Overview
ISO 9177-3:2022 specifies bending strengths and a test method for black leads of HB hardness used in mechanical pencils for technical drawings. The standard defines minimum bending strength values by nominal lead diameter, a standardized three-point bending test procedure (including loading rates and anvil spacing), the formula for calculating bending strength, and the information required in the test report. It applies specifically to leads intended for technical drawing use (not general‑purpose leads).
Key Topics and Requirements
- Scope: Bending strength of HB black leads for mechanical pencils for technical drawings.
- Minimum bending strengths (σ, MPa) by nominal diameter:
- Polymer (P): 0.35 mm - 240 MPa; 0.5 mm - 190 MPa; 0.7 mm - 160 MPa; 1 mm - 95 MPa
- Ceramic (C): 2 mm - 70 MPa
- Note on diameters: 0.35 and 1 mm correspond to older designations 0.3 and 0.9 mm respectively.
- Test method (three‑point bend):
- Arrange lead on two supporting anvils and apply central load with a loading anvil.
- Loading rates (±10%): 0.1 N/s for 0.35, 0.5, 0.7, 1 mm leads; 0.5 N/s for 2 mm leads.
- Verify loading rates using a rigid test piece prior to testing.
- Anvil spacing (l): Polymer leads (0.35–1 mm) - 20 to 40 mm; Ceramic 2 mm - 40 to 60 mm.
- Calculate bending strength using σ = (8 F l) / (3 π d), where F is breaking load (N), l is span (mm), d is actual diameter (mm).
- Test report must include: reference to ISO 9177-3, lead type (P or C), actual diameter, test values F and l, calculated bending strength, test centre/name/date/signature.
Applications and Users
ISO 9177-3:2022 is relevant to:
- Mechanical pencil and lead manufacturers for product specification, quality control, and conformity with technical‑drawing performance expectations.
- Laboratories and test centres that perform lead strength testing and issue compliance reports.
- Procurement and specification writers in engineering, architecture, CAD/drafting tool supply chains who require durable HB leads for precise technical drawings.
- Regulatory/standards teams and product designers ensuring compatibility with ISO 9177 series requirements.
Using this standard helps ensure consistent lead durability, reliable performance in technical drawing applications, and clear, reproducible test results for product certification and quality assurance.
Related Standards
- ISO 9177-1: Classification, dimensions, performance requirements and testing (mechanical pencils for technical drawings)
- ISO 9177-2: Black leads - Classification and dimensions
- ISO 20318-1 and ISO 20318-2: Mechanical pencils and leads for general use (classification, dimensions, test methods)
Open preview (PDF)Mechanical pencils for technical drawings — Part 3: Black leads — Bending strengths of HB leads
