Self-identifying
Load Cells
Pull-out force on a single cord is commonly in the low kilonewtons. A cell sized for the cord rather than for a belting frame is what keeps the initial and aged figures distinguishable from each other.
SpecificationsTesting standard
ISO 7623
Steel cord conveyor belts — Cord-to-coating bond test — Initial test and after thermal treatment
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 7623 pulls a single steel cord out of the rubber coating of a conveyor belt and reports the bond strength, first on the belt as made and again after a thermal treatment. It applies to metal-carcass belts only. Both ISO 7623:2015 and ISO 7623:2022 are in circulation, so confirm which edition a specification names.
At a glance
- Test type
- Peel & adhesion — a bonded joint is pulled apart
- Published by
- ISO
- Edition
- ISO 7623:2022
- Material
- Conveyor & transmission belts
- Runs on
- Series 7200 and Series 9000
What the test does
A specimen is cut from a steel cord conveyor belt so that individual cords are exposed at one end. A single cord is gripped and pulled out of the surrounding rubber coating while the rubber block is restrained, and the force needed to strip it is recorded. The test is run on the belt as made, and again on specimens that have been through a thermal treatment, so the bond can be compared before and after heat ageing.
What it measures, and why it matters
A steel cord belt carries its entire load in the cords. The rubber does not carry tension; it holds the cords in place, transfers load into and out of them at the splice, and keeps water away from the steel. Cord-to-coating bond decides whether it can.
The consequence of a poor bond is not a broken belt but a failed splice. A steel cord splice works by overlapping cords in rubber and relying on the bond along their length to transfer force from one belt end to the other. If the bond is weak, the cords pull through the rubber under tension, the splice elongates and then separates, and the belt fails at the one place that cannot be repaired quickly.
The after-thermal-treatment result is the more commercially interesting of the two. Heat ageing simulates service in a hot environment or the effect of a hot splice cure, and a compound whose initial bond is excellent and whose aged bond falls away is a compound that will disappoint in service. Reporting only the initial figure hides exactly the behaviour the method was written to find.
Specimen, and the cord that must not be damaged
The bond being measured is a few millimetres of interface around one cord. Anything that disturbs it during preparation is invisible in the result and changes it.
- Material
- Steel cord conveyor belt — metal carcass only
- Conditions tested
- Initial, and after thermal treatmentBoth are part of the method. Reporting only the initial figure hides the behaviour the standard was written to find.
- Specimen
- Cut with the cords running in the direction of pull
- Embedded length
- As specified in the standard
- Ageing
- To the schedule the standard sets, before test
- Both sets from the same belt sample
- DakOtherwise the comparison is between two belts, not between two conditions, and the ageing conclusion is worthless.
- Sample across the belt width
- DakCord embedment varies with position, particularly near the edges.
Rate, and what is recorded
- Loading
- The cord is drawn out of the restrained rubber block
- Rate
- As specified in the standard; not publicly quotable
- Reported
- Bond strength, initial and after thermal treatment
- Discard cords that break rather than strip
- DakA cord that breaks in the grip measured the cord, not the bond. It is a rejected specimen, not a low result.
What the two numbers say
Cord-to-coating bond—
The force required to strip one cord from its coating
Reported for the initial condition and again after the thermal treatment.
Retention after ageing—
Aged bond strength compared with initial bond strength
- initial
- bond on the belt as made
- aged
- bond after the specified thermal treatment
The commercially interesting figure. A compound with an excellent initial bond and poor retention will disappoint in a hot duty or after a hot splice cure.
Why the splice is what is really being tested—
A steel cord splice transfers force through this bond alone
Weak bond means the cords pull through the rubber under tension: the splice elongates, then separates. It is the one place on a steel cord belt that cannot be repaired quickly.
How the test runs
- 01Cut specimens from across the belt width, cords along the pull direction.
- 02Prepare the specimens so one cord is free at one end and its embedded length is intact.
- 03Set aside one set for the initial test and one for thermal treatment.
- 04Age the second set to the specified schedule.
- 05Restrain the rubber block squarely around the cord with the support plate.
- 06Grip the free cord end without crushing or nicking it.
- 07Pull at the specified rate until the cord strips from the coating.
- 08Record the force; reject any specimen where the cord broke instead.
- 09Repeat for the aged set.
- 10Report both conditions, and the retention between them.
The support plate hole is the detail that decides whether this test measures anything. Too large and the rubber bulges through it, so the effective embedded length is no longer what the specimen was built to.
