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This has been nagging me lately. Why 2 twists? I mean one twist seems enough to keep the door essentially locked. If the extra twist is for strength, wouldn't it be a better idea to implement a longer travel of the locking mechanism for single twist. I have no knowledge whatsoever when it comes to houses and carpentry and the like so forgive me if the question seems dumb.

closed as too broad by Machavity, ThreePhaseEel, The Evil Greebo, Daniel Griscom, mmathis Jun 15 '18 at 14:44

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • On some locksets, the first twist locks the latchbolt (the part that springs in to latch the door). The second twist sets a separate deadbolt. – bib Jun 13 '18 at 19:35
  • Do you have a specific lockset that you're inquiring about? Is it a Key-In-Knob lockset or a mortise lockset, for that matter? (It would not surprise me if there are locks that require a multiple movement simply to set the actuator to begin with...) – ThreePhaseEel Jun 14 '18 at 6:54
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It's for security, making the lock harder to pick. About twice as hard. First you have to pick it to rotate the cylinder. Once it travels 360 back to the upright, the pins pop out from behind the rotatory sheath and must be re-picked to continue the final leg of the journey. With a key this is instant, but with a single-pin pick it doubles the (at least) the typical pick time.

Depending on the mechanism (if the key is "stuck in" when unlocked "halfway") it can prevent bumpkey attacks as well since you can't pull back the bumper once the key is affixed. Some of what isherwood notes applies as well, tangentially, because high-security locks that have such a mechanism also have thicker/longer bolts and more robust mechanisms that require additional torque, compared to a crappy kwikset or schlage a residential customer might get at a box store.

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The gearing of the bolt throw mechanism needs to be appropriate so that enough force can be applied on the bolt. Since some bolts drag on the bore slightly, they need to be geared low. This allows a light force on the key to impart a stronger force on the bolt.

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