This article contains everything you need to know about latches.
Read further to learn more about topics such as:
- What are latches?
- Door latches
- Different types of latches
- Cam locks
- And much more…
Chapter 1: What are Latches?
Latches are mechanical components that allow the temporary joining of parts that are moving relative to each other. They are actuated to release the coupling and allow the two parts to separate. Latches are mostly seen on doors and enclosure openings. They are also used in devices such as seat belts, clamps, straps, and objects with retractable parts. Different designs feature various methods of actuation, holding style, and mounting.
Chapter 2: Door Latch
Door latches are a general type of latch used for keeping doors and gates closed. They can be in the form of a sliding bolt that prevents the door from swinging open, or a hook for maintaining a closed position for both sliding and swinging motions.
An example of a typical door latch is a doorknob. This type is commonly used for architectural purposes. It is composed of a latching mechanism, a locking mechanism, and two knobs on the interior and exterior sides. The latch is a spring and bolt-type mechanism that is activated by turning the knob on either side of the door. They are designed to be quarter-turn mechanisms and are fully activated when rotated by 90°.
Other designs use levers instead of knobs for aesthetics and convenience. Using levers creates an improved appearance as levers meld well in some architectural designs. The grip is also better with levers. For doors used as emergency exits, latches called panic bars are used. Panic bars (also known as crash or push bars) are opened by pushing a large bar fitted to the handle of the door. They are designed to be opened quickly and effortlessly. It aims to prevent people from getting trapped due to panic during emergencies.
All doorknobs also have a locking mechanism that prevents one side from releasing the latch. Modern door knobs are made with electrical and electronic parts that feature sophisticated lock systems and electrically actuated latch release.
Door latches can also be categorized as deadbolts, deadlatches, and deadlocks. Their mechanisms differ on how the latch is locked and unlocked. Their construction and characteristics are explained below.
Deadbolts are door latches that can only extend or retract by turning the knob or a key. It does not have a spring to keep the bolt extended. Thus, it does not automatically latch when the door is closed. When extended, the bolt cannot be retracted by pushing the bolt. Releasing the latch from the outside requires a key while doing the same from the inside only needs turning the knob. Generally, deadbolts have a more robust construction than deadlatches.
Deadlatches are the common mechanism seen in doorknobs. A deadlatch has two bolts: the main bolt for keeping the door closed and the smaller deadlatch bolt for preventing the main bolt from being tampered with. Both bolts are extended by springs. The main bolt is a cylinder with a rounded edge on its outer end while the deadlatch is a small D-shaped bolt placed adjacent to the main bolt. Closing the door causes the rounded edge of the main bolt to slide against the lip of the strike plate. This motion temporarily retracts the bolt. Once the main bolt is positioned in the box, the bolt locks in place. The latch is released the same way as that of deadbolts.
The unique feature of the deadlatch is the smaller deadlatch bolt. The strike plate of a deadlatch is sized only to accommodate the main bolt and not the deadlatch bolt. Thus, when the door is closed, the deadlatch bolt is retracted. A retracted deadlatch bolt has a mechanism that prevents the main bolt from being pushed or retracted.
These types of door latches offer the highest level of security. They function the same way as deadbolts. The only difference is both sides of the door need a key for unlocking the latch. The inside still has a knob but is only used for extending the bolt. Deadlocks are great for providing increased security but pose the danger of potentially trapping crowds during an emergency since a key is needed on either way.
Other types of door latches are simple sliding bolts and hooks. They are used on doors that do not get much traffic. They can be made stronger than door knobs since their bolts and latching mechanisms can be made thicker and more robust. These types of door latches are usually accessible only on one side of the door.
Chapter 3: Types of Latches
A simple latch is composed of a base, a bolt, and a box or hole. The base which contains the bolt is typically installed on the door. The box, on the other hand, is installed on the frame. Sliding or moving the bolt into the box activates the latch (such that one end of the bolt holds the door while the other holds the frame).
This basic design is further modified which brought forth different types of latches. The most distinct types are discussed below.
Bolt latches or latch bolts are simple latching mechanisms that consist of a bolt, a handle, a backplate, a set of barrels, and a box called a striker. The barrels hold the bolt in place and provide locking for the latch. They are formed into a non-continuous loop which allows the handle to pass through. A bolt latch can be locked or unlocked by sliding the bolt into the socket.
The most common type of latch bolt is a barrel bolt. They are used for closing gates, doors, and windows.
A spring latch is similar to a bolt latch but is equipped with a spring for automatic bolt extension. Some designs use the spring to keep the bolt retracted.
Spring latches can have different constructions. A simple spring latch has an L-shaped bolt called a plunger that has a small hole for inserting a clip or pin. Its backplate is formed with two holed lips for holding the plunger. To keep the plunger in an extended position, a spring is fitted between the lip and pin.
