Often used for applications that desire the hinge to be hidden, piano hinges are never fully hidden; as a result, the effect of a truly hidden door cannot be actualized with this type of hinge. If the hinge is intended to be seen, such as on a piano, a brass piano hinge or stainless steel piano hinge is used; they are also used for many other aesthetic applications.
The common piano hinge is made with aluminum; however, for heavy duty piano hinge applications such as tool boxes, electronic enclosures, barn doors, fire truck compartments, storm doors and ATM machines, materials such as brass, galvanized steel and stainless steel are often used. Most commonly used in structural applications, continuous piano hinges serve wide-ranging industries such as furniture, construction, marine, automotive, appliance and electronics.
A piano hinge is one of the two most common types of recessed hinge; recessed hinges require the wood, or other materials, to be removed to allow the hinge to be mortised, or bored, into the material. A piano hinge consists of two flat independent leaves; the interior of the leaves are curled into alternating “knuckles” that fit into one another and are secured by a pin that runs the entire length of the joint.
Knuckles in the two leaves are fabricated so that there is a gap, or “end play,” between each knuckle and between knuckles and leaves. This allows the hinge to swing freely without rubbing against itself and causing friction. Most piano hinge manufacturers provide piano hinges without holes; however slotted piano hinges are also available.
Piano hinges without holes allow for custom hole drilling for its specific application or spot welding, which is the process of creating an overlapping joint by welding at small points. Slotted piano hinges have their benefits as well, as they allow for precision alignments. Once the slots, or holes, have been determined, close-spaced fasteners are used to connect both leaves of the hinge to the objects that they are trying to connect.
Piano hinges are most commonly made through the extrusion process; although roll forming and stamping are also possible. However, since piano hinges are longer than other hinges, the extrusion process works best because extrusion increases the tensile strength of the piano hinge, or object being extruded.