A conveyor belt spec is four decisions written as one line: how many plies, how much rated tension, how much rubber on each face, and what compound that rubber has to be. Read it in that order and most of the guesswork disappears. Read only the price per foot and you will buy the same belt twice.
The Spec Line, Field by Field
A belt spec is shorthand. "3-ply 330 PIW, 3/16 x 1/16 covers, Grade 2" packs four separate decisions into one line, and every one of them changes what shows up on your dock. Read it field by field before you compare it to anything.
| Field | Example | What it controls |
|---|---|---|
| Plies | 3-ply | Fabric layers in the carcass. Drives stiffness, troughability, and minimum pulley diameter. |
| Tension rating | 330 PIW | Rated working tension, in pounds per inch of belt width. The strength number. |
| Covers | 3/16 x 1/16 | Top cover, then bottom cover, in inches. Your wear budget. |
| Cover grade | Grade 2 | The minimum rubber properties the cover compound has to meet. |
Four fields, four ways to get burned. Wrong tension rating and the belt either stretches or you overpaid. Wrong covers and you are back at the splice press early. Wrong grade and the cover chunks out under material it was never compounded for.
330 PIW: Pounds Per Inch of Width
PIW is how belt strength is rated in North America: rated working tension per inch of belt width. A 330 PIW belt 24 inches wide is rated to pull 7,920 pounds. That number covers the material, the belt itself, the friction of the bed or the idlers, and the lift on an incline.
Working tension is not breaking strength. The ratio between the two is the service factor, commonly 8:1 to 10:1 on quality domestic belting. A 330 PIW belt does not part at 330 pounds per inch. It is rated to run there continuously, and the margin above it is already spent on startup shock, plugged chutes, and the splice.
On a multi-ply belt the rating is the plies multiplied out: a 3-ply 330 is typically three fabric plies rated 110 PIW each. Ply count is not only strength, though. More plies means a thicker, stiffer carcass, and a stiffer carcass needs a larger pulley to wrap without damaging itself. Minimum pulley diameter, the subject of ISO 3684, climbs with ply count, with the percentage of rated tension you run, and with cover thickness.
Too Low and Too High Both Cost You
Under-rate the belt and you run it at or above working tension. Fabric belting stretches under load: roughly 1 to 1.5 percent at rated working tension for a polyester carcass, more for all-nylon, and more still above the rating. Take-up travel runs out, tracking goes sour, and the splice absorbs the difference. Over-rate it and you paid for strength you cannot use and bought a stiffer carcass with it. A belt too stiff for its pulleys will not wrap cleanly, and on a troughing idler conveyor it will not seat into the idlers either. Size the belt to the calculated tension of your conveyor, not to the biggest number on the price list.
3/16 x 1/16: Cover Thickness Is a Wear Budget
Those two fractions are cover thickness in inches, top cover first. Everything between them is the carcass, and the carcass is the thing you are protecting.
The top cover takes the impact and the abrasion. It is the part you are allowed to lose. Once it wears through and the material starts working on the fabric, the belt is finished. So read the top cover as a budget: 3/16 gives you three times the rubber to spend as 1/16, in the same place, on the same material.
The bottom cover does a different job. It is the friction surface that drives and tracks the belt, riding the drive pulley, the snub and bend pulleys, and the return rolls. A 1/16 bottom is fine when the return run stays clean and often too thin when it does not. Carryback packs onto return rolls, the rolls stop turning, and the bottom cover gets sanded by a stationary steel drum. On a slider bed conveyor the bottom cover also slides the full length of the bed every pass. Remcon builds slider beds with replaceable 3/8-inch flat-bar steel slats so the slats are the sacrificial part, but the belt still has to be specified to live on them.
The Ratio Between the Covers Matters Too
You cannot just add top cover and leave the bottom alone. Rubber shrinks as it ages, and a much larger mass of rubber on one side shrinks more than the smaller mass on the other. The belt cups, loses bottom-cover contact with the idlers, and stops tracking straight.
