This is the most common confusing question for new players in Conveyor belts business. If you are a new seller, receiving a inquiry from customer: 2ply 220PIW 3/16X1/16. You will be confused about 220PIW, because you are used to use metric unit like EP1000N/mm. Then you find “PIW”= pound/inch-width in web. You are happy to say it is easy, 1PIW=0.175N/MM, so 220PIW=39N/MM, but soon you realize you are wrong and the problem is deeper than it first appeared, because I’m sure you never see carcass strength below 50N/MM!
To resolve this common problem we must be aware that most of the world (excluding the USA) classifies belts with the metric system and ultimate breaking strength. So normally you say 1000N/MM means breaking strength. But USA system’s “PIW” means max. Operating strength (or working strength).
To further complicate matters, fabric belting is referred to in letters such as EP, PP or other combinations of steel cord belts as ST. So let’s begin simplifying this discussion by referring to Table 1 for definitions of the yarns used in most belt carcasses for either system.
Table 1. Fabric Designations
As you can see, we must be careful when using letters to designate the yarns. The reason for the apparent conflict in acronyms can be explained as follows:
In the USA “P” stands for Polyester and “N” for Nylon. In the metric countries Polyester (E) comes from Ethylene glycol and Nylon from Polyamide (P). So if you wanted an equivalent to a Metric EP belt you would select Poly-Nylon in USA terms or P-N.
Besides defining the carcass yarn, the EP designation is different from PIW in two other important ways:
1. EP means breaking strength (Not rating).
2. EP means metric units (N/mm).
Carcass Tension Rating
In the USA, the term carcass tension has many definitions but the NIBA version is probably the most appropriate.
NIBA: “Maximum safe working tension recommended by the manufacturer.”
This means the belt manufacturer determines the maximum tension that can be applied to the belt after reviewing the physical properties of the carcass. The tension units are PIW or Lb/In-width when they are all nylon or some other fabric combination.
In the metric system all tension values are in BREAKING STRENGTH not operating tension. For example, an EP 800 construction is 800 N/mm breaking strength. The operating tension, after converting to metric (assuming a 10:1 safety factor) would be 80 N/mm or 457 PIW.
In the USA a steel cord belt is in rated PIW just like a fabric belt. In the metric world the steel cord belt is designated ST (Steel) and like the EP designations is in breaking strength (N/mm). See Table 2
|PIW||LB/In-Width||Max. Oper. Strength||USA|
|EP||N/mm or Kn/m||Breaking Strength||Metric|
|ST||N/mm or Kn/m||Breaking Strength||Metric|
Table 2. Carcass Designations
Method for Conversion
To convert from EP to PIW, multiply by 5.71 to give breaking strength. This number must then be divided by the Safety Factor (SF) to give the rated belt PIW. This formula can be used for both fabric and steel cord belts.
Example: Convert EP1000 to PIW (Assuming 10 SF)
(1000 N/mm x5.71)
10 SF = 571 PIW
Likewise to convert from PIW to EP, do the reverse:
(571 PIW x 10 SF)
5.71 = 1000 N/mm
One last point for reference. Safety factors generally vary per Table 3.
|PIW||8 to 12|
|ST||5 to 8|
Table 3. Generally accepted safety factors
For you speed readers that find the above discussion too tedious to wade through, just clip out table 4 and place it in your planner for reference.
It will provide an easy method for quickly converting back and forth with the metric System. Simply locate the safety factor of the belt in question, move horizontally to the column description you wish to convert, and multiply by the indicated number.
|Safety Factor||EP to PIW||PIW to EP||ST to PIW||PIW to ST|
|Multiply by||Multiply by||Multiply by||Multiply by|
Table 4. Conversation between English and Metric.
Notice that 6.67 was included since it is the most common SF for steel cord belts around the world.
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