Surface Durability of Forged and Machined Steel Gears

Abstract

Forged and machined straight bevel gears have been tested in order to compare the surface durability or pitting fatigue characteristics. The gears were prepared with identical geometric properties to the extent possible with two different manufacturing processes. The machined gears were cut with a two-tool generator. A static torque four-square recycling power gearbox apparatus was used for the tests. The simulated transmitted power was 22.4 kW (30 hp) for a constant life of 201 hours or 14.5 × 106 cycles. The damage was compared based on the number of pitted teeth per gear and the pits per tooth. Eight straight bevel gears with nineteen teeth each or a total of 152 teeth were used for each of the ten tests. The machined gears had 30.6 percent more pitted teeth per gear and 38.5 percent more pits per tooth than the forged gears. The improvement in the forged gears may in part be due to slightly better microstructural characteristics resulting during the heat treatment operation. However, the strongest evidence would attribute the difference to the surface roughness. Without special processing, machined gears have more adversely oriented pitting nucleation sites resulting from tool marks.