rigger 47 Posted August 11, 2010 Share Posted August 11, 2010 Spectra LBA Posted: Tue Jan 13, 2009 7:41 pm Could someone give a confused oldie an answer. On trademe there is someone selling spectra. They state the breaking strain but then say that you should take 20% off for safe working load. I have always thought that the swl is 20% off the breaking strain but maybe that is just for the normal yacht braid. Am I totally confused? -------------------------------------------------------------------------------- W.T Posted: Wed Jan 14, 2009 5:01 am The usual approximation for SWL is as you say, Break load -20%. But the break load stated in the spec sheet for rope is usually an average break load for 5 tests. This is also tested on new rope in ideal conditions. So no knots, splices, salt, UV damage etc. As i understand the SWL could be taken as a reduced safe BL factor to allow for 'non-perfect' operating conditions. i.e. if the line took full SWL with a knot tied in the middle. Depending on the intended application for the rope, the SWL may not be the ideal figure to base a 'sizing' on. If it's for a tow rope then SWL will be fine. But if it's for backstays, rigging or a kite brace/guy then a larger diameter may be needed to reduce stretch even though the BL is way in excess on the predicted load case. Either that or go for a similar size, but higher spec fibre. -------------------------------------------------------------------------------- MrWolf Posted: Wed Jan 14, 2009 6:32 am Certainly different rope manufacturers specify their ropes properties in different ways. Generally it is very unwise to use a rope too close to its ultimate break load. (A safety factor of 3-4 is very appropriate) -------------------------------------------------------------------------------- Knot Me Posted: Wed Jan 14, 2009 10:55 am The SWLs of ropes vary depending on use. Generally they are 20% of the break load or 10% if a people is hanging off it. On a yacht work on 20%. Notes: Most (but not all) manufacturers publish break load numbers which DO NOT include a termination. Allow 15% for a good splice and 40-50% for a knot. More again if knoting fibres like Spectra/Dyneema/Vectran and the like. They like big radius's so knots are not their friend. Some manufacturers publish the maximum load they get in tests, some use the minimum and some use the average. There is no standard test so that's why some ropes look a lot stronger than other even though they are exactly the same construction. The '5 tests' WT mentioned above is for climbing ropes and they are called 'Drop tests'. Basically they drop a big weight to simulate a climber scratching his nose and falling off a cliff. As mentioned, rope loads are found on new rope using a slow steady pull on a test bed. SHOCK LOADS kill things. As soon as you start to use a rope it starts to degrade. Some ropes get weaker when they get wet, nylon is one. Drops 10% in loads when wet as opposed to dry. UV kills some ropes faster than others. Stretch and creep will become more of a noticeable factor once you start to exceed 30% odd of the break load. All depends on wallet size and end use. If you race and have a big wallet use the smaller end sizes and replace often. If you don't use the bigger end options as they will last a lot longer. Link to post Share on other sites
Recommended Posts
Archived
This topic is now archived and is closed to further replies.