-N curve behavior of case hardened, peened gears and the appearance of different fracture modes. Furthermore, the test results confirm the published values of the standard ISO 6336-5 [7] regarding tooth root load-carrying capacity of typically cleaned (case hardened) gears even for fatigue cycle limit up to 100 x 106 cycles. Nevertheless, especially for case hardened, shot-peened gears. additional influences on the high cycle fatigue should be taken into account.
In conclusion, the theoretical and experimental results of the research project have shown that the initiation of internal cracks and high cycle fatigue of case hardened gears not only depends on the residual stress condition or the presence of inclusions. It seems rather to be an interaction of state of load stress, state of residual compressive stress and critical inclusions in the material.
Acknowledgements
This research project was sponsored by the Forschungsvereinigung der Arbeitsgemeinschaft der Eisen und Metall verarbeitenden Industrie e.V. (AVIF) with an equity ratio of the Forschungsvereinigung Antriebstechnik (FVA).
References
1. ANSI/AGMA 2001 -C95, Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth. AGMA, Alexandria, 1994.
2. Bretl, N.: EinflUsse auf die Zahnfur3tragfähigkeit einsatzgehârteter Zahnräder im Bereich hoher Lastspielzahlen. Dissertation. TU MOnchen. 2010.
3. Bretl, N.: Spate Zahnful3brüche — ZaIrntuBbruch mit Rissausgang unterhalb der Oberflãche an einsatzgehärteten Zahnrãder. Forschungsheft Nr. 851, Forschungs-vereinigung Antriebstechnik e.V., Frankfurt, 2008.
4. DIN 50602: Mikroskopische PrUfung von Edelstãhlen auf nichtmetallische EinschlUsse mit Bildreihen. Beuth Verlag. Berlin, 1985.
5. FVA-Merkblatt Nr. 0/5- Ergãnzung: Empfehlungen zur Vereinheitlichung von Pulsatorversuchen zur Zahnful3tragfâhigkeit von vergOteten und gehãrteten Zahnrädern. Forschungsvereinigung Antriebstechnik e.V., Frankfurt, 2009.
6. Hirsch, T.: Kugelstrahlen. Untersuchungen zur Zahnful3festigkeit kugelgestrahiter Zahnrãder. Forschungsheft Nr. 126, Forschungsvereinigung Antriebstechnik e.V., Frankfurt. 1983.
7. ISO 6336: Calculation of load capacity of spur and helical gears. 2006.
8. Nishijima, S.; Kanazawa K: Stepwise S-N curve and fish-eye failure in gigacycle fatigue. Fatigue & Fracture of Engineering Materials & Structures, Volume 22, Issue 7, S. 601-607, 1999.AGMA 13FTM09 pdf download.