This invention relates to a flat key cylinder lock with anti-burglar features.
Known are flat key locks where, in addition to the usual bolt pins and counter-pins slided by the key dents, auxiliary side pins are provided which are slided by the key flanks and provide an added measure of burglar-proof ability.
Locks of this type are known, for example, from German Patent No. 2003059, in which the auxiliary pins are arranged to extend perpendicularly to the plane of the key. The pin action results from that, on the pins being slided sideways by the key being pushed in, they move out of the plug outer surface to engage in recesses formed in the cylindrical casing in which the plug is received rotatably. The proper key is formed with notches which, on the key being inserted into the lock, enable the auxiliary pins to retract by reaction effect as the plug is rotated. The pin retraction is facilitated by the crowned shape of the recesses.
When a bogus key is used, the notches and pins mismatch, and accordingly, the pins are held in a position of engagement with the recesses and prevent the plug from turning.
The burglar resisting ability of the known auxiliary pins is, in actual practice, rather unreliable because it can be easily overcome by effecting removal thereof lengthwise to the key profile at the areas abutting the auxiliary pins, that is by milling out the key flanks longitudinally, thereby the pins are allowed to slide freely in their seats. Such a deceptively machined key (referred to in the jargon as a "relieved" key) allow unrestricted sliding for the auxiliary pins which are, therefore, free to retract as the plug is rotated.
The published German Patent Application No. 31 23 511 discloses a lock provided with holes for the auxiliary pins perpendicular to the usual pins and counter-pins and having the same or a larger diameter than the latter. With a "relieved" key, the counter-pins, after a 90.degree. rotation, can fit into the holes for the auxiliary pins and make the lock fast. With a non-relieved key, the plug rotation would be hindered by the auxiliary pins being held engaged with the notches in the cylinder casing.
A lock according to the above disclosure has the disadvantage that when the lock is made fast in the rotated position, the bogus key cannot be removed and the lock is rendered unreusable.