The design of a spring latch is also seen on deadlatches and slam latches. Deadlatch and slam latches have their unique features which differ them from ordinary spring latches.
Slam latches are also known as push-to-close latches. They are used for doors that require automatic latching when closed. Their design uses cams instead of bolts for latching onto the striker or door frame. They have a rounded or chamfered edge to allow retraction upon impact onto the striker. A spring-loaded mechanism is used to keep the cam raised. When actuated, the cam moves either by turning or sliding.
A slam latch is more robust in construction when compared to an ordinary spring latch. They can resist more opening and closing cycles without developing damage to their parts. Slam latches are mostly used in high traffic doors seen in commercial and industrial areas. Emergency exits also feature a slam latch that can easily be released by pushing the handle.
Other types of push-to-close/slam latches include:
- Spring bolt latches
- Pin latches
- Explosion venting latches
- Tipper latches
- Toggle latches
- Push-button latches
- Slam action strike latches
- Cab latches
- Paddle handles
- Compartment latches
- Cane bolt latches
Swinging latches, or swinging door latches, are similar to slam latches. They are push-to-close latches that automatically activate when the door is shut. They feature a spring-loaded mechanism that keeps the bolt in the extended position. The edge of the bolt that impacts the striker is rounded to allow retraction.
Cam latches are commonly used in furniture such as drawers and cabinets. These types of latches work by turning a knob or key which also rotates an arm fitted on the other side of the door. Like other knob-activated latches, they are quarter-turn devices. Their assembly does not include a strike plate, box, or hole installed on the frame for receiving the arm. They lock the door or enclosure by barring only one side. Thus, they are not suitable for doors that swing or move past the frame.
Other types of cam latches include:
- Quarter turn latches
- Defeater handles
- Swing handles
- Actuator handles
- Folding T Handles
- Cam locks
Aside from hinges, handles, and guide rails, furniture such as cabinets and drawers always has a latch installed. Cabinets latches can be of various forms. The most popular are bolt, spring, cam, and slam latches.
Cabinet latches are usually made from stainless steel, brass, copper, and dyed anodized aluminum. These materials give better aesthetic quality while being durable and resistant to corrosion.
Different metalworking processes such as casting, cutting, and forging are done to produce cabinet latches. As a decorative product, cabinet latch dies and molds are prepared with patterns and shapes much more complex than ordinary latches.
As the name suggests, gate latches are used for securing gates and large doors. They are made much stronger and thicker than cabinet latches. One thing in common with cabinet latches is that they can also be made with architectural design features.
Common gate latch forms are barrel bolts, bars, and heavy-duty industrial latches. In some instances, slam and spring latches are also used. They are made with thicker gauge sheets or plate metal and are formed by larger compression forces.
Gate latches are usually made with galvanized steel and stainless steel, particularly grade 304. These materials are suitable for the high strength and corrosion resistance required by their application. Gate latches are seen in every industry, may it be industrial, commercial, or residential.
This type of latch is similar to a cam latch. It has a simple design wherein the latch is activated when the knob is turned 90°. It uses an arm or a cam that turns or rotates when the knob is rotated.
Some turn latches feature a keyed knob design for additional security. Other designs incorporate additional actions for actuation such as pushing or pulling These actions are done before turning the knob to release the latch. These double-action devices add a layer of safeguard to prevent accidental releasing of the latch.
A compression latch is a type of cam latch but with a modified arm. They are used to provide a certain amount of compression between the door and frame when closed. Compression is made by pushing the door and activating the latch, or by turning the latch multiple times creating screw-action. In contrast to ordinary cam latches, compression latches have arms that are formed in a way that allows a slight deflection when mated against the frame of the enclosure. Also, their arm is thicker than that of ordinary cam locks.
Compression latches are used on devices in which a certain level of ingress protection is needed. Examples of these applications are electronic and control equipment enclosures. Gaskets or rubber trims installed along the door or frame are squeezed from the compression provided by the latch. This provides a tight seal that protects the internals from dust and moisture. Aside from ingress protection, compression cams also prevent vibrations that can produce noise and fatigue on door hinges.
Other types of compression latches include:
- Lift and turn latches
- Spring panel latches
- Compression QT latches
- Compression folding Ts
- Trigger latches
- Lever latches
- Tension latches
Magnetic Door Catch:
A magnetic door catch consists of a catch body fitted with a permanent magnet and a ferromagnetic strike plate. They are used to provide automatic and convenient latching of furniture doors. Other designs employ two magnets each fitted on both the door and frame. These types are used for light-duty and low-traffic doors.
