Testing standard
Conveyor belts — Adhesion between constitutive elements — Test methods
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 252 peels the layers of a conveyor belt apart on a testing machine and reports the adhesion strength as force per unit width. Two interfaces are covered — ply to ply inside the carcass, and cover to carcass — and two test methods, A and B, are defined. The current edition is ISO 252:2023, superseding ISO 252:2007.
A strip is cut from a conveyor belt and the layers inside it are separated. One layer is clamped in the fixed grip, the other in the moving grip, and the machine peels them apart at a controlled rate while recording the force. Two test methods are defined, A and B, and both report an adhesion strength as force per unit width.
Two interfaces matter and both are covered: ply-to-ply, which is the bond between the fabric layers of the carcass, and cover-to-carcass, which is the bond holding the rubber cover onto the fabric beneath it.
Adhesion is what stops a conveyor belt coming apart in service, and it is a different property from tensile strength. A belt with ample longitudinal strength and poor interply adhesion will fail by delamination long before it fails in tension: the plies separate at a pulley, the cover lifts at a damaged edge, and moisture and dust get into the carcass and finish the job.
The peel record is read for a mean force over a length of peel rather than a single peak, because delamination is a progressive process and one point on the trace says nothing about the bond as a whole. One of the defined procedures makes a record of the force required to peel off enough length to obtain eight or more peak force points, at a driven-grip speed of 100 ± 10 mm/min.
Adhesion figures are also process figures. Low interply adhesion points at the calendering or the curing rather than at the fabric, and it is one of the few belt properties that can be traced back to a specific step on the production line.
Adhesion is not uniform across a belt. Where the specimen came from is part of the result, not an administrative detail.
Mean peel force divided by specimen width
Reported per unit width so specimens of different widths compare.
Delamination is progressive, not a single event
One point on the trace says nothing about the bond along the belt. A peak reported as an adhesion figure is an anecdote.
The calendering and the cure, not the fabric
Interply adhesion is one of the few belt properties that traces back to a specific step on the production line.
A wide vice-action face holds a rubber strip across its full width without cutting it. Slippage on rubber shows as a creeping specimen rather than a jump, so a grip that holds the whole width is worth more here than clamping force alone.
SpecificationsPeel forces on belting are commonly a few hundred newtons. A cell sized for the specimen rather than the frame is what makes eight peaks readable rather than eight ripples on a flat line.
SpecificationsPeel forces on conveyor belting are moderate — commonly a few hundred newtons per specimen, more on heavy multi-ply constructions. A frame of 5 to 50 kN is more than adequate and resolution at the low end matters far more than capacity, so the load cell should be chosen for the specimen rather than for the frame. Force accuracy to ISO 7500-1 Class 1 is the normal requirement.
The machine needs a driven grip capable of a steady 100 ± 10 mm/min and, critically, continuous logging over a long travel. The result is a mean over eight or more peaks, so the trace has to be recorded through the whole peel and not sampled at intervals. Grips must hold a rubber-faced strip without slipping and without cutting it: vice-action jaws with a face that grips the rubber are the usual choice.
A long crosshead travel is worth having. Peeling far enough to collect eight peaks on a wide specimen consumes stroke, and a short-stroke frame forces the operator to restart the peel, which puts an artefact in the record.
Slipping grips are the commonest fault, and on rubber they are easy to miss because the specimen creeps rather than jumping. Peeling too short a length and averaging over two or three peaks gives a number with no stability. Starting the separation with a knife that cuts into the ply below weakens the interface before the test begins. And reporting a peak instead of a mean turns a progressive measurement into an anecdote.
| Adhesion (ISO 252) | Tear propagation (ISO 505) | Cord bond (ISO 7623) | Fastening strength (ISO 1120) | |
|---|---|---|---|---|
| Belt type | Textile carcass | Textile | Steel cord | Textile, mechanically fastened |
| What fails | The interface between layers | A tear that is made to run | The cord-to-rubber bond | The joint |
| Result | Force per unit width | A force | A pull-out force | A peak force |
| Force needed | Low | Low to moderate | Low | High — tens to hundreds of kN |
| Predicts | Delamination in service | Longitudinal ripping | Splice performance | Whether a fastener is fit to consider |
These four are not alternatives and a specification naming one is not satisfied by another. A belt can have excellent adhesion and poor tear resistance, or the reverse.
ISO 252 is the international method for the adhesion between the constitutive elements of a conveyor belt. A strip is cut from the belt, the layers are separated, and one is peeled from the other on a testing machine while the force is recorded. It covers both the ply-to-ply bond inside the carcass and the cover-to-carcass bond, and defines two test methods, A and B. The current edition is ISO 252:2023.
Because they are different failure modes and adhesion usually arrives first. A belt with ample longitudinal strength and poor interply adhesion delaminates: the plies separate at a pulley, the cover lifts at a damaged edge, and dust and moisture get into the carcass and finish it. Tensile strength describes what the belt could carry; adhesion describes whether it stays in one piece while carrying it.
Because delamination is progressive. The peel force rises and falls as the front passes through the structure of the fabric, and a single peak is a point on that oscillation rather than a description of the bond. Averaging across eight or more peaks gives a figure with some stability. Peeling too short a length and averaging over two or three is the commonest way to produce an adhesion number that will not repeat.
The published procedure description gives a driven-grip speed of 100 ± 10 mm/min, and the record is taken over enough peel length to give eight or more peak force points. That speed and that length together are what make the result comparable between laboratories; changing either invalidates the comparison even though the test still produces a number.
No. Steel cord belts have their own method, ISO 7623, which pulls a single cord out of the surrounding rubber rather than peeling layers apart. The geometry, the fixture and the failure being measured are all different. ISO 252 is for textile-carcass constructions, where there are genuine layers to separate.
A modest frame with good low-end resolution and a long stroke. Peel forces are commonly a few hundred newtons, so a 5 to 50 kN frame with a load cell sized for the specimen and force accuracy to ISO 7500-1 Class 1 is right. The two requirements that catch people out are a driven grip that will hold a steady 100 mm/min and enough travel to peel far enough for eight peaks without restarting.
Usually that the problem is in processing rather than in materials. Interply adhesion is set by the calendering and the cure, so a low figure points at compound tack, dwell, temperature or pressure rather than at the fabric or the cover formulation. It is one of the few belt properties that traces cleanly back to a single step, which is why it is a routine production check and not only an acceptance test.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Low — commonly a few hundred newtons per specimen, more on heavy multi-ply constructions | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ISO 7500-1 Class 1 | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Gripping | Vice-action grips holding the two separated layers, with a driven grip at controlled speed | Our vice-action grips or peel and adhesion fixtures, built to the specimen |
| Environment | Standard atmosphere for conveyor belt testing; conditioned and tested in it | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
