Double key axial split-pin tumbler-type lock

Claim 1

In combination with an axial split-pin tumbler-type lock having a lock cylinder, a three-part plug assembly inserted in the cylinder, and a plurality of tumbler assemblies carried by the plug assembly, said plug assembly including a rear sleeve part fixed to the cylinder, a rotatable intermediate spindle part including a lock shaft extending through the sleeve part, a rotatable front drive part, and a post fixed to the drive part and having means drivingly engaging a key for rotation of the drive part by turning the key, said plug assembly parts each having a plurality of longitudinal bores arranged for alignment with the bores in the remaining parts in an initial disposition of said parts, said aligned bores being asymmetrically disposed about the longitudinal axis of the plug assembly, whereby the drive part may be rotated relative to the other parts and provide an inoperative second disposition of the parts wherein a blank area of the drive part is disposed over aligned bores in the spindle and sleeve parts and prevents access thereto, while at least one bore of the drive part is disposed in alignment with aligned bores in the spindle and sleeve parts, one of said tumbler assemblies being disposed in each series of aligned bores in said initial disposition of the parts and each tumbler assembly including spring means, a rear tumbler pin, an intermediate tumbler pin, and a front tumbler pin, said spring means yieldingly urging the several pins forwardly in the bores in said initial disposition of the parts, whereby a first interfacial plane between the sleeve and spindle parts, and a second interfacial plane between the spindle and drive parts are bridged by tumbler pins to secure the parts against relative rotation, and the tumbler pins in each series of aligned bores may be shifted longitudinally to cause the interface between the rear pin and the intermediate pin to coincide with said first plane while said second plane is bridged by at least one pin to thereby free the drive and spindle parts for conjoint rotation relative to the sleeve part, or alternatively to cause the interface between the intermediate pin and the front pin to coincide with said second plane while at least the interface between said aligned bores having said blank area thereover in said second disposition of the parts is bridged by one of said pins to thereby free the drive part for rotation relative to the spindle part and the sleeve part: first and second keys each adapted for drivingly engaging said post and having different bittings engaging said front tumbler pins, the bittings of the first key being adapted for shifting the tumbler pins longitudinally in each series of aligned bores so as to free the drive and spindle parts for cojoint rotation by the key from said initial disposition of the parts to operate the lock, the bittings of the second key being adapted for shifting the tumbler pins longitudinally in each series of aligned bores so as to free only the drive part for rotation by the key from said initial disposition of the parts to said inoperative second disposition of the parts, and said second key being removable from the lock in said second disposition of the parts to leave the parts secured against relative rotation, whereby the lock is rendered inaccessible for direct picking and the second and first keys must be employed successively in that order to operate the lock.