Using Multiplex Key Systems

Commercial locks are designed to limit the types of keys that can enter the lock cylinder.

This is accomplished by uniquely slotting the plug component within the lock cylinder. This unique slot is the “keyway.” The keyway allows a predictable subset of different key shapes to be properly inserted into the plug. Each of the different key shapes is a “key section.”

Some keyways accommodate a single key section while other keyways are strategically widened to allow whole groups (or families) of key sections, to access the plug.

The main application for keyways having more than one key section is to enhance or expand the number of change key possibilities for a master key system.

Creating and machining sectional keyways is part mathematics and part artistry. The keyways require precise machining. The keyway must be accommodating enough to allow all the intended key sections while being restrictive enough to limit unauthorized key sections.

Keyways are slotted into the lock cylinder plug, primarily using broaching tools. The broach is a hardened piece of tool steel that features graduated heights of teeth. As the broach is forced into the brass plug, the slot widens until the width meets specification. The complexity of the keyway is determined by different broaches that are run through the plug.

Basic key blanks are stamped out of flat stock and then run through several milling steps. Each step relieves more of the key until the specified overall profile is accomplished.

Most lock companies design their lock cylinder components including the plugs to be interchangeable. If a lock cylinder needs to be rekeyed to a different key section, a new plug can sometimes be ordered and retrofitted into the lock cylinder.

Multiplex Key Systems

A finite number of change keys are available in a two-step quadratic master key system. In a 10-depth, five-pin system the yield is 1024; and in a six-pin system it is 4096. With large and multiple buildings, these numbers may not be sufficient.

Lock companies can provide lock cylinders with keyways whose profiles can accommodate specific keyways, while limiting other keyways within the associated group (or family) of key sections.

Figure 5 is an example an example of a “multiplex” key system. Each different key section in the family can support an entire (but parallel) master key system.

In a multiplex key system there are multiple levels of access. Each higher level is thinner, enabling it to pass more or different keyways.

The “all-section” key blank for the multiplex key system in Figure 5 is identified as GST.

On the left of Figure 5 is the hierarchy of key sections; on the right is the GST key section overlaid in red, onto each key section. The black areas not covered by the red reveal the remainder of the keyway not filled by the GST key section.

A standard 10-depth six-pin system has 4096 change keys. Each of these key sections (in Figure 5) can yield 4096 change keys: GA; GB; GC; GD; GE; GF; GG; and GH. That produces 32,768 change keys.

Multi-section keys in the multiplex key system are: GAH; GBG; GCE; and GDF. GAH sections can access GA and GH; GBG sections can access GB and GG; GCE sections can access GC and GE; and GDF can access GD and GF.

Lock cylinders can be ordered for multi-section GAH; GBG; GCE; and GDF keyways. This provides for even more expansion as each of these key sections can also support a key system.

The GMK level can access GAH; GBG; GCE; and GDF. Again lock cylinders can be ordered with a GMK keyway. Only the GMK and GST sections can access GMK keyways.

Here is a typical example how multiplex key systems are used. Buildings at a college campus are each setup to different single-section keyways: GA; GB; GC; GD; GE; GF; GG; and GH. All custodial closets are keyed alike. The custodian's key operates in a given building that uses GD keyways would have a change key cut on a GD key blank. A custodian that works in all buildings would have the same key combination cut on a GMK key blank.