Wire rope length change as it twists

[trottiey]

I'm looking for an equation or chart that could tell me how much a wire rope shortens if you twist it. I'm not talking about the length change of the strands during manufacture of the rope. If you take a finished rope and twist it further, how much will it shorten per turn? I've looked through several reference books without success.

 

[Compositepro]

I think it is generally assumed to be zero. There are many wire rope constructions, but most are a balance of S- and Z-twist. So while the strands may twist tighter the wire in the strands loosen. Different layers will also twist in different directions. Of course, with extreme twisting there will be changes and the rope will also be destroyed.

 

[trottiey]

My rope is 7X19 aircraft cable. It's not right and left hand balanced; not rotation resistant. I'm not expecting extreme twisting, but I am interested in length changes on the order of 0.1%.

 

[Compositepro]

Well, look at it this way - does the length of a steel rod change with twist? If the rope is fully compacted (big if) then it should behave as a rod. "constructional stretch" is usually greater than 0.1%. Steel belts are often used in high precision applications because cables are not that precise.

 

[trottiey]

I'm not sure that a solid rod is a good analogy. A solid steel rod does change length with twist - it gets longer! But my practical experience with rope is that as you twist it, it gets shorter, until it eventually hockles. I guess I haven't been working with fully compacted rope?

Aircraft cable is pre-stretched, and we calibrate our length while the full operational load is applied, so I think that should take care of constructional stretch. A steel belt might have been a good idea, but this is an existing construction I have to deal with.