On the determination of the bending fatigue strength in and above the very high cycle fatigue regime of shot-peened gears

Demands on modern gearboxes are constantly increasing, for example to comply with lightweight design goals or new CO2 thresholds. Normally, to increase performance requires making gearboxes and powertrains more robust. However, this increases the weight of a standard gearbox. The two trends therefore seem contradictory. To satisfy both of these goals, gears in gearboxes can be shot-peened to introduce high compressive residual stresses and improve their bending fatigue strength. To determine a gear’s tooth root bending fatigue strength, experiments are conducted up to a defined number of load cycles in the high cycle fatigue range. However, investigations of shot-peened gears have revealed tooth root fracture damage initiated at non-metallic inclusions in and above the very high cycle fatigue range. This means that a further reduction in bending load carrying capacity has to be expected at higher load cycles, something which is not covered under current standard testing conditions. The question is whether there is a significant decrease in the bending load carrying capacity and, also, if pulsating tests conducted at higher load cycles—or even tests on the FZG back-to-back test rig—are necessary to determine a proper endurance fatigue limit for shot-peened gears. This paper examines these questions.

Effect of metal and polymer mating gears on the bending fatigue performance of asymmetric polymer gears

The material of the mating gear influences the fatigue life of polymer gears. The bending fatigue characteristics of polyamide 66 asymmetric gears (34°/20° and 20°/34°) corresponding to steel–polymer and polymer–polymer material combinations were investigated. The performance of symmetric gear pairs (20°/20°) was determined to serve as a comparison. Quasi-static numerical simulations were performed in a finite element analysis tool to predict root bending stress, load sharing ratio, and tooth deflection. The bending fatigue strength of steel–polymer and polymer–polymer pairs of each test configuration was determined using bending fatigue tests. The load sharing ratio and root bending stress of polymer–polymer pairs decreased substantially compared to steel–polymer pairs. The extent of deflection-induced load sharing was greater in polymer–polymer pairs. The bending fatigue life of polymer–polymer pairs was lower than that of steel–polymer pairs owing to the higher operating temperature. In polymer–polymer pairs, polymer driving and driven gears increased the heat generated and diminished the heat dissipation to the environment. In steel–polymer and polymer–polymer pairs, the configuration with the highest bending fatigue strength was 34°/20° and 20°/34°, respectively. This divergence was caused by the increase in temperature difference between the two configurations for polymer–polymer pairs. Analysis of hysteresis loops indicated that the loop area was higher for polymer–polymer pairs, signifying the increased amount of dissipated energy. No noticeable variation was observed between the failure modes of steel–polymer and polymer–polymer pairs despite the significant difference in the operating temperatures. The bending stress and operating temperature were the dominant factors affecting the performance of steel–polymer and polymer–polymer gear pairs, respectively.

Experimental Analysis of Residual Stress and Bending Strength of Gear Tooth Surface after Shot Peening Treatment

The fatigue strength of a gear tooth surface is affected by various factors, which subsequently impacts the transmission performance of gears. Usually, shot peening treatment is carried out during processing to improve the performance of gears. Most current studies focus on theoretical descriptions and simulation analyses of shot peening treatment. However, in this paper, the relationships among shot peening treatment, residual stress, and bending fatigue strength of a gear tooth surface are discussed, through experimental methods. Based on X-ray stress analysis, at select locations on the test samples, the residual stresses on gear tooth surfaces with and without shot peening treatment are determined and contrasted. The results show that shot peening treatment can effectively increase the residual stress on gear tooth surfaces. In addition, an electromagnetic resonance fatigue tester is used to analyze the bending fatigue strength of gear tooth surfaces. The test results indicate that the bending fatigue strength of the gear teeth with shot peening is higher than that of the gear teeth without shot peening. The obtained conclusions lay the foundation for further practical engineering applications of gears.

Influence of Shear Cutting Process Parameters on the Residual Stress State and the Fatigue Strength of Gears

Shear cutting is used for manufacturing various parts ranging from e.g. simple washers to complex gears. The latter are typically subjected to cyclic loading and fail foremost due to fatigue damages. Hereby, the parts lifetime is mainly determined by: the geometry, the applied load, the material, the hardness, the roughness and the residual stress state. While numerous research works deal with the influence of the process parameters on the hardness and the parts geometry, the influence of the process parameters on the residual stress state and on the resulting fatigue strength has not been investigated in detail, yet. In an earlier publication, suitable shear cutting techniques, which allow to achieve a high amount of clean-cut and a favorable residual stress state were compared. In this paper, the influence of the process parameters on the residual stress state and the resulting bending fatigue strength are addressed. To simulate the bending stress occurring in the tooth root, C-shaped specimens were manufactured by different blanking techniques. The die-clearance and punch and die edge radii were varied with these blanking techniques. After measuring the cut-surface geometry, the hardness distribution and the surface roughness, the fatigue strength was determined in a pulsating test rig. By carrying out residual stress measurements using x-ray diffraction and simulating the material flow behavior using the Finite-Element-Method, basic mechanisms, which are influencing the residual stress state and the resulting bending fatigue strength, were identified and will be presented and discussed in the paper.