A nut includes base portion forming an inner threaded bore and a first sun gear. cap forms a second sun gear of which the teeth is different from that of the first sun gear. The cap is rotatably mounted to the base with the first and second sun gears opposing and coaxial with each other. A control ring carries planetary gears engaging both the first and second sun gears. The nut secures a rotatable member of a rotary machine to a threaded driving axle. The nut is initially and loosely tightened on the axle by manually rotating the control ring. The cap has a high friction surface engaging the rotatable member. When the axle starts to rotate, a relative rotation occurs between the rotatable member and the axle which causes the cap to rotate with respect to the base. The rotation of the cap is transmitted to the base by the planetary gears to securely and completely tighten the nut. To release the nut, manual rotation of the control ring causes the base to rotate therewith in a speed reduced fashion due to the difference in teeth between the cap and the base. A small torque applied to the control ring is then converted into a large torque acting upon the base to readily loosen the nut.