Guide
What Is Inside a uPVC Door Lock Mechanism, and What Goes Wrong
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Naming the Parts of a uPVC Door Lock
A uPVC door lock is really three parts stacked together, and most confusion about it comes from treating them as one. There is the strip that runs up the door edge and carries the locking points; there is the gearbox at its centre, which moves those points; and there is the cylinder, the part the key fits. Because all three get called the lock, one firm may quote for a cylinder while another quotes for the whole strip on the very same door.
Locking Points and the Strip in the Door Edge
What you see along the edge is the faceplate, the cover of the multipoint mechanism. Behind it, linked bars carry the locking points up and down the door. A hook is a curved arm that swings out and grips. A roller is a round, adjustable stud that governs compression against the seals. A mushroom is a stud with a flared head that resists levering. A pinbolt and a deadbolt both shoot straight out, the deadbolt usually being the heavier of the two. The keeps are their partners, fixed into the frame to receive each point.
The Cylinder, the Gearbox and the Handle
Movement starts at the handle, which turns the spindle, a square steel bar. The spindle fits into the follower, the square hole in the gearbox, and the gearbox drives the whole strip. The euro cylinder sits through the mechanism but belongs to none of it; its cam turns the gearbox when you turn the key. More context sits in the full range of UK door lock types.
The Four Kinds of Mechanism in uPVC
How does the trade classify a uPVC lock? Into four groups: full lock, gearbox centre case on its own, overnight lock and auto slam shut. Those are the mechanism types listed by UPVC Spares 4 Repairs, and they reflect how a supplier arranges its range, not a formal rule. Most homeowners have never heard two of the names, yet one of them almost certainly describes their own front door. Knowing which one you have tells you both what can be ordered and how your door locks.
Gearbox Centre Case and Full Lock
A full lock is the strip in its entirety. A centre case is the gearbox unit taken out of the middle, sold alone. Where a mechanism has no removable centre case, a gearbox fault means a whole strip.
Overnight Lock and Auto Slam Shut
Shut an overnight lock and it is merely latched; lifting the handle throws the points and the key locks them. An auto slam shut throws its points as it closes, so it cannot be left on the latch.
| Type | Behaviour | Repair implication |
|---|---|---|
| Full lock | Depends on the strip's design | The strip is replaced as one piece, matched to the door |
| Gearbox centre case | The door behaves exactly as it did | Driving unit only, where the mechanism permits |
| Overnight lock | On the latch until locked by handle and key | Replaced with another overnight lock |
| Auto slam shut | Locks itself on closing | Replaced with another auto slam shut |
None of the four is the right or wrong answer; each is simply what is in the door.
Usually, What Fails and in What Order
Most of what goes wrong with a uPVC mechanism is wear or alignment, and the one component that genuinely fails, and fails most often, is the gearbox. It sits where all the force goes, taking the strain of each lift, so it is the natural weak point. Failures seldom arrive without warning. A door that has grown stiffer over the past few weeks is usually giving notice that the gearbox is near the end.
| Part in the door edge | What it feels like |
|---|---|
| Gearbox | A loose handle, or one that catches midway |
| Follower and spindle | Movement in the handle, an imprecise feel before failure |
| Rollers and keeps | Shoving to lock, or only a few points engaging; alignment, not damage |
| Euro cylinder | A key issue rather than a handle issue |
| Faceplate screws | Rare; a loose faceplate makes the whole door feel worse |
For the diagnosis itself, follow the guide that matches: the door will not lock, the door that will not open at all, or a key that will not turn at all.
A Mechanism, Identified and Matched
A uPVC mechanism is identified, and a replacement matched, on four measurements together with the number and type of locking points, and an error in any one of those means the wrong part arrives. The single most useful marker is the faceplate stamp. Manufacturers usually press their name into the faceplate, and knowing it narrows the rest of the search; it is a way of recognising the part, not a recommendation.
The Measurements, All Four
The four specifications and their ranges come from UPVC Spares 4 Repairs; the Home Secure changeover guide supports the backset and PZ definitions.
| Measurement | Definition | Stocked range |
|---|---|---|
| Backset | From the front of the faceplate to the euro cylinder cut out centre | Several sizes stocked |
| PZ | Between the cylinder centre and the spindle centre | From 48mm to 117mm |
| Faceplate width | The faceplate measured across | From 16mm to 45mm |
| Screw centres | The gap between the fixing screws | 215mm and 240mm most often |
Matching also depends on overall length, taken along the full strip.
