Troubleshooting Window Gearbox Problems: Handle Moves but Lock Won't Engage

Troubleshooting Window Gearbox Problems: Handle Moves but Lock Won't Engage

If the handle on your casement or tilt-and-turn window moves freely but the lock does not engage, the issue is often the window gearbox. This fault is common with espagnolette window locks and multipoint systems: the handle rotates, but the internal gears, follower or locking rods do not operate the keeps. This post explains how to diagnose the problem, when a simple DIY fix will do, and when you should call a trade specialist.

How the window gearbox works

The window gearbox is the mechanical heart of an espagnolette or multipoint locking system. The handle turns a square spindle; the spindle engages the gearbox follower which converts rotation into movement of locking rods, hooks or cams. If any part of that chain fails the handle will turn but nothing will lock. Knowing how the parts interact helps you pinpoint the fault quickly.

Common causes when the handle moves but nothing locks

• Stripped gearbox teeth or follower: internal plastic or metal cogs can wear or break.
• Disconnected or broken spindle: the spindle may be snapped or the grub screw loose; the handle spins without driving the gearbox.
• Detached or broken locking rods / links: the linkage between the gearbox and the top or bottom locking points can break or slip.
• Seized or heavily corroded gearbox: dirt, grit or corrosion can stop the mechanism moving even though the handle rotates.
• Misaligned keeps or frame movement: if keeps are out of alignment the locking action may jam and the gearbox will slip.
• Manufacturing or installation faults: wrong gearbox handedness or incorrect spindle length can prevent engagement.

Tools and supplies you will need

• Screwdrivers: Pozidriv and flat head.
• Allen keys for some handles and fixings.
• Replacement grub screw or handle if stripped.
• Silicone spray or PTFE lubricant; avoid oily lubricants that attract dirt.
• Replacement gearbox or follower if required; know spindle size (commonly 7mm square) and screw positions.
• Rivets and pop rivet tool only if gearbox is riveted; a drill may be required for removal.

Step-by-step diagnosis

• Check the handle: remove the handle cover cap and inspect the grub screw. Tighten the grub screw; if it is rounded, replace the handle assembly. If the handle turns freely on the spindle, the handle is the immediate fault.
• Inspect the spindle: remove the handle and try turning the spindle with a screwdriver. If the spindle rotates but the gearbox does not respond, the problem lies inside the gearbox.
• Operate the gearbox manually: with the sash open, remove any easy-to-access trim to reach the gearbox faceplate screws. Carefully remove the retaining screws and slide the gearbox out enough to see the follower end. Rotate the spindle again; if the follower does not move the gearbox is faulty.
• Check locking rods and connections: with the gearbox out, inspect the rods, clips and sliders for fractures or disconnection. Some systems use rivets to secure rods; if these have sheared they will need replacing.
• Examine keeps and frames: if the gearbox works but the lock still won’t engage fully, close the sash and check the alignment of keeps. A misaligned keep can stop engagement and strain the mechanism.

Practical DIY fixes

• Tighten or replace the grub screw: many cases are solved by securing the handle to the spindle.
• Lubricate moving parts: apply a silicone or PTFE spray to the gearbox, keeps and locking rods; operate the handle several times to work lubricant in. Do not use oil or heavy grease on the window seals.
• Reattach disconnected rods: if clips have worked loose, refit them or replace broken clips. If rivets have sheared a professional repair may be easier.
• Replace the follower: if the follower (the part that connects the spindle to the internal mechanism) is damaged it can sometimes be bought as a separate part and swapped without replacing the whole gearbox.
• Adjust keep position: minor misalignment can be corrected by loosening keep screws, moving the keep slightly and retightening.

When you need to replace the gearbox

Replace the gearbox when internal gears are stripped, the housing is cracked or the follower has failed internally. Many modern gearboxes are modular; you can remove the sash, take out the old gearbox and fit a like-for-like replacement. Important measurements before ordering: spindle size and length, distance between fixing screws, gearbox depth and whether the unit is left or right handed. If the original is riveted you may need to drill the rivets out and secure the replacement with pop rivets or coach screws where appropriate.

Trade tips for installers

• Always carry common spindle sizes (7mm, 8mm) and universal followers; these reduce revisit trips.
• When replacing riveted gearboxes, use stainless pop rivets to prevent future corrosion and to maintain strength.
• Keep a selection of keeps for adjustments; a slightly longer or deeper keep can fix an alignment issue without sash adjustment.
• Document sash handedness and screw centres before removing parts; this avoids ordering incorrect replacements.
• Advise customers about the lifespan: plastic internal gearboxes last less time in coastal locations due to salt corrosion; recommend stainless components where possible.

Safety and cautions

Be careful when removing glazing beads or working near sealed units; improper handling can damage the glass or invalidate warranties. If you need to remove rivets from a sash with bonded glass, consider a professional to avoid damage. Never force a seized gearbox as this can damage the frame or keeps.

Ordering parts and further information

When ordering a replacement window gearbox or follower, provide clear photos of the existing unit, the spindle size, and screw centre dimensions. For technical information on espagnolette window locks and compatible gearboxes see our pillar resource: /guides/espagnolette-window-locks. Our trade team can advise on part compatibility and commonly fitted alternatives.

Recommended Products

Based on this article, you may find these products helpful:

• ERAB Espagnolette Window Lock Universal Lock Replacement
• Avocet Slimline Offset Window Espag
• Multipoint Gearbox GU Old Style
• In-Line Window Mechanism
• Offset Window Mechanism

Conclusion and next steps

If the handle moves but the lock will not engage: first check the handle connection and grub screw, then inspect and lubricate the gearbox and locking rods. Replace the gearbox or follower where parts are stripped or broken; measure accurately before ordering. For riveted units or signs of corrosion, or if you are unsure about removing glazing beads, contact a qualified window fitter. If you need parts or technical advice, get in touch with our trade support team or visit our guides page to find the right espagnolette components.