Anatomy of a Safe Lock: Three-Wheel Combination Locks Old and New

Tips for servicing three-wheel combination safe locks, old and new.


In order to do great work with safe locks, perhaps the most important thing to know is how the locks work.  Whether you’re diagnosing a malfunction, opening a safe with an unknown combination or performing maintenance or repairs, a thorough understanding of lock functionality is likely to...


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ILCO 6731
The Ilco 6731 is a much more modern safe lock  –a UL Group 2 combination lock that is currently in production.  Figure 13 shows an Ilco 6731 that I am currently in the process of embellishing.  The lock’s cover is as butt-ugly as ever, but I’m working on it.  The inside of the lock can be seen in Figure 14, and an exploded view in Figure 15.  This lock’s size and mounting footprint are the same as the Diebold lock, but the dial spindle is thicker, at a diameter of 5/16”.  It fastens to the drive cam in the same way as the Kromer and Diebold locks.  The ornamental finishes and the red interior paint are my work.  This lock’s parts are labeled by letter in the exploded view. This Ilco lock is operated in the same way as the Kromer and Diebold locks examined earlier.  It is very similar to the Diebold lock, but there are at least five noteworthy differences.
First, this lock has a wave washer on its wheel-post to compress the wheel-pack.  This produces somewhat of an increase in the torque required to rotate the wheels.  This helps to prevent the wheels from being aligned through vibration, as the Jacobs brothers’ safe-opening device (also known as the “Jacobs Jiggler”) was designed to do on older combination locks.  The increased torque requirement is small, though, so the lock remains easy to operate by hand.
Second, the flies do not ride directly on the wheel-post.  Instead, each wheel has its own fly-post. This lock has fewer isolation washers, as it does not isolate the flies from the wheels in which they are installed.  In the Kromer and Diebold locks shown earlier, this would have caused wheel-dragging, but not in this lock.  The fly-posts prevent the wave washer from pressing the flies against the wheels.  The flies therefore move significantly more easily than the wheels do, eliminating the need for some of the isolation washers.
Third, the Ilco relock trigger functions differently than the Diebold.  When a burglar breaks off the dial and either punches the spindle or somehow removes it and punches through the lock, the lock’s cover will bend outward and then break.  When the cover is not firmly fastened flat against the case, one end of the spring-loaded relock trigger blocks the lock-bolt from retracting.  In this way, the relock trigger attempts to thwart any burglarious efforts to retract the lock-bolt.
Fourth, the lever-screw spacer is built into this lock’s lever screw (N), whereas in the Diebold this was a separate part that the lever-spring wrapped around.  Figure 16 shows the lever spring wrapped around the lever screw, with one end hooked in a hole in the lock-bolt and the other hooked around lever.
Fifth, this and other modern safe locks incorporate a lock-bolt detent (J).  This detent is a spring-loaded steel ball that protrudes into either of two dimples in the lock-bolt, which are shown in Figure 17.  One dimple corresponds with the locked position, in which the lock-bolt is extended; while the other corresponds with the unlocked position, in which the lock-bolt is retracted.  When the lock-bolt is in one of these positions, the detent tends to keep it there.
It only takes a small amount of force from the user to retract or extend the lock-bolt in spite of the lock-bolt detent, but the detent resists more minor forces such as Earth’s gravitational pull on the lock parts.  This feature can prevent the fence lever from being jammed against the lever stop by the weight of the lock-bolt when the lock is mounted with the lock-bolt pointed up.  It could also prevent the lock-bolt from being extended by gravity when the lock is mounted with the lock-bolt pointed down, I suppose, but that gravity usually wouldn’t be enough to overcome the friction in the wheel-pack unless a heavier lock-bolt or a lock-bolt extension were installed.  Anyway, this lock can handle being mounted in vertical orientations, whereas some older locks might have been prone to malfunction if mounted that way.
While the functionality of these locks is similar, there are low-level differences that are quite important.  When working with a combination lock, it becomes helpful to pay close attention to these details and to understand how all the parts work together to provide the security functionality.  This understanding increases the assurance that the lock will get good and proper service, which in turn increases the assurance that the lock will remain secure and operational for a long time.  In the event of a lock malfunction, it facilitates diagnostics. (One often cannot figure out what’s wrong with a mechanism without first understanding very well how it is intended to operate.)  It also makes possible the development of new methods to overcome those malfunctions.  It is what makes a safe technician good at his job. 

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