What Is a Window Gearbox? Understanding Your uPVC Window Lock

What Is a Window Gearbox? Understanding Your uPVC Window Lock

What a window gearbox is

A window gearbox is the internal mechanical component inside a uPVC window handle that converts the handle rotation into linear or rotary movement to operate the lock. It is sometimes referred to as the handle mechanism or follower assembly. On casement and uPVC outward-opening windows the gearbox interfaces with the spindle and the espagnolette or locking mechanism, moving locking rods, shootbolts or keeps to secure the sash.

How the gearbox works: the basics

The gearbox contains gears and a follower that transfer the square spindle movement into the action required by the lock mechanism. Typical behaviour includes:

• 90 degree travel: most uPVC handles rotate 90 degrees between closed and open positions.
• Mechanical advantage: the gearing can increase torque so a short handle can operate multiple locking points.
• Indexing and detents: many gearboxes have a neutral position and raised detents so the handle returns to the correct position for locking and for tilt.

Common spindle sizes in the UK are 7mm square for many euro-style handles, with 8mm used on some older or specialist hardware. The spindle length and the position of the handle relative to the lock face will determine whether replacement parts are compatible.

Where you will find a gearbox in a uPVC window

On a typical uPVC casement or top-hung window the gearbox sits behind the handle rose or plate. It is often part of a modular handle unit, which can be replaced without removing the whole locking mechanism. For tilt-and-turn windows the gearbox shape and spindle positioning differ to suit the handle and the multi-point tilt mechanism.

Common signs of gearbox failure

Knowing the symptoms helps trade professionals and DIYers diagnose problems quickly. Look for:

• Handle spins or rotates without resistance: likely stripped gears or a broken follower.
• Handle feels loose on the rose: grub screw or gearbox retention failure.
• Window will not lock or latch: the gearbox may not be engaging the espagnolette.
• Grinding, clicking or stiff operation: damaged teeth or debris in the mechanism.
• Handle returns to an odd angle: missing detent or worn spring in the gearbox.

Practical DIY diagnosis and maintenance

Before ordering parts, carry out a quick diagnosis and some routine maintenance. These simple checks often restore function and save a call-out:

• Check the handle grub screw: access the screw on the underside of the lever and tighten gently; do not overtighten as this can strip the thread.
• Remove and inspect the handle: most handles detach after loosening the grub screw; check the gearbox and spindle for visible damage or wear.
• Clean and lubricate: remove debris and apply a light lithium or silicone-based grease to moving parts; avoid heavy oils that attract grit and become gummy.
• Check locking keeps and keeps adjustment: misaligned keeps can overload the gearbox; adjust striker plates using the screws provided on the keep.

Replacing a window gearbox or handle: step-by-step

If the gearbox is damaged or worn it is often quicker and more economical to replace the handle/gearbox unit. Here is a typical procedure for a competent DIYer or trade operative:

• Confirm the correct replacement: measure spindle size (use callipers), spindle length and screw centres. Note whether the handle is left or right handed, and whether the handle is tilt-and-turn compatible.
• Ensure the window is in the closed position: this makes alignment easier and reduces strain on the lock.
• Loosen the grub screw and remove the handle: some roses have a cap that hides screws; carefully prise caps off using a flat screwdriver.
• Remove rose or backplate screws and withdraw the gearbox assembly: keep screws and cover parts in a tray so they do not go missing.
• Fit the replacement gearbox or handle: slide the new unit onto the spindle, align screw holes and tighten evenly; avoid overtightening as this can deform the rose or strip plastic parts.
• Test operation: operate the handle through its full travel, check locks engage fully, and check that the handle returns to the correct rest position.

Trade tips for installers and specifiers

For window installers and repair specialists, efficiency and first-time-fix rates matter. Consider these practical pointers:

• Stock commonly used sizes: 7mm square spindles in several lengths, plus a selection of roses and replacement handle sets to cover quick swaps.
• Carry a small tool kit: slim-line screwdrivers, metric callipers and sample spindles for on-site measurement reduce guesswork.
• Check compatibility with espagnolette systems: some gearboxes require a specific follower or adapter to engage with multi-point locking rods; confirm with manufacturer datasheets.
• Offer upgrades where appropriate: premium gearboxes and handles with anti-snap or reinforced followers improve longevity for high-use or high-security installations.

Safety and compatibility considerations

Not all gearboxes and handles are interchangeable. Match the gearbox to the window type: casement, flush casement, or tilt-and-turn. When replacing parts on escape routes or fire-safety critical windows ensure the replacement maintains means of escape. If the window is part of a fire escape or is a first-floor emergency exit, consider advising the homeowner to have work carried out by a qualified installer.

Further information and next steps

For more detail on espagnolette window locks and the wider locking system consult our pillar page on espagnolette window locks: /guides/espagnolette-window-locks. If you are unsure about measurements or compatibility, take clear photographs of the handle, gearbox and the lock face and supply them to your supplier; this will speed identification.

Conclusion: A window gearbox is a small component but essential to secure, smooth operation of uPVC windows. For minor issues try tightening, cleaning and lubricating first. If the handle still spins freely or the gearbox shows damage, replace the handle/gearbox assembly with the correct size and type. For trade professionals, stocking common sizes and carrying measurement tools will increase first-time fixes. If you need help identifying the correct replacement part contact a reputable supplier or a qualified window technician.

Recommended Products

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

• TSS Cockspur Window Handles
• TSS Offset UPVC Window Handles
• ERA Universal Inline Window Handle
• In-Line Window Handle - White
• ERAB Espagnolette Window Lock Universal Lock Replacement