Abstract: A rotary unit such as a disk file, retained in a dynamically balanced condition. In each end of the unit, there is an annular channel concentric with the rotational axis encircled by a flexible rim. A plurality of balls of substantially identical size and weight are inserted in an outer portion of each channel in which the balls are freely movable. While the unit is rotated at a speed above resonance, a balancing instrument is used to determine the mass magnitude and angle necessary to dynamically balance the unit. Then the balls are circumferentially relocated by vector balancing to provide mass at points equivalent to the determined mass and angle. The relocated balls are then forced axially past an interference ridge and into an inner portion of the respective channel in which the rim is flexed and the balls are held against movement.