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How locks work: key facts about how to obtain closure

In this exclusive extract from NBS Shortcuts, written by noted industry figure Austin Williams, we show how the humble door lock has changed.

Whichever crime statistics you choose to believe, the chances of an average household being burgled are around once every 35-50 years. Not bad odds, but still we try to devise more and more ingenious ways to keep over-hyped intruders out of our houses.

Actually, domestic front door locks haven't changed much for a hundred and fifty years; literally true if you examine the Easter Island display at the Science Museum's magnificent lock exhibition. Even in the West, the materials are different and their appearance is more varied, but locks' essential operation is unchanged. Yale even promotes itself as emulating the traditions of Egyptian locks and keys - dating back to Nineveh and its palaces, four thousand years ago.

Mind you, security consciousness has its downsides. In the 16th century, Ivan Vasilyevich, Tsar of all Russia, locked his wife in her room while he went off to war. To make sure no one could use a duplicate key to "gain entry" he had the locksmith beheaded. Tragically she fell ill and, unable to get medical help, she died - reputedly turning a relatively benign ruler into the legendary Ivan the Terrible.

It the turn of the 19th century, British locksmiths Robert Barron and Joseph Brammah developed the idea of having a small key that didn't reach the bolt but acted through intervening moving parts. In these, the lock turns a cam that pulls a latch back, allowing the door to open. A spring usually pushes the bolt back out again. A non-spring-loaded latch called a deadbolt (requiring a key to open and close the bolt) is usually more secure as it is more difficult to push the bolt in from the side of the door.

Jeremiah Chubb of Portsmouth patented the detector lock in 1818, which won a government challenge for a lock that could not be opened by any other than its own key. Even a professional locksmith who was imprisoned on a ship in Portsmouth Docks - with the incentive of a promise of a pardon and £100 - failed to open it after trying for three months.

The cylinder pin tumbler lock, by Linus Yale of New York in 1848, uses Barron's double tumbler principle invented seventy years earlier. It has rows of two sets of pins one above the other - the 'double tumblers' - which stop the cylinder from moving. The correct key raises the pins such that the joint lines between them lines up with the cylinder edge. This frees the cylinder to move within the barrel, allowing it to move and turn the cam. The cam moves the locking bolt - or latch - in and out. With minor modifications, this remains the basis of Yale locks today.

Returning to the opening paragraph, the risk of burglary is fairly remote. Statistically, at least, if you left your doors open you'll probably find that no one has bothered to take advantage.

Thanks to The Guild of Architectural Ironmongery and particularly to Master Locksmith, Mr Lewis Beadle of Abilock Lock Technicians.

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The full text of this NBS Shortcut is exclusively available as part of a subscription to NBS Building Regulations. 

NBS Shortcuts is a new series of illustrated "how-to" articles and guides, covering a wide range of practice, regulatory and design guidance. The easy to follow text and detailed hand-drawn graphics will aid any building designer. They are available online as part of your subscription to NBS Building Regulations.

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