ISO 9856

What the test does

A 50 mm wide strip cut through the full thickness of a conveyor belt is clamped at both ends, with at least 300 mm of free length between the jaw faces. A clip-on device spans a marked reference length of 100 mm or more on the axis of the strip. The machine then cycles the strip sinusoidally between two force levels — 200 cycles at 0.1 Hz — and records elongation throughout.

What it measures, and why it matters

Three figures come out: elastic elongation, permanent elongation and elastic modulus in newtons per millimetre of belt width. Permanent elongation sizes the take-up, since the tensioning gear on an installed conveyor has to absorb whatever stretch the belt keeps after running in. Elastic elongation and modulus feed belt-tension, sag and starting-and-braking calculations for the drive, and they separate steel-cord carcases from textile ones at specification stage rather than after commissioning.

Specimen

The test piece is a full-thickness strip 50 mm wide, cut long enough to leave at least 300 mm free between the clamps. Covers are skimmed back so the remaining cover is between 0.5 mm and 1 mm — cover rubber carries little of the load but adds thickness scatter. The sample is taken no sooner than five days after the belt is manufactured, because the carcase is still relaxing before that. Conditioning follows ISO 18573, at least 16 h in the agreed atmosphere. Two extensometer grids are marked on the axis at a known reference length. The sources consulted for this page do not fix a replicate count; in practice a run is discarded when the clamp creeps or the extensometer loses its reference, both of which show as a step in the elongation record rather than as a plausible curve.

What the machine must be capable of

Force demand scales with the belt rating, not with the frame's break capacity. The upper reference force is 10 % of nominal tensile strength multiplied by test-piece width and the lower is 2 %, so a 200 N/mm belt on a 50 mm strip cycles between roughly 200 N and 1 kN, a 3150 N/mm textile belt between about 3.2 kN and 15.8 kN, and a heavy steel-cord belt can carry the upper force toward 30–50 kN. An initial force of 0.5 % of nominal tensile strength times width is applied first to take up slack.

The binding capability is control, not capacity: closed-loop force control holding an approximately sinusoidal waveform at 0.1 Hz — six cycles a minute — for 200 consecutive cycles at those loads. Force indication is required to ISO 7500-1 Class 3 or better, with Class 2 given as an example of better, per the 2003 text and not confirmed here for the 2016 edition.

Strain measurement is set by the method itself rather than by an extensometer class: measuring length of at least 100 mm, accurate to 0.1 mm or better. Total strain stays inside about 5 %, so resolution matters far more than travel. Clamps must hold a 50 mm full-thickness strip without creeping over 200 cycles; the face pattern is not prescribed. Conditioning follows ISO 18573 — atmosphere A at 20 °C and 65 % RH, B at 23 °C and 50 % RH, or tropical C at 27 °C and 65 % RH, as the parties agree.

What goes wrong in practice

Grip creep is the dominant error: a strip walking a millimetre through the clamps over 200 cycles is read as permanent elongation the belt does not have. Cutting the sample inside five days, or skipping conditioning, leaves the carcase still relaxing and drifts permanent set high. Extensometer clips walking on soft cover rubber shift the reference length, depressing elastic elongation and inflating modulus. Stopping short of 200 cycles understates permanent elongation, because the belt has not yet settled.

Related and equivalent standards

EN ISO 9856:2016 is the identical European adoption, so results transfer directly between the two designations. ISO 18573 supplies the conditioning atmospheres and ISO 7500-1 the force-verification classes; neither is a test method in its own right. The superseded ISO 9856:2003 and ISO 9856:1989 editions still appear in older belt datasheets, and the 1989 edition reported modulus only. No ASTM counterpart is identified in the sources consulted, so elongation figures obtained by any other method should not be assumed comparable.