Gas Struts & Flap Fittings — Choosing the Right Force
A gas strut lifts cabinet flaps in a controlled manner and holds them open, with the required force in Newtons (N) matching the weight and height of the flap precisely. Anyone working in cabinet making or repairing overhead wall units with upward-opening fronts must select the correct hardware with precision. Incorrect sizing leads either to flaps that will not stay open, or to doors that require immense effort to close and eventually tear the hinges out of the cabinet carcase.
This guide explains the technical principles of gas struts and flap fittings, demonstrates how to calculate the correct force, and provides advice on professional installation.
Types of Flap Fittings and Gas Struts
The mechanics used to move a cabinet front horizontally upwards or downwards differ fundamentally from conventional side-hung doors. Depending on the direction of movement, front weight, and desired convenience, different systems are used.
The Classic Gas Strut
The gas strut (often referred to as a gas spring) is a hydropneumatic adjustment fitting. It consists of a pressure tube and a piston rod. The interior is filled with nitrogen compressed under high pressure. When opening the flap, the gas pushes the piston outwards, assisting the lifting movement. Standard models cover a force range from 50 N to 150 N and are suitable for typical kitchen wall cabinets made from chipboard or MDF. Visually, these components can match the furniture design. Common options include neutral variants such as the Gas Strut Silver, for dark carcases the Gas Strut Black, and for light interiors the Gas Strut White.
Heavy-Duty Gas Struts
When large fronts—such as those made from solid wood or fitted with mirrors—need to be moved, standard models are insufficient. This is where Heavy-Duty Gas Struts come into play. These feature a larger cylinder diameter and operate with forces of 200 N, 250 N, or more. For large chests or bed storage boxes where heavy panels are lifted, this reinforced design is also strictly required.
Mechanical Flap Stays
Not every flap opens upwards. For downward-opening fronts (such as on bar compartments or bureau desks), gas struts are often unsuitable because they would push the flap down forcefully rather than braking it. In these cases, mechanical Flap Stays are used. These usually operate via friction. The braking effect can be adjusted precisely to the weight of the flap via an adjustment screw, ensuring the front stays in any position or glides smoothly into the horizontal resting position.
Across all designs, these systems fall into the category of Flap Fittings, which must be combined with suitable concealed hinges depending on the motion geometry (lifting up, swinging up, or bi-folding up).
Calculating the Correct Force: Determining the Newton Value
The most common source of error when replacing or building flaps is selecting the wrong spring force. The force of a gas strut is specified in Newtons (N). As a rough rule of thumb in everyday use: 10 N corresponds to a lifting capacity of roughly 1 kg. However, this simplification is not enough for furniture making, as the leverage of the flap must be taken into account.
A gas strut functions much like a shock absorber on a car: if set too stiff, the flap springs open uncontrollably; if set too soft, it falls shut immediately.
Determining Flap Weight
To determine the correct force, you must first know the weight of the cabinet front. For existing fronts, they can be removed and weighed. For new planning, the weight can be calculated from the volume and the density of the material.
Formula: `Height (m) × Width (m) × Thickness (m) × Material Density (kg/m³) = Weight in kg`
Typical density values in cabinet making:
- Melamine-faced chipboard: approx. 650 kg/m³
- MDF (Medium-Density Fibreboard): approx. 750 kg/m³
- Solid wood (Oak): approx. 800 kg/m³
In addition, the weight of the handle (usually 0.1 kg to 0.3 kg) must be added, as it sits at the furthest leverage point and has a significant impact.
Newton Calculation
Alongside weight, the height of the flap is the decisive factor. The taller the flap, the greater the leverage acting on the hinge and strut. Fitting manufacturers generally provide charts from which the required Newton value can be determined.
General guidance for standard kitchen flaps (using two struts, one on each side):
- Flap height 300 mm, weight 1.5 kg to 2.0 kg: 50 N (per strut)
- Flap height 400 mm, weight 2.5 kg to 3.0 kg: 80 N
- Flap height 500 mm, weight 3.5 kg to 4.0 kg: 100 N
- Flap height 600 mm, weight 4.5 kg to 5.0 kg: 120 N or 150 N
It is good practice to install two struts (left and right) for any flap wider than 400 mm. If only one strut is used, the front will warp over time when closed, leading to uneven reveal gaps and damaged concealed hinges.
Technical Parameters: Stroke and Installation Dimensions
Besides force in Newtons, the component geometry is crucial for compatibility.
The Stroke
The stroke refers to the distance the piston rod can travel out of the cylinder (stated in mm). Together with the mounting points, it defines the maximum opening angle of the flap. Common opening angles on wall units are 75°, 90°, or 110°. A longer cylinder with a greater stroke generally allows a wider opening angle, but also requires more clearance inside the carcase.
Overall Length
Overall length is usually measured in the fully extended state from centre-to-centre of the mounting hole eyelets. Typical lengths for standard Gas Struts lie between 240 mm and 270 mm. When replacing a defective strut, this dimension must match exactly; otherwise, existing drill holes in the carcase cannot be reused and the movement kinematics will change.
