PIW means “Pounds Per Inch of Belt Width” in RMA American standard, described as EP(“PN” in the U.S) rubber conveyor belt’s “Max.Oper.Working Strength” or “Breaking Strength”, and is widely used in the United States of America.

The PIW figure is indicating the working tension an EP rubber conveyor belt is intended to be used for, irrespective of the breaking strength of the splice. Usually, the EP conveyor belt’s breaking strength and the safety factor are not given.

Outside of the USA, the breaking strength of the EP rubber conveyor belt is used, rated in N/mm, i.e. Newton per millimeter of EP rubber conveyor belt width. The belt breaking strength is determined precisely, depending on the operating conditions, the splice type, etc. The safety factor is related to the weakest point of the EP conveyor belt, which is the splice section of the belt.

How PIW and N/mm convert to each other?
Without consideration of a safety factor, PIW is converted into N/mm by multiplying by 0.175.

Example: 2284 PIW x 0.175 = 400 N/mm(400kN/m)
N/mm converted into PIW are multiplied by 5.71. Example: 400 N/mm x 5.71 = 2284 PIW

If a safety factor of 10 is assumed for both terms, then a 400 PIW belt would equal a 700 N/mm(700kN/m) conveyor belt.

Practical Converting Example:
Conveyor belt with tensile strength of belt 630N/mm(630kN/m),3ply, described as EP630/3, converted to PIW would be 630 X 0.5716 = 360 PIW (10:1 safety factor). So EP630/3 in US terms would be (3 plies 360 PIW). Where it is EP630/3 is actually 3 plies of EP200 fabric. EP1000/3 is 3 plies of EP315 fabric. EP800/4 is 4 plies of EP200 fabric. You may also convert EP200 X 0.5716 = 114 PIW per ply, EP315 X 0.5716 = 180 PIW per ply and so on.

Let’s take it clearly as below:

Steel Cord Belt Converting Example:
The above applies to textiles such as EP rubber conveyor belts or NN(Nylon) conveyor belt. For steel cord conveyor belts, the rating in N/mm breaking strength is used in the USA too. The test procedure can be different, for example:

Assume steel cord conveyor belt that has a standard safety factor = 6.7:1, steel cord is rated in ST- XXXX N/mm, this is the breaking strength of steel cords/ mm, assume Steel Cord rating is ST-1000 N/mm breaking strength, then working strength is:

ST-1000 N/mm / SF=6.7 = 149.25 N/mm working strength

Then we convert metric to English for ST-1000 N/mm to 852 PIW:

ST-1000 / 6.7 = 149.25 N/mm x ( 1 P/4.448 N) x (25.4 mm / 1 inch) = 852 PIW or 852 pounds/inch.

Muti-ply EP Fabric Grade (Tensile Strength kg/cm, PIW):

Muti-ply PN Fabric Grade (Tensile Strength kg/cm, PIW):

Muti-ply PP Fabric Grade (Tensile Strength kg/cm, PIW):

Muti-ply NN Fabric Grade (Tensile Strength kg/cm, PIW):

Conversation between USA and Metric under different safety factor: