Creating a Steel Rope Joint: Tips & Techniques

[mawais15]

hello friends, i am currently working on a project in which a steel rope is used to carry an object in a loop just like a chair lift. what i want to ask is what method can i adopt to make a joint of the rope that can pass through or over the pulleys and not de-track too.
The rope diameter is 8 mm, the tension in the rope is about 15-20 kN.

 

[Pkruse]

For about twenty years I was the lead engineer of a group that designed and maintained some very sophisticated wire rope systems. We had about 20 very well trained wire rope tech's to work the paper we wrote.

One of the most important and basic rules that we always followed was to never design a system like you are talking about. The rope must be continuous and we never let a wire rope termination run through any sheave. (In the rigging industry, nobody ever uses the term "pulley.")

That was a rule we treated as seriously as any of the laws of thermodynamics. This was a rule that cannot be violated at any time.

The only exception is when replacing a wire rope with a new rope. We would then attach the end of the old rope to the new rope so that the old rope could be used to pull the new rope through the system. In that case we would normally just weld the two ropes together, or you can use a cheap "Chinese Finger" sort of device for the same purpose. Or there are various hitches the riggers would use to lash the two ropes together, using parachute cord. For small rope, duct tape works well if the rope is clean; but we always preferred it to be very well lubricated. The only customers who insisted on clean rope were those working on spacecraft in clean rooms. But then clean unlubricated rope causes all sorts of serious potential problems, so their rope needed to be changed out much more often, and it generated wear particles that cause problems in their clean room.

 

[Pkruse]

A wire rope is a precision piece of machinery with hundreds of moving parts. Every wire must be free to slide across the other wires in any system of running rope. If any of the wires become locked such that they cannot slide freely, then the rope will quickly fatigue and break.

Common ways of accelerating this fatigue process is to have kinks in the rope, or any other damage or lack of lubrication or corrosion.

But nothing locks all the wires down like a termination fitting. The part of the rope near the termination must therefore never actually work around any radius. If possible, the system should also be designed such that the termination sees little or no load, but that is not always possible.

 

[Pkruse]

There is a solution to your problem that was common in olden days. I’ve never seen it used in any modern system. It is a serious compromise. It will cut the strength of your rope by perhaps half, and it will eventually lead to a fatigue failure. So if you use it, then you must inspect your rope frequently and replace it at the first sign of a pending fatigue failure. That implies that you have an inspector who fully understands all the signs of fatigue.

It is the long splice, which will pass through a sheave.

Or if your rope is not terribly long, a single strand grommet will work even better.

 

[mawais15]

the rope is 100 meters long. and the traction system is such that we can't install a rope in it which has no termination.

so there had to be a joint which can carry equal tension and pass over those sheaves as well.
the good thing is that the joint will not pass through end sheaves as in the case of chair lifts and cable car.. it will move only to and fro through a span of 500 meters.

the only thing i want from that joint is to bear the tension and pass through sheaves.

one idea that I've in my mind is to use some sort of shuttle which can slide over the pulley and rope will be friction locked in that shuttle with the help of bolts.

 

[Pkruse]

Then you are stuck doing it like on a ski lift. Use a long splice, and make it very long.

This is a skill that very few diggers have. Call up a local mobile crane rental company and ask who makes their wire rope rigging. Then give them a call.

 

[Pkruse]

Sorry for the typo.

Diggers should be riggers.

 

[marcusl]

The wire ropes used in San Francisco's cable car system are joined with long woven joints. Each end is unraveled and then rewoven together. The joints are slightly fatter than the rope, but nowhere twice the diameter. The splices are inspected regularly. I've never seen how the joints are made, however...

 

[Pkruse]

Sounds like a long splice with a Kellum grip installed as a protective sheath. I was thinking that might be a good solution, but it would require a larger sheaves groove, which would degrade performance.

 

[mawais15]

Each end is unraveled and then rewoven together.

great idea.. this idea worked great for us... both the ends were untwisted and inserted into each other over a span of about half meter.. then applied pressure and a little brazing. the joint is now perfect although a little more in dia but okay!

 

[mawais15]

Sounds like a long splice with a Kellum grip installed as a protective sheath. I was thinking that might be a good solution, but it would require a larger sheaves groove, which would degrade performance.

the sheaves we're using are small and have very small groves as are used in case of airborne chair lift systems

 

[mawais15]

the rope is 100 meters long. .

its 1100 actually :-s

 

[Pkruse]

I was wondering about the idea of brazing the long splice to secure the ends. That would of course reduce the strength of the rope, but if done carefully the reduction would probably not be any more than the reduction caused by the splice.

If you use a short splice, then you will have an increase in diameter. It will normally be about ten rope lays in length. A long splice in an application like this probably wants to be about a hundred rope lays in length.

If you use a long splice, then there will be no increase in diameter. But then the trouble is how to terminate the ends of the strands. If you are willing to braze and test, then that would solve that problem.

Keep in mind that anything that prevents the individual wires from sliding freely against each other will shorten the fatigue life of the rope, and will also reduce the strength. I probably would not plan on a splice efficiency greater than 50%--knowing full well that on a new installation it will be much better than that, but that the efficiency will reduce over time with fatigue.