Conveyor Belt Steel Cord Specifications: Tensile Strength, Thickness and Carry Capacity Explained

A steel cord belt enquiry lands on our desk as one line, most days. ST1600, 1,200 mm, 6 m/s, 400 m long. By then somebody upstream has already chosen the grade, so the rest of the conversation is price and delivery. Then the same buyer reappears, maybe two years on, holding a splice that has started to walk, a drive pulley whose lagging has shark-finned, or a cover worn through to the cables across the first 180 m of carry. The grade they picked? Probably right. What sat behind it was thin.

We're Ningbo Sinoconve Belt Co., Ltd., trading as SINOCONVE. Belting has been our business since 1988, and ten production lines run here today, eight on fabric core and two devoted to steel cord. In that time more than 1,500 industrial customers across mining, ports, cement, steel, power and EPC have bought from us. What this page covers is the three numbers that decide whether a conveyor belt steel cord purchase holds up once it is in service. Tensile strength, thickness, carry capacity. Not three boxes to tick, but one engineering decision looked at from three sides.

The job of a real conveyor belt manufacturer is to work backward from your duty to a grade, then show you the arithmetic on paper. If your supplier only echoes the grade you already named, you are doing the design work and carrying the risk for them. We would rather lose a quotation for asking too many questions than win one that fails at the splice.

Send Your Duty, Tension and Pulley Sizes - We Will Return the Full Steel Cord Specification

Everything below is written for whoever signs off the specification, not for whoever reads the brochure. We'll show you how an ST grade gets derived instead of assumed. We'll show why cord geometry sets the splice and pulley dimensions. Cover grade and cover thickness follow the material you carry, and carry capacity is a chain of four variables, never one tonnage figure.

01What a Conveyor Belt Steel Cord Carcass Actually Contains

Take a steel cord carcass apart and you find one layer of parallel galvanized cables running lengthwise, bonded in rubber, no woven fabric anywhere. The cables are the load path, full stop. Everything else in the belt is there to hold them in position, protect them, and let them bend around pulleys without the rubber tearing away from the wire.

On a datasheet that carcass is defined by four quantities. Cord diameter, in millimetres. Cord pitch, centre to centre between adjacent cables. Rated breaking strength, quoted in newtons per millimetre of belt width, and that is the number which becomes the ST designation. Then cord construction, written in wire-rope shorthand such as 7x7 or 7x19, describing how the strands inside each cable are laid up. That last one governs fatigue life on small pulleys, and buyers almost never ask for it.

When the duty leaves no margin, steel cord is the answer. At working tension the carcass stretches roughly 0.2 to 0.4 percent; an EP fabric belt of comparable rating goes 1.5 to 3 percent. Put that on a 3 km overland conveyor and the gap shows up in your take-up travel, in the energy the belt swallows at every start, and in whether it survives a choke-fed stop. We measured a fabric carcass on a 1,200 t/h coal line creeping 4.6 m across eighteen months of thermal cycling. Same route, steel cord replacement, 0.9 m of the same take-up stroke.

Short duty, light load, spliced in the field? A rubber conveyor belt in EP construction costs less and serves you just as well. Steel cord earns its price on long centres, high tension, high lift, and anywhere a fabric splice would be the weakest link in the circuit. There is no prestige in paying for cables you don't need.

So where does an industrial conveyor belt specification sheet go wrong most often? Hardly ever in the grade. It goes wrong in the four values nobody bothers to write down. Cord pitch, minimum pulley diameter, splice length, take-up travel. Geometry fixes all four of them, we cover that geometry in the next two sections, and it is the reason two belts carrying the same ST number are not interchangeable.

The Sub-Specifications Buyers Skip on a Steel Cord Datasheet

A good datasheet prints cord pitch; a lazy one leaves it off. Ask anyway, because pitch decides how many cables cross your belt and how much work the splice becomes. The minimum pulley diameter the supplier will guarantee in writing? Get that too. So the recommended splice length and the number of stages. And the belt mass per square metre, since that number reaches your drive sizing and your idler loading. Any supplier who can't answer those four in a single email isn't a belt maker worth the freight.

