Abstract
The contact fatigue tests were first carried out using Ni-Mo pre-alloyed sintered steel (46F4H) spur gears with different densities in the range of 7.30~7.54 Mg/m3. The sintered gear specimens were machined from sintered steel packs made from the single-press single-sinter route and some were surface-rolled using CNC form rolling machine. All test gears were case-carburized. The experimental results showed that the surface-rolled sintered gears with a density of 7.40 Mg/m3 or more had sufficiently high contact fatigue strength to replace gears made of typical Cr-Mo case-carburized wrought steel. These experimental results were then analytially investigated by examining the contact stress distributions around the operating pitch point of P/M pinion using a simplified two-dimensional finite element model considering voids. The peak of the maximum shear stress τmax increased with the decrease of density of the as-sintered gears and decreased as the amount of fully densified depth δ increased. There was only a slight difference in the distribution profile of τmax for every P/M pinion when δ reached to a specified depth of approximately 0.3 mm. These simulation results appear to explain the gear-running test results well and reveal the appropriate surface densification level.
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