Magnetic recording disk cartridge

A magnetic recording disk cartridge comprising a recording disk having a circular flexible recording medium and a hub, having an axis and a peripheral surface, secured to the central portion of the recording medium, and a casing, having a base wall, in which the recording disk is accommodated, the casing being provided with a hub opening, at the center of the base wall, for permitting means for rotating the recording disk to have access to the hub and a magnetic head receiving opening extending radially outwardly from the hub opening for giving a magnetic head access to the recording medium, in which said hub is provided with a plurality of axially extending engaging grooves on the peripheral surface thereof, said base wall of the casing is provided with a plurality of projecting walls spaced from each other along the periphery of the hub opening to surround the hub, said projecting walls being disposed substantially perpendicularly to said base wall, said projecting walls having an inner perimeter and an outer perimeter, and a hub locking device is provided in the casing to releasably lock the hub to prevent rotation of the recording disk and to close the magnetic head receiving opening when the cartridge is not in use, the hub locking device comprising a hub locking member having an annular bearing portion which slidably engages the outer perimeter of said projecting walls, said locking member being rotatable about the outer perimeter of said projecting walls, a shutter portion integrally extending from the bearing portion and an engaging portion formed integrally with the bearing portion to resiliently project toward the hub, and an operating member for swinging the hub locking member about the central axis of the hub between an operative position in which said shutter portion closes the magnetic head receiving opening and said engaging portion projects toward the hub through the space between the adjacent walls and within the inner perimeter of said projecting walls to engage with the engaging grooves of the hub, and an inopertive position in which said shutter portion is retracted from the magnetic head receiving opening to open it and said engaging portion is disengaged from the engaging grooves of the hub whereby the disk is free to rotate about said axis.