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I understand how a pin and tumbler lock stops you from opening the door unless you have the key that aligns all the pins with the edge of the plug, allowing it to turn. But I can't find anywhere how the opposite side of the lock works!

How does turning the lock from the inside unlock it if the pins are blocking the plug?

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  • $\begingroup$ You can find very detailed drawings of simple door locks all over the internet. $\endgroup$
    – Carl Witthoft
    Apr 5, 2018 at 17:58
  • $\begingroup$ @CarlWitthoft: ...and they all happily skip the part about the clutch. Coming up with an information about this specific feature is disproportionally hard, considering it's so simple comparing to safety pins, anti-bump pins, and the whole range of ingenious safety features. $\endgroup$
    – SF.
    Apr 6, 2018 at 9:46

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The parts responsible are the clutch and cam.

enter image description here

As the key bottoms out, besides aligning tumblers, it engages the clutch. As result, as you turn the key (and the core) - the cam turns too, moving the rest of the lock mechanics.

With dual-core locks (openable with a key from either side) the clutch is just friction-based, so once a key pushes it into "engaged" position, it stays in it until pushed to the position engaged for the other side once a key from the other side is inserted. For locks that just have a simple knob for opening from inside, the clutch is actuated with a spring - in absence of a key it engages the knob side and the core (key side) is disengaged.

As a side effect, it's a slight improvement of lock safety - when picking such lock, even if all tumblers are latched, all you achieve is turning the core uselessly while the deadbolt doesn't move. You need to depress the clutch to engage the cam and transfer the rotation to the rest of the mechanism.

Also, if you want to lock someone in, with the lock only lockable from your side, but openable with the knob from the other, after locking it, insert a wrong key. This way the clutch engages for the key side, and the tumblers block the core - the lock can't be opened from any side.

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  • $\begingroup$ This holds for one specific subset of tumbler designs. $\endgroup$
    – Carl Witthoft
    Apr 6, 2018 at 11:52
  • $\begingroup$ @CarlWitthoft: As I wrote in my prior comment - this specific feature, for reasons completely incomprehensible to me, is practically absent from the articles on locks available publicly. I wrote all I knew - I can't even give a good rundown on how exactly the clutch works. If you know more, feel free to post an answer, or edit mine and add. $\endgroup$
    – SF.
    Apr 6, 2018 at 14:53

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