a head drum assembly comprises a stationary drum , a rotary drum , and a self - compensating dynamic balancer . the stationary drum is fixed to a shaft , and the rotary drum is rotatably mounted to the shaft so that it faces the stationary drum . the rotary drum supports a magnetic head . the self - compensating dynamic balancer automatically compensates for any eccentricity between the rotational axis and the centroid of the rotary drum by using the centrifugal force of the rotary drum being rotated .