BASIC LOCK GROUPING

Locks can be divided into two very general classes: (1) those that oper-ate on purely mechanical principles and (2) those that are electrical and combine electrical energy with mechanical operations and are commonly associated with automated access control systems.

Mechanical Locks

A mechanical lock utilizes some barrier arrangement of physical parts to prevent the opening of the bolt or latch. In such a lock, the functional assemblies of components are:

1. The bolt or latch that actually holds the movable part (door, win-dow, etc.) to the immovable part (jamb, frame, etc.).

2. The keeper or strike into which the bolt or latch fits. The keeper is not an integral part of the lock mechanism but provides a secure housing for the bolt when in a locked position.

3. The tumbler array, which constitutes the barrier or labyrinth that must be passed to move the bolt.

4. The key or unlocking device, which is specifically designed to pass the barrier and operate the bolt.

In most mechanical locks, the bolt and barrier are found in the perma-nently installed hardware or lockset, and the key or unlocking device is separate. However, in some mechanical locks that use physical logic devices, the entire lock is a single assembly. Examples of these would be locks with integral digital keypads, which mechanically release the bolt if the correct sequence is entered, and dial type combination locks.

The Warded Lock. The mechanical lock longest in use and first devel-oped is the warded lock. Figure 1 illustrates a typical warded lock. This lock is exemplified by the open, see-through keyway and the long, barrel-like key. In the illustration, six different keys are shown plus a master or skeleton key that will open all six locks. Still found in older homes and farm buildings, and even in older inns, the warded lock, as used in the U.S., is a very simple device.

The greatest weaknesses of this type of lock are its vulnerability to spring manipulation by any key that is not stopped by the wards, and cor-rosion due to weathering and age. A well-planned, modern locking program does not include warded locks. In any installation where extensive warded

Figure 1. Cutaway of a typical warded lock and the principle of operation of the warded lock key.

locks are already present, phased replacement or augmentation with other locks is recommended.

The Lever Lock. A significant lock improvement after the warded lock came in the 18th century with the perfection of the lever principle. (Termi-nology note: a lever lock should not be confused with a lever handle on a lockset.) Figure 2 illustrates the operating principle of lever locks. Note that the lever tumblers are flat pieces of metal held to a common pivot and retained in place inside the lock case by the tension of spring wire. Each lever is cut on the edge opposite the pivot to accommodate a lug or appendage attached to the bolt and is designated as a fence. When all the levers are positioned so that the fence slides into the spaces cut into the levers, the bolt can be withdrawn.

By varying the position of the cut in each lever, it becomes necessary to lift the levers to different heights to bring the cuts into proper alignment for insertion of the fence. This is accomplished by designing the lever lock key so that the cut or bit depth for each lever tumbler is matched to the fence cut on that tumbler. When inserted in the keyway, the flat key is turned and, as it rotates, the varying depths of the tumbler cuts lifts those tumblers until the fence cuts are all aligned. Continuing the key rotation then draws the fence into the aligned cuts and withdraws the bolt. In locking, the action is reversed and the bolt is thrown until the fence leaves the recess.

Returning the key to the original entry position allows the lever tum-blers to drop into their home positions, retained by the spring tension, and disarranges the fence cuts, thus preventing the bolt from being withdrawn.

The lever lock offers more security than the warded lock. Moreover, by placing two or more fence cuts on each lever tumbler, it is possible for two or more keys, cut to different dimensions, to operate the lock. This permits

“master keying,” which is discussed later in this chapter.

The lever lock finds continued application today in such varying situa-tions as desk, cabinet and locker installasitua-tions, bank safe deposit boxes, and U.S. mail boxes. Although the lever lock is inherently susceptible to picking, it can be designed to provide a high degree of lock security through resistance to picking.

The Pin Tumbler Lock. The most important development in the his-tory of mechanical locks to date has been the invention of the pin tumbler in the 19th century by Linus Yale, an American who also developed the dial-type combination lock. The pin tumbler is probably the most widely used lock in the U.S. for applications such as exterior and interior building doors. A number of very useful refinements have been added to the basic pin tumbler in recent years so that now a very high level of lock security can be achieved with many models.

The pin tumbler is illustrated in Figure 3. It consists of the same basic ele-ments as all mechanical locks: the bolt moving device, the maze or labyrinth, and the keyway. It is in the maze or obstacle segment that it is different from the others.

