Hinges

Chapter 1: What are Hinges

A hinge is a machine element that connects two bodies allowing angular movement about a fixed axis of rotation, all the while preventing translations and rotations on the remaining two axes. A hinge can be considered as a journal bearing with only one degree of freedom where a shaft is rotating in a hole. Hinges only allow one rotation, either yawing (the most common), pitching, or rolling.

Hinges are commonly used in doors, enclosures, containers, furniture, jewelry, construction, and electronics. For every application, there is a suitable design of hinge available. For a unique application, it is also possible to design and manufacture a custom hinge at a low cost.

Chapter 2: Types of Hinges

There are a variety of types of hinges to choose from for every application. For indoor furniture, either barrel or concealed hinges are used. For main entrance doors, sheds or wooden gates, butterfly and strap hinges provide decorative styles. For industrial electrical enclosures, friction and geared continuous hinges are used. If none of the hinge types fit your requirement, it is always possible to design a custom hinge. Below are the hinge types available in the market.

• Ball-bearing Hinges:

  Instead of the usual journal-bearing type, knuckle-and-pin assembly, ball bearings are used primarily to reduce friction. Ball bearings minimizes friction by decreasing the point of contact between rubbing surfaces while providing support for radial and axial loads.

  

  
  

• Barrel Hinge:

  A barrel hinge is composed of two hollow cylinders or barrels with a cut in the middle where the pivot is attached. These hinges are typically used in light-duty and concealed applications where aesthetics is highlighted such as wooden furniture doors.

  

  
  

• Butt Hinge:

  A butt hinge, also known as a mortise hinge, is the most common type of hinge used on doors. It is composed by a pair of leaves with several knuckles attached by a shaft or pin. One leaf is attached to a moving component (the door) while the other is attached to a fixed body (the door jamb). Usually, the hinge is flushed to the wood surface through a mortise, hence the name. This hinge can be full or half mortise. Full mortise means both leaves are flushed to the surfaces of the door and door jamb, while half mortise involves only one leaf is flushed (usually the moving component).

  

  
  

• Butterfly Hinge:

  Butterfly hinges are like butt hinges but with added aesthetics to the design. They are usually used in furniture doors for adding decorative features where strength is not particularly important.

  

  
  

• Case Hinge:

  Case hinges are similar to butterfly hinges but are commonly used in suitcases, briefcases and so forth.

  

  
  

• Concealed Hinges:

  Like barrel-type hinges, this type of hinge is usually used in furniture doors. This hinge consists of two parts, the hinge cup, and the arm. Concealed hinges, also called cup or Euro hinges, are complicated in construction and have several design features. They cannot be seen from the outside making the furniture have a continuous appearance. Also, this type can be adjusted after installation making viable for constructions with slight imperfections. Aside from being adjustable, some designs also have self-closing features and damping systems.

  

  
  

• Continuous Hinge or Piano Hinge:

  Continuous hinges are a variation of the butt hinge but are constructed longer and narrower. This hinge spans the entire length of the door or lid providing strong support. It is sometimes referred to as a piano hinge since this is commonly used to attach piano lids to its body. Continuous hinges can also be mortised to the surface they are attached to.

  

  
  

• Flag Hinge:

  The flag hinge consists of a pair of leaves (male and female) with the pin permanently attached to one leaf. The male leaf has the pin permanently attached to the plate which resembles a flagpole, hence the term flag hinge. The main advantage of using flag hinges is its easy dismantling. The male end is secured to the other only by gravity. Flag hinges are also classified as lift-off hinges.

  

  
  

• Floating Hinge:

  This is a type of hinge that allows some translation of one part relative to the other aside from rotating. While behaving like a butt hinge, the door or cover can move away from the fixed part.

  

  
  

• Flush Hinge:

  Flush hinges are designed in such a way that the leaves sit flush against each other. This is achieved by fitting one leaf inside the other. This is an advantage against butt hinges since flush hinges do not need a mortise or a recess on the surface of the body they are attached to. These types of hinges are used in light load applications.

  

  
  

• Friction Hinge:

  Friction or torque hinges are designed to function like a butt hinge but hold its position over extended periods of time. Friction is developed into the barrel which resists motion caused by gravity or other external force. These hinges are particularly useful in display devices and other electronics.

  

  
  

• Geared Continuous Hinge:

  A geared continuous hinge differs from a butt hinge by using gears and a vertical cap instead of knuckles and a pin for movement. This is useful in joining two heavy objects while allowing one or both to partially rotate. Despite its strength, a major downside of this type is its limited range of movement.

  

  
  

• H-Hinge:

  H-hinges are also mortise hinges with an “H” shape. This shape is due to the smaller length of the knuckles and pin as compared to the length of the leaves.

  

  
  

• HL-Hinge:

  HL-hinges are the same as H-hinges but with one leaf shaped like an “L”. This is used when the door or the moving part is heavier.

  

  
  

• Heavy Duty Hinge:

  Heavy duty hinges can be of any hinge design but with an added thickness to the material making it stronger. These hinges can be mortised, surface mounted, or welded depending on the application.

