Lever tumbler lock

A lever tumbler lock is a type of lock that uses a set of levers to prevent the bolt from moving in the lock.[1] In the simplest form of these, lifting the tumbler above a certain height will allow the bolt to slide past.

Animation of a lever tumbler lock mechanism

The number of levers may vary, but is usually an odd number for a lock that can be opened from each side of the door in order to provide symmetry. A minimum number of levers may be specified to provide an anticipated level of security (see five-lever locks section).

History

"Double acting" lever tumbler locks were invented in 1778 by Robert Barron of England.[2][3] These required the lever to be lifted to a certain height by having a slot cut in the lever, so lifting the lever too far was as bad as not lifting the lever far enough. This type of lock is still used today, on doors in Europe, Africa, South America and some other parts of the world.

A five-lever lock which is designed to be mortised into a door. The faceplate has been removed to see the inner workings.

Design

The lock is made up of levers (usually made out of non-ferrous metals). Each lever needs to be lifted to a specific height by the key in order for the locking bolt to move.[1] Typically, the belly of the lever is cut away to various depths to provide different combinations, or the gate is cut in a different location, to provide differs. A lever will have pockets (or gates) through which the bolt stump (or post or fence) moves during unlocking.[1]

There has not always been universal agreement about which variants of the basic design merit the terms "lever lock" or "detainer lock" or both.[4][5][6][7] Some authors use the term "detainer lock" to refer specifically to variants where the gates are "open" (i.e. at the edge of the lever), rather than "closed" (i.e. entirely surrounded by the lever).[8]

Lever locks generally use a bitted key. Some locks used on safes use a double-bitted key, as do some door locks of a type often used in Southern and Eastern Europe.

Three-lever locks

A three-lever lock is a common type of lever lock, but is generally used for low security applications such as internal doors[9] as their tolerances are much lower (there are fewer combinations of key available, so they are likely to unlock doors they shouldn't).

Five-lever locks

A five-lever lock is often required for home insurance and often recommended by the police for home security.[10] There are various grades but the current British Standard (BS3621:2007) is usually required for insurance purposes. Locksmith Valerie Olifent notes that, "The doors of many historic churches still carry an old wooden lock although often you find that a modern 5-lever mortice lock has been installed alongside it to meet insurance requirements."[11] BS3621:2007 requires a bolt throw of 20 mm rather than the 14 mm of the earlier British Standard.

Most BS3621 locks have anti-pick devices built in to reduce the chance of lock picking, along with hardened bolts and anti-drill plates to reduce risk of physical attack.

Vulnerabilities

A type of lock pick used to pick lever tumbler locks

Lever tumbler locks can be picked with a tool called a curtain pick which is inserted into the keyway of the lock, and a force is applied to the locking bolt. The pick is then used to lift each lever inside the lock to the correct height so that the locking bolt can pass.

Higher security lever locks (such as the five-lever) usually have notches cut into the levers. These catch the locking bolt and prevent it from moving if picking is attempted (similar to the security pins in a pin tumbler lock).

The Chubb detector lock is a variation of the lever lock which was designed to detect and prevent picking attempts.

Lever locks can be drilled, but usually a template or stencil is required to mark the drilling point, as the lock mechanism is commonly mortised into the door and so it is harder to determine the point at which to drill.

See also

• Warded lock
• Mortise lock
• Lock picking

References

1. Rathjen, Joseph (1995). Locksmithing: From Apprentice to Master. McGraw-Hill. pp. 90–93. ISBN 0070516456. Lever tumbler locks are built and operate much differently than the conventional and more popular pin tumbler lock. Although they have their place in high-security applications, such as safe deposit boxes with a double-set tumbler design, they are used mostly for applications that allow a more lenient state of security. Today, they are used mostly for mailboxes, luggage, lockers, and other light security applications...
   Their biggest disadvantage, however, is that they do not allow for as many master-key combinations as pin tumbler locks.
   In a basic, fixed, lever tumbler lock design, the entire lock is enclosed in a case... The simple lever lock has three or more lever tumblers that pivot on a post and are pushed into the 'locking' position by springs, which are bound against the case and attached to the tail end of each lever tumbler. When the proper key is inserted into the lock and turned, it raises each tumbler to the point where the fence of the lock's bolt can clear the obstruction or face of each tumbler, which also blocks each tumbler's gate. When all the tumblers are in this position, the fence can be thrown through each tumbler's gate, which pushes the bolt into the retracted position.
   In other lever lock designs, a post is connected directly to the bolt. With the lever tumbler lock, when the tumblers are raised to the proper position the post of the bolt will pass through the front gating to the rear gating and hold the bolt in the retracted position.
   There are two different ways that the lever lock bolt can be thrown. In some lever locks, the end, or uncut, portion of the key that acts as a bolt throw will make contact with a bolt throw notch cut into the bolt. The other method is to use a trunnion with a talon built on the bottom of it. In both instances, when the key is turned contact is made with the bolt throw notch of the bolt, which allows it to be retracted.
   Different types of lever locks have different tumbler designs. [Simple] lever [tumblers] with one gate [rely] on the obstruction built onto the face or front of the tumbler and its tumbler trap to lock the bolt into either the locked or unlocked position. [Other types] of lever tumbler [use] a front and rear gate to trap the bolt into either position.
   Some lever tumblers have serrated saddles, or bellies, others have false gates. These varying designs make it more difficult to pick a lever tumbler lock. Lever tumblers with false gates usually are reserved for high-security units.
2. "The history of locks". London Locksmiths Ltd. Retrieved 8 November 2011.
3. Bellis, Mary (8 November 2011). "The History of Locks". About.com Inventors. About.com. Retrieved 8 November 2011.
4. Priess, Peter J. (26 September 1979). A Study of Surface-Mounted Door Locks from a Number of Archeological Sites in Canada. National Historic Parks & Sites. ISBN 9780660101804 – via Google Books.
5. Price, George (1856). "A Treatise on Fire and Thief-proof Depositories, and Locks and Keys". Simpkin, Marshall and Company – via Google Books.
6. Friend, Mick (2004). The Encyclopaedia for Locksmiths. Authors On Line Ltd. ISBN 9780755201174 – via Google Books.
7. Pulford, Graham (17 October 2007). High-Security Mechanical Locks: An Encyclopedic Reference. Butterworth-Heinemann. ISBN 9780080555867 – via Google Books.
8. Tobias, Marc Weber (1 January 2000). LOCKS, SAFES, AND SECURITY: An International Police Reference Two Volumes (2nd Ed.). Charles C Thomas Publisher. ISBN 9780398083304 – via Google Books.
9. "Lockshop Warehouse". 2 & 3 lever mortice locks are generally only recommended for use on internal doors as they do not provide adequate levels of security externally
10. "Insurance company requirements | The Crime Prevention Website". thecrimepreventionwebsite.com. Retrieved 13 March 2016.
11. "The Ancient Art of the Locksmith" http://www.buildingconservation.com/articles/locks/locks.htm