A conveyor belt spec is four decisions written as one line. The four decisions are how many plies, how much rated tension, how much rubber on each face, and what compound that rubber has to be. If you read the spec in that order, most of the guesswork disappears. If you read only the price per foot, you will buy the same belt twice.
The Spec Line, Field by Field
A belt spec is shorthand. The line "3-ply 330 PIW, 3/16 x 1/16 covers, Grade 2" puts four separate decisions in one line. Every one of them changes what arrives on your dock. Before you compare the spec to anything, read it field by field.
| Field | Example | What it controls |
|---|---|---|
| Plies | 3-ply | Fabric layers in the carcass. Determines 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 allowance. |
| Cover grade | Grade 2 | The minimum rubber properties the cover compound has to meet. |
Each of the four fields can go wrong in a different way. If the tension rating is wrong, either the belt stretches or you overpay. If the covers are wrong, you have to splice the belt again early. If the grade is wrong, the cover breaks out in chunks under material that its compound was never designed for.
330 PIW: Pounds Per Inch of Width
PIW is how belt strength is rated in North America: the rated working tension per inch of belt width. A 330 PIW belt that is 24" wide is rated to pull 7,920 lb. 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. The service factor is commonly 8:1 to 10:1 on quality domestic belting. A 330 PIW belt does not break at 330 lb per inch. The belt is rated to run at that tension continuously. The margin above that rating is already used by startup shock, plugged chutes, and the splice.
On a multi-ply belt, the rating is the number of plies multiplied by the rating of each ply. A 3-ply 330 is typically three fabric plies rated 110 PIW each. Ply count is not only about strength, though. More plies means a thicker, stiffer carcass. A stiffer carcass needs a larger pulley to wrap around without damaging itself. Minimum pulley diameter (the subject of ISO 3684) increases with ply count, with the percentage of rated tension you run, and with cover thickness.
Too Low and Too High Both Cost You
If you under-rate the belt, you run it at or above working tension. Fabric belting stretches under load. The stretch is roughly 1 to 1.5 percent at rated working tension for a polyester carcass, more for all-nylon, and more still above the rating. The take-up travel runs out, tracking gets worse, and the splice takes the extra strain. If you over-rate the belt, you pay for strength that you cannot use, and you buy a stiffer carcass with it. A belt that is too stiff for its pulleys will not wrap cleanly. 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 Allowance
Those two fractions are the cover thickness in inches, top cover first. Everything between the covers is the carcass. The carcass is the part that you are protecting.
The top cover takes the impact and the abrasion, and it is the part you are allowed to lose. When it wears through and the material starts to damage the fabric, the belt is finished. So treat the top cover as your wear allowance. A 3/16 cover gives you three times as much rubber to lose as a 1/16 cover, 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. The bottom cover rides on the drive pulley, the snub and bend pulleys, and the return rolls. If the return run stays clean, a 1/16 bottom cover is fine. If the return run does not stay clean, that cover is often too thin. Carryback packs onto the return rolls, and the rolls stop turning. A stationary steel drum then sands the bottom cover. On a slider bed conveyor, the bottom cover also slides the full length of the bed on every pass. We build slider beds with replaceable 3/8" flat-bar steel slats, so the slats are the sacrificial part. The belt still has to be specified to last on them.
The Ratio Between the Covers Matters Too
You cannot add top cover and leave the bottom cover alone. Rubber shrinks as it ages. A much larger mass of rubber on one side shrinks more than the smaller mass on the other side. Then 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", 2:1 on belts from 42" to 60", and 3:1 only above 60". A 3/16 x 1/16 belt is 3:1. That ratio is also the one that belting distributors stock most. On a 24" belt, 3:1 is double the recommended ratio. Many of them run fine anyway. If you already have problems with tracking or cupping, you should ask about the cover ratio before you buy the same belt again.
Grade 1 vs Grade 2: The Part Most People Get Wrong
Here is the correction, and it is the most important point on this page. You will hear people say that Grade 2 resists cutting while Grade 1 resists abrasion. We have said this ourselves. The claim is wrong. There is no cut-versus-abrasion trade-off to choose between. Grade 1 is the higher-spec cover. Grade 2 is the general-purpose cover. The grades come from ARPM, the Association for Rubber Products Manufacturers. ARPM publishes the conveyor belt standards that the industry still calls Rubber Manufacturers Association (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 gives cut and gouge resistance. The rubber has to be pulled harder before it tears. The higher tensile strength is the difference that the grade certifies. Read the abrasion column with more caution. Published industry guidance states clearly that the grade rating does not by itself indicate abrasion resistance. The guidance also states that a lab abrasion value has practical relevance mainly to grinding-type wear. Grade 1 is the tougher cover, but it is not a wear-life guarantee.