  

  
  

• Lift-off Hinge or Slip Joint Hinge:

  Lift-off hinges, like flag hinges, have male and female leaves in which the male leaf has a permanent pin attached. The leaf length of lift-off hinges can be the same as the length of the pin, unlike the flag hinge.

  

  
  

• Living Hinge:

  Living hinges are different from the other types since it does not rely on knuckles and pins for attachment. It is made from the rigid body itself where a portion is cut or thinned to allow bending. This type of hinges is usually seen in plastic containers and packaging materials. Since these hinges are made of plastic, they are inherently protected from corrosion. Polyethylene and polypropylene are the best materials for containers with living hinges because of their excellent fatigue resistance.

  

  
  

• Offset Hinge:

  An offset hinge is a modified butt hinge primarily used to increase the width of doorways. This is achieved by moving, or offsetting, the pin away from the door jamb. Opening the door at a 90° angle sets the door about two inches from the doorway, leaving more space for passage. This simple modification of a butt hinge is a cost-effective way for increasing access space as compared to dismantling and constructing a wider doorway.

  

  
  

• Overlay Hinge:

  This type of hinge has its leaves folding into each other, like a flush hinge. These hinges are commonly used in cabinets where insert, full overlay, or partial overlay is required.

• Pivot Hinge:

  A pivot hinge has its pins (the pivot) attached to the top and bottom side of the door against the door frame. This hinge is different from the butt hinge where the pins are located at the sides. Pivot hinges allow a wider range of rotation which makes it suitable on doorways with high traffic on both directions.

  

  
  

• Self-closing Hinge:

  A self-closing hinge is a type of spring-loaded hinge designed to automatically close doors by means of a spring. The spring exerts a force to close the door while a damper, either mechanical or hydraulic, causes the rotation of the door to slow down as it closes.

  

  
  

• Spring-loaded Hinge:

  Spring-loaded hinges are hinges fitted with a spring to assist opening and closing of the door. One type of this hinge is the self-closing hinge which was mentioned earlier. Another is the double action spring hinge. Its movement is similar to the pivot hinge, but with springs attached. The door can swing in both directions while the springs automatically close the door in its center position.

  

  
  

• Strap Hinge or Tee Hinge:

  Strap hinges are like butt hinges but with a shorter pin and wider leaves (straps). This type of hinge is not intended to be concealed, but for aesthetics and decoration. These are commonly used on garage and shed doors.

  

  
  

• Weld-on Hinges:

  Weld-on hinges are used in metal enclosures or gates where the hinge is needed to be fixed permanently through welding. Their construction is similar to a butt hinge.

  

  
  

Chapter 3: Hinge Terminologies

This section defines the terms mentioned such as leaves, barrels, and pins. Aside from the component terminologies, hinge characteristics are also discussed. These characteristics define the specifications for manufacturing or purchasing a hinge.

Components:

• Leaf

  This is the plate that extends laterally from the knuckle or barrel. This is the part that is being attached to the fixed or moving body either by screwing, welding, or gluing. The leaf attached to the moving part revolves around the pin.

• Knuckle or Barrel

  This is the hollow cylinder attached to the leaf where the pin is slid through. Considering the hinge as a machine element, the knuckle can also be referred to as a bearing.

• Pin

  The pin is the rod or shaft that runs through the knuckles which holds the leaves together.

  

  
  

Characteristics:

• Leaf Length

  This is the dimension of the leaf as measured to the side parallel to the pin’s axis.

• Joint Length

  This is the length of one knuckle or barrel.

• Barrel Length

  This of the overall length of the barrels as measured parallel to the pin’s axis.

• Leaf Width

  This is the dimension of the leaf as measured to the side perpendicular to the pin’s axis.

• Open Leaf Width

  This is the overall width of the hinge as measured across the pin.

• Paint Clearance

  The paint clearance is the dimension of the gap between the outer face of the knuckles and adjacent edge of the leaf.

• Side Play

  Side play is the degree of movement of the leaves perpendicular to the pin.

• End Play

  This is the degree of axial movement between the leaves.

• Pitch

  Pitch is the distance between the end of a knuckle and the end of the adjacent knuckle on the same leaf.

• Leaf Material

  Common leaf materials are steel, stainless steel, aluminum, brass, and bronze.

• Gauge

  This is defined as the thickness of the leaves.

• Slop

  Slop is the loose angular movement of the leaves relative to the pin.

• Standard Assembled

  In this assembly, the leaves lay flat on the same plane when in an open position.

• Reverse Assembled

  In this assembly, the leaves extend laterally from opposite sides of the pin. Reverse assembled hinges cannot close with its leaves in parallel.

Chapter 4: Design Considerations

There are certain factors to consider when purchasing a hinge. Since the hinge has moving parts and carries loads exerted both by the moving body and the external loads, the hinge most of the time becomes the weakest point. Dynamic loads applied to the hinge will cause it to fatigue over time causing sagging on the door or gate. Moreover, hinges can only have a maximum thickness because of manufacturing limitations or space restriction. Hinges must be specified properly with the right dimensions, material, and mechanical properties fit to the application.

