Rubber Conveyor Belt Tensile Testing: Standards, Sample Requirements, and Test Methods

Rubber conveyor belt tensile testing is a critical quality control procedure that evaluates the tensile strength, elongation at break, and elongation at a reference force of conveyor belts under standardized laboratory conditions. This article provides a comprehensive technical overview of rubber conveyor belt tensile testing, covering applicable international standards such as ISO 283 and GB/T 7984, sample preparation requirements, testing procedures, and typical performance parameters. Whether you are a belt manufacturer, procurement engineer, or quality assurance professional, this guide will help you understand how tensile test results determine the reliability and service life of conveyor belt systems.

Technical Overview

Rubber conveyor belts are the load-carrying backbone of bulk material handling systems in mining, ports, power plants, cement works, and metallurgical facilities. During operation, a belt is continuously subjected to longitudinal tension from the drive pulley, impact loading at loading points, and bending fatigue around pulleys. The tensile properties of a conveyor belt — including its full-thickness tensile strength, elongation at break, and elongation at a specified reference force — directly determine whether the belt can safely withstand the maximum working tension without excessive stretch or catastrophic rupture.

Rubber conveyor belt tensile testing is performed in accredited third-party laboratories or in-house quality control facilities using universal tensile testing machines. The test pulls a rectangular specimen of the full belt construction (or a defined sub-component) to failure while continuously recording force and elongation. The results are compared against the specifications of product standards such as ISO 283, ISO 14890, GB/T 7984, GB/T 10822, DIN 22102, and EN ISO 283 to verify conformity of the belt carcass and cover grades.

• Purpose: verify carcass strength class, elongation behavior, and overall structural integrity
• Primary standards: ISO 283, GB/T 7984, GB/T 5754, DIN 22102, EN ISO 283, ISO 14890
• Equipment: constant-rate-of-extension (CRE) universal testing machine with suitable grips and extensometry
• Typical deliverables: test report listing tensile strength, elongation at break, and elongation at reference force

Testing Samples

Full-Thickness Tensile Specimens

The standard specimen for full-thickness tensile testing is a rectangular strip cut from the finished belt, containing all textile plies and both covers. According to ISO 283, the preferred specimen width is 50 mm with a gauge length of 200 mm (a 100 mm gauge length may be used for belts whose elongation is high, provided it is stated in the report). Specimens are cut in both the longitudinal (warp) direction and, when required, the transverse (weft) direction, with a minimum of three specimens per direction taken from positions evenly distributed across the belt width and at least 50 mm from the belt edge.

Cover Rubber Dumbbell Specimens

To characterize the rubber compound itself, cover samples are sliced from the top and bottom covers and die-cut into Type 1 or Type 2 dumbbell specimens in accordance with ISO 37, ASTM D412, or GB/T 528. These specimens are used to measure the tensile strength and elongation at break of the cover rubber, which relate to cut resistance, tear resistance, and impact performance in service.

Steel Cord and Steel Cord Belt Specimens

For steel cord conveyor belts conforming to DIN 22131, ISO 7622, or GB/T 9770, additional specimens include individual steel cords extracted from the belt for cord breaking strength determination, and full-thickness belt sections for longitudinal and transverse tensile evaluation. The adhesion between the steel cord and the surrounding core rubber is evaluated by cord pull-out (H-pull or similar) test pieces per ISO 7623 or DIN 22131-4.

Sample Conditioning

All specimens must be free of visible defects, and the cut edges must be clean and parallel. Specimens are conditioned at a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 16 hours prior to testing. Fabric belts should not be tested within 5 days of vulcanization unless otherwise agreed.

Testing Items

A complete rubber conveyor belt tensile evaluation typically includes the following test items:

• Full-thickness longitudinal tensile strength (kN/m): the maximum force per unit width sustained by the complete belt construction
• Full-thickness transverse tensile strength: measured when specified by the belt class or purchase agreement
• Elongation at reference force: the elongation measured at 10% of the nominal full-thickness tensile strength, critical for belt sag and take-up design
• Elongation at break: total extension at the moment of rupture, expressed as a percentage of gauge length
• Cover rubber tensile strength and elongation at break: measured on dumbbell specimens from the top and bottom covers
• Interply adhesion strength (related tensile-shear evaluation of the carcass structure)
• Steel cord breaking strength and cord-to-rubber adhesion strength for steel cord belts
• Reference force verification for belt strength classes such as EP 400/3, EP 800/4, NN 200, ST 1250, and similar designations

