Sidebar Locks: A Second Bar That Checks Every Tumbler

How a Sidebar Works

In a basic pin tumbler lock, the plug turns when every pin stack is split at the shear line. A sidebar adds a different kind of test. The bar sits in a groove between the plug and the outer shell and blocks rotation. It can only move out of the way when every tumbler has a gate or notch lined up with it.

Lockwiki describes the bar as typically spring-biased toward the cylinder, and says it is "allowed to retract (through a variety of methods)" when the correct key is used. Those methods vary by design:

• In a disc detainer, the key rotates each disc until its gate lines up with the sidebar.
• In a Medeco cylinder, each pin must be lifted and also turned to the right angle.
• In a Kwikset SmartKey cylinder, a set of small racks must line up.

Lockwiki adds that "more advanced sidebars have dedicated locking components that must be properly raised or rotated," which it calls side pins or sliders.

Primary and Auxiliary Sidebars

Lockwiki draws a useful line. Sidebars are "primary locking mechanisms in many wafer and disc-detainer locks, and (usually) … auxiliary locking mechanisms in many pin-tumbler locks".

• Primary. The sidebar is what actually holds the plug. Lockwiki calls the sidebar "the primary locking component of most disc-detainer locks."
• Auxiliary. The cylinder still has standard pin stacks at a shear line, and the sidebar is an additional, independent condition. Schlage's catalog describes Primus cylinders as having "independent sidebar and finger pin" locking mechanisms.

Early Sidebars: Briggs & Stratton in the 1930s

Some of the earliest sidebar patents came from Briggs & Stratton, a major maker of automobile locks.

• U.S. Patent 1,965,336. Filed March 12, 1934 and granted July 3, 1934 to John W. Fitz Gerald (listed by Google Patents as "Gerald John W Fitz"), assigned to Briggs & Stratton. One of its figures shows "the spring provided to project the auxiliary tumbler or locking bar to its inoperative position."
• U.S. Patent 2,004,433. Filed October 17, 1934 and granted June 11, 1935. It describes tumblers with notches cut into them that receive "a locking member carried by the cylinder."

The sidebar quickly reached cars. Locksmith Ledger reports that "the GM locking sidebar system … first appeared on 1936 GM models." It adds that from 1936 to 1966, GM used a six-cut sidebar lock system. Lockwiki also credits Chicago Lock with producing sidebar wafer locks in the 1930s. That claim rests on Lockwiki alone. The wafer lock article covers this period.

Medeco: Rotating Pins and a Sidebar

Roy Spain's Medeco design made the sidebar central to high-security pin tumbler cylinders. U.S. Patent 3,499,302 was filed March 1, 1967 and granted March 10, 1970. Its pins must reach three levels and three angles. Only when every pin is both lifted and rotated correctly can a fence, Medeco's sidebar, be "cammed out of engagement with the cylinder shell."

Later Medeco platforms added further elements around the sidebar. The company's catalog describes the Medeco 4 (M4) as having three finger pins and one lift pin that work with the sidebar. The Medeco article covers the Biaxial, M3 and M4 history, along with the M4's UL 437 listing.

Schlage Primus: Finger Pins and Side Milling

Schlage's Primus line pairs ordinary pin tumblers with a sidebar controlled by "finger pins." In Schlage's catalog, the sidebar is "operated with a patented side bit milling design such as found in the Primus XP cylinder mechanism." The key carries a second set of cuts along its side, separate from the top cuts, and these position the finger pins.

The catalog lists Primus among products that require a letter of authorization for keys. That links the sidebar to key control and restricted keyways, because the side milling is part of what is patented.

Kwikset SmartKey: A Side Locking Bar in a Rekeyable Lock

Kwikset's SmartKey is a consumer example. In a 2011 press release, Kwikset said that "the patented BumpGuard side locking bar technology replaces a traditional pin & tumbler design." The same release described the lawsuit it settled with Schlage as a defense of its "SmartKey re-key technology patents."

One patent in that rekeyable-cylinder family, U.S. 7,878,036 B2, shows how such a bar works. It was published February 1, 2011, continues an application filed in 2002 and is now assigned to Kwikset. It describes "a spring-loaded locking bar" that sits in a groove in the cylinder body and stops the plug from turning. Inside the plug, small racks carry grooves for the bar. When the right key is inserted, the racks line up and "the locking bar is free to cam out of the groove" as the key turns.

The patent does not use the trade name SmartKey. Linking this specific patent to the retail product relies on the press release's wording about "re-key technology patents". By Kwikset's own description, the side locking bar replaces the pin-and-tumbler shear line rather than adding to it, so in SmartKey the bar is the primary lock.

What a Sidebar Does and Does Not Do

A sidebar changes what the lock checks. It does not, by itself, make a lock high security. Ratings such as UL 437 and EN 1303 apply to tested products, not to a feature. Lockwiki notes that sidebars can also enforce key profiling, citing EVVA's DPS. Patents add legal key control for as long as they last. For how these pieces combine, see high-security locks.

Related Topics

• Disc detainer locks
• Wafer locks
• Pin tumbler locks
• Medeco
• Kwikset
• Abloy
• UL 437
• EN 1303
• High-security locks
• Restricted keyways
• Key control
• Dimple locks

Sources