Mounting Types
Fixing to the carcase and door front is done via mounting brackets. These are generally made from die-cast zinc or steel. The connection between the strut and mounting bracket is achieved using a ball socket or clip mechanism. This allows quick, tool-free removal of the spring if the door needs to be detached. Brackets are fixed using wood screws (usually 3.5 mm or 4.0 mm diameter) with PZ or TX drives. For MDF or hardwood fronts, pilot drilling is strongly recommended to prevent the material from splitting.
Application Areas in Furniture Construction
The choice of fitting depends heavily on where it is installed. Different room concepts require specific opening mechanisms.
Kitchen Wall Units
This is where standard gas struts are most commonly found. Wall cabinets above worktops require fronts that swing upwards out of the work area and line of sight. Because kitchen tasks often involve wet or greasy hands, a strut that automatically lifts the door to its end position after a gentle initial lift is ergonomically sensible.
Bathroom Furniture and Mirror Cabinets
In damp environments, corrosion resistance is vital. Strut cylinders are usually painted or powder-coated (e.g. as Gas Strut White), while the piston rod is chrome-plated. Due to the glass, mirror fronts are noticeably heavier than plain timber fronts. Here, the calculation for Newton force must include the high specific density of glass (approx. 2500 kg/m³).
Chests and Storage Benches
For upward-opening chest lids, the leverage characteristics change. While a cupboard flap starts vertically, a chest lid begins horizontally. The maximum weight bears on the fitting right at the start of the opening movement. For this application, dedicated chest stays or heavy-duty Gas Struts for Heavy Flaps are needed. Precise positioning is essential here to guarantee slow, controlled closing over the final centimetres to prevent trapped fingers.
Living Room and Office Furniture (Drop-Down Compartments)
Sideboards or media units often have drop-down flaps to house devices or serve as a temporary surface. As noted, gas struts are physically the wrong choice here. Instead, mechanical Flap Stays or special cable-stay flap fittings should be installed to support the weight of the front during descent and lock it securely at the 90° position.
Installation: Fundamentals and Positioning
Accurate installation determines the service life of the hardware. Even a perfectly calculated strut will fail if the mounting points are misplaced.
- Preparations and Hinges: The front must first be hung on the carcase with suitable concealed hinges. For flaps that need to open wider than 90°, hinges with an opening angle of at least 110° are required.
- Marking Dimensions: The position of the mounting brackets depends on the desired opening angle. Manufacturers supply installation diagrams for this. A typical dimension for a 90° opening angle on a 245 mm strut, for example, is: carcase fixing point 220 mm down from the hinge axis, front fixing point 90 mm from the hinge axis.
- Carcase Fixing: The elongated mounting plate is screwed to the carcase interior.
- Front Fixing: The round or oval plate is secured to the inside of the flap.
- Clipping In: The cylinder is clipped onto the ball studs (usually with the thicker pressure tube pointing upwards/to the rear). Orienting the pressure tube upwards ensures that the internal seal remains lubricated when closed by the small quantity of oil inside the cylinder.
Distinction: Gas Strut vs Soft-Close Damper
In trade practice, there is often confusion between gas struts and furniture dampers. These components perform opposite functions:
- A gas strut generates an active outward force (pressure). It opens flaps and holds them open against gravity.
- A furniture damper (soft-close) decelerates movement. It generates no opening force of its own, but absorbs the kinetic energy of a closing door or drawer to allow silent, cushioned closure.
These two systems must not be confused during project planning.
FAQ – Frequently Asked Questions
What does the N rating on a gas strut mean? The "N" stands for Newtons and specifies the extension force of the gas strut. 10 N equates theoretically to roughly 1 kg of lifting force. However, the exact choice depends on the weight and height of the cabinet front.
Can I replace a faulty 80 N strut with a 100 N strut? This is not technically recommended. If the force is too high, the flap will require excessive effort to close. Additionally, high pressure is exerted on the concealed hinges when closed, which over time can pull the screws out of the carcase.
How many gas struts do I need per flap? For flaps 400 mm wide or more, two struts (left and right) should always be installed. A single strut creates uneven loading, causing the door front to twist and warp when closing.
Why can I not compress my new gas strut by hand? This is completely normal. A gas strut rated at 100 N, for instance, requires a force of over 10 kg applied strictly in the axial direction to compress. Without the leverage provided by an installed cabinet flap, compressing it by hand is almost impossible.
How do I measure the stroke of a gas strut? The stroke is the exposed, polished metal section of the piston rod when fully extended. It defines the maximum travel distance the strut covers during opening and closing.
Are gas struts maintenance-free? Yes, these furniture fittings are generally maintenance-free. The piston rod must never be oiled or greased, as this attracts dust and debris that will damage the internal seals when the rod retracts.
Why is my flap no longer closing flush? If a flap stands proud, the gas strut is often rated too high, or the mounting points were not positioned strictly according to manufacturer specifications. Misaligned hinge adjustments can also cause door gaps.
Related Categories and Further Guides
To finish wall units and storage boxes professionally, the combination of fittings is key. Alongside selecting the correct spring force, suitable hinges must also be chosen. For our full range of motion mechanisms, explore our comprehensive selection of Flap Fittings. If you want to introduce specific colour accents inside the carcase, Gas Strut Black or Gas Strut Silver provide ideal solutions for modern furniture designs. For downward-opening compartments such as bureau desks, selecting dedicated Flap Stays is essential to ensure proper braking performance.