A type of hinge that prefers one position over the other.
Where in a conventional hinge, the gaps between
knuckles are perpendicular to the pin of the hinge, so the hinge can rotate freely without the leaves moving along the axis of the pin:
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Rising butt hinges are most commonly used on doors, but could be used anywhere there is a force acting along the axis of the pin. The gaps between the knuckles are spiraled in such a way that when the hinge is opened, one leaf is forced upwards, the other down.
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Gravity (or whichever force is acting on the hinge) acts downwards on the top leaf, and if it is free to rotate it will rotate around (and down) the
pin until the hinge is closed again.
Rising butts are often used in
lavatories, where if the
lock on the cubicle door fails, the door will tend to close of its own accord. Unlike a spring, which exerts more force the further it is stretched, a rising butt hinge exerts a constant force at all times, closing the door gently, instead of slamming it shut.
'Rising butt' sounds dirty, but really isn't.