Complete Guide to Window Mechanisms
Modern window mechanisms are central to the security, weather performance and ease of use of UK homes. Whether you are a trade installer, maintenance contractor or a competent DIY customer, understanding how a window gearbox, espagnolette, multipoint lock and other window lock mechanisms work will save time, reduce call-backs and help you choose the right replacement parts first time.
This guide from Window Hardware Store (windowhardwarestore.co.uk) explains the main types of window mechanisms used in PVCu, aluminium and timber windows in the UK. It covers how they operate, common faults, how to diagnose issues, and the correct process for measuring and replacing components using products from our window mechanisms category.
Basic principles of window locking
Although designs vary between manufacturers and window styles, most modern UK window locks operate on the same basic principles:
The handle turns a square steel spindle (typically 7 mm) that passes through the window gearbox.
Inside the gearbox, this rotation is converted into linear movement.
This movement drives locking points (mushroom cams, rollers, bolts or shootbolts) along an espagnolette strip or multipoint rail.
The locking points engage into keeps or striker plates fixed to the window frame.
When locked, the handle is usually secured with a key, preventing the spindle – and therefore the locking points – from moving.
The combination of a central gearbox and multiple locking points creates a strong, secure seal around the sash, helping to resist forced entry and improve weather tightness. Many modern windows are designed to comply with enhanced security standards such as PAS 24 when used with suitable hardware.
Key components in a typical modern window lock mechanism
A standard outward-opening PVCu casement window with an espagnolette or multipoint locking system will usually include:
Handle – sometimes called an espag handle or locking handle; commonly key locking with 43 mm screw centres and 7 mm spindle.
Gearbox – the central locking unit operated by the spindle; often replaceable without changing the full mechanism.
Espagnolette rail / multipoint strip – metal strip running along the edge of the sash with locking points attached.
Locking points – mushroom cams, rollers or shootbolts that engage into frame keeps when locked.
Keeps / striker plates – fixed to the frame; designed to suit the specific type and position of locking points.
Hinges – friction stays or butt hinges that must be set correctly to align the sash with the frame and keeps.
Common UK brands supplying these components include Yale, ERA, Mila, Avocet, Maco, Roto, GU, Sigenia and others. While the basic principle is similar, small dimensional differences between brands make correct identification and measurement essential when ordering spares from our replacement window gearboxes and related hardware ranges.
Security and regulatory context in the UK
Window locking mechanisms do more than simply keep the sash closed. In UK domestic properties, they contribute to:
Security against intrusion
Child safety (restricted opening where required)
Means of escape in case of fire
Compliance with Building Regulations
Key standards and guidance relevant to window locks include:
Approved Document Q (Security) – for new dwellings in England, requires “reasonable resistance to unauthorised access”. Windows near ground level typically need secure, key-lockable multipoint systems.
PAS 24: Enhanced security performance – many new-build windows are tested to PAS 24 using specified hardware combinations, often including key-locking handles, multipoint espagnolette systems and specific keeps.
BS 8213-1/2 – provides guidance on design, installation and maintenance of windows, including safe operation and means of escape.
When replacing window lock mechanisms in existing properties, you are not usually obliged to upgrade to PAS 24 hardware. However, it is good practice to use quality branded components and maintain at least the same level of security as originally supplied – often easily achieved with our range of multipoint window mechanisms and secure locking handles.
Casement vs tilt-and-turn vs other window types
The details of how locks work vary by window type:
Standard casement windows (outward opening) – normally use espagnolette or multipoint locks operated by an espag handle on the inside. The sash secures to the frame along one or more edges.
Tilt-and-turn windows – use more complex multipoint gear operated by a single lever handle; different handle positions (closed, tilt, turn) move the mechanism through multiple functions.
Reversible windows – often use specialised locking and hinging systems that allow the sash to rotate 180° for cleaning while remaining supported.
Older aluminium or timber windows – may use cockspur handles with simple wedge-type keeps, or separate locking devices rather than a continuous multipoint system.
