This standard specifies the torque test and minimum failure torque for bolts and screws with nominal diameters from 1 mm to 10 mm. It ensures fasteners meet mechanical performance requirements under torsional loads, critical for applications in machinery, automotive, and construction industries. The standard references ISO 898-7:1992 and applies to property classes 8.8, 9.8, 10.9, and 12.9.

Understanding this standard helps engineers select appropriate fasteners, preventing failures due to insufficient torsional strength. It outlines precise testing methods to verify that bolts and screws can withstand specified torques without fracturing prematurely.

Principle

The torque test involves clamping the bolt or screw in a testing device and applying torque until failure, measuring the failure torque. This assesses the fastener’s torsional resistance, ensuring it meets minimum requirements without breaking before the specified value.

• Focuses on pure torsional loading to isolate shear strength.
• Essential for validating material integrity and manufacturing quality.

Apparatus

The testing setup includes a torque testing device as illustrated in typical diagrams (e.g., Figure 1 in the standard). Key components ensure accurate measurement:

1. Torque wrench or meter with capacity not exceeding five times the minimum failure torque of the specimen.
2. Maximum error of the torque meter should be ±1% of the minimum failure torque.
3. Clamping fixtures to hold the fastener without inducing axial loads or friction that could skew results.

Calibration of the apparatus is vital to maintain reliability in industrial testing environments.

Test Conditions

Tests must apply only torsional forces, with values as per Table 2. The specimen should not fracture before reaching the minimum failure torque. Avoid friction between the head and threaded parts to ensure accurate outcomes.

• Ambient temperature conditions typically apply unless specified otherwise.
• Suitable for fasteners with thread tolerances 6g, 6f, and 6e.
• Multiple samples recommended for statistical confidence in batch quality.

Procédure

Insert the bolt or screw into the fixture with at least two full threads engaged, leaving at least one thread diameter between the fixture and head. Apply torque continuously and steadily until failure.

1. Secure the specimen properly to prevent slippage.
2. Record the torque at which fracture occurs.
3. Inspect fracture surfaces for defects like inclusions or improper heat treatment.

This method provides reproducible results, guiding quality control in production.

The minimum failure torque (M_{B min}) is calculated using the formula:

M_{B min} = τ_{B min} × W_{P min}

Where:

• W_{P min} = π/16 × d_{1 min}³ (torsional section modulus)
• τ_{B min} = X × σ_{b min} (minimum torsional strength)
• d_{1 min}: Minimum minor diameter of external thread
• σ_{b min}: Minimum tensile strength
• X: Strength ratio (from Table 1)

This calculation ensures fasteners can handle design loads safely, factoring in material properties and geometry.

Strength Ratio X

These ratios adjust torsional strength based on tensile properties, ensuring conservative design values.

Minimum Failure Torque Values

Note: Values apply to threads with tolerances 6g, 6f, and 6e. These ensure safe installation torques in assemblies.