02How an ST Grade Is Calculated, Not Chosen by Habit

ST means steel cord, and the number behind it is the nominal breaking strength of the finished belt, in newtons per millimetre of width. ST1000 is rated to break at or above 1,000 N for every millimetre of width, which puts a 1,200 mm wide ST1000 belt at roughly 1,200 kN in the test frame. Not a marketing figure, that. It is a measured value from a cured sample under ISO 15236, and it is the ceiling your splice and pulley layout have to respect.

Watch the direction of the logic here, because it is where most specifications come apart. The grade is an output, not an input. Deriving it takes the maximum steady-state running tension your conveyor develops, expressed in newtons per millimetre, and the safety factor your duty justifies. Multiply one by the other and out comes a required breaking strength. Round up to the next standard grade, and no further. Two grades above the calculation adds mass, adds pulley diameter, adds cost, and adds nothing to service life.

Maximum tension comes out of a conveyor calculation, never off a catalogue page. It is the highest tension anywhere on the carrying run, usually just behind the drive pulley when the belt is loaded and inclined, and it depends on how much tonnage moves, how much lift the route has, how long the centre is, how much friction the moving parts carry, how the idlers drag, how hard you accelerate at start-up, and how much friction the loading point adds. Nudge the incline by three degrees on a 1.5 km conveyor and that maximum can move twenty percent. Ask the designer for the tension diagram, not one number on a general arrangement drawing.

What the calculation can't cover, the safety factor does. Transient shock where the material hits the belt. Sag out between the idlers. A splice quietly degrading through ten years of cycles and four hundred thousand starts. For steel cord in normal service we treat a steady-state factor of 6.7 as a practical floor, and we work at 8 where the duty brings heavy lump impact, frequent starting, or a belt expected to run two decades rather than five. On a conveyor with an engineered soft-start and belt changes already written into scheduled stops, some operators push to 5. We wouldn't sign that off on a downhill regenerative conveyor, where the belt is holding back a loaded incline.

Read those same numbers the other way round and Table 1 does the work. Divide the nominal breaking strength of a class by the maximum operating tension published against it. ST630 and ST1000 land near 8 to 1. From ST1250 upward the ratio tightens to about 6.7, and that is why ST1600 at 1,600 N/mm carries a published maximum of 240 kN/m for every metre of width, while ST4000 carries 600 kN/m. The ratio, not the class number, is the safety margin you are actually buying, and the standard sets it, not whoever happens to be quoting.

Worked Example: From Tension to Grade in Three Lines

Say a 1,400 mm wide overland conveyor comes back from the calculation with a maximum running tension of 168 N/mm on the loaded incline. Tension times width gives 235 kN of pull on the belt. ST1600 allows 240 kN/m, so across 1,400 mm it permits 336 kN, and the class holds with real margin left for a choked restart. Now extend the route by 600 m and let the tension climb. At 320 N/mm that same 1,400 mm belt is carrying 448 kN, and 448 kN is past the 420 kN ST2000 allows on that width, so the class moves up to ST2500 at 525 kN. Nothing about the material changed. One number did, and the grade followed it.

That is the whole method, then. Tension out of the calculation, factor out of the duty, product out of arithmetic, grade out of the standard series. Everything after this is about making sure the belt you receive can actually deliver the number you bought.

Table 1 lays out the grades we build most often, with the cord geometry and belt mass that travel with each one. Read it as a family of products, not a price list.

Go back and read that table a second time, because three of its columns drive everything after this. Belt mass nearly doubles between ST630 and ST4000, and every kilogram per square metre you add is mass the drive has to accelerate on each start, mass the idlers carry for twenty years. Cord diameter climbs with the class, and cord diameter is what sets your pulley diameters and splice lengths. Cord pitch widens more slowly, and it decides how many cables end up inside your splice and how much rubber is left between them. Cover thickness rises with the class too, since a thicker cable needs more rubber above and below to embed properly. That is the only reason a belt gets thicker at all.