This guide focuses mainly on modern PVCu and aluminium casement-style mechanisms, particularly espagnolette and shootbolt systems, as these are the most common in UK residential properties and the most frequent reason for customers visiting our window mechanism pages.
What is a window gearbox?
The window gearbox is the central component that converts the rotary movement of the handle into the linear motion required to operate locking points along the espagnolette or multipoint strip. In many designs, the gearbox is the part most likely to fail over time due to wear, lack of lubrication or excessive force on a stiff mechanism.
A typical gearbox will include:
Square spindle socket (usually 7 mm) to receive the handle spindle
Internal gears or cams that translate rotation into movement
Drive points where espag rods, shootbolts or connecting bars attach
Fixing holes (often two or three) to secure the gearbox to the sash
A locking facility (in some designs) that engages when the handle is key-locked
Because most brands design their own specific gearboxes, replacements must usually be like-for-like. The good news is that for many systems, particularly from brands like ERA, Mila, Avocet, Yale and Maco, standalone replacement gearboxes are readily available from our window gearbox range, avoiding the need to change the complete espagnolette strip.
Main types of gearboxes used in UK windows
In UK market practice, several broad gearbox types are encountered:
1. Standard espagnolette gearboxes
These are found on the majority of modern PVCu casement windows. Key characteristics include:
Central body typically 30 mm to 35 mm wide
Backset usually 20 mm (distance from faceplate edge to spindle centre), sometimes 22 mm
Front faceplate width 16 mm or 20 mm, occasionally 24 mm for timber or aluminium
Connection points for espagnolette rods above and below the gearbox
Brands such as ERA, Yale, Mila, Avocet and GU all supply standard espagnolette gearboxes compatible with their own locking strips. When measuring a standard espag gearbox, the most important dimensions are:
Backset – typically 20 mm or 22 mm on PVCu; measured from the front of the faceplate to the centre of the spindle hole.
Case width – the overall width of the gearbox body (commonly 30–35 mm).
Faceplate width – usually 16 mm or 20 mm.
Spindle hole position – both vertically (relative to fixing holes) and horizontally (backset).
If you are unsure, take clear photos and note measurements in millimetres before ordering from the replacement gearbox category.
2. Shootbolt gearboxes
Shootbolt gearboxes are similar to standard espag gearboxes but are designed to drive top and bottom shootbolt extensions. You will find these on higher-security windows and often on larger sashes.
Typical characteristics:
Provision for attaching telescopic or fixed-length shootbolt extensions
Sometimes a different drive action (e.g. 90° vs 180° handle rotation)
May have multiple fixing points to reinforce the centre of the sash
Brands such as Yale, ERA, Maco and Roto all provide shootbolt systems that use specific gearboxes. These may not be interchangeable with standard espag gearboxes, even within the same brand, so exact identification is essential.
3. Tilt-and-turn gearboxes
Tilt-and-turn windows use specialised gearboxes that:
Operate multiple locking points on both the vertical and horizontal edges of the sash
Switch between “tilt” and “turn” modes depending on handle position
Often incorporate safety features such as mishandling devices
Because tilt-and-turn gear is more complex, replacing only the gearbox is less common. Usually you will replace a complete gearing set. However, understanding the gearbox function helps in diagnosing issues such as the sash being stuck in the wrong position or the handle not turning fully.
4. Older cockspur and offset gear arrangements
Older aluminium and timber windows may not have a modern espagnolette gearbox at all. Instead:
A cockspur handle clamps over a wedge or spur on the frame
Some aluminium systems use offset gearboxes with external rods
Many rely on simple “tab” or “peg” locks rather than true multipoint locks
Upgrading these older arrangements to a modern espagnolette or shootbolt system can greatly improve security and ease of use, but typically involves some joinery or fabrication work. Trade installers often specify retrofit espag kits and our range of universal espagnolette sets to bring older windows up to a better standard.
Measuring a gearbox accurately
Correct measurement is critical when ordering a replacement window gearbox. Use a tape measure in millimetres and, if possible, remove the mechanism from the window before measuring. Key dimensions include:
Backset – measure from the front edge of the faceplate to the centre of the spindle hole. Typical values: 20 mm, 22 mm, 25 mm.
