Those who are new to rope climbing may assume that tensile and safe working load are the same. Such an assumption will prove dangerous for those who are using ropes because there is a considerable difference between these two concepts. Tensile strength is the necessary force for breaking a rope, and manufacturers figure it out by scientific testing.
The working load limit, however, pertains to the maximum load applied to a rope. It is the tensile strength’s friction that lets for a generous safety margin when using a rope. Rope manufacturers usually provide the working load limit for their products to keep the users informed.
Although there are general agreements regarding the safe working load standard for rope, there are some discrepancies among various professional groups and associations regarding the safety factor acceptable for the safe working load.
Mountaineers, cavers, and other groups engaged in outdoor activity generally accept the 10:1 ratio between tensile strength and safe working load as the acceptable ratio standard. Thus, if the rope’s tensile strength is 5000 lbs, you can load it with a maximum working load of around 500 lbs.
However, the National Fire Protection Association generally recommends a safety ratio of 15:1 safety factor, which means that if you have 5000 lbs tensile strength, then the maximum working load of that rope is 333 lbs.
Tensile strength, as a definitive quantity, measures the force necessary to break a rope. However, the working load limit takes into account a wide range of factors and variables. Tensile strength, of course, is greater than the working load limit.
Differentiating Between Tensile Strength and Working Load Limit
So, if your job requires you to use ropes more often, you need to know the following differences between tensile strength and working load limit:
Tensile strength pertains to the breaking point of a new rope. Ropes are tested for their approximate strength using D-6268 of the ASTM testing system. To come up with the new rope’s minimum tensile strength, testers only need to reduce the approximate average by around 20%.
Some factors limit the tensile strength, such as the rope’s age, how it has been used, and type of knots used on it, and many other factors.
Ropes vary in their characteristics. They come in various breaking strengths. For example, the Manila line is the standard breaking strength used to measure other rope’s strength. Experts have assigned tensile strengths relative to that of the manila line.
To get the manila line’s strength factor, it is best to multiply by 900 lbs the line’s circumference square. Thus, they use the formula 900 lbs. x circumference² is equal to breaking strength. Thus, they use the formula 900 lbs. x circumference² is equal to breaking strength.
When you are buying a line, you would usually buy it based on diameter. Yet, if you would engage in USCG license exams, you would find all lines measured by circumference.
Thus, you will need to use the formula Circumference=p PI (3.14) x diameter. To figure out synthetic lines’ breaking strength, you need to consider another factor. This factor is the comparison factor developed for comparing the breaking strength of manila and synthetics. Thus, your calculation will go like this: comparison factor by 900 lbs by circumference² + breaking strength.
The tensile strength refers to the average strength of a new rope as experts test it under laboratory conditions. Experts wrap the rope around two capstans with large diameters. Then, slowly apply tensions on the rope until it reaches its breaking point.
Working Load Limit
The tensile strength is not synonymous with the working load limit, and it would be good to understand the difference between the two. The working load limit considers various factors like friction, abrasion, vicissitudes of temperature, effects of harmful substances, knots on the rope, and many other factors that may affect the rope’s tensile strength.
Strictly speaking, the working load limit refers to the maximum load that you can apply to the rope uniformly without it having to break. The working load limit is always less than the tensile strength. As a fraction of the tensile strength, it allows for a reasonable margin of safety.
Wire ropes usually have a working load limit of 20% of the tensile strength. You don’t need to engage in the meticulous calculation of this 20%. You will often see the working load limit of a product on its package, and manufacturers provide the working load limit for their products.
Manufacturers determine the working load limit by dividing the tensile strength by the factor that accurately reflects the highest load that a rope can comfortably and safely handle. Of course, the factor differs depending on the fiber type and the weaving style and construction.
Yet, other factors may affect the tensile strength of a rope, and these factors may not be within the manufacturers’ control.
Important Factors to Consider
Generally, most ropes’ working load limits range between 15% and 25% of their tensile strength. When you tie a rope in half, you reduce the tensile strength by almost half. Moreover, when you apply tension on the line with a knot, you don’t need to expect the rope to have the functional tensile strength as determined by manufacturers.
Various knots may have different effects on tensile strength. Yet, it will be useful to bear in mind that the 50% tensile strength loss, as a general rule is manageable. Moreover, applying an eight knot on a rope would reduce its tensile strength by almost 35% instead of 50%.
Knots and Splices, and Other Considerations
If you are engaged in outdoor climbing or other jobs that use rope, it will be of great help to bear in mind the difference between tensile strength and working load limit. Moreover, it will be useful to bear in mind that every knot you do on the rope will reduce the rope’s overall tensile strength. Nevertheless, some knots do not substantially affect the tensile strength, while others will have a radical effect on its tensile strength.
As mentioned above, a knot may reduce the tensile strength by 50%. Thus, when calculating the working load limit, make sure that you factor in the knots you have on your rope, especially if you will use the string to haul critical loads. More often, you can’t avoid knotting your rope, for the occasion demands it, and knots, of course, are useful.
On the other hand, splices will also affect the tensile strength of your rope. It will be helpful to remember that a simple distortion on the cord will impact its outer strands, making the outer strands bear more weight than that of the inner strands. This situation may not be right for you if you will use the rope for risky jobs.
It is also good to consider that some knots can cause fibers to compress while overstretching the others. These effects on the rope’s fibers may cause issues and may substantially affect the rope’s integrity and tensile strength.
Yet, you would seldom notice these effects on the rope, for some knots are not damaging. Moreover, some loads are not significant enough to cause the cord to snap. Additionally, some ropes like those made of Dyneema, polypropylene, and other fibers show more resiliency than traditional ropes.
Elasticity & Dynamics
Another factor worth considering when dealing with tensile strength and working load limit is the dynamics or elasticity of the line subjected to dynamic loads. Ropes come with a design that allows them to recover from elastic hysterics after being subjected to dynamic loads. However, some cords may not recover from elongation, or some may take several minutes or hours to regain their form, while some quickly return to their original state.
Ropes come in many designs and weave forms, and clusters of fibers make up a single strand, and several strands are woven together to form a rope. Ropes can be three-strand or four-strands twisted ropes. Twisted cords under strain usually convey a rotational force on each end. Yet, for heavy lifting, non-rotational ropes are often preferable. Plus, there are ropes with a single braid, some with hollow braid, and solid braid.
There are also double braid ropes that you can use for dinghies and yachts. Moreover, you will find a kernmantle for climbing that comes with a tightly braided sheath or mantle.
When choosing a rope, you should also consider whether the rope has a high modulus or low modulus. High modulus means it has little elasticity, while low modulus may indicate higher elasticity.
Furthermore, when selecting a line or rope, you need to understand how a rope performs. Some applications require a lower stretch for limiting movement, while others require a higher degree of elasticity. Elasticity, of course, refers to the degree a rope would stretch given varied load.
Elasticity, as a concept, is a complicated thing to figure out. It may include elastic elongation, hysteresis, constructional elongation, extension while relaxed, and extension while working. Knowing the rope’s elastic stiffness will also help you compare how cords will perform when subjected to a load. Elasticity will also factor well in the tensile strength and working load limit of a rope.
A comprehensive discussion of the tensile strength and working load limit of a rope will be beneficial if you are a person who engages in outdoor activities that use ropes. Although related to each other, these two concepts are different from each other. Thus, it would be useful to know these two concepts to understand better the ropes you are using.
Being cognizant of the rope’s tensile strength and its working load limit will enable you to choose the right kind of cord you would use for various applications. Lastly, understanding these two concepts will help you stay on the safe side when using ropes for multiple applications.