Two suppliers can both quote ST1600 and hand you belts that aren't interchangeable. One runs 6.5 mm cords on a 13.5 mm pitch, the other 7.5 mm cords on a 15 mm pitch, and suddenly the mass differs, the pulley requirement differs, and so does the splice. So insist that cord diameter and cord pitch are printed on the steel cord conveyor belt datasheet, and treat the ST number as a family name rather than a full specification.

Inside the safety factor there is a second trap. Buyers inherit a factor from an old fabric belt quotation, apply it to steel cord, and the grade climbs one or two steps for no benefit at all. What it buys you is extra mass, extra pulley diameter, extra cost. And a conveyor belt supplier who gains from that oversizing is not the right party to check the arithmetic on your behalf, so ask for the calculation rather than the conclusion.

The cut end of a steel cord belt shows what the grade really is. Count the cables, measure the pitch, and compare them with the datasheet.

03Cord Diameter and Pitch Decide Your Splice Length and Pulley Diameters

Cord pitch is the distance from the centre of one cable to the centre of the next, measured across the belt width. A small number, normally somewhere between 10 and 18 mm, and it quietly controls three things that dominate both cost and service life on a steel cord installation.

First, it sets how many cables the splice has to couple. A 1,200 mm wide belt on a 12 mm pitch carries about 100 cords across its width. On a 16 mm pitch, only 75. Every one of those cables gets stripped of rubber, cleaned, overlapped with its neighbour and cured on its own, and each overlap needs a bond length that grows with cord diameter. Thicker cables on a wider pitch leave you a longer and far stiffer splice zone.

Second, the finished length of the splice. On a steel cord belt a single-stage splice typically finishes between 1.0 and 1.5 m. Once the bond length each cord needs outgrows what one stage can hold, you move to two stages or three, and the finished length roughly doubles or triples. Extra press time and extra consumables are the least of it. You now have a longer stretch of belt that is thicker and stiffer than the parent belt, and it has to bend over every pulley on the route. We have seen a 2.6 m three-stage splice crack along the stage boundary inside a year, simply because the head pulley could not accept it comfortably.

Two rules from the splice standards keep that geometry honest. On a single cord, overlap should run to at least 100 times the cord diameter, which means a 5 mm cord needs 500 mm of bond and a 7 mm cord needs 700 mm before you add any margin. Between adjacent cords leave at least 2 mm of rubber, or 0.3 times the cord diameter, whichever is greater, and that is what stops a tight pitch from squeezing the bond rubber out of the joint. Class drives the number of stages too. Single stage up to ST2000 is our practice, two stages from ST2500 to ST3150, three stages from ST3500 to ST4500. Every stage you add brings length and stiffness, so pulley diameter and splice design have to be settled together rather than one after the other.

Third, minimum pulley diameter, and this is the specification error we are asked to correct most often. Steel cables do not tolerate bending below a certain radius. Bend them tighter and the outer wires of each strand work harden and fatigue, the rubber can no longer hold them, and you find the failure as individual cord breakage near the belt edge, or a pocket of loose cables just downstream of a pulley. ISO 15236-1 deals with pulley sizing as a function of belt rating, pulley group and how much of the belt breaking strength you actually use. Our working rule is roughly 160 to 200 times nominal cord diameter for a main drive pulley at full utilization, and about 80 to 125 times cord diameter for a snub, bend or take-up pulley. The class tables our desk works from put the minimum drive pulley for a 4.9 mm cord at 1,000 mm, and for a 7 mm cord at 1,250 mm.

Run the arithmetic once and the constraint jumps out. A 7 mm cord on a fully utilized drive pulley wants somewhere between 1,120 and 1,400 mm of diameter, which lands you on a 1,000 mm or 1,250 mm pulley, not the 800 mm that may have served a fabric belt of the same width. If the head pulley on site is 800 mm and the new belt is ST2000, no amount of competent splicing rescues the cable. Either the pulley diameter changes, or the grade comes down.

Table 2 is the ratio sheet our designers reach for before any pulley size gets confirmed to a customer.