Case width – width of the main gearbox body, excluding faceplate; usually 30–35 mm.
Faceplate width – 16 mm or 20 mm for most PVCu, wider for timber/aluminium.
Fixing screw positions – distance between fixing screws both vertically and horizontally; match to the existing gearbox and sash preparation.
Spindle size – almost always 7 mm square on UK espag handles, but confirm if you are dealing with unusual or imported systems.
Trade tip: When in doubt between two sizes, take clear photos including a ruler against the gearbox and keep the existing part until the new one arrives. This reduces the risk of misordering and unnecessary downtime on site.
What is a shootbolt system?
A shootbolt window mechanism is a type of multipoint lock that uses steel bolts which extend into keeps at the head and sill of the window sash, in addition to standard side locking points. The term “shootbolt” refers to the action of the bolts shooting out from the sash into the frame when operated by the handle.
Shootbolt systems are widely used in:
Higher-security PVCu windows
Larger casement sashes that need extra strength and anti-warping support
New-build projects seeking PAS 24 compliance
Windows marketed under “Secured by Design” schemes (when combined with appropriate keeps and glazing)
Components of a shootbolt mechanism
A typical casement shootbolt system consists of:
Central gearbox – as described earlier, but configured to drive shootbolts.
Shootbolt extensions – telescopic or fixed-length bars running towards the corners of the sash.
Shootbolt heads – hardened steel bolts that project into corresponding keeps at the top and bottom of the frame.
Side locking points – mushrooms or rollers on the espag strip that engage into side keeps.
Keeps – specifically shaped receiver plates fixed to the frame to accept the bolts and cams.
Systems from brands like Yale, ERA, Mila, Avocet, Maco and Roto may use different shootbolt head profiles and keep designs, so it is important to match like-for-like when replacing components via our shootbolt window mechanisms selection.
Benefits of shootbolt mechanisms
Shootbolt systems provide several performance advantages over basic espagnolette-only locks:
Enhanced security – multiple engagement points around the entire sash perimeter make it harder for an intruder to jemmy the sash away from the frame.
Improved sash stability – top and bottom engagement helps prevent bowing or twisting in larger sashes.
Better compression – allows fine adjustment of gasket compression at multiple points, improving weather and acoustic performance.
Compliance with security standards – often required for PAS 24 window specifications.
For trade installers, specifying a recognised shootbolt system from a major brand via our window lock mechanism range is often the simplest way to meet both customer security expectations and regulatory requirements on new installations.
Common shootbolt configurations
Shootbolt window mechanisms come in various formats:
Telescopic shootbolts – adjustable length to suit different sash sizes; often used with a standard central gearbox and cut-to-length espag strip.
Fixed-length shootbolts – supplied in specific sizes; typically used in factory-fitted systems by particular window manufacturers.
Top-hung only – shootbolts at the bottom of the sash only, used on top-hung windows.
Side-hung – shootbolts at head and sill, plus side locking points, used on side-hung sashes.
When replacing a failed shootbolt mechanism, measure:
Overall sash height – to select the correct telescopic range or fixed length.
Backset and faceplate width of the central gearbox.
Position and type of shootbolt keeps in the frame.
Our technical team at Window Hardware Store can assist with cross-referencing older or discontinued systems to modern equivalents within our multipoint lock and gearbox categories where a direct replacement is no longer available.
Installation and adjustment considerations
To achieve optimum performance from a shootbolt system:
Ensure hinges are correctly set so the sash sits square in the frame before adjusting keeps.
Adjust keeps so that the sash compresses the gasket firmly, but without excessive force on the handle – aim for a smooth, firm action.
Lubricate moving parts with a light oil or suitable spray lubricant, particularly around the gearbox and bolt heads.
Avoid overtightening frame keeps, which can distort PVCu frames and cause binding of the mechanism.
Trade tip: On large or heavy sashes, consider upgrading standard hinges to heavy-duty friction stays when fitting shootbolt gear, to reduce long-term sagging and ensure the locking points remain correctly aligned.
