Knots vs. sewn end terminations
Recently we were at a large department out west and the question came up as to why we are sewing the ends of the rope and not tying knots to secure the anchor or attaching device.
Knots have been tied in rope as far back as rope has been made. With the implementation of Fire Rated “Bailout” systems which use Arimid ropes (Kevlar, Technora, Spectran etc.) in these systems it was determined that the Arimid ropes would not hold there tensile strength like Nylon or Polyester.
NFPA 1983-06 Edition Standard on Life Safety Rope and Equipment for Emergency Services added a manner of function test to the standard. This test required the entire Bailout system be tested for overall strength from attachment anchor point to attachment to the belt or harness. 8.6 Manner of Function Tensile Test
This new test created challenges for manufactures to make or design ropes and or descending devices to meet this test. The properties of Arimid ropes, having little or no stretch reduces the breaking strength from 45-60% depending on the design. This is due to Arimid rope or webbing placed into a tight bend will use only the outer portion of the rope as Nylon will stretch and allow for more of the fibers working to display the load. This requirement severely reduced the number of personal descenders and ropes on the market that would actually pass the test.
RIT safety Solutions actually had the first pre-rigged fire rated bailout on the market. Since 1998 RIT has designed and marketed the system using Arimid ropes, webbing, belts and harnesses. Having several systems third party certified by UL and in house testing, we have a great deal of experience utilizing Arimid systems. With this said RIT also had difficulties at first to make and design a system that would pass the manner of function testing.
Testing for the NFPA 1983 standards require a minimum of 5 samples which the average breaking strength is measures. Once an average is calculated a standard deviation is also calculated to reach the certified braking strength. Standard Deviation can actually make or break (no pun intended) your results. We as I am sure many manufactures have had a great average breaking strength but failed the minimum because the Standard Deviation requirement. You can take most Excel programs and type in STD Dev in the formula and run your own results, you may find it interesting to see how simple it is to change the final result.
I will be posting several videos on our You Tube channel in the next few weeks that may help understand the process and why Arimid ropes and webbing if not dramatically over built will require the ends sewn instead of tying knots. As always if you have questions about our product please give me a call or e-mail.