Patent ID: 8806908

Claim:
A dual key bi-step lock comprising: a shell, a cylindrical plug in the shell, the plug having a primary keyway and a secondary keyway, at least one set of primary pin stack and at least one set of secondary pin stack provided between the shell and the plug, respectively coupling with the primary and a secondary key, each set of the primary and the secondary pin stacks having a compression spring, a driver pin, and a key pin, the key pin being outermost and being supported on the driver pin, which is seated on the compression spring, which assists in forcing the key pin across a shear line and into the corresponding keyway formed in the plug, a longitudinal stage formed on a cylindrical surface of the plug, an involute side face extending from the stage to the cylindrical surface of the plug, the primary key for the primary keyway, and the secondary key for the secondary keyway, wherein when the primary key is inserted in the primary keyway and rotated, the primary key is able to drive the plug to a position of first step open at which the stage is stopped by the key pin of the secondary pin stack, and when the secondary key is inserted in the secondary keyway and rotated together with the primary key, the secondary pin stack is unlocked by the secondary key and the plug is able to be further driven to a position of second step open by the primary and the secondary keys from the first step open.