Overview of common window lock problems
Most issues with modern window lock mechanisms fall into a few familiar categories. Correct diagnosis saves time and avoids unnecessary replacement of whole systems when a single component is at fault. Typical complaints from customers include:
“The handle turns but the window will not open.”
“The handle will not move at all – it feels jammed.”
“The key will not turn, or the handle will not lock.”
“The window closes but is draughty or rattles in the wind.”
“I have to slam the window to get it to close.”
These symptoms can usually be traced to issues with the gearbox, espagnolette strip, shootbolts, hinges, keeps or frame alignment.
Because the gearbox handles the main operating forces, it is a common point of failure. Indicators of a faulty gearbox include:
Handle rotates freely through 360° with little resistance, but locking points do not move.
Grinding, clicking or crunching noises from the central area of the sash.
Handle moves part-way then stops abruptly, with no obvious obstruction along the espag strip.
Window stuck shut: handle seems to move but locking points remain engaged in keeps.
In many cases, internal components have broken or become displaced inside the gearbox. Once this happens, the unit cannot usually be repaired reliably and should be replaced with an equivalent from our window gearbox range.
Trade tip: If the handle spindle has rounded off inside the gearbox, always replace both the handle and gearbox. A new gearbox used with a badly worn handle spindle may fail prematurely.
Espagnolette and shootbolt strip problems
Faults in the espag strip or shootbolt extensions are less common but do occur, particularly on older or heavily used windows. Common signs:
One side of the sash appears to lock while the other side does not.
Visible bending or warping of the espag strip.
Loose or detached locking cams or rollers along the strip.
Shootbolts failing to fully retract into the sash, catching on keeps.
These issues typically require replacement of the full espagnolette strip or shootbolt bar assembly. Where possible, fit a branded replacement from the same manufacturer to ensure correct cam heights and keep engagement. We stock a wide range of espagnolette and shootbolt sets in our window lock mechanism category.
Handle and key issues
Not all locking problems originate in the mechanism itself. Handles can also cause faults:
Worn or rounded spindle – handle turns but does not reliably operate the gearbox.
Broken internal spring – handle does not return to its resting position, leading to drooping handles.
Key cylinder wear – key difficult to turn, or key will not come out in locked position.
Incorrect handle type – espag handle fitted where a cockspur handle is needed, or the wrong spindle length causing binding.
These can usually be resolved by fitting new handles from our dedicated handle ranges. Measure:
Screw fixing centres – typically 43 mm on modern espag handles.
Spindle length – measure from the back of the handle to the tip of the spindle (e.g. 10 mm, 15 mm, 30 mm, etc.).
Handing – some handles are handed (left/right), others are inline (universal).
For security, we recommend replacing non-locking handles with key-lockable versions, particularly on ground-floor or easily accessible windows, in line with Approved Document Q’s emphasis on secure, key-operated hardware.
Many reported “lock faults” are actually alignment problems caused by:
Sash dropping on the hinge side due to wear or incorrect friction stay specification.
Frame or sash expansion/contraction due to temperature changes (common with dark-coloured PVCu).
Improperly set or loose frame keeps.
Building settlement affecting frame squareness.
Symptoms include:
Difficulty engaging the handle into the fully locked position.
Visible rubbing or catching between sash and frame.
Uneven gaps around the sash perimeter.
Draughts and rattling even when locked.
In these cases, first check:
That hinges are secure and not excessively worn.
That the sash is square and level when closed.
That keeps are correctly aligned with cams and shootbolts, adjusting where possible.
Trade tip: On older PVCu windows, slight packing or re-fixing of hinges can often restore alignment without the need for full mechanism replacement, especially when combined with gentle adjustment of the keeps.
Diagnosing a stuck-shut window
A particularly challenging situation arises when a window is stuck shut and cannot be opened in the normal way due to a failed gearbox or jammed locking points. In such cases:
Avoid excessive force on the handle, which may break the spindle or handle.
From inside, carefully locate the locking points around the sash by eye or by feeling along the edge.
Use a stiff plastic card or thin tool between sash and frame to try to manually retract individual cams or bolts while gentle pressure is applied to open the sash.