The tougher Grade 1 cover does not make Grade 2 the wrong answer. Grade 2 is the general-purpose grade. It is what most suppliers keep in stock. Grade 2 handles the majority of bulk applications without problems. The published application lists divide them about like this. Grade 1 is for hard sharp ore, riprap, granite, glass cullet, scrap metal, and demolition debris with rebar in it. Grade 2 is for sand and gravel, limestone, crushed rock, slag, coke, and coal.
The Grade Is a Minimum, Not a Ranking
Both numbers are minimums. A cover that tests at 120 mm³ volume loss and a cover that tests at 60 mm³ both qualify as Grade 1. In an abrasive stream, the 60 will outlast the 120 by a wide margin. The words "Grade 1" on a quote do not tell you which cover you are getting. If wear life is the only reason for the purchase, ask for 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. The two systems do not match one to one, because DIN splits the properties that 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. That person 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 restarts production in much less time. Splice efficiency is an assumption in the belt calculation. Before you order, decide the splice efficiency.
- 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. Cleats, sidewalls, and V-guides change pulley requirements and how the belt is spliced. Our guide to maximum conveyor incline angle covers when you need them.
The Mistake: Comparing Belts on Price Per Foot
The most expensive mistake we see is comparing two quotes on price per foot without anyone reading the covers. A 1/8 x 1/16 belt and a 3/16 x 1/16 belt are not the same product. The thicker belt carries fifty percent more rubber on the side that wears. In a sharp stream, the thin cover is not cheaper. It makes you splice the belt again sooner. The cost of that splice is the downtime, the crew, and the tons you did not run.
Divide the belt price by the months of service that you expect. Compare that figure instead. A cover that costs more per foot but outlasts the cheap one by a wide margin gives two benefits. The cover costs less per month of service. You also skip a change-out. The same mistake appears at the conveyor level, when people buy conveyors. That mistake 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. We also specify and supply replacement conveyor belts and replacement parts. To specify a belt, we need the width and the center distances, and what kind of conveyor it is. We also need the material. Name the worst item in the stream, not the average. We need the incline and lift, the drive horsepower, and the belt speed. If you have the old belt's spec, we need that too.
Then tell us where the old belt failed. That information is worth more than all the rest together. A top cover worn through in one spot is a loading problem. Damaged edges are a tracking or alignment problem. A bottom cover worn down over the full length is a bed, return roll, or carryback problem. 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 a problem.
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 lb for every inch of its width. So a 24" belt at that rating carries 7,920 lb 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. The difference is not a cut-versus-abrasion trade-off. ARPM (formerly RMA) Grade 1 requires a minimum cover tensile strength of 2,500 psi. Grade 2 requires 2,000 psi. Both require a minimum elongation at break of 400 percent. That higher tensile strength gives Grade 1 better cut, gouge, and tear resistance. The grade alone does not reliably indicate abrasion resistance. When wear life is what matters, ask for tested cover values. Grade 2 is the general-purpose grade most suppliers stock.
What does 3/16 x 1/16 mean on a belt spec?
The two fractions are cover thicknesses in inches: a 3/16" top cover and a 1/16" bottom cover, with the fabric carcass between them. The top cover protects the carcass from impact and abrasion and is the wear allowance that you use up over the life of the belt. 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. The stiffness raises the minimum pulley diameter that the belt can wrap without damage. The stiffness also 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. ARPM, the Association for Rubber Products Manufacturers, now publishes the conveyor belt standards. Most of the industry still says RMA Grade 1 and RMA Grade 2, and suppliers use the names interchangeably. When wear life is what matters in the purchase, ask which published minimums the quote is written to. Do not rely on the grade number alone.
Is a thicker top cover always better?
No. A thicker top cover gives more wear life, but it adds cost, weight, and stiffness, and it also changes the balance between the two covers. If the top cover is far thicker than the bottom cover, the belt can cup as the rubber ages. The cupping lifts the bottom cover off the idlers and makes the belt hard to track. Match the cover thickness to the material and the belt width, not to the largest number offered.