Friction locking on aluminium profiles explained
Friction locking (a friction fit) describes a type of joint in which forces are transmitted by friction between two surfaces pressed together. In aluminium profile technology this is the dominant principle for joining profiles flexibly and safely with T-slot nuts and screws.
What is friction locking?
Alongside positive locking and material bonding, friction locking is one of the three elementary types of joint in mechanical engineering. Physically it is based on the principle of static friction. A joint counts as a friction fit when the frictional force produced by a normal force (bolt preload, for instance) is greater than the external tensile or shear force trying to displace the parts relative to each other.
In the context of modular aluminium construction profiles (for example Maunsystem, compatible with item or Bosch Rexroth), friction locking is essential. When a bracket or a plate is bolted on, the screw pulls the T-slot nut against the inner flanks of the profile slot. The resulting clamping force prevents slipping under load.
How the types differ:
- Friction locking: holds through friction (bolted joints, clamping).
- Positive locking: holds through geometric interlocking (key, pin, bolt in bearing).
- Material bonding: holds through a molecular bond (welding, adhesive bonding).
Purpose & key questions
For designers and maintenance teams, understanding friction locking is safety-critical.
How much load does the joint take?
That depends on the tightening torque and the friction coefficient of the material pairing (anodised aluminium against zinc-plated steel, for example).
Does the joint stay tight permanently?
Vibration and the settling behaviour of aluminium can reduce the preload.
When is a pure friction fit no longer enough?
Under extremely dynamic loads or in crash scenarios, a combination with positive locking is often used.
Relevance in an industrial context
- Infinitely adjustable: components can be positioned along the profile slot to the millimetre.
- Economy: no machining is necessary – assembly is fast and inexpensive.
- Safety to standards: bolted joints can be calculated and documented to VDI 2230.
Key components / technical elements of friction locking
- Preload: the clamping force produced by tightening the screw – the basis of every friction joint.
- Normal force: acts perpendicular to the joint face and produces the frictional force.
- Friction coefficient: depends on the surface and the material pairing. Steel on anodised aluminium (dry) is typically μ ≈ 0.3–0.6. Lubricants reduce the holding force.
- Tightening torque: the tool (a torque wrench) applies a moment which the thread pitch converts into preload.
Factors influencing selection & application
| Factor | Effect on the friction fit | Recommendation for practice |
|---|---|---|
| Surface | Anodised layers are hard and smooth; oil lowers the friction coefficient. | Keep contact surfaces free of oil and grease. |
| Settling behaviour | Aluminium creeps slightly under pressure and the preload drops. | Check and retighten the screws after commissioning. |
| Vibration | Dynamic loads can cause screws to work loose. | Use chemical thread locking or ribbed washers. |
| Temperature | Aluminium expands more than steel screws. | With large fluctuations, consider disc springs or expansion sleeves. |
| Profile geometry | Heavy series offer thicker flanks and a larger contact area. | Choose the profile cross-section to match the load case. |
Benefits for companies & engineering teams
For the company:
Non-destructive joints allow profiles and assemblies to be reused. That supports sustainability and reduces stock costs.
For design & production:
High flexibility: release, slide, tighten. Prototypes can be built in hours and adapted quickly.
Practical tip for design & purchasing
State the tightening torques for safety-relevant joints bindingly, in the drawing or the bill of materials.
| No. | Assembly step |
|---|---|
| 1 | Accessory check: do the T-slot nut and the profile slot match (slot 8, for example)? |
| 2 | Cleaning: is the profile slot free of oil or emulsion? |
| 3 | Positioning: align the parts precisely. |
| 4 | Tightening: use a torque wrench (M6 approx. 10 Nm, M8 approx. 25 Nm). |
| 5 | Marking: mark the screw with a paint pen for visual inspection. |
The guide values given are below the maximum permissible torques for 8.8 screws and are regarded as conservative practical values.
A current example (2024–2026)
In cobot integration, mobile base frames are made from aluminium profiles and have to absorb dynamic acceleration forces. Extended or high-strength T-slot nuts combined with heavy-duty brackets enlarge the friction area and distribute the normal force better – for a secure friction fit that keeps its flexibility.
Future trends 2026+
Smart fasteners are gaining importance. Screws or washers with integrated sensors measure the preload in real time. A drop in clamping force is reported straight to the control system – the basis for predictive maintenance and digital twins.
Conclusion
Friction locking is the functional heart of modular aluminium construction kits. It combines stability with maximum flexibility. Anyone who has preload, friction coefficient and assembly quality under control builds durable and safe equipment.
FAQ – the 5 most important questions about friction locking
What is the difference between friction locking and positive locking?
Can a friction joint work loose?
What tightening torque do I need?
Do I need grease or oil during assembly?
What does slip mean?
Sources
- VDI 2230 – Systematic calculation of highly stressed bolted joints
- Engineering Toolbox – Friction Coefficients
- VDI 2230 teaching material
- Engineering Toolbox – Thermal Expansion
- Metric Bolt Torque Chart
- Würth Industrie – Technical guide
- Fraunhofer IPA – Assembly and connection technology
- Strainlabs – Smart Fasteners
- Interbolt – Intelligent Bolting Solutions