Counting Up the Locking Points
Count with the handle lifted and the door open. Listings from the same supplier show hooks from none to five, rollers none, two or four, mushrooms none or two, pinbolts none or two, and deadbolts none to three. No combination is better than another; it is simply what the door has.
Write it all down: stamp, measurements, length and points, plus one photo of the edge with everything out.
Gearbox Alone or the Whole Strip?
Gearbox alone or the whole strip is not a judgement call by whoever turns up; the mechanism makes that decision. What decides it comes down to three things.
- A mechanism with a removable centre case can take a new gearbox while every hook and the faceplate remain fitted.
- A mechanism without that feature needs a full strip when its gearbox fails, matched on all four measurements and every locking point, which are the specifications UPVC Spares 4 Repairs uses.
- Home Secure's changeover guide points out that full multipoint locks are too long to ship, so they have to be matched and bought in person, while a gearbox can be sent by post.
The consequence for you is easy to state. A gearbox can often be sourced without much delay. A full strip usually involves someone at your door, the old strip in hand, matching it piece by piece. The longer route is a property of the part.
What Keeps a Mechanism Working
The single most useful habit is to lift the handle fully every time you lock the door, because a gearbox driven only half way takes the load without completing its travel. After that, four things help:
Lubricate the door edge, not the keyhole.
The moving parts are the hooks, rollers and bolts along the strip, so that is where a product made for locks belongs.
Keep the keeps and the threshold clear.
Grit or debris in either makes every locking point work harder.
Deal with stiffness as soon as it starts.
Stiffness is the notice period before most mechanism failures.
Hang nothing heavy on the handle.
Bags and coats add strain to the spindle and gearbox every day.
Be realistic about what this achieves: maintenance postpones a gearbox failure but does not prevent one, because the part wears by design. Doors also move a little with the seasons, and the figures for that live in the guide to a uPVC door that will not lock.
Have Your Mechanism Identified
Since identification is the whole job, that is what we offer first: ring Eds Locksmiths Runcorn, tell us the faceplate stamp and what the door edge shows with the handle lifted, and in most cases we can name the mechanism and say whether its gearbox is removable on its own.
Three things make the call quicker. Read out the stamp. Give the four measurements if you have them. Say what the handle is doing when you lift it.
No call-out charge applies at any time of day. Emergency lines are staffed through nights, weekends and bank holidays as well as normal hours. You get a price on the call based on what you describe, and if something on site would change it, you are told before any work begins.
Describe It and Get It Identified: 01928 611001
Prefer to make contact another way? Call or message Eds Locksmiths Runcorn.
uPVC Mechanism Questions
My handle has gone floppy and the door will not lock. Is that the gearbox?
The symptoms often suggest trouble with the gearbox, so that is a reasonable suspicion. They cannot confirm the answer on their own. Before arranging a particular replacement, seek an assessment from a qualified locksmith. The aim is to establish what needs attention, using the symptoms as a clue.
Can I just replace the gearbox?
Only a mechanism with a detachable centre case allows that approach. Start by confirming this detail rather than assuming the gearbox can be changed separately. Once the mechanism has been identified, a qualified locksmith can establish whether it meets that condition and advise on the replacement.
Is the cylinder part of the mechanism?
No. The cylinder is a separate component from the mechanism. Keep that distinction clear when describing a fault or discussing replacement parts. If advice mentions one of them, ask which component is meant before proceeding, so the proposed work relates to the part actually under discussion.
How do I find out which mechanism I have?
Start with the manufacturer's name stamped into the faceplate, then record all four measurements. Count the locking points while the handle is lifted. Keep those details together when asking for identification. The useful description includes the maker, all four measurements and the locking-point count.
Why can my door not be left on the latch?
This probably comes from a mechanism with an automatic slam-shut action. Closing the door causes that type to throw its locking points, which would explain the latch behaviour described. Treat this as a likely identification until a qualified locksmith has checked which arrangement the door has.
Why does a full strip take longer to source than a gearbox?
Full strips are long enough to create a shipping issue, as Home Secure explains. Its account is that they are too long to ship and are instead matched and bought in person. That purchasing process is why obtaining the whole strip can take longer than sourcing a gearbox.
How long should a uPVC mechanism last?
No universal replacement age applies here because the lifespan is not fixed. For the mechanism on a particular door, the useful next step is an individual assessment. A qualified locksmith can help with that decision without treating the unit's age as an automatic instruction to replace it.