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Types of Locks

This Web site focuses on two types of locks: key-operated locks and combination locks. In normal operation, key-operated locks require a key to operate them. Combination locks require that wheels be aligned so that the lock can open. Some combination locks (notably locks on school lockers) also can be opened with a key.

Key-operated locks include:

    Image of a standard cylinder lockset for a door.
  • Cylindrical Locksets—Often called key-in-knob or key-in-lever locks. They are normally used to secure offices and storerooms. The locking cylinder in the center of the doorknob distinguishes these locks. Some cylindrical locksets have keyways in each of the opposing knobs that require a key on either side to lock and unlock them. Others unlock with a key, but may be locked by pushing or rotating a button on the inside knob. These locks are suitable only for very low-security applications. Rooms secured with these locks may require additional locks on containers within the room. Cylindrical locksets are easily defeated with a pipe wrench or water-pump pliers.

  • Image of a cylinder lock that does not have handles, but does have a removable core.
  • Dead-Bolt Locks—Sometimes called tubular dead bolts. They are mounted on the door much like cylindrical locksets. The primary difference is in the bolt. When the bolt is extended (locked), the dead bolt projects into the doorframe at least 1 inch. It cannot be forced back (unlocked) by applying pressure to the end of the bolt. The dead-bolt lock can provide acceptable levels of protection for storerooms and other areas where more security is desired. In situations where glazing is in or beside the door, a double-cylinder dead-bolt lock (that requires a key to open from either side) should be used. Double-cylinder dead-bolt locks must never be used on designated emergency exit doors or on any door that will be used as an exit in an emergency. It is better to eliminate the vulnerability created by the glazing than to require someone to insert and operate a key during an emergency.

  • Image of a remote control monitor box with alarm lights and toggle switch.  It is linked to a wall mount junction box that connects to zones being monitored.
  • Mortise Locks—Mortised or recessed into the edge of the door. The most common variety of mortise locks has a doorknob on each side of the door. Entrance doors often have an exterior thumb latch rather than a doorknob. The mortise lock can be locked from the inside by a thumb turn or a button. Mortise locks are considered low-security devices because they weaken the door structure in the mortised area. This is particularly true in wood doors, even solid-core wood doors. If mortise locksets must be used, they should be used only in heavy-gauge metal doors.

  • Drop-Bolt Locks (often called jimmy-proof locks)—Auxiliary locks similar to dead bolts. Both the drop-bolt lock body and the strike plate have interlocking leaves similar to a door hinge. When closed, locking pins in the lock body drop down into the holes in the strike plate, securing the locking system. Because the lock body and the strike plate are interconnected with locking pins when they are closed, the lock essentially becomes a single unit and is extremely difficult to separate.

  • Rim-Cylinder Locks—Mounted to the door's inside surface and secured by screws in the door face. These locks generally are used with drop-bolt and other surface-mounted locks and latches. They consist of an outer barrel, a cylinder and ring, a tailpiece, a back mounting plate, and two mounting screws. The tailpiece screws are usually scored, allowing the lock to be tailored to fit varying door thicknesses.

  • Image of the front of a mounted digital combination lock.
  • Unit Locks—Frequently seen in heavily traveled facilities such as hospitals or institutional buildings. These locks are a complete, one-piece unit that slides into a notch cut into the door's latch edge. The cutout of the door edge simplifies the preparation for the lock. However, the unit lock must be rigidly mounted in the notch.

  • Access Control Devices—Mechanical, push-button (push buttons numbered 1 through 9) combination locks or digital combination door-locks used to limit access by individuals who are not authorized or cleared for a specific area. These devices normally are used for access control and should be backed up by door locking devices when the facility is unoccupied. These devices must be well maintained and have their combinations changed frequently to reduce their vulnerability to operational failure and covert manipulation.

  • Image of a combination disk padlock which has four numbered wheels on the bottom.
  • Padlocks—Detachable locks typically used with a hasp or other specialized hardware. Low-security padlocks, sometimes called secondary padlocks, are used to deter unauthorized access. They provide minimal resistance to force. Low-security locks are made with hardened steel shackles. Precautions must be taken to avoid confusing these locks with similar brass or bronze locks. The brass or bronze locks do not meet the security requirements of the locks with hardened shackles. High-security padlocks may be used to secure assets with high value or high threat. They provide the maximum resistance to unauthorized entry when used with a high-security hasp.

Some locks have interchangeable cores, which allow the same key system to include a variety of locks. Padlocks, door locks, cabinet locks, and electrical key switches can all be operated by the same key system. Because these cores can be removed by a special key, this system allows rapid rekeying of locks when an unauthorized user is suspected of having access to the key.

Once the special key has been used to remove the interchangeable core, the lock usually can be operated by a hand tool such as a screwdriver. Although interchangeable core locks increase both the convenience and flexibility of locks, they also increase their vulnerability if the core removal key is not properly controlled.

Locks are keyed in several different ways. When several locks are keyed differently, each is operated by its own key. When they are keyed alike, one key will open them all. Locks that are master keyed are keyed differently, yet have one key that will open them all. Master keying can be done on several levels. Master keying is done for convenience and represents the controlled loss of security. Effectively designing a master keying system requires detailed planning by the facility manager and a qualified locksmith.

Combination locks are available as padlocks or as mounted locks. They are low-security padlocks with combinations that are either fixed or changeable. Combination locks may be either mechanical or electronic. They are operated by entering a particular sequence of numbers. When the correct combination is entered, the lock's bolt is retracted. Combination locks used for securing classified material must meet Federal Specification FF-L-2740.


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