Axial split-pin tumbler-type lock mechanism

In an axial split-pin tumbler-type lock mechanism, said mechanism including a lock cylinder, a barrel assembly secured within said cylinder and having a longitudinal axis extending between front and rear ends thereof, said barrel assembly including a forwardly disposed operating part rotatable about said axis and a rearwardly disposed stationary part adjoining the operating part at a transverse interfacial plane, means forming longitudinal bores in said operating and stationary parts, respectively, and disposed radially outwardly of said axis therearound, said bore-forming means in respective parts being relatively movable into and out of longitudinal alignment of their bores upon rotation of said operating part, tumblers each including a forwardly disposed driver element carried in one of said operating part bores and a separate rearwardly disposed follower element carried in one of said stationary part bores with the elements adjoining each other when in aligned bores, said tumblers each being reciprocally movable in the axial direction in aligned bores between respective positions wherein the joint between said elements thereof is disposed on opposite sides of said interfacial plane, said operating and stationary parts being secured against rotation relative to each other when at least one of said tumbler elements bridges said interfacial plane and being rotatable relative to each other when said tumbler joints coincide with said interfacial plane, and spring means yieldingly urging said tumblers in aligned bores forwardly to positions wherein said interfacial plane is bridged by a plurality of said follower elements, said driver elements having front ends engageable with a key, whereby rearward movement of the key moves said tumblers in aligned bores rearwardly to positions wherein said tumbler joints coincide with said interfacial plane, the improvement for obtaining increased pick-resistance which comprises providing said tumblers in substantially equal overall lengths, said tumblers including at least two tumblers having driver elements differing in length, and providing said spring means in at least two different spring rates respectively to act upon said tumblers having elements differing in length, one of said spring means which has a relatively high spring rate acting upon a tumbler having a relatively short driver element, and another of said spring means which has a relatively low spring rate acting upon a tumbler having a relatively long driver element.