Overview
ISO 898-7:1992 specifies a standardized torsional test and tabulated minimum breaking torques for bolts and screws with nominal thread diameters from 1 mm to 10 mm. The part applies to fasteners of property classes 8.8 to 12.9 (per ISO 898-1) for which tensile breaking or proof loads are not given, and to short bolts and screws (3 mm to 10 mm) that cannot be subjected to tensile testing. Hexagon socket set screws are explicitly excluded from the minimum breaking torque values.
Keywords: ISO 898-7, torsional test, minimum breaking torque, bolts and screws, fasteners, property classes 8.8–12.9
Key Topics and Requirements
- Scope and applicability
- Bolts and screws nominal diameters 1 mm–10 mm.
- Property classes 8.8, 9.8, 10.9, 12.9 (per ISO 898-1).
- Thread tolerances 6g, 6f and 6e considered for the tabulated values.
- Test principle
- Determine breaking torque by applying torsion until rupture.
- The fastener must be subjected exclusively to torsion - results must not be influenced by head or thread friction.
- Test apparatus
- A torsional test device that clamps the fastener (example layout provided in the standard).
- Torque meter with scale not exceeding five times the relevant minimum breaking torque; maximum permitted inaccuracy is ±7% of the minimum breaking torque to be tested.
- Test procedure
- Clamp over at least two full threads.
- Maintain a free thread length of at least one thread diameter between head and threaded insert.
- Apply torque in a continuously increasing manner and record the breaking torque.
- Evaluation and calculation
- A fastener passes if it does not rupture before the tabulated minimum breaking torque.
- The standard gives a formula linking minimum breaking torque to thread minor diameter, tensile strength and polar moment of resistance, and provides strength ratio X for each property class (e.g., X = 0.84 for 8.8, 0.815 for 9.8, 0.79 for 10.9, 0.75 for 12.9).
- Tabulated data
- Minimum breaking torques are provided in tables for common thread sizes (M1–M10, including fine pitches).
Applications
- Quality control and acceptance testing in fastener manufacturing.
- Test laboratory procedures for small or short fasteners that cannot undergo tensile tests.
- Engineering specification and procurement when torsional strength is a functional requirement.
- Product development and failure analysis for threaded components used in precision assemblies, electronics, medical devices and miniature mechanical systems.
Related Standards
- ISO 898-1 - Mechanical properties of fasteners: definitions of property classes and tensile/proof loads (referenced normative standard).
- Annex A of ISO 898-7 is informative and provides explanatory context about test limitations and the rationale for torque values.
Use ISO 898-7 when you need a reproducible, internationally recognized method and acceptance criteria for torsional strength of small or short fasteners.
Open preview (PDF)Mechanical properties of fasteners — Part 7: Torsional test and minimum torques for bolts and screws with nominal diameters 1 mm to 10 mm
