Why Is a Door Easier to Push at the Edge? A Fixed Line to Turn About

A free object can move in six independent ways, three of translation and three of rotation, and almost all mechanical design consists of removing some of those and permitting others. A hinge removes five and leaves one, so the attached part can rotate about a single fixed line and cannot move in any other respect. That single remaining freedom is exactly what a door, a lid, a gate, a laptop and a folding chair require. The constraint is also what carries the load, since the fixed line takes the weight and the forces applied, which is why hinges fail by wear at that line and why their size is determined by load rather than by the size of what they carry.

Where the line sits determines the forces involved:

• •Turning force equals the applied force times its distance from the line
• •So a handle at the far edge needs a fraction of the force one near the line does
• •A door pushed near its hinges barely moves however hard it is pushed
• •Weight of the door acts through its centre, which is why hinges pull outwards at the top
• •Long doors need more hinges to spread that load
• •Setting the line slightly off the face lets a door close flush

Different problems have produced quite different designs. A butt hinge is two leaves and a pin, is recessed into the edge, and is the ordinary door arrangement. A piano hinge runs the full length and spreads the load continuously. A concealed cabinet hinge allows adjustment in three directions after fitting, which is why modern kitchen doors line up. A rising hinge lifts a door as it opens, so it clears a carpet. A spring hinge closes automatically, which fire regulations require. A living hinge is a thin flexible section of a single moulded part, with no pin at all, which is how a shampoo bottle lid works and which can survive millions of cycles in the right plastic.

Several arrangements achieve controlled motion without a fixed pin and they solve different problems. A slide constrains an object to move along a line rather than to rotate, which is what a drawer does. A four-bar linkage moves a part along a curved path while keeping it at a chosen angle, which is how a car bonnet lifts clear and how an anglepoise lamp holds its head level. A ball joint permits rotation in every direction while preventing translation, which is what a hip and a tow hitch need. A flexure bearing bends a thin metal section instead of sliding anything, which eliminates friction and wear entirely and is used in instruments where a fraction of a micrometre matters.

Failures follow a small number of patterns that are diagnosable from the symptoms. A door binding at the top and gapping at the bottom indicates the hinges are pulling out of their fixings, which happens because the weight of the door acts at a distance and levers the top screws outwards, and it is cured by longer screws reaching the frame rather than the lining. Squeaking indicates dry or worn bearing surfaces. A door swinging open or closed by itself indicates the line is not vertical, which is a frame problem rather than a hinge one. Wear in the pin produces play that grows with use. And too few hinges for the weight overloads each one, which is the commonest specification error.

A hinge removes five of the six ways an object can move and leaves rotation about one fixed line, which is exactly what a door or a lid needs, and that line carries the load. Turning force is the applied force times its distance from the line, which is why a handle sits at the far edge. Concealed, rising, spring and moulded flexible designs each solve a different problem, and most failures are fixings rather than the hinge.