If necessary, remove internal glazing beads and carefully deglaze the sash to create a little movement, taking great care not to crack the glass.
Once access is gained and the sash can be opened, the faulty gearbox or mechanism can be removed and replaced. For difficult cases, many trade professionals carry thin-blade release tools designed for espagnolette systems.
General preparation and safety
Whether you are a professional installer or a capable DIYer, replacing a window gearbox or espagnolette mechanism follows broadly the same process. Before you start:
Confirm the window type – outward-opening casement, tilt-and-turn, reversible, etc.
Identify the mechanism brand if possible – look for stamped logos or numbers on the faceplate.
Measure key dimensions in millimetres – particularly backset, faceplate width and sash size.
Order suitable replacement parts from our window mechanisms and gearboxes ranges.
Safety considerations:
Use appropriate PPE (gloves, safety glasses) when handling glass and metal components.
If working above ground level, ensure safe access and avoid leaning excessively out of windows.
On tilt-and-turn or large sashes, support the sash properly when removing hardware to prevent it dropping or twisting.
For most PVCu casement window mechanism replacements, you will need:
Pozi (PZ2/PZ3) screwdriver or bit
Flat-blade screwdriver (for bead removal in some systems)
Tape measure (metric, in mm)
Marker pen or pencil
Lubricant (light oil or silicone-based spray as appropriate)
Replacement handles and screws if required
Trade installers may also use:
Cordless drill/driver with clutch
Thin-blade release tools for stuck locks
Glazing paddles and suction cups for deglazing if absolutely necessary
Step-by-step: replacing a standard espagnolette gearbox
This procedure covers a typical PVCu outward-opening casement window with espagnolette locking and a central gearbox.
1. Open and secure the sash
Unlock the window and open it to a comfortable angle.
If necessary, use a wedge or temporary support to prevent the sash from moving while you work.
2. Remove the handle
Open the handle cover cap (if present) to reveal the two fixing screws.
Unscrew both screws and pull the handle away from the sash, withdrawing the spindle from the gearbox.
Set the handle aside; inspect the spindle for wear.
3. Access the gearbox and espag strip
Locate the fixing screws along the espagnolette faceplate on the sash edge.
Remove the screws in and around the gearbox area first.
In some windows, additional screws may be hidden under gasket lips; gently pull the gasket back if necessary.
4. Remove the mechanism
With all relevant screws removed, carefully pull the espagnolette strip and gearbox assembly away from the sash.
You may need to slide the strip slightly to disengage it from the corners or from shootbolt extensions.
Take care not to bend or kink the espag strip excessively if you intend to reuse it.
5. Compare old and new gearboxes
Lay the old gearbox next to the new one from our gearbox range.
Confirm that backset, faceplate width, case size, spindle position and fixing hole locations all match.
Check that the drive connections to the espag strips or shootbolts are compatible.
6. Transfer espag strips/shootbolts
If your replacement is gearbox-only, detach the old gearbox from the espagnolette strip (usually by sliding or unclipping).
Attach the new gearbox in the same orientation, ensuring full engagement of drive lugs or hooks.
Confirm that the strip moves smoothly when the gearbox spindle is turned by hand.
7. Refit mechanism to sash
Offer the complete assembly back into the sash edge, aligning it with the original position.
Refit the fixing screws through the faceplate into the sash reinforcement.
Do not overtighten; screws should be snug but not deform the PVCu profile.
8. Reinstall the handle
Insert the handle spindle into the gearbox spindle hole.
Align handle fixing holes with those in the sash and refit the screws.
Check that the handle moves through its full range and that locking points operate correctly.
9. Test operation and adjust keeps
Close the sash and operate the handle to engage the locking points.
If the handle is stiff or will not fully lock, identify any cams or bolts that are binding on keeps.
Adjust or realign keeps as needed, or slightly adjust hinge positions to improve alignment.
Lubricate the gearbox and all moving parts lightly as a final step.
