Rotary plug cylinder lock

A cylinder lock of the rotatable disk tumbler type comprising a stationary cylinder casing, a rotatable plug assembly in said casing including an inner shell member rotatable in the casing and a plurality of locking disk tumblers encircled within said shell member and rotatable about a common axis therein, an elongated locking bar paralleling the axis of rotation of said disk tumblers adjacent the periphery thereof normally restrained by the disk tumblers at a position traversing the shear line between the inner shell member and the casing for locking the inner shell member against rotation relative to the casing, the disk tumblers being formed with substantially circular body portions having true gate recesses of appropriate depth alignable with and adapted to receive said locking bar to accommodate radial inward movmement of the locking bar relative to the disk tumblers to a position permitting rotation of the inner shell member relative to the casing and said tumblers having key openings therein collectively defining a forwardly opening keyway and shaped to be engaged and angularly moved by a key inserted therein for aligning the true gate recesses with the locking bar, said tumblers each having a tab formation projecting from the periphery of the circular portion thereof, said inner shell member having an inwardly opening, circumferentially elongated sector recess for receiving the tab formations projecting from each of the disk tumblers over a selected range of angular movement of the tumblers, the sector recess having an end wall defining stop surfaces for engaging the tabs and angularly positioning the tumblers to align their key openings at proper positions to receive a key therein, and the tab formation of each disk tumbler being a thin frangible member of less thickness than the circular body portion designed to be sheared from the thickness circular body portion of associated disk tumbler by engagement with end walls of the sector recess when torque forces exceeding a predetermined threshold value are applied to the tumblers.