Testing Methods

Full-Thickness Tensile Test (ISO 283 / GB/T 5754)

The specimen is clamped in the grips of a CRE-type universal testing machine with the gauge length set to 200 mm. A pre-tension of 1% of nominal tensile force (or as specified) may be applied to remove slack. The machine is set to a constant separation speed of 50 mm/min (or 100 mm/min where agreed), and the specimen is pulled to rupture. The maximum force is recorded and converted to tensile strength in kN/m by dividing by the specimen width. Elongation at the reference force and elongation at break are calculated from the recorded force-extension curve. The median of at least three valid results is reported.

Cover Rubber Tensile Test (ISO 37 / ASTM D412 / GB/T 528)

Dumbbell specimens are marked with bench marks at the standard gauge length, clamped in pneumatic or manual grips, and extended at 500 mm/min for Type 1 specimens until failure. Tensile strength is calculated from the maximum force and the original cross-sectional area, and elongation at break is calculated from the extension between bench marks. Three specimens per cover are tested and the median values are reported.

Steel Cord Belt Tensile Test (ISO 7622 / DIN 22131)

For steel cord belts, longitudinal tensile strength is determined on full-width samples containing a known number of cords, and the breaking strength is normalized to kN/m of belt width. Individual cord breaking force and cord pull-out adhesion are tested separately to verify the load-carrying capacity of the carcass and the integrity of the cord-to-rubber bond.

Result Evaluation

The measured values are compared with the minimum requirements of the declared belt type. For example, a belt designated EP 800/4 must achieve a full-thickness longitudinal tensile strength of at least 800 kN/m, and the elongation at reference force must fall within the typical limit of 4% or less for polyester-nylon carcasses, as specified by the relevant product standard. Failure to meet any single requirement constitutes non-conformity of the belt type.

Applications

Tensile testing data for rubber conveyor belts is used throughout the product lifecycle:

• Type approval and certification: demonstrating conformity of new belt designs to ISO, EN, DIN, ASTM, JIS, and GB standards before market release
• Factory quality control: batch-by-batch verification of carcass strength and cover compound properties during manufacturing
• Incoming inspection: end users in mining, ports, power plants, and cement plants verifying supplier declarations before installation
• Engineering design: providing elongation-at-reference-force values for calculating take-up travel, belt sag, and drive tension requirements
• Failure analysis: comparing retained strength of a degraded or damaged belt against original specifications to assess remaining service life
• Purchase contracts and tenders: objective acceptance criteria between belt suppliers and purchasers

FAQ

What is the difference between tensile strength and elongation at reference force?

Tensile strength is the maximum force per unit width that the belt can sustain before rupture, expressed in kN/m. Elongation at reference force is the stretch measured when the belt is loaded to 10% of its nominal tensile strength; it indicates how much the belt will permanently extend under normal working tension and is essential for correct take-up and conveyor design.

Which standard should be used for fabric conveyor belt tensile testing?

ISO 283 (also adopted as EN ISO 283) and the corresponding GB/T standards are the most widely applied methods for full-thickness tensile testing of textile carcass conveyor belts. Belt performance requirements are specified in ISO 14890, GB/T 7984, and DIN 22102 depending on the target market.

How are specimens cut from a finished belt?

Square-edged rectangular strips are cut with widths of 50 mm, with the long axis parallel to the belt length. At least three specimens are taken from positions distributed across the belt width, away from the edges and any splices or joints, and the covers are retained intact unless the standard specifies trimming.

How long does a typical tensile test take?

The mechanical test itself is usually completed within 10 to 20 minutes per specimen. Including specimen preparation, conditioning, and reporting, a full set of longitudinal and transverse tests is typically completed within 3 to 5 working days at an accredited third-party laboratory.

Why did a belt pass tensile strength but fail elongation at reference force?

This occurs when the fabric construction or the rubber-fabric adhesion allows excessive stretch at low loads, even though the ultimate breaking force is sufficient. Excessively high elongation at reference force causes belt creep, mistracking, and insufficient take-up reserve, so the belt is still considered non-conforming even if the breaking strength meets the minimum requirement.