Figure 2. Various lever tumblers with single key and master key applications.

The pin tumblers are cylindrical metal sections that fit into matching cylindrical holes in two complementary parts of the lock. The first part, called a plug, rotates with the key when properly actuated and permits the bolt to be thrown or drawn by that rotary action. The second part is the shell or immovable housing into which the plug is fitted. Because the pin tumblers extend from the shell into the plug, they prevent the plug from turning. The pin tumbler cylinder lock operates on the principle that the pin tumblers must be manipulated into a position in which they are entirely contained in the plug, leaving no interference with the free turning of the plug.

Figure 3 shows that each pin tumbler actually consists of at least three elements: (1) the pin, (2) a driver or separate metal cylinder, and (3) a spring. The spring provides tension against the driver, which in turn pushes against the pin, forcing it down from the shell into the plug. The pin is retained in the plug and prevented from falling into the keyway by the fact that the keyway is narrower than the diameter of the pin.

When a properly designed key is inserted into the keyway, each pin is raised so that the line at which the pin and driver meet is brought exactly

Figure 3. Conventional pin tumbler lock.

even with the edge of the plug. When this is done for each pin tumbler, a shear line is created between the plug and the shell and the plug is free to turn. To complete the bolt or latch action, the plug usually makes a com-plete revolution so that when the key is removed, the plug is locked into the shell, with the bolt in either the locked or unlocked position. Unless a separate device is present on the particular cylinder lock, it is necessary to use a key both to lock and unlock the bolt.

To enhance security in some models of pin tumbler locks, the pins and driver are interlocked so that random movement of the pins by lock picks or keys not specifically coded for the lock will not properly align the pins and drivers. In such locks, although the individual pins might be aligned at a shear line, the interlocking feature on the driver will prevent the plug from turning in the shell (see Figure 4). In this type of lock, the keys are cut at precise angles, as well as depths, so that when inserted into the plug, the key will both raise the individual tumbler array of driver and pins to a shear line and, at the same time, turn each pin so that the interlocking mechanism is positioned to pass through a special groove at the base of the plug, thus permitting the entire plug to rotate enough to move the bolt.

Figure 4 illustrates the operating principles involved, as exemplified in the Emhart (Corbin) high-security cylinder.

A variant of this principle is found in the Medeco high-security cylinder.

In the Medeco lock, instead of grooves at the bottom of the plug through which the interlocking feature of the pins pass, a side bar is moved into a cutout housing in the shell or withdrawn into grooves in the pins. The side bar or fence would otherwise prevent the plug from turning. In some locks, the pin tumblers are not interlocked but are “mushroom” shaped so that failure to align them correctly will cause them to bind in the housing, again preventing the plug from turning. In both types of high-security locks, the keys are specially cut at specific angles, as indicated in Figure 4, thus mak-ing routine duplication of keys quite difficult, except on special equipment used by the manufacturer.

Another variant of the pin tumbler lock utilizes multiple tumblers on dif-ferent axes and requires an entirely difdif-ferent type of key. In locks of this type, the spacing between the pins and the lengths of the pins are both varied to establish the individual lock coding. In Figure 5, the multiple-axes-tum-blers technique is illustrated in the Sargent Maximum Security System.

The Wafer Tumbler Lock. A fairly late development, the wafer tumbler lock utilizes flat tumblers fashioned of metal or other material to bind the plug to the shell. A properly designed key raises the wafers out of the lower portion of the shell until they are all contained within the plug, thus creat-ing a shear line, with the plug free to turn inside the shell. Sprcreat-ing tension keeps each wafer locked into the shell until lifted out by the key. By varying

Figure 4.Pin tumbler cylinder lock with interlocking pins.

the height of the central hollow portion of the wafer through which the key passes, withdrawal of the wafer from the shell can be matched to varying key bit depths. Figure 6 illustrates a typical wafer tumbler assembly.

If the hollow center is divided in half, with the left and right sides being of different heights, keys can be designed to operate on either side of the wafer tumbler, raising it to different positions. This permits master keying of wafer tumbler locks by using one side of the wafer for the master key bit position and the other side for the operating or change key bit position.

