A lever tumbler lock is a type of lock whose mechanism uses a set of levers to prevent the bolt from moving in the lock. It does not mean a locking lever handle incorporating a cylinder locking device. In the simplest form of lever lock, lifting the tumbler above a certain height will allow the bolt to slide past. Lever locks developed throughout the nineteenth century, but those used in some parts of the world were of modest security, with large keys. When Yale's pin tumbler lock was developed in the late nineteenth century, that became a more popular lock. It has a small key, many differs, and an easily interchangeable cylinder locking device, and in much of the developed world has become the most popular key mechanism for doors and padlocks. In some parts of Europe, and elsewhere, secure lever locks continue in use for doors, and safes.
History In 1778 Robert Barron of Britain invented [British patent GB1200] the principle of all modern mechanical security locks: the double-acting movable detainer. As examples, his patent described two realisations (of several possible) of what would now be called a lever lock. 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.
The word 'tumbler' originally meant 'something which turns; it later also gained the connotation of 'falling', and was the term used for the dead-locking member invented by the Romans for use with a turning key in a warded lock. It was still the term used by Barron. However, by the mid-nineteenth century, 'lever tumbler' had commonly been shortened to 'lever'. The British Standard BS3621 originally entitled Thief resistant locks for hinged doors and now Lock assemblies operated by key from both the inside and outside of the door uses the term 'movable detainer' for any type of movable blocking member in a lock.
Design The lock is made up of a plurality of flat levers (usually made of non-ferrous metal and impelled to the rest position by a spring). Any number of levers may be used, though some two-sided designs require an odd number. A minimum number of levers may be specified to provide an anticipated level of security for door locks (see § Five-lever locks). Each lever needs to be moved (typically, lifted) to a specific position (height) by the key in order for the locking bolt to move. Typically, the belly of the lever is cut away to various depths to provide different combinations, or the gate is cut in a different height in the fence, to provide differs. A common form of lever has two pockets separated by a bar or fence, having a gap or gate, through which the bolt stump moves during unlocking or locking. Some levers eliminate the boundary of the outer pocket, becoming end-gated levers. Lever locks generally use a bitted key. Some locks used on safes use a double-bitted key (a key with cuts on both sides, increasing security), as do some door locks of a type often used in Southern and Eastern Europe and elsewhere. Levers are generally arranged to pivot, but in some realisations the levers slide. Notably in France, there are locks related to lever locks, which use a push key. The Bauche Monopole lock, for example, is based on Aubin's Vibrating guard lock. There has not always been universal agreement on which variants of the basic design merit the terms "lever lock" or "detainer lock" or both. 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). The Chubb lock company chose to use the term "detainer" in its patent for the lock design deriving ultimately from Strutt's 1818 lock, in which a probe (originally, one end of a bell-crank) briefly interrogates the edges of end-gated levers.
Three-lever locks A three-lever lock is a common type of lever door lock now generally used for low security privacy applications such as internal doors in Britain as their tolerances are lower. There are fewer key differs available, so they are might unlock doors they should not. In the past, two- and four-lever door locks were also common.
Five-lever locks A five-lever lock is often required for home insurance in Britain for wooden exterior doors and recommended by the police for home security. There are various grades but the current British Standard (BS 3621:2017+A1:2024) is usually required for insurance purposes. Such locks have features, in addition to five levers, to resist a variety of attacks. Locksmith Valerie Olifent states 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." BS3621:2007 requires a bolt throw of 20 mm rather than the 14 mm of the earlier British Standard. Most BS3621 lever locks incorporate anti-pick devices built in to reduce the chance of lock picking, along with physical robustness such as hardened bolts and anti-drill plates to reduce risk of physical attack. With less standardisation of lock sizes, lever locks can be more expensive to fit than modern tubular cylinder locks.
Other lever locks There are many realisations of lever locks, but only a few have achieved lasting success in the market place. In France, for example, four-lever door locks are used, but as in many countries, are losing popularity to pin tumbler cylinder locks. In Italy, double-bit lever locks using sliding levers are used.
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