Abstract
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.
Highlights
Over the last 10 years, new registrations of passenger cars increased by approx. 21% in Germany for the segment above 100 kW
When performing tooth root bending studies, the limiting number of load cycles for tests conducted on the pulsating test rig is commonly either 3 million or 6 million
No value could be assigned for a limiting number of load cycles that is sufficient for determining the endurance fatigue limit
Summary
Over the last 10 years, new registrations of passenger cars increased by approx. 21% in Germany for the segment above 100 kW (see Fig. 1). Over the last 10 years, new registrations of passenger cars increased by approx. 21% in Germany for the segment above 100 kW (see Fig. 1). The trend is not restricted to Germany but is apparent worldwide. A second trend is that more and more lightweight passenger cars are being constructed, for example with the aim of improving CO2 figures and to satisfy EU regulations. The gearbox and powertrain have to be made more robust, which, increases the weight of common gearboxes. To achieve both goals, gears in gearboxes can be shotpeened. The high compressive residual stresses introduced by the shot-peening process have the effect of increasing the tooth root load carrying capacity, because the strain
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