Square shaped axial split pin tumbler lock

In an axial split pin tumbler lock mechanism having increased pick resistant of the type including a lock cylinder, a barrel assembly secured within the lock cylinder and having a longitudinal axis extending between front and rear ends thereof, barrel assembly including a lock shaft having a forward and a rear end and a forwardly disposed operating part rotatable about the longitudinal axis formed by the lock shaft carrying a cylindrical head portion adjacent the forward end thereof, a stationary sleeve member carried on the lock shaft adjacent the rear end thereof and adjoining the cylindrical head portion at a transverse interfacial plane, a plurality of longitudinal bores formed in each of said cylindrical head portion and sleeve member, the bores being disposed radially around the longitudinal axis thereof and being relatively movable into and out of longitudinal alignment upon rotation of the operating part, tumbler means each including a forwardly disposed driver tumbler carried in the cylindrical head portion and a rearwardly disposed follower tumbler carried in the stationary sleeve member, the tumbler means being axially reciprocable to provide a rotationally movable position when the respective driver tumblers and follower tumblers coincide with the interfacial plane between the cylindrical head portion and the sleeve member, and a lock position when at least one of the tumblers bridges the interfacial plane, spring means carried in one set of the bores for yieldingly urging the tumblers in aligned bores into the lock position and the driver tumblers having front ends engagable with a key such that rearward movement of the key moves the tumblers in aligned bores rearwardly to positions wherein tumbler joints coincide with the interfacial plane, the improvement comprising in combination,

• said lock cylinder formed by a hollow cylinder body carrying a lock head at the forward end thereof and the hollow cylinder body adapted to carry the barrel assembly therein,
• said lock head provided with a substantially square shaped keyhole formed therein, said substantially square shaped keyhole defining a front face on said lock head which includes a series of thickened face sections alternating with a series of thinner face sections, and said keyhole defining an inner face defined by said substantially square-shaped keyhole,
• said inner face of said keyhole providing a tumbler ledge forming a riding surface for the front ends of said driver tumblers positioned within said cylindrical head portion whereby said driver tumblers ride in a path alternating between thickened sections and thinner sections of said tumbler ledge,
• said cylindrical head portions of said operating part having a forward end provided with a substantially square shaped cut-out section presented therein,
• said substantially square shaped cut-out section radially traversing the tumbler bores formed therein,
• said substantially square shaped cut-out section of said cylindrical head portion being in axial alignment with and disposed immediately rearward of said substantially square shaped keyhole formed in said lock head,
• and a tubular key having an inner circular core and an outer substantially square shaped face adapted to matingly engage within said substantially square shaped keyhole in said lock head and said substantially square shaped cut-out section in said cylindrical head portion,
• said outer substantially square shaped key face having bitted sections around the periphery thereof for engagement with corresponding driver tumblers located in said cylindrical head portion,
• whereby only a properly bitted key having an outer substantially square shaped face may be inserted in said substantially square shaped keyhole and into said substantially square shaped cylindrical head portion to manipulate the driver tumblers and operate said lock mechanism,
• said driver tumblers otherwise being at least partially shielded from manipulation by a lock pick by the thickened sections of said tumbler ledge.