Two more numbers follow from the same geometry. Take-up travel at the tail must cover the elastic stretch of the belt, the permanent creep that accumulates over the first year, and the allowance the splice consumes when the belt is shortened. On a steel cord belt the elastic component is small in absolute terms, but it is not small in design terms, because a take-up sized for a fabric belt is usually generous and a take-up sized by habit is usually wrong in the other direction. Belt length tolerance must also be tighter than what a fabric supplier would accept, since you cannot recover two metres of length from a take-up engineered for one.

Transition distance from the last troughed idler to the pulley deserves the same care. It has to be long enough for the belt to flatten out of a 35 degree trough into a flat wrap without pinching the edges. Steel cord belts are stiffer across the width than fabric belts, so they need more distance rather than less. Short transitions are one of the commonest sources of edge cracking on machines that were converted from fabric to steel cord.

When we quote, this is the exact sequence our engineering desk runs on every belt that leaves the conveyor belt factory. Cord diameter first, then pitch, then splice, then pulley clearance, then take-up. Only afterwards does anyone discuss price.

04Cover Grade and Cover Thickness Follow the Material You Carry

The cover is the only part of the belt that touches the material, and it is the part most often chosen from memory. Cover grade is not one single scale. DIN 22102, ISO 15236, AS 1332 and the RMA grades each publish their own letters and their own test limits, and meeting one of them does not make a belt compliant with another. One point of housekeeping before you compare them. DIN 22102 covers the rubber on textile carcass belts, while DIN 22131 is the companion designation standard for steel cord belts, so an enquiry written to German practice should name the second one. A quotation that says "wear resistant cover" and stops there tells you nothing you can verify, and it is usually a sign that the compound exists in a looser tolerance band than the one you need.

Three numbers define any cover grade. Tensile strength of the cured compound in megapascals. Elongation at break in percent. Abrasion loss in cubic millimetres, measured on a rotating drum. Of those three, abrasion loss is the number that decides how long the belt lasts on a sharp, dry material, and it is the number most often left off a datasheet. Hardness in Shore A tells you how a sample felt on the day it was tested, and nothing about how it will behave under a 400 mm lump landing from four metres up.

Table 3 compares the grades across the standards you will meet most often on a steel cord enquiry.

Note what kind of limit each column carries. Tensile and elongation are minimums the compound must reach. Abrasion loss is a maximum it must not exceed, so a lower figure is the better figure. When a supplier quotes an abrasion number with no units, it could be a volume loss in cubic millimetres or a weight loss in milligrams, and the two are not interchangeable.

Look closely at the two most common DIN grades, because the relationship between them is counterintuitive and it regularly catches out experienced buyers. DIN-W has the lower tensile strength at 18 MPa, yet it carries the best abrasion ceiling in the table at 90 mm3. DIN-X is the stronger compound at 25 MPa, and its abrasion ceiling of 120 mm3 is worse. On dry, sharp, screened ore the W compound will outlast the X. The X earns its place where the cover must also resist gouging and tearing, or where the rubber has to survive contact heat on top of sliding wear. Choosing the grade with the highest megapascal figure is not the same as choosing the grade that wears longest.

Region changes the answer again. A buyer in Australia specifies to AS 1332 and asks for an A grade, which caps abrasion at 70 mm3 and is the toughest cover in that standard. A buyer in North America writes RMA-I, where the ceiling is 150 mm3 and the tensile demand is 17 MPa. Those two specifications are not equivalents, and swapping them on a sharp iron ore duty shows up in the wear record within a year. Where a project makes the local standard contractual, insist that the datasheet also quotes the nearest ISO 15236 or DIN equivalent so the compound can be compared like for like instead of by letter. Our own abrasion resistant conveyor belt sheets print both sets of values for that reason.

We keep a returned section from a basalt quarry as a teaching sample. The belt was specified DIN-X because the buyer equated the higher tensile figure with durability. It had lost 9 mm of a 10 mm top cover in fourteen months. The DIN-W belt on the parallel line was still at 6 mm of its original 10 mm after two years. Both covers were sold as wear resistant, and only one of them was.

Where the material arrives hot, cover grade has to be split into two demands that pull against each other. Heat resistance and abrasion resistance are formulated differently, and a compound optimized for 200 C service at the transfer point is rarely the compound with the lowest abrasion figure. A heat resistant conveyor belt for clinker or sinter therefore needs a stated continuous temperature, a stated peak temperature and a stated duration at that peak, not a single word on a brochure. Ask for all three.

