A large number of variables will determine the maximum working load of a rope, including the age and condition of the rope too. It’s determined by taking the tensile strength rating and dividing it accordingly, making a figure that’s more in-line with an appropriate maximum load, taking factors such as construction, weave, and rope longevity into the mix as well. Working LoadĪ rope’s working load is a different measurement altogether. A rope’s minimum tensile strength is calculated in the same way, but it takes the average strength rating and reduces it by 20%. However, a manufacturer may also test a rope’s minimum tensile strength. The average number will be quoted as the rope’s tensile strength. Note that all of these tests will use the ASTM test method D-6268. This test will be repeated on numerous ropes, and an average will be taken. Rather than adding weight to a line, the test is performed by wrapping the rope around two capstans that slowly turn the rope, adding increasing tension until the rope fails. These tests are done by incrementally increasing the load that a rope is expected to carry, until the rope breaks. Tensile StrengthĪ rope’s tensile strength is the measure of a brand-new rope’s breaking point tested under strict laboratory-controlled conditions. Both terms refer to rope strength but they’re not the same measurement. It’s very important to understand the fundamental differences between the tensile strength of a rope, and a rope’s working load. The Difference Between Tensile Strength and Working Load For more information, we have discussed the safe working load of ropes made of different materials in this article here. Without straining the line until it parts, you don’t know if that particular piece of line was above average or below average. The breaking strength formula was developed on the average breaking strength of a new line under laboratory conditions.
Just being able to calculate breaking strength doesn’t give one a safety margin. However, a splice is stronger than a knot. The weakest point in the line is the knot or slice. Knots and splices will reduce the breaking strength of a line by as much as 50 to 60 percent. First, convert the diameter to the circumference as we did above and then write the formula including the extra comparison factor step.Ģ.5 X 1.57 2 X 900 = 5, 546 pounds of breaking strength Using the example above, letÂ’s find the breaking strength of a piece of ½” nylon line. X circumference 2 = breaking strength)įollowing is a comparison factor chart for synthetic lines. Since synthetics are stronger than manila an additional multiplication step is added to the formula above. As mentioned above, a comparison factor has been developed to compare the breaking strength of synthetics over manila. To calculate the breaking strength of synthetic lines you need to add one more factor. (.5 X 3.14 = 1.57) Then using the formula above:ġ.57 2 X 900 = 2,218 pounds of breaking strength To convert use the following formula.Īs an example, if you had a piece of ½” manila line and wanted to find the breaking strength, you would first calculate the circumference.
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However, for purposes of the USCG license exams, all lines must be measured by circumference. When you purchase line you will buy it by its diameter.
The basic breaking strength factor for manila line is found by multiplying the square of the circumference of the line by 900 lbs. Synthetic lines have been assigned “comparison factors” against which they are compared to manila line. Natural breaking strength of manila line is the standard against which other lines are compared. How Do You Measure Rope Strength?Įach type of line, natural fiber, synthetic and wire rope, have different breaking strengths and safe working loads. Here’s all you need to know about rope strength. Each of these measurements has different uses, and here we’re going to give a brief overview of what’s what.
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Both of these are important measurements, and it’s worth learning how to measure and understand them. One boater will talk about tensile strength, while the other will talk about working load.
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