# How a Door Hinge Works

By Primordial One

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/DBUBx1u7H6hf)

## Guide

### 1. The quiet helper on every door

Every time you open a door, a small metal hinge does the work\. Most doors hang on three of them\. Let us find out how such a simple thing lets a heavy door swing so easily\.

Drag anywhere to turn the room\. Pinch or scroll to look closer\.

### 2. Two leaves

A hinge has two flat plates called leaves\. One leaf is screwed to the door frame and never moves\. The other is screwed to the edge of the door\. Each sits in a shallow pocket, called a mortise, so the door closes snugly\.

This common hinge is 3\.5 inches \(89 mm\) tall and as wide when opened flat\.

### 3. Knuckles that take turns

Each leaf curls into round tubes called knuckles\. This hinge has five: three belong to the frame leaf and two to the door leaf\. They stack and take turns, like the fingers of two hands laced together\.

### 4. One pin does the turning

A steel rod called the pin slides down through all five knuckles\. It holds the two leaves together and becomes the axle the door turns around\. Watch it lift out and drop back in\.

### 5. Every point moves in a circle

The frame leaf stays still while the door leaf turns around the pin\. So every part of the door moves in a circle around that one line\. The gold arc on the floor shows the path of the door's far corner\.

Slide to swing the door\. Most doors open about 90 to 110 degrees before they meet a wall or a stop\.

### 6. Three hinges share the weight

The door's weight hangs to one side of the hinges, so it tries to tip away from the frame\. The top hinge holds it in, like a hand pulling \(orange arrow\)\. The bottom hinge pushes back against it \(blue arrow\)\.

The top hinge works hardest\. Loose top screws are the usual reason a door sags and rubs\.

### 7. Pull the pins, lift the door

Because the pin is the only thing joining the leaves, taking it out separates them\. With all three pins out, the door lifts straight off its frame\. That is how people take doors off to move furniture\.

Outside doors often use pins that cannot be removed, so nobody can lift the door away\.

### 8. Hinges all around you

Two leaves, interlocking knuckles and one pin: that is the whole trick\. You will find the same idea in cupboards, laptop lids and box lids\. Your elbows and knees are called hinge joints because they bend the same way\.