Now the thickness half of the decision. Cover thickness on a steel cord belt is not a free variable. ISO 15236 ties the minimum cover thickness to the cord diameter, and the logic is simple. Wear removes rubber from the outside in, and the cord layer is a fixed distance below the surface. A 10 mm cable under 4 mm of rubber is partly exposed from the first day of wear, and once the outer cables appear the belt is on a countdown to a splice failure. As a working rule for high wear duty we want a top cover at least equal to the cord diameter, plus the wear allowance the duty justifies, and a bottom cover of at least half the cord diameter to protect the pulley side.

Thickness has to be measured over the cables, not at the belt edge, because the edges of a steel cord belt carry more rubber by construction than the centre. Two belts can both measure 20 mm at the edge and differ by 3 mm over the cord plane. If the thickness certificate does not say where the measurement was taken, it does not tell you what you think it tells you.

Table 2 in the previous section dealt with tension and pulley geometry. Table 3 above dealt with cover compounds. Table 4 below is the one to bring to a site meeting, because it links a real material condition to a grade and a thickness.

05Belt Thickness and Belt Mass, and What They Do to Your Drive

Total Thickness Is Three Layers, Not One Number

Thickness on a steel cord belt is the sum of three layers. Top cover, then the cord layer, then bottom cover. The cord layer is roughly 1.5 to 1.8 times the diameter of the cable inside it, because the rubber has to bury that cable on both sides. That is why a class carrying 5 mm cords lands near 18 mm of total thickness behind 6 mm of top cover and 6 mm of bottom cover, while a class carrying 8.1 mm cords lands near 33 mm.

The standards set a minimum cover thickness for each class rather than one figure for the whole range. Our own sheet runs from 3.5 mm at ST630 and ST800, through 4.0 mm at ST1000 and ST1250, 5.0 mm from ST1600 to ST2500, 6.0 mm at ST3150 and ST3500, and 6.5 to 8.0 mm at the top of the series. Those are the minimums that keep rubber over the cable. The wear allowance a particular job needs is a separate decision, and it is the one Table 4 deals with.

Mass per square metre is the number that reaches the rest of the plant. Reference belt weight runs from about 18 to 20 kilograms per square metre at ST630 up to 48 to 55 at ST4000. Three consequences follow, and they arrive in different places. The drive has to accelerate that mass on every start. The idlers have to carry it, alongside the material, for the life of the machine. On an inclined section the belt's own weight also works against you inside the tension calculation rather than helping you.

On a 3,200 m overland conveyor we swapped a 26 kilogram per square metre belt for a 30 kilogram belt of the same class, in order to buy impact resistance under the transfer chute. The extra 4 kilograms per square metre, across 1,200 mm of width and roughly 6,400 m of belt loop, put about 31 tonnes of additional mass into every single start. The impact idlers we installed at the same time did more for that belt's survival than the extra cover weight did, and they cost a small fraction as much.

That is the trade thickness asks you to make. More rubber buys wear life and impact protection. It also buys bending stiffness, which makes the belt harder to trough, shortens the acceptable transition distance and adds rolling resistance on the return strand. A 3 mm thicker bottom cover on a 1,400 mm belt is not free, and it is seldom the cheapest way to solve a wear problem.

Thickness is not a quality score either. Two belts can read identically on a gauge and differ completely in how well the rubber is bonded to the cable underneath. That bond is what a documented quality assurance programme exists to prove, and it is checked with a cord pull-out test rather than with a caliper.

One more layer sits behind the belt instead of inside it. The drive that turns the head pulley, from motor through gearbox to the sheaves, usually runs on V-belts, and a worn set of those costs you throughput long before the conveyor belt does. We hold ours to the same evidence standard we expect from any V-belt manufacturer, and we quote belt and drive together, because a transmission belt manufacturer who cannot match a datum length to the pulley groove is a problem waiting to happen. A conveyor belt distributor worth keeping will hand you the datum length and the groove angle before you have to ask for them. Buyers consolidating a whole plant order usually pull the fabric belt, the steel cord belt, the rollers and the drive belts from a single wholesale conveyor belts list for exactly that reason.

