in "inertia friction welding" the drive motor is disengaged, and the workpieces are forced together by a friction welding force. the kinetic energy stored in the rotating flywheel is dissipated as heat at the weld interface as the flywheel speed decreases. before welding, one of the workpieces is attached to the rotary chuck along with a flywheel of a given weight. the piece is then spun up to a high rate of rotation to store the required energy in the flywheel. once spinning at the proper speed, the motor is removed and the pieces forced together under pressure. the force is kept on the pieces after the spinning stops to allow the weld to "set".