Mechanical Locks

There are two basic types of mechanical locks, each with variations. The oldest and simplest is
the warded lock, which is essentially a spring-loaded bolt in which a notch has been cut. The key
fits into the notch and slides the bolt backward and forward. The lock takes its name from the
fixed projections, or wards, inside the lock and around the keyhole. The correct key has notches
cut into it that match the wards, which block the wrong key from operating the lock. The ward lock
is the easiest to pick and now is used only for cheap padlocks.
The tumbler lock contains one or more pieces of metal (called tumblers, levers, or latches) that
fall into a slot in the bolt and prevent it being moved. The proper key has serrations that raise the
metal pieces to the correct height above the slot, allowing the bolt to slide. There are three types
of tumbler locks, pin-tumbler, disk-tumbler, and lever-tumbler. Pin-tumbler locks are the most
common. The tumblers in this type of lock are small pins. The modern door lock is a compact
pin-tumbler cylinder lock of the type developed (1860) by the American inventor Linus Yale. Door
locks on automobiles and most high-security locks have pin tumblers. Disk- or wafer-tumbler
locks, use flat disks, or wafers, instead of pins. When the proper key is inserted, the disks retract,
releasing the bolt. Disk-tumbler locks are often used in desks and file cabinets. Lever-tumbler
locks employ a series of different-sized levers resting on a bolt pin to prevent the bolt from
moving. When the proper key is inserted, all the levers are raised to the same height, enabling
the bolt pin to release the bolt. Lever-tumbler locks are often used in briefcases, safe-deposit
boxes, and lockers.
The first of the keyless locks was the combination lock, developed at the beginning of the 17th
cent. In it a number of rings inscribed with letters or numbers are threaded on a spindle. To open
the lock the rings must be turned to form a code word or number, which causes the slots inside
the rings to align and permits the spindle to be drawn out. A variant of the combination lock
employs a movable dial with a series of numbers around it in place of the rings. The dial must be
turned clockwise and counterclockwise in the proper sequence of numbers to align disk tumblers
and open the lock. Once used only for padlocks, combination locks began to be used in safes
and strong-room doors during the last half of the 19th cent. The time lock, first used successfully
c.1875, has a clock mechanism that is set to permit opening only a certain time.
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