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A letter combination lock.
A combination lock is a type of lock in which a sequence of numbers or symbols is used to open the lock. The sequence may be entered using a single rotating dial which interacts with several discs or cams, by using a set of several rotating discs with inscribed numerals which directly interact with the locking mechanism, or through an electronic or mechanical keypad.
From a mathematical standpoint, "combination" lock is a misnomer, as such locks typically do not require a combination of values to open. A related misconception is that the unlocking code is a permutation; however, the correct term for describing the set of values used to open the lock is sequence.
Exploded view of the rotating discs. The notches on the disc correspond to the numerals in the correct combination. In this case, the combination is 9-2-4.
The discs are mounted on one side of the lock, which may in turn be attached to the end of a chain or cable. The other side of the lock, or the other end of the cable, has a pin with several protruding teeth.
When the toothed pin is inserted and the discs are rotated to an incorrect combination, the inner faces of the discs block the pin from being extracted.
Contents
1 History
1.1 Multiple-dial locks
1.2 Single-dial locks
1.3 Other designs
2 Manufacturers
3 References
4 External links
//
History
The earliest combination lock was excavated in a Roman period tomb on the Kerameikos, Athens. Attached to a small box, it featured several dials instead of keyholes.[1] In 1206, the Muslim engineer Al-Jazari documented a combination lock in his book al-Ilm Wal-Amal al-Nafi Fi Sina'at al-Hiyal (The Book of Knowledge of Ingenious Mechanical Devices).[2] Muhammad al-Astrulabi (ca 1200) also made combination locks, two of which are kept in Copenhagen and Boston Museums[3].
Gerolamo Cardano later described a combination lock in the 16th century.
In 1878 a German man by the name of Joseph Loch was said to have invented the modern combination Lock for Tiffany's Jewelers in New York City, and from the 1870s to the early 1900s made many more improvements in the designs and functions of permutation locks.[4]
Multiple-dial locks
One of the simplest types of combination lock, often seen in low-security bicycle locks and in briefcases, uses several rotating discs with notches cut into them. The lock is secured by a pin with several teeth on it which hook into the rotating discs. When the notches in the discs align with the teeth on the pin, the lock can be opened.
This lock is considered to be one of the least secure types of combination lock; many locks of this type can be quickly opened without knowledge of the correct combination. Opening one in this fashion depends on slight irregularities in the machining of the parts. Unless the lock is machined precisely, when the pin is pulled outward, one of the teeth will pull more strongly than the others on its corresponding disc. This disc is then rotated until a slight click is heard, indicating that the tooth has settled into the notch. The procedure is repeated for the remaining discs, resulting in an open lock, and a correct combination, in very little time.
A single-dial padlock.
The component parts of a Stoplock combination padlock.
Single-dial locks
Combination locks found on padlocks or safes may use a single dial which interacts with several parallel discs or cams. Customarily, a lock of this type is opened by rotating the dial clockwise to the first numeral, counterclockwise to the second, and so on in an alternating fashion until the last numeral is reached; the cams typically have an indentation or notch, and when the correct combination is entered, the notches align, allowing the latch to fit into them and open the lock
Depending on the quality of the lock, some single-dial combination locks can also be defeated relatively easily. Typical padlocks are manufactured with generous tolerances, allowing two, three or even more digits of 'play' in the correct access sequence. Given a 60-number dial with three cams and three digits of play, the search space is reduced from 60 60 60 to 20 20 20, a 96% reduction in potential combinations.
Additionally, if testing the mechanism to open the lock does not modify the state of the lock, multiple combinations can be tried sequentially, drastically reducing the brute force search time. The first two digits are entered normally once, then, starting from the second digit, the dial is rotated sequentially through the digits, testing the lock on each. If it takes three seconds to input the first digit,...(and so on)
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