ISO 7040:2025

Overview

ISO 7040:2025 - Fasteners - Prevailing torque hexagon nuts - Regular nuts (with non‑metallic insert) defines the characteristics, dimensions and requirements for prevailing torque hexagon regular nuts with non‑metallic inserts (commonly known as “nylon insert” or polyamide locking nuts) in steel and stainless steel, for metric coarse pitch threads M3 to M39. The fourth edition updates dimensional rules, product grades and references to related ISO documents, and adds specific guidance for very small sizes (D < M5) in Annex A.

Key topics and technical requirements

• Scope and design principle
  • Applies to prevailing torque hexagon regular nuts with a non‑metallic insert, product grades A (D ≤ M16) and B (D > M16).
  • Nut overall height = mmin (per ISO 898‑2 / ISO 4032 for style 1) plus a prevailing torque feature; the insert height (hmax − mmin) is identical across regular, high and thin nuts for a given diameter.
• Dimensions and tolerances
  • Detailed dimensional tables for M5–M39 (Tables 1 & 2); Annex A provides conforming dimensions for M3 and M4 designed per ISO/TR 16224 (style 1).
  • Thread tolerance class 6H (6G optional for coated nuts) and thread standards per ISO 965‑1.
• Materials and mechanical properties
  • Steel nuts: property classes and heat‑treatment requirements referenced to ISO 898‑2.
  • Stainless steel nuts: grades and property classes per ISO 3506‑2 (A2, A4, D‑grades, etc.).
  • Insert material example: polyamide (manufacturer’s choice for shape of prevailing torque feature).
• Functional and acceptance requirements
  • Functional tests and prevailing torque functional aspects refer to ISO 2320 (steel nuts); for stainless steel, performance and test methods should be agreed between purchaser and supplier.
  • Surface finishes, coatings and corrosion protection per ISO 4042, ISO 10683; inspection/acceptance per ISO 3269 and surface discontinuities per ISO 6157‑2.
• Marking and labelling
  • Product marking follows ISO 898‑2 (steel) or ISO 3506‑2 (stainless); package labelling must include ISO 7040 reference, thread size, material/property class, finish, manufacturer ID, lot number and quantity.

Applications and users

• Typical applications: assemblies requiring vibration‑resistant locking (machines, automotive subassemblies, industrial equipment, construction, maintenance).
• Primary users: fastener manufacturers, design and mechanical engineers, procurement/specification writers, quality/inspection teams and maintenance personnel who need standardized prevailing torque hexagon nuts for reliable, interchangeable locking performance.

Related standards

• ISO 898‑2 (mechanical properties - nuts)
• ISO 3506‑2 (stainless steel nuts)
• ISO 2320 (prevailing torque steel nuts - functional)
• ISO 965‑1, ISO 8992, ISO 4759‑1, ISO 3269, ISO 4042, ISO 10683, ISO 6157‑2, ISO/TR 16224 (style 1 for small sizes)

Keywords: ISO 7040:2025, prevailing torque hexagon nuts, non‑metallic insert, metric nuts M3–M39, fastener standards, ISO fasteners.