The fixture this method needs
What the report has to contain
- Reference to ISO 7623 and the edition — 2015 or 2022
- Belt construction, cord diameter and pitch
- Coating compound where known
- Embedded length
- Where across the belt width the specimens were taken
- Thermal treatment schedule
- Bond strength for each initial specimen
- Bond strength for each aged specimen
- Any specimen rejected because the cord broke, and why
- Mean for each condition and the number of specimens
What the machine must be capable of
Pull-out force on a single cord is modest — commonly in the low kilonewtons — so a frame of 5 to 50 kN is ample and low-end resolution matters far more than capacity. Force accuracy to ISO 7500-1 Class 1 is the normal requirement.
The fixture is what makes this test work. The rubber block has to be restrained squarely around the cord while the cord itself is gripped without being crushed or nicked, because a damaged cord breaks in the grip rather than stripping out of the rubber. That means a cord grip suited to the cord diameter and a support plate with a hole that clears the cord and bears on the rubber. Where nothing standard fits the construction, the fixture is made to the specimen and to the method.
A heating oven or an ageing cabinet is required for the thermal treatment; that is separate equipment from the testing machine, and a laboratory offering this method needs both.
What goes wrong in practice
Cords nicked during specimen preparation break instead of stripping, and the result is discarded — or worse, reported. A restraint plate whose hole is too large lets the rubber bulge and the effective embedded length changes. Grips that crush the cord do the same thing at the other end. And running only the initial test when the specification asks for both conditions is a reporting failure that is easy to miss, because the initial figure on its own looks complete.
Steel cord belts against textile belts
The two constructions fail differently and are tested by different documents. Substituting one method for the other is not possible.
| Steel cord belt | Textile belt | |
|---|---|---|
| Adhesion method | ISO 7623, cord pulled from coating | ISO 252, layers peeled apart |
| Tensile method | ISO 7622-1 and -2 | ISO 283 |
| Joint | Vulcanised splice, ISO 8094 | Splice or mechanical fastener, ISO 1120 |
| Load carried by | The cords alone | The woven carcass |
| Rubber does | Holds cords, transfers splice load, excludes water | Carries part of the structure |
ISO 252 has no meaning on a steel cord belt — there are no plies to separate. Specifications occasionally call for it anyway, and the right answer is to query the specification rather than to improvise a specimen.
Questions we are asked about this test
What is ISO 7623?
ISO 7623 is the international method for the cord-to-coating bond of steel cord conveyor belts. A single cord is pulled out of the surrounding rubber while the rubber block is restrained, and the force is recorded — once on the belt as made, and again on specimens that have been through a specified thermal treatment. It applies exclusively to metal-carcass belts.
Which edition is current?
Both ISO 7623:2015 and ISO 7623:2022 appear in publisher and distributor catalogues, and the national adoptions follow both: DIN EN ISO 7623:2016 carries the 2015 text and DIN EN ISO 7623:2023-02 carries the 2022 text. In practice this means checking which edition a customer specification names rather than assuming the newest, because both are being cited in the field.
Why is the after-ageing result the important one?
Because it is the one that predicts service. The thermal treatment stands in for a hot operating environment and for the heat of a splice cure, and a compound whose initial bond is excellent but whose aged bond falls away will fail in exactly the situation it was bought for. A report giving only the initial figure looks complete and has left out the finding.
What does a poor cord bond actually cause?
A failed splice rather than a broken belt. A steel cord splice works by overlapping cords in rubber and relying on the bond along their length to transfer force from one belt end to the other. If that bond is weak the cords pull through the rubber under tension, the splice elongates and then separates. It is the one part of a steel cord belt that cannot be fixed quickly, so the consequence is unplanned downtime.
Why do some specimens have to be discarded?
Because a cord that breaks in the grip has measured the cord, not the bond. This happens when the cord was nicked during specimen preparation or crushed by the grip. It is a rejected specimen, and recording it as a low bond figure corrupts the mean. The report should say how many were rejected and why.
What fixture does it need?
A support plate that restrains the rubber squarely around the cord, with a hole that clears the cord without letting the rubber bulge, and a cord grip matched to the cord diameter that holds without crushing. Both are dimension-specific to the belt construction. Where nothing standard fits, Dak builds the fixture to the specimen and to the method rather than adapting an unrelated one.
Can ISO 252 be used instead on a steel cord belt?
No. ISO 252 peels apart layers, and a steel cord belt has no plies to separate — it is cords in a rubber matrix. The two methods measure different things on different constructions. Where a specification asks for ISO 252 on a steel cord belt the correct response is to query it, not to invent a specimen that fits the words.
Running ISO 7623 on the Series 7200 and Series 9000
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 kN on a single cord | 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 | A support plate restraining the rubber block around the cord, with a cord grip matched to the cord diameter | Our a fixture built for this method, built to the specimen |
| Environment | An ageing oven is required for the thermal treatment set, in addition to the testing machine | 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.