Method of imparting compressive residual stress to balls

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

• a) placing the first plurality of balls between a first body having a first surface and a second body having a second 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,
• b) imparting a compressive stress along a first diameter of the first plurality of balls by pressing the first body toward the second body or the second body toward the first body or the first and second bodies toward one another with a force, and
• c) causing relative movement between the first surface and the second surface while maintaining the force at or above a minimum level to impart the compressive stress along other diameters of the first plurality of balls different than the first diameter,
• wherein the contact portion is substantially planar, and
• wherein the second surface includes at least one circular groove facing the smooth contact portion, the at least one groove having an entrance in a groove sidewall and an exit in the groove sidewall and wherein placing the first plurality of balls between the first body and the second body comprises feeding the first plurality of balls into the at least one groove through the groove entrance and wherein causing relative movement comprises rotating one or both of the first and second bodies to cause the first plurality of balls to move around the circular groove.

A method of producing compressive residual stress in at least a surface portion of a first plurality of balls having an initial surface hardness using a machine having a first support and a second support wherein the first support or the second support or both the first support and the second support are rotatable about an axis of rotation, the method comprising:

• placing the first plurality of balls in a circular groove in a first plate attached to the first support, the first plate being attached to the first support such that a center of the circle of the circular groove lies on the axis of rotation,
• placing a second plate having a smooth contact surface having a hardness greater than or equal to the initial hardness against the first plurality of balls in the circular groove, the second plate being attached to the second support and the contact surface being substantially flat or convex,
• applying a compressive force to the first plurality of balls by pressing the first support toward the second support or the second support toward the first support or the first and second supports toward one another, and
• causing relative rotation between the first plate and the second plate while maintaining the force at or above a minimum level to move the first plurality of balls around the circular groove and produce the compressive residual stress in at least the surface portion of the first plurality of balls.