Published guidance recommends a top-to-bottom ratio of 1.5:1 on belts up to 36 inches, 2:1 from 42 to 60 inches, and 3:1 only above 60 inches. A 3/16 x 1/16 belt is 3:1, and it is also the ratio belting distributors stock most. On a 24-inch belt that is double the recommended ratio. Plenty of them run fine anyway, but if you are already fighting tracking or cupping, the cover ratio is worth asking about before you buy the same belt again.
Grade 1 vs Grade 2: The Part Most People Have Backwards
Here is the correction, and it is the most important thing on this page. You will hear it said that Grade 2 resists cutting while Grade 1 resists abrasion. We have said it that way ourselves. It is wrong. There is no cut-versus-abrasion tradeoff to pick between: Grade 1 is the higher-spec cover and Grade 2 is the general-purpose one. The grades come from ARPM, the Association for Rubber Products Manufacturers, which publishes the conveyor belt standards the industry still calls RMA grades. The published minimums:
| Cover property | Grade 1 | Grade 2 |
|---|---|---|
| Minimum tensile strength | 2,500 psi (about 17 MPa) | 2,000 psi (about 14 MPa) |
| Minimum elongation at break | 400% | 400% |
| Maximum abrasion volume loss | 125 mm³ | 175 mm³ |
| What it is for | Premium cover for sharp, heavy, gouging material | General purpose; the grade most suppliers stock |
Higher tensile strength at the same elongation is what buys cut and gouge resistance: the rubber has to be pulled harder before it tears. That is the difference the grade actually certifies. Read the abrasion column with more caution. Published industry guidance is blunt that the grade rating does not by itself indicate abrasion resistance, and that a lab abrasion value has practical relevance mainly to grinding-type wear. Grade 1 is the tougher cover. It is not a wear-life guarantee.
That does not make Grade 2 the wrong answer. It is the general-purpose grade, it is what most suppliers keep on the rack, and it handles the majority of bulk applications without complaint. The published application lists split them about like this: Grade 1 for hard sharp ore, riprap, granite, glass cullet, scrap metal, and demolition debris with rebar in it. Grade 2 for sand and gravel, limestone, crushed rock, slag, coke, and coal.
The Grade Is a Floor, Not a Ranking
Both numbers are minimums. A cover that tests at 120 mm³ volume loss and one that tests at 60 mm³ are both legitimately Grade 1, and in an abrasive stream the 60 will outlast the 120 by a wide margin. "Grade 1" on a quote does not tell you which you are getting. If wear life is the whole reason you are buying the belt, ask for the tested cover values, not the grade number.
If the Quote Comes Back in DIN Letters
Imported belt is often written to DIN 22102 letters instead of ARPM grades. They do not map one to one, because DIN splits the properties ARPM bundles together:
| DIN 22102 grade | Min. tensile | Min. elongation | Max. abrasion |
|---|---|---|---|
| X — cut and gouge | 25 MPa | 450% | 120 mm³ |
| Y — general purpose | 20 MPa | 400% | 150 mm³ |
| W — abrasion | 18 MPa | 400% | 90 mm³ |
| Z — economy | 15 MPa | 350% | 250 mm³ |
If somebody offers a DIN-graded belt as the equivalent of an ARPM grade, ask which property they matched, because they cannot have matched both.
What the Spec Line Leaves Out
- Carcass material. Polyester warp with nylon weft, sold as EP, is stiffer and stretches less. All-nylon troughs better and absorbs shock better. The PIW number does not tell you which one you got.
- Splice. Mechanical or vulcanized. A vulcanized splice is stronger and runs smoother under scrapers; mechanical lacing gets you back in production in a fraction of the time. Splice efficiency is an assumption in the belt calculation, so decide it before you order.
- Specialty compounds. Oil, heat, and flame resistance are separate from the grade. A stream carrying motor oil or solvent will swell a standard cover no matter what grade is stamped on it.
- Cleats, sidewalls, and V-guides. Steep runs usually need them, and they change pulley requirements and how the belt is spliced. Our guide to maximum conveyor incline angle covers when you actually need them.
