When designing a conveyor belt, there is far more to consider than just the rubber cover compound. At the core of every belt is the carcass, the internal structure that delivers the strength, stability, and performance required for reliable operation. Getting the carcass design right is the first step to ensuring belt success in the field.

As conveyors in mining and bulk materials handling increase in capacity and grow in length, steel cord belt is emerging as the most common construction type. These belts can vary significantly in their arrangement and characteristics. For many OEMs, carcasses are often defined to customers only by their Steel Thread Rating (ST Rating). However, this does not guarantee a belt that’s compatible.

This understanding has become more important with the release of AS1333-2024, which explicitly describes some ST Ratings with optional carcass constructions, that use different Cord Pitches. Those familiar with the Standard will also know that the Cord Diameter is not defined. This means even if the Pitch is aligned with the Standard, Cord Diameters can vary between manufacturers.

When purchasing a replacement Belt it is best practice to specify not only the ST Rating, but also the Number of Cords in the Belt, the Cord Pitch and the Cord Diameter, as these variables do impact Belt performance, and ensure the Splice Design does not change.

When designing a steel cord belt, one fundamental requirement must be met: the number of cords and individual cord strength must result in a minimum belt breaking strength that meets or exceeds value defined in the ST Rating. Because cord strength is closely linked to diameter, this results in a carcass where total cord cross‑section is relatively consistent across constructions with the same ST Rating.

However, as highlighted below, there is a range of other factors that are considered when designing a steel cord belt carcass. Each of these factors contributes to the overall performance of a belt, and our ability to take them into account is improved by every piece of detail provided during the application process.

Cord Pitch and Number of Cords

Cord Pitch is the centre distance between the Cords in the Belt, and the Number of Cords is decided in part by both the Cord Pitch and Belt Width, while also ensuring a reasonable thickness protective of protective rubber, known as the Free Edge, remains between the Belt Edge and the outer Steel Cords.

The Cord Pitch and Number of Cords are usually aligned with one of the AS1333 options, or variants defined in the ISO / DIN standards. For bespoke applications, such as Overland Conveyors, and very high strength belts, a customised Cord Pitch and Number of Cord may be provided.

Alignment to Standards where possible is motivated by manufacturing considerations and to promote a degree of standardisation in the market, allowing for interchangability of belting across conveyors.

Cord Diameter

The ST Rating and Number of Cords in a belt tell us the minimum required Cord Strength. Cord Diameter is selected based on identifying a Cord that meets this Minimum Strength requirement.

Generally, Cord Diameter will increase with the required Cord Strength. So the larger the Cord, the fewer Cords required in the Belt.

Be aware that the specific Cord Diameter for a given ST Rating and Cord Pitch may vary depending on Cord Quality, Cord Construction, and a Manufacturers or Customers desire to use preferred (common) Cord Diameters.

Belt Mass, Thickness and Cost

Minimising belt mass is a key priority for applications where weight impacts power demand or where strict mass limits apply, common on stackers, reclaimers, and shiploader conveyors.

For any two versions of a specific ST Rated belt, the one with the smaller Cord Diameters will be lighter, thinner and often more cost effective. This because the Cord Diameter dictates the Belt Carcass Thickness, so a larger diameter Cords result in a thicker carcass and in turn a thicker Belt.

This increased thickness also leads to the need to add more Rubber to form the Carcass, and this increases the Belt Mass and also the Belt Cost in most cases.

Increasing Belt Mass can limit the length of Reels, requiring more splices to install belt on conveyors requiring multiple Rolls of Belt.

Where the Conveyor Structure is limited a heavy heavier belt may require Covers to be reduced, reducing the service life of a belt.

Pulley Diameters

Larger Diameter Cords will require larger Diameter Pulleys.

When consulting Datasheets, this effect is not always obvious, as the like of the ISO Standard for Pulley Diameters applies significant rounding to align with preferred Pulley Sizes.

Splice Length

There is no standard relationship between Cord Diameter, Cord Pitch and Splice Length due to the complexities of Splice Design.

However, the influence can be understood to a degree by considering two fundamentals of Splice Design.

• Step Length

Belts with larger Diameter cords will generally require a longer Step Length. This will mean a longer Splice if the Number of Stages remains the same.

• Number of Splice Stages

For a given Cord Pitch, larger Diameter Cords can require the number of Splice Stages to be increase making the Splice Longer.

But also, a smaller Diameter Cord will have a smaller Cord Pitch, so for a given Cord Diameter, a smaller Cord Pitch can also require the Number of Splice Stages to be increased making the Splice Longer.

Other Considerations

• Larger Diameter cords may be desirable in situations exposed to high localised impact loads due to their greater individual strength

• Many small Diameter Cords, Pitched closely together can result in ore loads being better distributed over a number of Cords.

• Smaller Diameter Cords may, in certain situations, result in a belt more tolerant of Short Transitions or Small Diameter Curves.

Is it better to have more small‑diameter cords or fewer large‑diameter cords?

This depends on the application and some situation may prefer large cords over small cords, and vice versa depending on the actual construction options available when aligning to Standards and preferred Cord Diameters.

While this may seem to overwhelmingly support the selection of smaller diameter cords, in practice this not the case, as Splice Design and Impact Performance are common decision drivers.

It is worth noting that these example constructions represent points on a range; any particular belt may lay anywhere upon that range, and its construction will depend on the needs of the application.

We also see in practice the alignment to preferred Cord Diameters can result in outcomes opposite to the above, where the nominal benefits are outweighed by the influence of rounding cord diameters upwards.

How to optimise a Steel Cord belt selection

At Fenner Conveyors, we optimise every steel cord belt design using the full range of factors outlined above. That’s why the level of detail provided during belt selection, application review, and quoting is so important, even information that may not seem relevant at first glance can influence the final carcass construction.

For replacement belts, critical construction details are often recorded on the original belt reels. If they are not, Fenner Diagnostics can perform Ultrasonic or X‑Ray inspection to determine carcass makeup non‑invasively. As a last resort, Fenner Service can conduct an invasive inspection and close the belt with a repair.