M
moray
Guest
This may be of some interest to the splicers out there...
Many modern ropes have a hollow center, and splicing them involves burying a core in that hollow space. The manufacturer's instructions for splicing such rope, and everthing you may read elsewhere, emphasize creating a smooth taper at the end of the "bury" to maximize strength. This instruction sometimes includes phrases such as "The most important thing is to create a smooth taper..., It is vital that..." and so on. The most extravagant language seems to appear in Internet postings, but there is never any evidence to accompany the advice. So how important is it?
I had wondered about this for some time. Now, with access to a homemade rope-testing rig, I decided to run a few tests. My rig can create and measure tensions up to 10,000 lbs with a 2-lb. resolution and an accuracy over the full scale of about +-10 lb. This is far better than what I would need.
For ease of splicing and ease on the wallet, I chose 5/16 in. Tenex Tec and 3/8 in. Tenex Tec for the tests. Even though these ropes are simple hollow braids, the principles involved should apply to all spliced ropes with a buried core. All the rope was new, and all tests were done on slings with spliced eyes on both ends. The splices were all substandard in length because I knew they would not pull apart. The tapers, however, were full length and carefully done. The untapered buries were just that--full diameter rope cut straight across.
Here are the breaking strengths, in pounds, for the 5/16 in. rope:
2 tapers - 4460
2 tapers - 4400
1 blunt - 4006
2 blunt - 4084
And for the 3/8 in. rope:
2 tapers - 5554
1 blunt - 5118
All ropes broke at the end of a splice bury. If a rope had both a blunt and a tapered bury (labelled above as '1 blunt'), it broke at the end of the blunt bury.
These data clearly don't support the use of glowing adjectives extolling the strength benefits of a tapered bury! The benefit is quite modest. In the 5/16 in. rope, the blunt buries preserve 91% of rope strength, and for the 3/8 in. rope, 92%.
The interesting question is why does a blunt bury reduce rope strength? The answer, I think, is pure geometry. The strands in rope--almost any rope--are not aligned with the load. Even though the strands themselves have a fixed tensile strength, the collective strength of the strands is less than their simple sum because of the alignment problem. When a hollow rope swells up to swallow a buried core, the angle of the individual cover strands becomes less favorable still, and the cover becomes weaker. This effect is masked by the presence of the core, whose strength more than makes up for the weakening in the cover.
But at the very end of a blunt bury, the weakening is unmasked! Here the cover strands have the poor geometry of the fat part of the splice, but they have to support the entire load alone.
I measured the angles of a lightly loaded, spliced piece of 5/16 rope. In the fat part of the cover the deviation angle was 31.1 degrees. In undisturbed rope it was 21.3 degrees. The ratio of the cosines of these angles should give us the relative strengths. It is 92%. The measured relative strength, as seen above, was 91%.
Many modern ropes have a hollow center, and splicing them involves burying a core in that hollow space. The manufacturer's instructions for splicing such rope, and everthing you may read elsewhere, emphasize creating a smooth taper at the end of the "bury" to maximize strength. This instruction sometimes includes phrases such as "The most important thing is to create a smooth taper..., It is vital that..." and so on. The most extravagant language seems to appear in Internet postings, but there is never any evidence to accompany the advice. So how important is it?
I had wondered about this for some time. Now, with access to a homemade rope-testing rig, I decided to run a few tests. My rig can create and measure tensions up to 10,000 lbs with a 2-lb. resolution and an accuracy over the full scale of about +-10 lb. This is far better than what I would need.
For ease of splicing and ease on the wallet, I chose 5/16 in. Tenex Tec and 3/8 in. Tenex Tec for the tests. Even though these ropes are simple hollow braids, the principles involved should apply to all spliced ropes with a buried core. All the rope was new, and all tests were done on slings with spliced eyes on both ends. The splices were all substandard in length because I knew they would not pull apart. The tapers, however, were full length and carefully done. The untapered buries were just that--full diameter rope cut straight across.
Here are the breaking strengths, in pounds, for the 5/16 in. rope:
2 tapers - 4460
2 tapers - 4400
1 blunt - 4006
2 blunt - 4084
And for the 3/8 in. rope:
2 tapers - 5554
1 blunt - 5118
All ropes broke at the end of a splice bury. If a rope had both a blunt and a tapered bury (labelled above as '1 blunt'), it broke at the end of the blunt bury.
These data clearly don't support the use of glowing adjectives extolling the strength benefits of a tapered bury! The benefit is quite modest. In the 5/16 in. rope, the blunt buries preserve 91% of rope strength, and for the 3/8 in. rope, 92%.
The interesting question is why does a blunt bury reduce rope strength? The answer, I think, is pure geometry. The strands in rope--almost any rope--are not aligned with the load. Even though the strands themselves have a fixed tensile strength, the collective strength of the strands is less than their simple sum because of the alignment problem. When a hollow rope swells up to swallow a buried core, the angle of the individual cover strands becomes less favorable still, and the cover becomes weaker. This effect is masked by the presence of the core, whose strength more than makes up for the weakening in the cover.
But at the very end of a blunt bury, the weakening is unmasked! Here the cover strands have the poor geometry of the fat part of the splice, but they have to support the entire load alone.
I measured the angles of a lightly loaded, spliced piece of 5/16 rope. In the fat part of the cover the deviation angle was 31.1 degrees. In undisturbed rope it was 21.3 degrees. The ratio of the cosines of these angles should give us the relative strengths. It is 92%. The measured relative strength, as seen above, was 91%.
