- Standard Number: NF E25-411 (sometimes seen as NFE25-411)
- Title: All Metal Reusable Dual Slotted Self-Locking Hexagon Nuts (metric, ESL/M type)
- Origin: French standard (NF = Norme Française), managed by AFNOR (Association Française de Normalisation)
- Latest Revision: NF E25-411-1985 (reconfirmed in 2001, R2001)
- Scope: Applies to metal hexagon nuts with two slots (double slotted) that provide self-locking functionality by elastic deformation of the slots. These nuts are reusable and designed for high-vibration or critical fastening applications, especially in railway, aerospace, and heavy machinery.
- Structure: The nut has two slots (cut into the top face) that act as elastic locking elements.
- Locking Mechanism: When tightened onto a bolt, the slots are forced open, generating radial tension and a counteracting force on the bolt threads. This prevents loosening under vibration.
- Reusability: Unlike nylock or insert nuts, these are all-metal and can be reused (subject to inspection and specified limits).
- Thread: According to ISO 965-2 (standard metric thread).
- Material: Carbon steel (commonly grades 8, 10, 12), stainless steel (A2-70, A4-70).
- Surface Treatment: Various (plain, zinc plated, Dacromet, etc.) depending on application.
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