Wafer tumbler locks may be designed for double-bitted keys by spring loading some wafers to protrude upward into the shell and others to pro-trude downward. The key, cut on both sides, will draw all wafers back into the plug, irrespective of which way they protrude.

Dial-Type Combination Locks. Dial-type combination locks, while not employing a key, resemble the lever tumbler lock in many respects. They operate by aligning gates on tumblers to allow insertion of a fence in the

Figure 5. Pin tumbler lock with multiple tumbler axes.

Figure 6.Wafer tumbler cylinder showing operation of the change key and the master key.

bolt. However, the tumblers are fully circular and are interdependent; that is, moving one results in moving the others. This makes the order of move-ment important and is really why these are true combination locks rather than permutation locks.

The number of wheels or tumblers in a combination lock determines the number of elements in the combination. For example, a combination of Right-10, Left-25, Right-9, Left-0 to open would indicate a three-tumbler lock. Dialing the first three numbers in the correct sequence would align the gates on each of the three wheels. Dialing to “0” at the end would move all aligned wheels to a position at which the fence would fall into it. The

“gate” and “fence” mechanisms are quite similar to those discussed previ-ously for the lever lock. The primary difference is that in a combination lock, the tumblers are circular and are not restrained by steel spring pressure.

To unlock a combination dial-type lock, the first tumbler to move is the one furthest away from the dial. This is designated as a “driver” tumbler and is the only one actually turned by the spindle. From it will protrude a drive pin or dog that will engage a stop on the next tumbler nearer to the dial.

That tumbler also has a drive pin that will engage the next nearer tumbler, and so on, until all tumblers have been engaged and moved to the correct opening position. The direction of movement alternates to permit picking up successive tumblers and moving them into proper alignment. This makes it clear why it is recommended practice when locking a combination type lock to scatter the combination by dialing randomly in several directions at least as many times as there are tumblers. This effectively changes their relative alignment so that no two tumblers will have their gates aligned.

With combination locks, the theoretical maximum number of combina-tions is the base number of posicombina-tions on each tumbler (typically 100 on a good-grade lock), raised to the power of the number of tumblers. Thus, a 4-tumbler combination lock, each of whose tumblers has 100 numbers of dial positions, would have a theoretical maximum of 100⁴ or 100,000,000 changes. A three-tumbler lock with 100 numbers to each tumbler would have a maximum of 100³ or 1,000,000. The theoretical maximum is not really available, however, as it is generally wise not to utilize the numbers immediately adjacent to the opening number on any single tumbler or to repeat the same number twice in a single combination. In a three-tumbler lock, the reduction in combinations would be from 1 million to 912,673 (97³) when the opening number and the two numbers adjacent to the open-ing number on each tumbler are dropped. Even so, when reduced, the available maximums are still very formidable to an attacker.

Most important in the use of combination locks are procedures for the selection and maintenance of the numerical combination. The selection should allow those who must know and use it to memorize it without having

to write it down. One suggestion that is included in Defense Investigative Service training manuals (for the protection of classified government mate-rials), is to select a six-character word that can easily be remembered; the characters are transformed into numbers by looking at a telephone key-pad. For example, the word “S-E-L-E-C-T” transforms into 73-53-28. For more or fewer tumblers, a longer or shorter word can be used. Procedures should also be in place for changing the combination, usually whenever a person who knows it is terminated or no longer requires access, or if there has been some form of compromise. In any event, the code should be changed at least once every one to two years.

Electronic Dial-Type Combination Lock. Recently, electronic combina-tion locks have been developed as direct replacements for dial combinacombina-tion locks on safes and secure document cabinets. These devices are powered by the user turning the dial; the combination numbers are displayed via an LCD rather than by gradations on the dial. The display is viewable only from a limited angle and the number being dialed bears no direct relation-ship to the position of the mechanical dial. Additional features include a time-out of a specified number of seconds between each number dialed, and a two-person rule where two numbers must be dialed before the lock will open. As each user is assigned an individual number, an audit trail of which combinations were used to open it and when it was opened is avail-able. The lock can memorize the number of unsuccessful attempts to open.

Because of the electronic precision of the system, there is no reduction in the number of combinations due to the unavailability of adjacent numbers.

These locks are claimed to be immune from all the typical defeat modes of a regular mechanical combination lock, as well as from electrical and mag-netic attacks.