Overview
ISO 21181:2013 specifies a laboratory test method to determine the relaxed elastic modulus of light conveyor belts (as per ISO 21183-1) or other belts where ISO 9856 is not applicable. The standard defines how to measure the change in tensile force under cyclic elongation and how to calculate the long-term, “relaxed” k value (tensile force for 1% elongation per unit width). This relaxed elastic modulus is a practical metric for predicting how belt tension will change in service due to relaxation and permanent stretch.
Key topics and requirements
- Scope and purpose: Measurement of the relaxed k value after cyclic stretching to model tension loss in belts that are initially tensioned and not re-adjusted.
- Principle: Cyclic elongation between two defined limits with tensile force recorded over cycles; logarithmic regression yields the relaxed modulus.
- Apparatus: Displacement-controlled tensile testing machine with force measurement calibrated to ISO 7500-1; machine must apply ±5 mm cycles at 0.5 Hz.
- Test pieces: Five longitudinal specimens, (50 ± 0.5) mm wide, 500 mm free length; selection and spacing specified.
- Conditioning: Pre-test conditioning per ISO 18573 (Atmosphere B) for 24 h (48 h for high-moisture-absorbent materials).
- Procedure:
- Cyclic elongation either between 1% and 2% (5–10 mm) or between 0.5% and 1% (2.5–5 mm) at 0.5 Hz for 500 cycles.
- Record tensile force vs. cycle number; read forces at z = 250, 350 and 500 cycles.
- Compute k values (N/mm) and perform logarithmic regression using ln(z) as the independent variable.
- Extrapolate relaxed k for z = 43 200 cycles (24 h at 0.5 Hz; ln 43 200 ≈ 10.67).
- Acceptance criteria: Correlation coefficient r ideally ≈1.0; r between 0.8 and 1.0 acceptable. If permanent elongation ≥ 1% the method is unsuitable-use ISO 9856 instead.
- Reporting: Include belt ID, test conditions, conditioning, procedure used, k results (mean of five specimens), and test date.
Applications and users
- Who uses it: belt manufacturers, conveyor system designers, quality assurance and testing laboratories, procurement/specification engineers, and maintenance planners.
- Practical value: predicting long-term tension loss, selecting belts for light conveyor applications, specifying tensioning procedures and maintenance intervals, and validating belt performance against ISO 21183-1 requirements.
Related standards
- ISO 21183-1 (Light conveyor belts - principal characteristics)
- ISO 9856 (elastic and permanent elongation test method; used when ISO 21181 is unsuitable)
- ISO 18573 (test atmospheres and conditioning)
- ISO 7500-1 (verification of tensile testing machines)
Keywords: ISO 21181:2013, light conveyor belts, relaxed elastic modulus, k value, cyclic elongation, tensile testing, belt relaxation, conveyor belt testing.