A large herringbone bevel gear awaits treatment in a shot-peening chamber. Multiple nozzles will direct shot at the gear teeth to increase their resistance to surface fatigue.

Subjecting a metal part to the shot peening process induces a residual compressive stress in its surface. This compressive stress counters the effect of tensile stresses during operation of the part, thereby improving its resistance to fatigue failure.

This method of inducing residual stress increases the fatigue life of PT components, such as rolling-element bearings and shafts. It has long been applied to the root fillet area of gear teeth to improve their bending strength. But until 1982, it was not used on the tooth face. Then NASA initiated two series of tests that showed how shot peening extends the surface fatigue life of gears in the tooth contact area.

NASA tests — Part 1

In 1982, researchers at the NASA Lewis Research Center, Cleveland, Ohio, tested two groups of spur gears to determine the effect of shot peening on the surface fatigue pitting resistance in the tooth contact area. The gears were manufactured from a vacuum-melted (CVM) AISI 9310 steel. Both sets were carburized and case hardened to Rockwell C 58. The test gears had 28 teeth and a 3.5-in. pitch diameter.

On one group, researchers shotpeened the gear tooth surfaces, using a normal shot peening intensity (measure of energy imparted to the component) of 7 to 9A. The second group was not shot peened.

Gears were tested at a temperature of 170 F and a speed of 10,000 rpm, with a maximum Hertz stress of 248,000 psi at the contact surface. Figure 1 illustrates typical pitting of the tooth contact area during the tests.

Residual compressive stresses in the shot-peened gears increased 40% over the standard gears at a depth of maximum shear stress (7 mil) for the load conditions, Figure 2. These compressive stresses improved the surface fatigue life, or pitting resistance, of the gears by 1.6 times compared to those that were not shot peened, Figure 3.

After these initial NASA tests, auto manufacturers began to use shot-peened gears in the gearboxes of cars and trucks. These gears are generally made from carburized chrome, nickel, or moly steels. Millions of these automotive gears were subsequently produced for both domestic and offshore use.

NASA tests - Part 2

In 1991, NASA expanded on its earlier study by investigating the effect of shot peening gears to a higher-than-normal intensity. The new tests involved two sets of spur gears that were manufactured from vacuum-induction-melted, vacuumarc- remelted (VIM-VAR) AISI 9310 steel. The gears were case carburized, hardened to Rockwell C 60, and ground.

Researchers peened one set of gears to a medium intensity (7 to 9A) and the second set to high intensity (15 to 17A). Both sets were honed after peening to improve the surface finish to 16μ min. rms. As in the earlier tests, gears had 28 teeth and a 3.5-in. pitch diameter. Test conditions were the same as in 1982: 170 F, 10,000 rpm, and a maximum Hertz stress of 248,000 psi.

Residual compressive stresses for the high-intensity shot-peened gears were 57% higher than that for the medium-intensity shot-peened gears at a depth of 5 mil, Figure 4. This is the calculated depth of maximum shear stress when the effect of surface friction is considered. The compressive stress in the high-intensity shot-peened gears was also much higher at 1 to 6 mil below the surface.

As a result, the high-intensity shotpeened gears exhibited a surface fatigue (pitting) life 2.13 times that of the gears that were peened to medium intensity, Figure 5.

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