combination lock

Background

The combination lock is one opened not by a key but by the alignment of its interior parts in a definite position. The most common types have an internal mechanism consisting of a series of three or four interconnected rings or discs that are attached to and turned by a central shaft. Manually rotating the outside knob or dial turns the discs, each of which is "programmed" to stop at a notched opening or gate. However, with a four-disc lock, certain preliminary spinning of the dial is necessary to get the lock to move the correct disc. The knob must first be turned to the right and spun past the first number four times before being allowed to stop beneath the marker. Next, rotating in the opposite direction, the knob must pass the second number three times. Reversing directions again, the user must spin the dial past the third number twice, and so on. When the apertures of all the rings align, they enable projections on a spring-loaded bolt to slide through, releasing the bolt and opening the lock.

Combination locks come in two varieties: hand and key change. One kind of hand combination lock that does not use internal wheels is the pushbutton lock, commonly installed in office doors and affording some measure of security. Pushing three or four buttons in order or together releases a shaft or deadbolt, allowing the door to open. The internal mechanism operates similarly to conventional padlocks.

Many people remember the simple padlocks that graced their school lockers. Picking these and other low-priced combination locks was frequently a game—and one often played successfully. With practice, an aspiring lockpicker could actually hear the audible clicks made when the protuberances on the bolt aligned with the notches on the discs. However, manufacturers of better locks design false gates in the discs to make cracking the lock extremely difficult. Only experts can distinguish between the three or more false gates and the true gate, and, since a lock with four discs can use any of 100,000,000 possible combinations, identifying the correct one by chance is unlikely.

The combination lock was invented in China, although historical records provide little specific information about its development. Combination locks came into popular use in the United States in the mid-1800s to secure bank vaults. The locks, integrated into the vault doors, are a colorful footnote to the history of the old West as western films testify. In 1873 James Sargent foiled many a real bank robber by perfecting a time lock that, coupled with a combination lock, kept everyone out of the vault until the clock or clocks that regulated the lock reached the time at which it was set to open, usually once a day.

Raw Materials

A typical combination lock of the padlock variety has twenty component parts, generally made of stainless steel or cold-rolled steel that is plated or coated to resist corrosion. Combination locks are constructed to last a lifetime, and their parts are not intended to require repair or replacement. In addition to steel, two other raw materials are essential to the combination lock. Nylon is used for the spacers that separate the discs, enabling them to turn independently, while zamak, a zinc alloy, is molded under pressure to form the bar, shaft, and outside dial.

Design

A combination lock's parts can be divided into two categories: internal and external components. Excepting the springs when extended, none of the internal parts exceed two inches (5.08 centimeters) in length. The internal works of the locking mechanism comprise the lever and supporting lever post and a disc shaft about which disc spacers and the combination disc turn. Two, three, or four combination discs are the key precision elements of the mechanism, but it is the combination cam, a notched disc, that generates the combination for the lock mechanism. The cam is also attached to the outside combination dial that is turned by the lock's user. The internal disc spring supports the combination discs under tension, enabling the combination to be dialed. Other internal components in the lock case include a shackle collar that holds the shackle (the U-shaped component that detaches from the case when the lock has been opened) in the locked position with a latch that fits into the shackle notch. An inner case encloses all internal parts and gives the lock body housing strength. External parts include the lock's outer case, the shackle, a back cover, and the combination dial.

The Manufacturing
Process

The twenty component parts of the typical combination lock are formed, drawn, cut, pressed, and molded on a variety of machines, both manual and automatic.

Making the internal components

• The lever, locking latch, and disc shaft are all made by injection molding, a process in which molten zamak is poured into a mold and subjected to heat and pressure until it solidifies into the shape of the mold. Although the post is shaped at room temperature, it is also formed under high pressure.

  The combination disc and the cam are made of cold-rolled—passed under huge rolls without being heated—flat strip steel; after being cold rolled, the steel is put in a blanking die, a sophisticated cookie cutter, which cuts (or blanks) out the properly shaped piece. The internal disc spring is made from stainless steel round wire and produced on a spring winder that automatically twists and turns the wire to form the traditional coil spring. The shackle collar, like the combination disc and cam, is made from cold-rolled flat strip steel that is blanked in a blanking die. The inner case is produced from flat steel strip and drawn to a cup configuration. This process requires great pressure to stretch and compress the material as it is pressed or drawn around a die, whose shape it takes.

Making the outer parts

• The outer case is manufactured similarly to the inner case but from stainless steel sheet instead of strip. The back cover, also stainless steel, is blanked in a blanking die. The durable shackle is made of round bar stock and machined on a screw machine, then formed to its U-shape and notched to accept the locking latch. It is finally annealed (heated to great temperature before being quenched in water) to make it resistant to hacksaws and bolt cutters. The combination dial, also zamak, is injection molded, then chromized. This process involves heating the part in a salt bath rich in chromium. The steel absorbs the chromium, which hardens on the surface as it cools rapidly. The dial is painted black and white wiped, which leaves the numbers highlighted against the black face.

