Grades 6–8 · Everyday Life
How do hidden wheels remember a lock’s combination?
A mechanical combination lock can retain a setting without a battery. Inside one common design, several wheels share an axle. Each wheel has a notch near its edge, called a gate. The lock responds to the wheels’ positions, not to printed numbers having a special physical power.
In the diagramEvery gate must leave room for the fenceThree wheel faces spread apart · shared axis omittedWheel 1Wheel 2Wheel 3Shared fenceAll three gates can receive the fence.Separate linkages control the bolt.This explains alignment, not a dialing procedure.
A dial and drive mechanism can pick up wheels in sequence as the dial turns. Changing direction allows particular wheels to be positioned while others remain where they were left. The details differ among locks, but the general result is a set of stored angular positions. This mechanical memory is why the order and direction of the authorized dial sequence matter.
A shared part called a fence must be able to enter every wheel gate together. If even one wheel presents its solid edge instead, that edge blocks the fence. When the gates line up at the fence, there is room for it to move; other linked parts can then allow the bolt to withdraw.
The illustration spreads three wheels apart so their gates are visible. It shows alignment only, not a real lock’s combination or opening procedure. Actual locks add drive cams, linkages and protective features. Three aligned notches explain the basic permission to move, but not a complete design or a guarantee of security.
About this explanation
Starting knowledge: Understands rotation, alignment and a solid edge blocking movement.
Sources
- United States Patent and Trademark Office — US2767572A: Combination lock
- Sargent and Greenleaf — Group 2M Mechanical Safe Lock