In mechanical assembly, flange nuts are often promoted as a solution that can "replace washers" thanks to their integrated flange design. So, in actual practice, do flange nuts really need an additional washer? The answer is: in the vast majority of cases, no – but in a few special scenarios, exceptions do apply.

The Essence of the Flange Nut

Built‑in Integrated Washer Function

The core design philosophy of the flange nut is to integrate the function of a washer directly into the nut body. One end features a wide flange face, which acts as an integrated washer to distribute the clamping force evenly over the surface of the joined parts, thereby reducing the risk of damage or deformation to the workpiece.

Flange nuts come in two main types, distinguished by their design details to suit different working conditions:

Smooth flange face: Used for general fastening, mainly to enlarge the bearing area and protect the contact surface.

Serrated flange face: The underside of the flange has radial anti‑slip serrations that bite into the surface of the connected part, providing strong frictional resistance and effectively preventing loosening due to mechanical vibration and operation.

Integrated Flange Face vs. Separate Nut + Flat Washer

Comparison Dimension	Flange Nut (Integrated Flange)	Standard Hex Nut + Flat Washer (Separate)
Pressure uniformity	Flange and thread are integrally concentric; force distribution is completely uniform	Flat washer tends to shift and misalign, leading to eccentric loading and uneven pressure
Relative sliding risk	No separate contact interfaces; no relative sliding after tightening	Two sliding interfaces exist: nut‑washer and washer‑workpiece
Workpiece surface protection	Rounded edges, no burrs, very low scratch risk	Washer stamping edges may have burrs that scratch coatings during tightening
Assembly coaxiality	Naturally concentric; very low eccentric load risk	Washer displacement can tilt the nut, causing thread wear and eccentric loading
Number of parts	Single part, no extra accessories	Two parts, requiring a matching flat washer size
Basic anti‑loosening performance	Greater static friction on the contact surface; stronger slip resistance	Two metal contact surfaces are prone to sliding; weaker anti‑loosening ability

Normal Working Conditions: Extra Washers Not Recommended

Weakened Anti‑Loosening and Anti‑Slip Effect

For serrated flange nuts, the anti‑slip serrations are designed to bite into the workpiece surface after tightening, forming a mechanical interlock that resists vibration and prevents rotational loosening.

If a flat washer is placed between them, the serrations will act entirely on the washer surface and cannot contact the workpiece itself. This completely eliminates the mechanical locking ability, leaving only the friction between smooth metal surfaces – a significant reduction in anti‑loosening performance.

Compromised Load Coaxiality

The integrated structure of the flange nut ensures absolute coaxiality between the flange face and the internal thread. When tightened, the axial force is evenly distributed over the entire flange circumference. Adding an extra flat washer introduces an additional clearance fit; the washer can easily shift or become eccentric, causing the nut to tilt and resulting in localised stress concentration on the threads.

Increased Total Cost

One of the core values of the flange nut is to simplify parts and improve assembly efficiency. Adding a washer adds a new SKU on the procurement side, requires separate inventory management, increases the risk of sorting and distribution errors, and adds an extra pick‑and‑place operation on the assembly line – reducing manual efficiency and, on automated lines, necessitating an additional feeding mechanism that raises equipment costs and failure risks.

When Is a Washer Actually Needed?

Excessively Large Mounting Holes

When the hole in the workpiece is oversized, has a positional deviation, or is a slotted hole with significant adjustment allowance, the standard flange outer diameter may not fully cover the hole. The nut risks sinking into the hole or having insufficient bearing support. In such cases, a large‑outer‑diameter flat washer can be added to expand the bearing area and ensure the nut face presses firmly on solid material.

Extremely Soft Workpiece Materials

For low‑hardness materials such as thin‑wall aluminium profiles, plastics, or foam sandwich panels, even the standard flange bearing area may cause crushing or deformation. Adding a larger‑size flat washer further distributes pressure and protects the workpiece from damage.

Assembly Gap Compensation

When machining errors or pre-existing gaps exist between two mating components—requiring the gap to be filled to ensure a tight fit—an appropriate number of flat washers can be installed to compensate for the height difference.

Special Functional Requirements (Insulation, Sealing, Conductivity, etc.)

If the assembly requires special functions that a standard flange nut cannot provide, corresponding washers can be added:

• For electrical equipment requiring insulation, add nylon insulating washers.

• For pipe or enclosure connections needing waterproof sealing, add rubber sealing washers.

• For lightning protection, earthing, or electrical connections needing conductivity, add specialised conductive washers.

Practical Recommendations for Correct Use

First, prioritise selecting the right nut from the start rather than adding washers. Choose serrated or locking types for anti‑loosening needs; select stainless steel or hot‑dip galvanised options for corrosion resistance.

Second, if a washer is truly necessary, follow the "minimum and matched" principle. Use as few as possible – one instead of two. Select washers with outer and inner diameters that match the flange nut to avoid eccentricity due to size mismatch. Also, be aware of galvanic corrosion risks when combining different metals – for example, do not pair a stainless steel nut with a carbon steel washer.

Third, after adding a washer, adjust the tightening torque accordingly. Changes in the friction coefficient mean that the torque must be recalculated or tested to ensure the preload meets design requirements.