A loaded steel cord belt in a 35 degree trough. Most of the cover wear happens in the first few metres after this point.

06Carry Capacity Is a Chain, Not a Single Number

Ask three suppliers what a 1,200 mm belt will carry and you will get three tonnages, all of them followed by the words up to. Capacity is not a property of the belt. It is the product of four variables that have to be right at the same time, and the belt is only one of the four.

The arithmetic is tonnage per hour equals 3,600 multiplied by the usable material cross-section in square metres, multiplied by belt speed in metres per second, multiplied by loose bulk density in tonnes per cubic metre. Each of those four can be argued about, and the argument is usually where the number goes wrong.

Cross-Section Comes From Troughing Angle and Material Behaviour

Cross-section begins with belt width and the troughing angle of the idler set, then loses whatever the material refuses to occupy. A 20 degree trough holds far less than a 35 degree trough on the same belt, and a 45 degree or deeper trough holds more again. After that, the material takes its own share of the space away. Dry rounded sand flows and settles into a shallow pile. Damp angular ore or crusher clinker heaps steeply and permits more load per metre. The usable fraction of the trough normally lands between about 70 and 90 percent of the full trough area, and quoting the full area is a reliable way to overstate capacity.

Belt Speed Is a Trade-Off, Not a Free Number

Speed multiplies tonnage directly, which makes it tempting. It also multiplies wear. Idler and pulley surface wear tracks roughly with the square of belt speed, so moving from 3.5 to 5 m/s doubles the wear rate on the rotating parts. Faster belts throw more dust, degrade friable material harder at the transfer and demand more energy at start-up. On coal and aggregate we work between 3.0 and 5.0 m/s, on hard rock lump ore often below that, and on fine dry material higher where the containment is good. Our port bulk material handling jobs are the exception, because there high speed is the design point rather than the risk.

Bulk Density Is Loose Density, and Sag Finishes the Chain

Bulk density has to be the loose, aerated figure as the material actually sits on the belt. Design sheets are sometimes filled in with the solid density of the mineral, and that one substitution can overstate a conveyor by 40 percent or more. Loose iron ore runs about 2.0 to 2.4 tonnes per cubic metre, coal 0.8 to 1.0, cement clinker 1.2 to 1.5, and dry sand 1.4 to 1.6. If your figure came from a geology report instead of a bulk sample, go and get a better figure.

The last link in the chain is idler spacing, and it belongs in a capacity discussion because of sag. Sag is the droop of the belt between two idlers, and it is driven by the weight the belt is carrying and the tension inside it. Hold sag under about 2 percent of the idler spacing and the material rides in a stable trough with the skirt rubber sealing against the belt. Let it run to 3 percent and you see the load bouncing at the load point, fines working under the skirt and material spilling at the edges. Beyond that the belt develops a ripple of its own and the idlers take shock instead of rolling load.

The relationship is simple enough to check by hand. Sag as a fraction of spacing equals the weight of belt plus material per metre, multiplied by the square of the spacing, divided by eight times the tension. Spacing is squared, so doubling it multiplies sag by four. That is why low-tension sections need short idler spacing, and why the same belt in the same trough can be spaced much further apart once the tension climbs.

Table 5 works those numbers at a 1,200 mm width with a 24 kilogram per square metre belt, gravity taken as 9.81 and sag measured at mid-span. Scale the spacing with the square root of your own figures and you will stay in the right range.

Read the last row again, because it surprises people. On the return strand the tension is higher and there is no material on the belt, so sag alone would permit six metres between idlers. Nobody builds it that way. Belt tracking, the need to keep an empty belt running centred, and the stiffness of a heavy steel cord belt hold the practical limit near 3.0 to 3.5 m. This is where a competent conveyor belt distributor earns a margin, because belt and idler layout have to be chosen as one system rather than two purchases. We keep our own idler and frame range beside the belt for that reason, and you can see those on our conveyor rollers and conveyor components pages.

