Line Strength Ratings

SYNTHETIC LINE STRENGTH RATINGS AND WHY THEY ARE IMPORTANT

Ultimate Tensile Strength (often referred to simply as “Tensile Strength”, or TS) is a measure of how much load a material can withstand when being stretched or pulled.  Because ropes are designed to be pulled, their strength is defined by their TS.  TS is determined by destructive testing – by applying a continuously increasing force (i.e., loading to failure) to a set of sample ropes one by one until they break, and noting how much force it takes to break each one (Breaking Strength, or BS).  After breaking the samples, the resulting data is used to calculate the Average Tensile Strength (ATS) and Minimum Tensile Strength (MTS).

MTS: is a calculated minimum load the rope must be able to hold (99% probability at 95% confidence when using Weibull A criterion) before breaking.  

ATS: is the average strength (i.e. the applied force) at which a sample set of ropes break – it is higher than MTS and by definition approximately half the sample ropes broke below the ATS.

BS: is the strength at which each individual sample rope broke (sometimes called Max Tensile Strength).  The highest recorded BS in a test group will exceed both the ATS and MTS.

MTS is the best measure of the actual strength of a synthetic line-based item, because it is a loading condition that all such items can achieve without breaking.  Safe-Xtract labels all its synthetic rope products with an MTS rating derived from Cordage Institute testing standards and the MTS Safe-Xtract uses is the lower spliced MTS, not the higher rope MTS. Safe-Xtract also uses the strongest McDonald Brummel Splice and also dips the eyes in a proprietary rubberized coating for additional protection. This ensures that all Safe-Xtract synthetic line products can be loaded to their respective MTS strength ratings without failure.  

Unfortunately, there is no regulatory body to standardize the way manufacturers determine the strength ratings of their synthetic line-based items.  Because of this, many other brands do not hold themselves to the MTS standard.  Instead, they go with different standards, some of which can be used to inflate the apparent strength of their products.  Strength ratings can be inflated in many different ways, including as follows:

1.     Item rated at ATS – if a manufacturer bases the strength of their item on Average Tensile Strength, it makes their rope sound stronger – but the problem with this is that by definition, approximately half their items will break below the ATS.  You can basically flip a coin as to whether the item will break below rated strength.

2.     Item rated at Max Tensile Strength – this method also yields a strength rating that sounds impressive, but means that virtually all items will break below rated strength.  

3.     Item rated at Break Strength – this means that the strength rating is based on a single break test.  Even if multiple articles are tested and broken, manufacturers can base their strength rating on only one test – the test that had the highest Break Strength.  Again, this method yields a strength rating that sounds impressive, but essentially means that virtually all will break below rated Break Strength.

4.     Item rated at rope strength – this means the strength rating is assumed to be equal to the strength rating of the rope used to make the item.  However, many items are spliced, and splices typically reduce the strength of the rope. For example, the pass-through splice is so prone to creep and without stitching to hold it in place, is weaker.  Manufacturers use these lesser splices because they are easier/quicker to do and use less rope and therefore save them labor and material costs – but they also reduce the strength of the item compared to the strength rating of the rope, and there is no guarantee that this is loss of strength is factored into the strength rating of their item.  

5.     Strength Rating simply made up – as stated earlier, there is no regulatory body that oversees how a strength rating is determined, and so it is entirely possible to simply inflate the strength rating of a product to make it stronger than the competition. 

Some companies use something called a Working Load Limit (WLL), derived by dividing their strength rating by a design factor/safety factor.  This can introduce confusion for two reasons: 1) the strength rating they are using could be influenced by the reasons stated above, and 2) design factors/safety factors vary based on application, and are therefore inconsistent.  

Bottom line, beware of any synthetic line products that do not come with an MTS rating.