Average breaking strength is determined by averaging the samples in a given test. Minimum breaking strength can be figured several ways, either using the true minimum number that was recorded during a testing sample or more commonly by using a statistical method such as 3-Sigma.

 

Safety factors are a ratio between the minimum breaking strength of rope or equipment and the maximum anticipated load. Safe working loads are calculated using an industry established safety factor deemed necessary. For example a 15 to 1 Safety Factor with a 600lb SWL would require a MBS of 9000lbs. Safety Factors vary by industry. NFPA 1983 specifies a Safety Factor of 15 to 1 for life safety rope.

 

There are a lot of publications available that contain information about knots and knot strength. Foe that reason we will not include much information here, except to say that knots reduce the overall strength of a rope. Therefore, whenever possible use a tensionless hitch around a cylindrical object with at least a 4" diameter. Four inches is the magic number for maintaining full strength in a rope up to 5/8". Any bend tighter than 4" reduces the strength of a rope because it stresses the rope in flexure, the fibers' weaker direction. Here is a list of some common knots and the translating remaining rope strength.

          Knot Strength Loss

Overhand                  60-65%            

 

In the United States yarn weight is classified by the term "denier". Denier is the weight in grams per 9000 meters of yarn. In Europe, they use a base ten system called "tex". Tex is the weight in the grams per 10,000 meters of yarn. More and more US manufactures are using the tex instead of denier, as it is much easier to convert.