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 adhesion as force per unit width — a mean adhesion figure and a minimum adhesion figure, both taken from the peel trace. 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 its layers are separated. One is clamped in the fixed grip and the other in the driven grip, and the machine peels them apart at a controlled rate while the force is recorded. Two methods are defined, A and B, both reporting adhesion 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.
Three exclusions sit in the scope: belts containing steel cord reinforcement, textile-reinforced belts with a full-thickness tensile strength of less than 160 N/mm, and light conveyor belts as described in ISO 21183-1. All other constructions of conveyor belting are covered.
Adhesion is what stops a conveyor belt coming apart in service, and it is a different property from strength. A belt with ample longitudinal strength and poor interply adhesion delaminates long before it fails in tension: the plies separate at a pulley, the cover lifts at a damaged edge, and moisture and dust finish the job.
The peel record is read as a whole, not as a single point, because delamination is progressive. Both methods record the force over enough peel to obtain eight or more peak force points, at a driven-grip speed of 100 ± 10 mm/min, and the trace is analysed to ISO 6133. Two quantities come out of it: the median peak, specified as the mean adhesion force, and the lowest peak, the minimum adhesion force. Each is divided by the nominal test-piece width, so a complete result is a pair of figures, not one.
Adhesion figures are also process figures: a low interply result points at the calendering or the cure rather than at the fabric.
Adhesion is not uniform across a belt. Where the specimen came from is part of the result, not an administrative detail.
Force divided by the nominal test-piece width
The trace is examined and analysed in accordance with ISO 6133. Both quantities are reported, each per unit width.
Delamination is progressive, not a single event
One point on the trace says nothing about the bond along the belt, which is why the analysis takes the median peak rather than any single one — and why the lowest peak is reported alongside it.
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.
Vice-action faces hold a 25 mm rubber strip square without cutting it. Slippage on rubber shows as a creeping test piece rather than a jump, so alignment and face condition matter more here than clamping force.
SpecificationsPeel forces on belting are commonly a few hundred newtons. A cell sized for the test piece rather than the frame is what makes the peaks readable rather than ripples on a flat line.
SpecificationsPeel forces on belting are moderate — commonly a few hundred newtons, more on heavy multi-ply constructions. A frame of 5 to 50 kN is ample; resolution at the low end matters far more than capacity, so the load cell is chosen for the test piece, not for the frame. Force accuracy to ISO 7500-1 Class 1 is the normal requirement.
The machine needs a driven grip that holds a steady 100 ± 10 mm/min and continuous logging through the whole peel, because the ISO 6133 analysis reads every peak and a logger that samples at intervals will miss them. Grips must hold a 25 mm rubber-faced strip square, without slipping and without cutting it: vice-action jaws with a face that grips the rubber are the usual choice.
Stroke is rarely the constraint: about 100 mm of peel is required, which any frame reaches. What matters is that the record runs unbroken from the first peak to the last.
Slipping grips are the commonest fault, and on rubber they are easy to miss because the piece creeps rather than jumping. Peeling too short a length and taking 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 the mean adhesion without the minimum leaves out half the result.
| 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 trace is analysed to ISO 6133, which gives a mean adhesion figure and a minimum adhesion figure, both reported per unit width. 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. Eight or more peaks give the ISO 6133 analysis enough of the trace to work with: the median peak is taken as the mean adhesion force and the lowest as the minimum adhesion force. Peeling too short a length and working from two or three peaks is the commonest way to produce an adhesion number that will not repeat.
Both methods specify a driven-grip speed of 100 ± 10 mm/min, with the record 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 are one of three exclusions in the scope, along with textile-reinforced belts with a full-thickness tensile strength of less than 160 N/mm and light conveyor belts as described in ISO 21183-1. 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.
A modest frame with good low-end resolution. Peel forces are commonly a few hundred newtons, so a 5 to 50 kN frame with a load cell sized for the test piece and force accuracy to ISO 7500-1 Class 1 is right. Stroke is rarely the constraint, since only about 100 mm of peel is needed. The two requirements that catch people out are a driven grip that will hold a steady 100 mm/min and logging continuous enough that every peak, and the lowest one, is actually in the record.
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 | Conditioned to ISO 18573 using atmosphere D or E — temperature control only, no humidity requirement — and tested immediately after conditioning | 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.