Now put the whole chain together on one figure. A 1,200 mm belt in a 35 degree trough has a usable cross-section of about 0.11 square metres once the surcharge angle of angular ore is accounted for. At 3.5 m/s and a loose bulk density of 2.2 tonnes per cubic metre, the tonnage is 3,600 times 0.11 times 3.5 times 2.2, which is roughly 3,050 tonnes per hour. Drop the troughing angle to 20 degrees and the same belt at the same speed carries closer to 2,000 tonnes per hour. Nobody changed the belt. The trough changed.

07Building a Specification Sheet That Can Be Checked on Delivery

A steel cord conveyor belt specification that survives a warranty argument has about a dozen lines in it, and the class is not one of them. What follows is the list our desk sends back when a buyer asks what we need before quoting, reordered so you can use it as your own enquiry template.

Start with the duty. Tonnage per hour, material description with the lump size range, loose bulk density, and the temperature the material reaches as it leaves the chute. Then the geometry. Belt width, centre distance, lift or drop, incline in degrees, and whether the route has horizontal curves or a vertical sag. Then the machine itself. Head pulley diameter, tail and take-up diameter, the take-up travel available, troughing angle, idler spacing on both strands, and belt speed. Everything after that is arithmetic.

Only now does the belt get written down, and the line reads roughly like this. ST1600, 1,200 mm wide, cord diameter 4.9 mm, cord pitch 12 mm, top cover 8 mm of grade W to DIN 22102 or grade A to AS 1332, bottom cover 6 mm, hot vulcanized splice in three stages, minimum head pulley 1,000 mm, belt length 2 x 320 m plus 1.5 m of extra for the splice and the take-up.

Every field in that line can be audited. Cord diameter and pitch are visible on a cut end with a rule and a magnifier. Cover thickness is measurable over the cables, which is the only place it means anything. Compound grade is verifiable from the batch record and the abrasion figure. Pulley diameter sits on a drawing. Splice length follows from the cord diameter by the 100 to 1 rule. Belt length is checked on the roll with a tape, and it is the field where disputes most often begin, because a short belt cannot be stretched to reach a long take-up.

Two documents belong in the delivery pack and are usually missing. A cord pull-out record for the batch, and the cure log for any splice made before dispatch. We have argued a claim on a mining and quarrying conveyor where the belt evidence was complete and the splice evidence had never been kept. The missing log cost the buyer every bit of leverage they had.

Site conditions change the pack more than they change the class. A cement plant needs the temperature rating written into the order, because clinker arrives hot and a cover that shrugs off abrasion may not shrug off 180 degrees. A quarry with one steep section is often better served by a chevron conveyor belt on that section alone, and a confined transfer house may want a sidewall conveyor belt lifting at 90 degrees instead of a long inclined run. Those are routing decisions, not steel cord decisions, and they are far cheaper to make at the enquiry stage than after the structure is poured.

The class table, the splice recommendation and the recommended pulley diameters for the whole steel cord range are reproduced on our steel cord conveyor belt page, so a buyer can check our figures against their own conveyor designer's before anything is ordered.

Final inspection on a finished steel cord belt. This is the last point at which a wrong cord pitch can still be caught cheaply.

08Steel Cord Conveyor Belt Price and the Levers Behind It

Buyers ask for a steel cord conveyor belt price as one figure per metre, and that figure can honestly vary by a factor of three for belts of the same width in the same class. Knowing which lever is moving is the difference between a fair quotation and a costly mistake, and price is the last thing on the page that should be compared.

Steel is the first lever and the largest single material cost in the belt. Cord diameter and pitch decide how many kilograms of galvanized cable go into every square metre, and cable is an alloy product with a market of its own. When a quotation sits noticeably under the field for a given class, the first two numbers to check are cord diameter and cord pitch, because those two set the steel content and they are the numbers a buyer can still measure after the belt arrives.

Cover grade and cover thickness form the second lever. Rubber is cheaper than steel per kilogram, but there is a great deal of it. Taking a 1,200 mm belt from a light Z grade at 5 mm up to a W grade at 10 mm adds cost, and on a sharp dry ore it also adds years. This is the lever where under-specifying happens most often, because cover grade is invisible in a photograph and difficult to argue about in a meeting six months later.

