Overview
ISO 21181:2025 specifies a test method for determining the relaxed elastic modulus of light conveyor belts (the “k 1% value”) used where ISO 9856 does not apply, and for belts covered by ISO 21183-1. The standard describes sample preparation, conditioning, cyclic stretching, data collection and a logarithmic regression procedure to calculate the long-term (relaxed) tensile stiffness that reflects both elastic relaxation and any small permanent stretch.
Key topics and requirements
- Scope and purpose: Measurement of the relaxed elastic modulus for light conveyor belts to predict how tension changes in service.
- Test principle: Cyclic elongation of a test piece between two defined limits for 500 cycles while recording tensile force; results extrapolated by logarithmic regression to a 24 h equivalent (43 200 cycles at 0.5 Hz).
- Apparatus: Displacement-controlled tensile testing machine with force measurement in accordance with ISO 7500-1, class 3 or better, capable of ±5 mm cycles at 0.5 Hz.
- Test pieces: Five longitudinal rectangular specimens, 50 ± 0.5 mm wide with 500 mm free length; cut from full belt thickness; not tested sooner than 5 days after manufacture.
- Conditioning and test environment: Condition specimens per ISO 18573 (Atmosphere B) for 24 h (48 h for high moisture-absorbent materials). Test room: 23 °C ± 2 °C, 50% ± 5% RH.
- Elongation ranges:
- Standard: 1% to 2% (5–10 mm on 500 mm gauge) at 0.5 Hz; or
- For high-modulus reinforcements: 0.5% to 1% (2.5–5 mm) at 0.5 Hz.
- Calculation: Measure tensile forces at cycles z = 250, 350, 500; compute k values, perform logarithmic regression (k = a + c·ln z), then calculate relaxed k at ln(43 200) = 10.67. Average results from five specimens.
- Validity check: If permanent elongation ≥ 1% after the cycle sequence, ISO 21181 is not suitable-use ISO 9856 instead.
- Reporting: Include belt designation, test conditions, conditioning, procedure used, results, deviations and date.
Applications and users
ISO 21181:2025 is used by:
- Belt manufacturers for quality control and specification,
- Independent testing laboratories and certification bodies,
- Design and maintenance engineers for conveyor systems (packaging, light manufacturing, baggage handling),
- Procurement professionals comparing belt performance.
Practical uses include predicting initial tension loss in light conveyor belts, selecting belts with appropriate long-term stiffness, and ensuring reliable conveyor performance without frequent re-tensioning.
Related standards
- ISO 21183-1 - Principal characteristics and applications for light conveyor belts.
- ISO 9856 - Determination of elastic and permanent elongation (alternative method for belts with large permanent stretch).
- ISO 7500-1 - Calibration and verification of tensile testing machines.
- ISO 18573 - Test atmospheres and conditioning periods.
