6 steps · 19 parts · 5 pin stacks · one shear line
How a key opens a lock: interactive 3D guide
A door lock’s pin tumbler cylinder cut open in 3D: why it stays locked, how the right key lifts five pin stacks to the shear line, and how the turning plug pulls the deadbolt back. Press play to watch a step, drag to look around, or switch to Drawing for a technical drawing’s black lines.
What is inside
A pin tumbler cylinder, the kind in most front doors, cut away down its middle: a brass plug that turns inside a brass housing, five stacks of pins crossing the gap between them, and a cam at the back that throws the deadbolt. Its 19 parts are the housing, the plug, 5 key pins, 5 driver pins, 5 springs, the key and the cam.
- A lock that turns stiffly needs a dry lubricant made for locks, not oil, which collects dust.
All 6 steps
- 1Inside the cylinder
Five pin chambers are drilled through the top of the housing and on down into the plug, lined up when the plug is straight. Each holds a stack: a key pin at the bottom, a driver pin on it and a spring on top. The thin gap where the plug meets the housing is the shear line: the plug can only turn when nothing crosses it.
- 2Locked without a key
With no key in, the springs push every stack down as far as it goes, until each key pin rests on the keyway’s ledge. Each driver pin then sits half in the housing and half in the plug, across the shear line, like five bolts. Try to turn the plug and it moves no more than its slack: the drivers hold it.
- 3Inserting the key
Pushed into the keyway, the key’s bevelled tip lifts each stack, and the pins ride up and down over the key’s ridges and cuts as it slides past them, each spring pushing its stack back down onto the key.
- 4Hitting the shear line common mistake
All the way in, the key’s shoulder against the plug, each key pin sits in its own cut. The right key’s cuts are exactly as deep as its key pins are long, so every key pin’s top lands right on the shear line and every driver pin sits wholly in the housing: nothing crosses the gap any more. Common mistake: A key cut wrong at one place, or a different key that nearly matches, lifts that pin too far: here the third cut is 1.2 mm shallower, so the third key pin pokes 1.2 mm up into the housing. Four pins line up, the fifth crosses the shear line, and the plug jams however hard it is turned. Forcing it only bends the key: use the right key, or have a copy cut again from the original.
- 5Turning the plug
Turned clockwise, the key turns the plug with it, and the key pins inside it. The driver pins, wholly in the housing now, slide onto the plug’s round side and stay up in their chambers, still pressed down by their springs.
- 6Retracting the deadbolt
Past a quarter turn, the pin on the cam at the plug’s back drops into the slot in the bolt’s lug, and as the key turns on to a half turn the cam pulls the deadbolt back into its case. Turning the key back the other way throws the bolt out again, and the pins drop back across the shear line when the key comes out.
What is in it
- Housing · the fixed brass body with five pin chambers
- Plug · the part that turns, with the keyway
- 5 key pins · of different lengths
- 5 driver pins · all the same length
- 5 springs · on top of the stacks
- Key · five cuts, each a different depth
- Cam · at the plug’s back, throws the bolt
Questions
How does a pin tumbler lock work?
Stacks of pins cross the gap between the turning plug and the fixed housing, so the plug cannot turn. The right key lifts each stack by exactly the right amount, so the gap between every key pin and its driver pin lines up with the gap between the plug and the housing, the shear line, and the plug turns.
What is the shear line?
The thin gap where the plug meets the housing. The plug can only turn when no pin crosses it.
Why will a wrong key not turn?
Its cuts are not the right depths, so at least one pin stack stops too high or too low and a pin still crosses the shear line, blocking the plug.
What are the springs for?
They push each pin stack down: onto the key when it is in, and across the shear line when it is out, so the lock locks again as soon as the key is taken out.
How does turning the key move the bolt?
A cam at the back of the plug turns with it and, past a quarter turn, catches the bolt and slides it back into its case.
Safety
Taking a lock cylinder apart lets its small springs and pins spring out, and a lock put back together wrong can fail to lock: have a locksmith service or rekey a door lock.
About this guide
The cylinder here is a common 5-pin design of our own, cut away down its middle so the pins show. Its key follows Schlage’s standard spacing and depths: cuts 3.97 mm (0.156 in) apart, each depth step 0.381 mm (0.015 in), cut with a 100-degree cutter. The plug, the pins’ lengths and the key’s other sizes are our own, in proportion.
The torque readings are illustrative, to show when the plug is blocked, turning freely or pulling the bolt; they are not measurements of a particular lock.
Sources: Allegion: Schlage’s combinating rule (cut spacing, depth steps, cutter angle), LockWiki: the shear line.
The 3D models, drawings and text are our own.