Plating the components

• Several plating and finishing processes can be used to protect the components against corrosion. The lever, disc shaft, combination cam, and dial are chromized. The inner case, shackle collar, and lever post are all cadmium-plated. The shackle and locking latch are copper nickel-plated. The outer case, of stainless steel, is mechanically polished to enhance luster.

Assembling the lock

• Assembly of the components is precise with the back plate disc shaft, combination cam, and spacers forming one subassembly. The outer and inner cases are riveted together and then pierced at the point where the shackle is inserted. The combination dial, outer and inner case unit, and combination cam are then fastened together. Finally, these sub-assemblies and the remaining parts are fitted together. The lock case is closed and the edges folded over and sealed. Conventional hardware fasteners that can be released with the proper tool are not used.

Labeling and packaging

• The remaining operation is the application of a removable tag or label to the lock. On this tag is the combination, determined randomly by machine draw. Lock manufacturers today jealously guard their combination setting procedures. A typical combination lock is sold in blister pack, a rigid molded plastic with cardboard backing, although locks may also be individually boxed.

Quality Control

Before any lock is packaged, many manufacturers completely test the locking and unlocking sequence. Other inspections and measurements are performed by individual operators at their stations during both manufacturing and assembly. Combination locks today enjoy a reputation for excellent reliability and durability.

Where To Learn More

Books

All About Locks and Locksmithing. Hawthorne Books, 1972.

Combination Lock Principles. Gordon Press Publishers, 1986.

The Complete Book of Locks and Lock-smithing. Tab Books, 1991.

[Article by: Peter Toeg]

This article is about a type of locking mechanism. For other uses, see Combination Lock (disambiguation).

The examples and perspective in this article may not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page.

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (June 2009)

A Wordlock 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 Internal mechanisms 3 Manufacturers 4 References 5 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.

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, two seconds for the second digit, and one second for the third digit, then the normal search time for a 60-number dial with three cams would be (3 + 2 + 1) × 60³. The reduced search time would be (3 + 2 + 60) × 60², a reduction of nearly 82% from 360 hours to 65 hours. This strategy can be extended to the second digit as well, slightly reducing the search time further.

When these two strategies are combined on a lock with the properties given above, the brute force search time is reduced by greater than 99%; a brute force search that would have taken 360 hours is reduced to an achievable 2.78 hours. This is still significantly better security than multiple-dial locks and many keyed locks, but unacceptable for high security applications.

Inexpensive padlocks are often also susceptible to direct mechanical attacks, such as the use of a padlock shim which can release the shackle without entering a combination. Early combination padlocks made by Master Lock could be cracked by pulling on the shackle of the lock and turning the dial until it stopped; each numeral in the combination could be revealed in this manner. More recent models of Master padlock with a 40-position dial have a mechanical weakness that can give away the last numeral in the combination, and the first two numerals have a mathematical relationship with the last number. This weakness reduces the number of possible combinations from 64,000 to a mere 100, which can be tried in a relatively short time.

Other designs

Many doors use combination locks which require the user to enter a numeric sequence on a keypad to facilitate entry. These special locks usually require the additional use of electronic circuitry. The chief advantage of this system is that if used for the door of a large office, each employee can be told the code number without having to supply a key to each person. However, if the code number is learned by someone outside the desired group or is not changed regularly, it could allow easy access to a potential intruder (the same could be said for all combination and keyed locks, and many other security measures, however).

Electronic combination locks, while generally safe from the attacks on their mechanical counterparts, suffer from their own set of flaws. If the arrangement of numbers is fixed, it is easy to determine the lock sequence by viewing several successful accesses. Similarly, the numbers in the combination (but not the actual sequence) may be determined by which keys show signs of recent use. More advanced electronic locks may scramble the numbers' locations randomly to prevent these attacks.

In The DaVinci Code, Dan Brown invents the term cryptex to identify a sort of combination lock that contains a small locked compartment inside, like a small safe.

Internal mechanisms

Please help improve this article by expanding it. Further information might be found on the talk page. (May 2009)

• A relock trigger, or internal relocker, is an integral part of the combination lock itself. It is usually designed to activate when the dial spindle is punched through. The trigger may consist of a spring-loaded lever or plunger that engages the bolt when the back cover is dislodged from the lock case. Some combination locks also are equipped with a thermal relock trigger that activates in the event of a torch attack. Nearly all safes made after World War II have relock triggers in their combination locks.

Manufacturers

• Master Lock
• Sargent and Greenleaf
• Wordlock

References

1. ^ Hoepfner, Wolfram (1970), "Ein Kombinationsschloss aus dem Kerameikos", Archäologischer Anzeiger 85 (2): 210–213 
2. ^ Paul Vallely, How Islamic Inventors Changed the World, The Independent, 11 March 2006.
3. ^ Hill, D.R. (1998) Studies in medieval islmaic technology, Ashgate.
4. ^ "IMPROVEMENT IN TUMBLERS FOR PERMUTATION-LOCKS" by Joseph Loch, U.S. patent 200070, Feb. 5, 1878. Found in Google Books. [1]

External links

• How Combination Locks Work HowStuffWorks.com
• Locksoft glossary

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