The magnetic catch has a hollow body typically made of non-ferromagnetic metal or plastic. Inside the non-magnetic body is a bar magnet. Attached on the bottom and top sides of the bar magnet are steel plates used to concentrate the magnetic flux. These steel plates contact the striker plate.
The magnetic catch is mounted on the door jamb or frame while the strike plate is placed on the edge of the door. They can be surface mounted or concealed. They are installed such that there is physical contact between the magnet and metal strike plate when the door is closed. This is needed to maintain a strong magnetic attraction.
A draw latch, also known as a toggle latch, is characterized by having a component that is under consistent tension. They are suitable in applications that require strong closing forces to maintain seals against leaks and ingress of dust and water. They are also designed to prevent accidental release of the latch. Examples of applications that use draw latches are electronic enclosures and equipment control panels.
A common design is composed of two hooks. One hook is called a receiver, striker, or catch. It is made from a formed metal plate or sheet metal. The other hook is called a blade or claw. It is also made from sheet metal formed into a hook. Other designs are made from a wire or rod bent to form a loop.
Latching is made when both hooks (or when the hook and the loop) clasps each other under tension. One hook is attached to a lever through a pin. Tension is created or released when the lever is pushed or pulled. The lever is carefully designed to produce a mechanical advantage so that sufficient tension can be created while requiring minimal force for releasing the latch.
Subtypes of draw latches include:
- Over center latches
- Under center latches
- Twist latches (link lock, butterfly latch, wing latch)
- Rubber T-handle latches
- Hood latches
- Living hinge latch
Over Centre Latch:
Over centre or over center latches are types of draw latches described by having minimal actuation force while providing strong closing forces. The lever is carefully designed to produce a mechanical advantage so that sufficient tension can be created while requiring minimal force for releasing the latch.
The over centre latch derived its name from its characteristic over center lock point. When the lever is moved past its over center position, the latch becomes activated. At this point, the closing force is provided by the hooks.
A rotary latch is composed of a latching mechanism, an actuator, and a cable. A bolt is usually installed on the opposite part that mates with the latching mechanism. When activated, the cams or rotors of a rotary latch turn or rotate. It has a spring-loaded mechanism that enables a push-to-close function, similar to that of a slam latch.
A unique feature of a rotary latch is its remote actuation. A cable connects the actuator and the latching mechanism. This allows the actuator to be located at a distance from the rotary latch. Pulling the actuator causes the rotors to rotate which releases the latch. The actuator only pulls in one direction. Thus, a spring-loaded mechanism is required to return the rotor to its position.
Rotary latches can have single or double rotors. A single rotor rotary latch provides the basic function of keeping a door or panel closed. On the other hand, double rotor rotary latches have two mirrored single rotors placed adjacent to each other. Intuitively, an advantage of a double rotor type is better rigidity attributed to the two rotors holding the bolt. Another advantage is their better tolerance to misalignment. Double rotor types have wider catch opening than single rotor types.
Another classification of rotary latches is the number of latching stages. They can either be single or two-stage. Again, a single-stage rotary latch only has the basic latching function. However, in a two-stage rotary latch, there are two latching positions. One is a secondary, partially closed (false close) position, while the other is a primary, fully closed latch. A two stage-rotor must always be fully closed. But in case the primary stage fails, the secondary stage prevents the door from suddenly opening. This type of rotary latch is frequently seen in cars and other vehicles.
A hasp is a simple latch that consists of a strap and a staple. The strap is a metal plate punched on one end with a slot that fits over the staple. Its other end has a hinge used to attach the strap to the door or door frame while allowing it to swing. The other part, called a staple, is a loop made of a bent metal rod or formed plate.
A pin is inserted through the loop of the staple to secure the strap. Padlocks are commonly used instead of a pin for better security. Other types of hasps, called hasp locks, use built-in locks in place of a staple. Latching is achieved when the lock is rotated which secures the strap in place.
Hook and Eye Latch:
This is one of the earliest forms of latches. A hook and eye latch are commonly made from bent wires and rods. They can also be made from cast metals. Through casting, they are formed into intricate shapes intended to be used as decorations. The hook becomes the arm of the latch that is attached to the door. The eye functions like a catch that receives the hook when the latch is closed.
A bar is another form of primitive latch. They are made from large wooden or metal beams that are much larger than bolts. Because of their ruggedness, they are used to hold gates and large doors. They are built to withstand high bending forces to prevent forcing the door open. The bar is held by a set of cleats or hooks that are equally strong. A door is closed by sliding the bar into the hole, or by lifting and manually placing the bar onto the cleats.
Chapter 4: Cam Locks
Cam locks are cam latches combined with a key instead of an ordinary knob. Their main feature is the added security provided by the lock. The base of the cam is a cylinder fitted into a hole on the door or cabinet. On the lateral side of the base, external threads are cut. A nut mates with these threads for holding the cam lock firmly on the door.