Step-by-step: replacing a complete espagnolette strip or shootbolt mechanism
In some cases, the entire espagnolette or shootbolt assembly must be replaced rather than just the gearbox. The initial steps (removing handle and mechanism) are the same, but additional considerations apply:
Measure overall length of the existing espag strip from end to end (in mm).
Note position and type of locking points relative to the gearbox centre line.
Measure sash height to choose appropriately sized telescopic shootbolts if upgrading.
When fitting the new strip:
Position the gearbox centre at the same height as the original to align with the handle location.
Mark and drill new fixing holes if the positions differ slightly, ensuring you drill into reinforcement where present.
Fit new frame keeps if the cam or shootbolt design differs from the original; old keeps may not be compatible.
Trade tip: When upgrading from a simple espagnolette to a shootbolt system, always replace the frame keeps as a set and check hinge capacity. Use branded kits designed to work together, available from our shootbolt mechanism and keep/striker categories.
Special cases: tilt-and-turn and aluminium systems
Tilt-and-turn and many aluminium systems use more complex or proprietary gearing:
Tilt-and-turn: Replacements often require ordering a matching set of components (gearbox, corner drives, scissors, keeps, etc.). Precise brand and system identification is crucial.
Aluminium: Many commercial aluminium systems are specific to the profile manufacturer; hardware may be branded by specialist suppliers and must be matched exactly.
In these cases:
Look for any reference numbers on the hardware or frame.
Measure critical dimensions (sash height/width, backsets, faceplate widths).
Photograph the hardware in situ and removed, including close-ups of corners and keeps.
Consult the technical data on our website or contact our support team with details to source the correct parts.
DIY vs professional replacement – what’s realistic?
Many competent DIYers can successfully replace a straightforward espagnolette gearbox or handle using parts from Window Hardware Store, provided they:
Are confident working with basic hand tools.
Can accurately measure and identify parts.
Take care not to damage glazing, gaskets or the frame.
However, consider engaging a professional installer or service engineer in cases such as:
Stuck-shut windows where deglazing or more advanced release techniques may be needed.
Tilt-and-turn or commercial aluminium systems with complex gearing.
Upper-storey windows where safe access is an issue.
Situations where the window also serves as a designated fire escape route; correct operation is critical.
Trade professionals using our trade window hardware ranges can streamline their on-site work by carrying a small stock of the most common gearbox backsets (20 mm and 22 mm) and a selection of telescopic shootbolt kits, allowing same-day repair in most domestic situations.
Post-installation checks and maintenance
After any mechanism replacement, complete a thorough check:
Operate the window several times to confirm smooth locking and unlocking.
Check gasket compression – there should be firm contact all around without excessive force needed at the handle.
Ensure any child-restriction or safety features operate correctly if present.
Confirm key locking works and keys are provided to the occupier.
Regular maintenance will extend the life of your new gearbox and mechanisms:
Lubricate moving parts once or twice a year with a light machine oil or specialist window hardware lubricant.
Avoid heavy oil or grease on friction stays, which can attract dirt; use a light oil instead.
Clean dirt and debris from around keeps and sash edges.
Check hinge screws for tightness and adjust as necessary.
In coastal or industrial environments, more frequent maintenance is advisable due to increased corrosion risk. Consider specifying stainless steel or enhanced-corrosion-resistant hardware from our premium window mechanism ranges for such installations.
Measuring checklist before ordering
To minimise the chance of ordering the wrong component, note the following measurements in millimetres before visiting windowhardwarestore.co.uk:
Window material: PVCu, aluminium, timber.
Window type: casement (side-hung/top-hung), tilt-and-turn, reversible.
Gearbox backset (e.g. 20 mm, 22 mm).
Faceplate width (e.g. 16 mm, 20 mm, 24 mm).
Overall espagnolette strip length (if replacing full strip).
Handle screw centres (usually 43 mm).
Handle spindle length (from handle backplate to spindle tip).
Sash height and width (to select correct shootbolt range if applicable).
Armed with this information, you can confidently select suitable replacements from our gearbox and window lock mechanism categories, whether you are maintaining a single domestic window or planning stock for regular trade repairs across multiple properties.
