Apparatus for producing compressive residual stress in balls

An apparatus for imparting compressive residual stress to at least a surface portion of a first plurality of balls, the balls having an initial surface hardness, the apparatus comprising:

• a first body having a first surface, the first surface including a smooth contact portion, the smooth contact portion being substantially flat or convex and having a surface hardness greater than or equal to the initial surface hardness of the balls,
• a second body having a second surface, the first surface overlying the second surface, and
• at least one drive operably connected to the first body or to the second body and configured to move the first body relative to the second body at a substantially fixed distance from the second body or to move the second body relative to the first body at a substantially fixed distance from the first body, the at least one drive also being configured to move the first body toward the second body with a force or to move the second body toward the first body with the force,
• wherein the second surface includes at least one circular groove facing the smooth contact portion, the at least one groove having an entrance and an exit, the exit being configured to allow the first plurality of balls to exit the groove through a sidewall or bottom of the at least one groove.