How does the lock in a uPVC or composite door actually work?

Almost every uPVC, composite and aluminium door in the country has the same kind of lock: a long steel strip down the door edge carrying several bolts, all driven by one gearbox in the middle. Understanding that one arrangement makes the faults readable rather than mysterious, and it is the difference between diagnosing your own door in five minutes and being sold a whole new door.

Step by step

1. Identify the strip on the door edge.Open the door and look at the closing edge. You will see a continuous steel faceplate running most of the height of the door, with bolts of different shapes projecting through it at intervals. That whole assembly is the lock. It is one part, not several.
2. Name the bolts.Curved claws are hook bolts: they swing out and pull the door in towards the frame. Small barrels are roller cams or mushroom cams: they turn off-centre and squeeze the door onto the weather seal. The square-ended bolt near the middle is the deadbolt. Some doors also have a sprung latch so the door holds shut without being locked.
3. Find the gearbox.Behind the faceplate in the middle sits the gearbox, the only part with moving gears in it. Everything else is driven by it. Two things enter it: the square spindle from the handles, and the cam on the tail of the euro cylinder.
4. Understand what the handle does.Lifting the handle rotates the follower in the gearbox, which pushes drive rods up and down the strip and throws every hook and roller at once. All the mechanical work of locking happens here. This is why lifting the handle feels heavy and why it should never be forced.
5. Understand what the key does.Turning the key rotates the cylinder cam, which throws the deadbolt and locks the gearbox so the handle cannot be pushed back down. The key does almost no work. On a door that has been lifted properly, the key should turn with two fingers.
6. Look at the frame side.Opposite each bolt is a keep, sometimes called a striker or a receiver: a shaped steel box with a slot for the hook and a ramped face for the roller to climb. Most keeps are adjustable in and out, which is how compression against the seal is set.
7. Read faults through that sequence.Handle will not lift when the door is shut, but lifts freely when open: the bolts are hitting keeps rather than entering them, so the door has moved. Handle lifts but the key will not turn: cylinder or cam. Handle goes floppy or spins with nothing happening: the gearbox has sheared internally.

Tips

• The whole lock is one orderable part, and so, on most modern locks, is the gearbox on its own. Which of the two you need depends on the make, and some brands do not sell the gearbox separately.
• The handle is not the lock. Handles snap, droop and wear out independently, and swapping a handle will not cure a gearbox fault.
• A door with a latch as well as hooks can be pulled shut without lifting. A door without one has to be held closed while the handle is lifted, which is normal and not a fault.

Common mistakes

• Forcing a handle that will not lift — The gearbox is the weakest link in the chain. A hook meeting a keep edge stops the mechanism dead, and the force goes into the gears. This turns a free hinge adjustment into a lock replacement more often than any other single act.
• Blaming the key or the cylinder for a stiff door — The key locks the gearbox; it does not move the bolts. If the door is stiff before the key goes in, the cylinder is not involved.

Questions people ask