Width and length form the third lever, and the relationship is not as linear as buyers expect. Long belts come off the press in fewer rolls, which improves yield, but they freight heavier and handle badly on a remote site. A belt split into short rolls to suit a restricted access road costs more than the same belt in one length, and it will need more splices in the field to put it back together.

The fourth lever is the splice, and buyers routinely forget to price it. A three-stage hot splice done on site, with a press, a trained crew and a cure log, is a service rather than a product. It carries its own labour, consumables and downtime. If it is not in the quotation it is not in the price, and it will return later as a separate invoice with nothing to compare it against.

The fifth lever is documentation and testing. A belt delivered with batch cord pull-out records, cover thickness measurements over the cables and a full cure log costs more than a belt delivered with a packing list. That gap is normally smaller than the cost of a single unplanned stoppage, and on a critical conveyor it is the cheapest line on the page.

So compare quotations sheet to sheet rather than price to price. Put the two specification sheets side by side. If the cord diameters differ, or the cover grades differ, or one quote bundles the splice and the other does not, you are not comparing prices at all. Where a project needs several belt types at once, transmission and timing belts included, one consolidated enquiry prices better than four separate ones and leaves you with a single conversation if anything goes wrong.

Send Us Your Conveyor Data and We Will Work Out the ST Class

09Frequently Asked Questions

How do I work out which ST class I need?

Take the maximum running tension from the conveyor designer, in newtons per millimetre of belt width, and check it against the maximum operating tension column for each class. ST1600 allows 240 kN/m of width, so a 1,400 mm belt in that class can carry 336 kN of pull. If your calculated maximum sits below that figure, the class holds. If it sits above it, step up one class and check again. Do not add a class for comfort, because the extra mass and the larger pulleys it then demands are real costs with no service benefit.

What is the minimum pulley diameter for a steel cord belt?

It depends on the class and on what the pulley does, not on the belt width. A class carrying 4.9 mm cords needs a minimum drive pulley of around 1,000 mm, and a class carrying 7 mm cords needs about 1,250 mm. Tail and take-up pulleys can be smaller because they see less tension, and light bend pulleys smaller again. Get the figure confirmed in writing against ISO 15236-1, because a pulley that is one size too small shows up as broken cables rather than as cover wear, and broken cables are invisible until the belt fails.

How long does a steel cord splice have to be?

Start from the cord diameter. The overlap on each cord should be at least 100 times that diameter, so a 5 mm cord needs 500 mm of bond and a 7 mm cord needs 700 mm before any margin is added. A single-stage splice for a mid-range class normally finishes between 1.0 and 1.5 m. From ST2500 upward we move to two or three stages, and the finished splice roughly doubles or triples, which is why pulley diameter and splice design have to be settled together rather than one after the other.

Which cover grade should I specify for crushed rock?

For blasted rock arriving from a primary crusher, with lumps up to 800 mm and drop heights of four metres or more, specify a W grade to DIN 22102 or an A grade to AS 1332, a top cover of at least 8 mm, and impact idlers under the load zone. For screened rock under 50 mm, the same abrasion grade at 6 to 8 mm is normally sufficient. The error we see most often is a buyer choosing the grade with the highest tensile figure instead of the grade with the lowest abrasion figure.

Can a steel cord belt go onto a conveyor that was built for fabric?

Sometimes, and the pulleys usually decide it. Measure the head, tail and bend pulley diameters and compare them with the minimums for the class you actually need. A fabric machine commonly carries an 800 mm head pulley, which is below the minimum for most steel cord classes above ST1000. If the pulleys and the take-up travel do not fit, one of them has to change. Converting without checking those two numbers is how a perfectly good belt fails inside a year.

What actually drives the price of a steel cord conveyor belt?

Five things move it. Steel content, which cord diameter and cord pitch set. Cover grade and cover thickness. Belt width and the number of rolls the length is supplied in. Whether the splice is included as a service. And how much testing and documentation travels with the belt. Compare two quotations line by line against the specification and most price gaps explain themselves before anyone has to argue about the total.