What an APG door lock is
APG stands for "all-purpose glass" — the slimline aluminium framing system used to build the all-glass entrance doors found on shopfronts, lobby doors, office reception entrances and the front doors of many modern restaurants. The frame is narrow because most of the door is the glass itself; the lock has to fit in a strip of aluminium often less than 30 mm wide at the bottom rail.
The standard locking arrangement is a hookbolt mortice lock — a steel hook that rotates 90 degrees out of the bottom rail into a slot in a floor-mounted keep plate. Hookbolts hold securely against being pulled or pried (which is why they replaced simpler bolt locks on retail doors years ago), but they are sensitive to alignment: a hookbolt that misses its keep slot by even a millimetre will not engage cleanly.
See our APG door lock replacement case study for a worked example of the symptoms, diagnosis and replacement.
The three reasons APG locks stick
First, the hookbolt wears. Repeated rotation against the keep gradually rounds off the hook. Once the hook profile has worn past its design tolerance, it will not fully engage on every cycle — sometimes locking, sometimes not. Customers describe it as "the lock has become temperamental" and that is exactly right: the mechanism is on the boundary of working.
Second, the lock case contaminates. APG locks are mounted close to the floor and are exposed to mop water, cleaning chemicals, road grit, and (in hospitality settings) sticky residues that work their way into the case through the keyway and the hookbolt slot. The internal mechanism gums up. Locking action becomes stiff, then partial, then refuses.
Third — and most commonly — the floor keep drifts out of alignment. The keep plate is bolted to the floor immediately below the door. The thread inserts in the floor work loose, the plate shifts a millimetre or two, and the hookbolt now arrives at a slightly offset slot. Force locks the door for a while; eventually the force itself rotates the keep further out and the door stops locking entirely.
Why APG faults appear at the worst time
APG locks are operated twice a day — once at opening, once at close. The opening cycle is forgiving (an awkward unlock during business hours is an inconvenience). The closing cycle is unforgiving: it happens after the building has been emptied, often by the last person on site, and a failure to lock at this point means an immediate decision between "leave the building unsecured" and "call an emergency engineer". This is why APG lock callouts are heavily concentrated in our late-evening and weekend-emergency slots.
The corollary: customers should treat the first occasional sticking as an urgent issue rather than a minor inconvenience. A daytime callout with the door still working is cheaper, less disruptive and gives the engineer a less-stressed door to diagnose.
Diagnosis and the universal-fit replacement
On site, the engineer checks three things in sequence. Hookbolt rotation by hand with the door open (rules in or out lock-case contamination and hookbolt wear). Keep alignment with a feeler gauge (rules in or out floor-keep drift). Cylinder rotation independent of the case (rules out cylinder wear, which is rare but possible).
For replacement, we carry universal-fit APG hookbolt cases that retrofit the large majority of installed APG doors regardless of original manufacturer. Direct brand-match replacements (genuine Dorma, Geze, Adams Rite, Briton) are stocked for premium installations where the existing keyway and cylinder need to be preserved.
The replacement work
A standard APG lock replacement runs three to four hours on site. The door is supported open. The lock case is removed from the bottom rail through a long access panel. The keyway and any associated cylinder are transferred to the new case (or replaced if also worn). The floor keep is checked, re-bedded if necessary, and the new hookbolt aligned against it with the door closed. Final test is at least 20 lock-unlock cycles to confirm consistent engagement.
For the full service description see door hardware repair; for cross-referenced case study see APG lock replacement.
Preventing the sticking-then-failing cycle
APG lock health is straightforward to maintain. A six-monthly lubrication of the lock case (graphite only — never oil-based; oil attracts dirt and accelerates contamination) and a yearly keep-alignment check is enough to keep most APG locks within tolerance for their full service life. Door hardware service contracts include this as standard.
For high-security buildings (jewellery retail, pharmacies, premium retail) we additionally fit anti-pick keep guards and reinforced strike plates as part of a one-off security upgrade. See commercial door lock and access control repairs for the boundaries between mechanical lock work and electronic access control.