What the ST Rating Means
ST stands for Steel (cord) Tensile. The number is the minimum break strength of the belt in Newtons per millimeter of width (N/mm), which is equivalent to kN/m.
So ST1250 means a steel cord belt with minimum break strength of 1,250 N/mm (1,250 kN/m).
This is significantly higher than EP belts β the strongest standard EP belt (EP630) has a break strength of 630 kN/m, while steel cord starts at ST630 and goes to ST5000 and beyond for specialist applications.
Standard ST Ratings
| Rating | Break Strength | Typical Cord Diameter | Typical Application |
|---|---|---|---|
| ST630 | 630 N/mm | 4β5mm | Port conveyors, shorter overland |
| ST800 | 800 N/mm | 5β6mm | Medium overland, heavy in-plant |
| ST1000 | 1,000 N/mm | 6β7mm | Overland mine conveyors |
| ST1250 | 1,250 N/mm | 7β8mm | Longer overland, heavy mining |
| ST1600 | 1,600 N/mm | 8β9mm | Major overland mine systems |
| ST2000 | 2,000 N/mm | 9β10mm | Long-distance high-tension |
| ST2500 | 2,500 N/mm | 10β11mm | Major mining overland |
| ST3150 | 3,150 N/mm | 11β13mm | Longest overland systems |
| ST4000 | 4,000 N/mm | 13β15mm | Extreme tension applications |
| ST5000 | 5,000 N/mm | 15mm+ | World's longest conveyors |
How to Determine Which ST Rating You Need
The ST rating is determined by a belt tension calculation. The key inputs:
- Conveyor length β longer conveyors build more tension in the belt
- Lift height β raising material against gravity adds tension
- Belt width β wider belts carry more load per meter
- Tonnage (t/h) β heavier loading increases tension
- Belt speed (m/s)
- Starting/stopping dynamics β transient tensions during start can be 1.5β2Γ steady state
The required ST rating = Maximum belt tension (N/mm width) Γ Safety factor (typically 6.7β7 for steel cord)
Without detailed calculation, rough guidance:
| Conveyor Length | Lift (approx) | Typical ST Rating |
|---|---|---|
| Up to 500m | Flat to slight incline | ST630βST1000 |
| 500mβ2km | Moderate lift | ST1000βST2000 |
| 2kmβ5km | Variable | ST2000βST3150 |
| 5km+ | Variable | ST3150βST5000 |
| Major Pilbara/Atacama overland | High tonnage, long | ST3150βST5000 |
These are broad estimates. A proper calculation is always required for new installations.
Key Differences Between ST Ratings
Cord diameter. Higher ST ratings use thicker steel cords. Thicker cords are heavier and require larger minimum pulley diameters.
Minimum pulley diameter. Steel cord belts require large pulleys because the cords cannot be bent sharply:
| ST Rating | Approximate Minimum Drive Pulley Diameter |
|---|---|
| ST630 | 630β800mm |
| ST1000 | 800β1000mm |
| ST1600 | 1000β1250mm |
| ST2500 | 1250β1600mm |
| ST4000 | 1600β2000mm |
This is a critical constraint. If you specify ST2500 on a conveyor with 1000mm pulleys, the belt will be damaged at every pulley revolution.
Belt weight. Higher ST rating means heavier belt per meter. This adds to conveyor running load and affects drive power calculations.
Splice requirements. Higher-tension steel cord splices are more complex and require larger overlap lengths and more careful preparation. ST5000 splicing is a specialist operation.
ST630 β Where Steel Cord Starts
ST630 represents the lowest steel cord rating. It overlaps with high-end EP belt (EP630) in terms of break strength but offers steel cord's main advantage: very low elongation (0.1β0.25% at working load vs 1.5β2% for EP).
Typical ST630 applications: - Port and terminal conveyor systems - Shorter overland conveyors (under 1km) where low stretch matters - Underground main haulage where EP stretch causes take-up travel problems - Replacement of EP630 belts where tension is adequate but stretch is the issue
ST2500 and Above β Major Mining Applications
ST2500, ST3150, and ST4000 are specified for the world's major overland conveyor systems:
- Chilean copper mines (Escondida, Collahuasi, Los Pelambres)
- Australian iron ore (Pilbara overland systems)
- Brazilian iron ore (Vale CarajΓ‘s)
- South African coal export lines
These ratings require specialist engineering, large-diameter pulleys (often 1600β2000mm), and highly trained splicing crews. The conveyor engineering for ST3000+ systems is not routine β it requires specialist conveyor engineers.
Frequently Asked Questions
Our EP500 belt keeps breaking at the splice. Should we switch to steel cord? Splice failure on EP belt usually indicates a poor splice, not inadequate belt rating. Steel cord doesn't solve splice problems β steel cord splices are actually more demanding, not easier. Investigate why the EP splice is failing before specifying steel cord.
Can we use a lower ST rating than the calculation requires to save cost? No. The safety factor already accounts for real-world loads above steady state. Using a belt below the calculated requirement means the safety factor is compromised, which increases the risk of catastrophic belt failure under starting load or overload.
Do you stock steel cord belts in high ST ratings? We stock ST630 and ST1000 in common widths. ST1600 and above are manufactured to order β production time 25β40 days depending on rating and width.
What is the maximum ST rating you manufacture? ST5000 as standard. Higher ratings are achievable on inquiry for specific projects.
Contact Elephant Rubber
We manufacture steel cord conveyor belts from ST630 to ST5000. Contact us with your conveyor tension calculation or conveyor dimensions for a specification.