A cam lock can only be actuated when the correct key for turning the cam. Cam locks are available with different forms of locks and keys. The lock assembly can be generally classified as mechanical or electronic.
An ordinary mechanical lock used in cam locks is a wafer tumbler lock. A wafer tumbler lock is operated by correctly aligning the wafers assembled inside. Each lock has a unique set of wafers. The wafers are pushed down by springs so that they are at the farthest position when there is no key inserted. By inserting a key with the right profile, the ends of the wafers become aligned to the sides of the plug. The plug is then allowed to rotate.
Another type of lock used in cam locks is a tubular lock. Instead of being positioned radially towards the axis of the plug, the pins of a tubular lock are arranged circularly. These pins also have varying lengths that match the profile of the key. A tubular lock key is cylindrical and has half-cylinder notches to align the pins.
Other types of cam locks feature a lock that can be released by a common or universal key. These types of cam locks are usually seen in utility cabinets and control boxes in which more than one person is permitted to access. Also, using these types allows multiple cabinets to be accessed with just one key. These keys have standard shapes and profiles. Examples of key profiles are square, triangle, and double bit keys.
Electronic cam locks have actuators powered by an electric current. These actuators can be in the form of motors or solenoids. They have an electronic control unit that accepts input from keypads and other sensors. The control unit then authenticates the provided input to release the lock. Common authentication keys are numerical codes, security tokens, and biometrics (fingerprint).
Electronic cam locks offer better features than mechanical types. First is convenience since it is easier to input a code or perform a fingerprint scan than by using a physical key. Another advantage is being programmable. The key can easily be changed without changing the lock. Other features of electronic cam locks are remote control, events history, and blacklist functions.
Chapter 5: Design Features and Specifications
Latches can have different design features which makes them more suitable for a particular application. They can be made with different types of materials, finishes, thicknesses, and other features. Summarized below are some of the general features and specifications of latches.
Latches are usually made from metals. Examples of these materials are carbon steel, alloy steels, stainless steels, aluminum alloys, and copper alloys. Metals are the first choice for producing latches because of their formability, machinability, and strength. Their external characteristics can also be improved by secondary processes such as finishing and coating. Other types of materials such as plastic and rubbers are added as trims or wraps to provide shock absorption or to reduce vibration.
Finishing and Coating:
Finishing and coating are secondary processes done on latches or any other products to give them enhanced surface qualities. Popular secondary processes for manufacturing latches are plating, polishing, powder coating, and painting. Plating is done to create a thin layer of metal such as zinc, silver, and chromium. Electrochemical deposition is used to set the metal onto the surface of the part. The result is a higher corrosion resistance allowing the latch to be suitable for outdoor use.
Polishing and buffing
Polishing and buffing are done to create a smooth surface and bright finish. Polished latches are free from burrs and other microscopic defects. Polishing can be done mechanically or through an electrochemical process called electropolishing.
Powder coating and painting
Powder coating and painting adds a layer of polymer material to the surface of the part. These processes add corrosion resistance to the metal latch, similar to metal plating. Moreover, coatings can have different colors for improving the appearance of the product.
Latches can be classified as light, medium, and heavy-duty. They differ in the thickness of the sheet metal or metal plate used to form backing plates, barrels, and rotors. The cams and bolts can also have different thicknesses. They are designed to withstand impact, shearing, and bending stresses.
There are different kinds of mounting used for installing latches. Latches used in architectural applications can be flush-mounted, mortised, or surface-mounted. A flush-mounted latch has both the latch and door surfaces on the same level. Mortise latches are concealed into the hollow cavity or pocket inside the door. Only the handle and keyhole are protruding from a mortise latch. Surface-mounted latches are installed by simply bolting the latch assembly over the surface of the door and frame. No bores or holes are needed.
Latches such as deadlatches and some designs of bolt, rotary, and slam latches are made to be either left-handed or right-handed. Most designs can be interchangeable with few modifications. But in cases when these are not options, handedness can be determined by looking at the position of the hinges. The position of the hinges, either left or right, suggests the handedness of the door and latch to be used.
- Latches are mechanical components that allow the temporary joining of parts moving relative to each other. They are actuated to release the coupling and allow the two parts to separate.
- A door latch is a general type of latch used for keeping doors or gates closed. A typical door latch is a door knob which is composed of a latching mechanism, a locking mechanism, and two knobs on the interior and exterior sides.
- Other types of latches are bolt, spring, cam, compression, slam, and rotary latches.
- Cam locks are cam latches combined with a key instead of an ordinary knob. Their main feature is the added security provided by the lock.