• Load

  The load to be carried is a key factor to consider when specifying a hinge. This will determine the dimensions and the material required for the application. In obtaining the right load specification, it is important to consider not only the weight of the moving part, which is usually the part being carried, but also external and dynamic loads. Also, the door’s center of gravity will cause a moment or torque onto the hinge. This moment may cause the door to eventually sag, especially when the door is wide and heavy.

  

  
  

• Required or Allowable End Play

  Some hinges have limited angular movement such as the geared continuous hinge. The part or may need to revolve a full 180° or just 90° depending on the application. Check if the space can accommodate the door swing and if the swing will affect access through the door. There are different types of hinges to choose from depending on the application.

  The required movement may also need friction or spring-loaded hinges for fixing the position of the moving part. The laptop covers, camera tripods, and ventilation screens are popular examples of devices that use friction hinges.

• Aesthetics

  Hinges used for furniture and doors usually have good aesthetics imparted on them. This may mean decorative designs of hinges as what can be seen on butterfly and strap hinges. Also, aesthetics may also require hinges to be concealed so that the furniture will have a continuous, uninterrupted look.

• Environment

  The type of environment, whether outdoor or indoor, will affect the material specification of the hinge. For applications requiring corrosion resistance and high strength, stainless steel is the material of choice especially in salt environments. Plastic hinges are corrosion resistant also, but without the high strength. If stainless steel is too expensive, galvanized steel works as well. Brass and bronze offer good resistance against outdoor conditions and are suitable for decorative purposes because of their natural patinas.

Chapter 5: Common Manufacturing Processes

Hinges can be manufactured either by casting, extruding, forming, or milling. Each process has its own advantage and is chosen based on the hinge profile, material, cost, and available technology. Take forming for example, which is a cheap and straightforward way to mass produce hinges with acceptable tolerance. However, since its products have low strength. one may opt to use milling or casting as a process instead if heavy duty hinges are required.

• Casting

  Casting is a process where liquid metal is poured into a mold cavity that contains the shape of the product. Casting is one of the earliest metalworking processes used to manufacture hinges. To create a cast hinge, a replica of the desired shape is made onto a piece of wood, metal, plastic, or plaster. For the barrel, an additional figure made of sand or metal shapes the internal to make it hollow. Next will be the preparation of the mold where a special type of sand is packed around the pattern contained in a frame called flask. Once the sand is packed, the hinge pattern can then be removed. After the mold is prepared, the molten metal is poured into the cavity and left to solidify. Once solidified, the mold is then broken, and the sand is removed from the casting. The cast is then cleaned, smoothened and heat treated to enhance its characteristics.

  

  
  

• Extrusion

  Extrusion is a metalworking process (although applied on plastics as well) where the metal is forced through a die with a desired cross-section. The resulting shape will have a fixed cross-sectional profile. To create a butt hinge, this extruded metal is then milled and cut according to the required leaf length and pitch. Geared continuous hinges, on the other hand, does not need further milling since its cross-section is fixed throughout its length. Aluminum is the material commonly used for extruded hinges. The gauge of the hinge can easily be made thicker by using a die with thicker hollow sections, thus making the hinge stronger.

• Forming

  Metal forming a hinge involves a thin blank sheet which is cut and subjected to plastic deformation using forming tools such as rollers and dies. Like casting, metal forming through hot forging is one of the earliest methods for producing metalworks. A common forming process is stamping where blanks are pressed onto a die. The dies are designed so that the required pressure exerted will accurately produce the required shape of the hinge. Stamped hinges have higher dimensional precision that can be produced at a lower cost than casted and extruded hinges. Furthermore, the hinges are cold worked (for cold forming processes) meaning the resulting material is made stronger and harder for its gauge. One downside of forming hinges is that the gauge of the hinge is limited. The thicker the plate, the more difficult it is to form.

  

  
  

• Cutting or Milling

  This process involves removing excess material from a metal workpiece using lathes, shapers, drills, or milling machines. Milling is commonly used for producing hinges with precise dimensions. Milled hinges can be made thicker, similar to the casting and extruding process.

After the process of shaping the hinge, secondary processes take place. These secondary processes are a combination of heat treatment, galvanizing, coating, painting, polishing, and so forth. Heat treatment gives the hinge the desired mechanical properties like toughness, strength, and hardness. Galvanizing, coating, and painting provides corrosion resistance for outdoor environments. Polishing, burnishing, and buffing is done for decorative hinges.

Conclusion:

• A hinge connects two bodies, usually one fixed and one moving, which allows angular movement along one axis while preventing any translation and rotation on the other two axes.
• There are several types of hinges available each with intended application. Most hinges are derived from the butt hinge type. Hinges with an entirely different construction are geared continuous and living hinges.
• The main components of hinges are leaves, knuckles (or barrels) and pin. From these components, characteristics can be defined such as leaf length, end play, pitch, and so forth.
• Hinges are the usual weak points in structures. Design of hinges are determined by the load, end play, aesthetics, and environment. These factors must be balanced to produce the most suitable hinge.
• Hinges are manufactured based on the hinge profile, material, cost, and available technology. The earliest metalworking processes to develop hinges is by forging and casting. As technology develops, mass production through extrusion, forming and milling are then used.