DIAGRAMS.INK / MECHANISM STUDIES

Why only one setting opens a combination lock

Each wheel hides one gate. A spring-loaded bar can only drop when all three gates sit under it, and only then does the latch let go of the shackle. Turn the wheels and watch the bar.

Teaching combination3·7·1
Section along the axleThe three wheels seen end-on. Drag a disc round like a dial.
Latch detail, seen from the right
Gates under the bar0 of 3needs 3 of 3
Bar travel0%drops only into gates
Latch boltIn the groovecam withdraws it
ShackleLockedspring lifts when free

No gate is under the bar.

Wheel 10
Wheel 20
Wheel 30
  1. 1Digit wheelsTen numbers each. Drag to turn.
  2. 2GateThe amber notch cut in a rim.
  3. 3Locking barSpring-pressed, tongues ride the rims.
  4. 4Latch boltCam-driven, sits in the shackle groove.
  5. 5ShackleSpring lifts it when the latch lets go.
  6. 6SpringsPush the bar down, the shackle up.

One wheel is enough to say no

The bar is a single rigid piece, so it can only fall as far as its highest tongue lets it. A wheel whose rim sits under one tongue holds the whole bar up, and a raised bar keeps the latch in the shackle's groove. The gate is cut a digit and a half away from the number you read, so no digit gives it away.

A teaching model, not a real lock

Real padlocks hide their gates on inner discs and use other latch layouts. This one is drawn open, with a plain cam and bolt, so every gate is visible. The combination 3 · 7 · 1 is made up for the lesson. Tolerance, wear and friction are left out.