The Trap: Comparing Belts on Price Per Foot
The most expensive mistake we see is two quotes lined up on price per foot with nobody reading the covers. A 1/8 x 1/16 belt and a 3/16 x 1/16 belt are not the same product. One carries fifty percent more rubber on the side that wears. In a sharp stream the thin cover is not cheaper. It is a faster trip back to the splice press, and the cost of that trip is the downtime, the crew, and the tons you did not run.
Divide the belt price by the months of service you expect and compare that instead. A cover that costs more per foot and outlasts the cheap one by a wide margin wins twice, because you also skipped a changeout. The same trap shows up one level up when people buy machines, which is why we wrote a separate guide on buying a conveyor.
What to Send Us When You Need a Belt
Remcon designs, fabricates, installs, and repairs custom conveyors, and we spec and supply replacement conveyor belts and replacement parts. To spec a belt we need the width and center distances, what kind of conveyor it is, the material with the worst thing in the stream named rather than the average, the incline and lift, the drive horsepower and belt speed, and the old belt's spec if you have it.
Then tell us where the old belt failed. That is worth more than the rest put together. A top cover worn through in one spot is a loading problem. Chewed edges are a tracking or alignment problem. A bottom cover ground down over the full length is a bed, return roll, or carryback problem, and a new belt will not fix it. Diagnosis is part of belt replacement and repair, and it is the difference between buying a belt and solving something.
Conveyor Belt Spec FAQ
What does PIW mean on a conveyor belt?
PIW stands for pounds per inch of width, the North American rating for a conveyor belt's working tension. A 330 PIW belt is rated to pull 330 pounds for every inch of its width, so a 24-inch belt at that rating carries 7,920 pounds of belt pull. Working tension is not breaking strength. On quality domestic belting the breaking strength is commonly eight to ten times the rated working tension.
What is the difference between RMA Grade 1 and Grade 2 conveyor belt?
Grade 1 is the higher-spec cover, and the difference is not a cut-versus-abrasion tradeoff. ARPM (formerly RMA) Grade 1 requires a minimum cover tensile strength of 2,500 psi against 2,000 psi for Grade 2, and both require 400 percent minimum elongation at break. That higher tensile strength is what gives Grade 1 better cut, gouge, and tear resistance. The grade alone does not reliably indicate abrasion resistance, so ask for tested cover values when wear life is the point. Grade 2 is the general-purpose grade most suppliers stock.
What does 3/16 x 1/16 mean on a belt spec?
Those are cover thicknesses in inches: a 3/16-inch top cover and a 1/16-inch bottom cover, with the fabric carcass between them. The top cover protects the carcass from impact and abrasion, and it is the wear budget you spend over the belt's life. The bottom cover is the friction surface that drives and tracks the belt against pulleys, return rolls, or a slider bed.
What does 3 ply mean on a conveyor belt?
A 3-ply belt has three layers of fabric in its carcass, bonded with rubber between them. On a 3-ply 330 PIW belt the plies are typically rated 110 PIW each. More plies add strength, but they also add stiffness, which raises the minimum pulley diameter the belt can wrap without damage and reduces how well the belt forms a trough on idler conveyors.
Is RMA the same as ARPM for conveyor belt grades?
For practical purposes, yes. The grades were originally published under the Rubber Manufacturers Association name, and the conveyor belt standards are now published by ARPM, the Association for Rubber Products Manufacturers. Most of the industry still says RMA Grade 1 and RMA Grade 2, and suppliers use the names interchangeably. When wear life is the point of the purchase, ask which published minimums the quote is written to rather than relying on the grade number alone.
Is a thicker top cover always better?
No. A thicker top cover buys wear life, but it adds cost, weight, and stiffness, and it changes the balance between the two covers. If the top cover is far thicker than the bottom, the belt can cup as the rubber ages, which lifts the bottom cover off the idlers and makes the belt hard to track. Match cover thickness to the material and the belt width, not to the largest number offered.