Force required to bend steel wire rope

[samanchek]

I have searched all over for the answer to this question and have had no luck. Maybe someone here can point me in the right direction.

I have a wire rope that is bent in a U shape and it needs to be removed. However, we are concerned that once it is cut from the restraints currently holding it in place there could be a dangerous spring action as it tries to straighten itself out. Does anyone have any suggestions for how to go about determining the spring force we could expect? Is there some approximation method or test data I could use? I figured the simplest way would be to determine how much force it would have take to bend the wire rope into the position in the first place but could not find any information on that either.

Thanks

 

[IRstuff]

> Don't get it; why not cut the restraint and the loop off together?

> Seems to me that you could easily find out stuff by bending a section of the rope and measuring its behavior.

TTFN

FAQ731-376

 

[MiketheEngineer]

What size rope - what kind of steel
How tightly bound

Can you grab it with a come-a-long or some such device. Cut the rope and then unload the come-a-along slowly.

Put a cutting torch on a very long wand, stand back and cut it.

Or dynamite??

Good Luck

 

[samanchek]

IRStuff: There wouldn't be enough room to cut it all out together, it needs to be cut into small sections and removed piece by piece. It is 4" thick wire rope which would be too expensive to buy just to test. I'm considering testing several smaller diameter ropes and maybe scaling up the results but would prefer to find an alternative method if I can.

MiketheEngineer: It is 4" diameter bound around a 25" radius. We are not sure of the exact material yet. I was considering some sort of come-a-long device but without knowing the forces that would be released it would be difficult to size properly.

 

[IRstuff]

What I was implying was that there ought to be sufficient length elsewhere on the rope that you could bend and get some measurements.

TTFN

FAQ731-376

 

[EngineerTex]

First:
If you *really* want to determine what type of spring force might be contained in the rope after bending it around a 50" diameter sheave, you should be able to get a ballpark approximation. Remember that the wire rope is intentionally designed not to experience permanent deformation in service. Therefore, yield and tensile strength should not play into this analysis. So, determine the diameter of each individual strand by measurement or looking it up in a wire rope manual (I don't have mine in front of me) and determine the strain energy required to bend a wire of the individual strand's diameter around a diameter of the wire rope assembly's neutral axis. This will allow you to calculate the spring rate of a single strand bent through 180 degrees on a 54" diameter (50" + 2" + 2") and then multiply that number by how many strands you have in the wire rope. This does not take into account various other factors such as manufacturing techniques used for purposely making the rope stiffer or more flexible and it wouldn't help you if for some reason the rope has a dozen revolutions of twist left in it from its days in servic.

Second:
I would ignore most everything I just wrote above. When wire rope is in service and you don't know its history, there are problems that you can run into that you didn't know existed. When a heavy load is suddenly released from a wire rope, the rope can take a permanent deformation called a birdcage which is not noticeable under load. The weight of your rope may prevent this from being seen. There could also be kinks in the rope that are in the reverse direction of the sheave which would make it very difficult to notice. In either of these cases, (and probably some others) the spring calculation you determine with the above method wouldn't take into account residual stresses from permanent deformations. With a 4" rope, I'd be concerned.

http://www.rigging.net/wire.html

Third:
This may be an elephant you have to eat one bite at a time. Assuming that you have no load on the rope, you would likely have an simpler time identifying one of the seven bundles (or maybe 19?) and cut that single group. Use a marlinspike to unwind it from the rest of the cable and do the same thing for the remainder of the bundles until you reach the last one. This would assume that you are going to dispose of this beast. Once you get to the last bundle, it should be pretty slack (and small) due to the fact that most of the rest of the rope is no longer there to hold it in its shape.


Engineering is not the science behind building. It is the science behind not building.

 

[cntw1953]

Can heat & straighten do the trick for this case similar to straighten a bend bar?

 

[hydtools]

Heat the bend to take all retained stress out of it so it will not spring when cut out.

Ted

 

[whammett]

The only problem with heating the bend is compromising the intergrity of the wire. We don't even know the exact specs of the wire, I would not risk it.
IMO I would consider using a come along as MiketheEngineer has stated. Just make sure all of the slack is taken out of the come along before you cut the steel wire.

"I came, I saw, I made it better."
-Ode to Industrial Engineers
Will ChevronTexaco Corp.

 

[Ron]

I agree with hydtools...heat it and relax the stress. Heat at top of the curve and watch it relax...then cut.

 

[unclesyd]

The wire rope has already been compromised by being wrapped around the pin and was going to be cut anyway. Normally one would selvage the wire on both sides of the cut to prevent the unwinding or springing out of individual strands. As this approach is problematical in the OP's situation even with heating I would go slow to ascertain the behaviour of the wire rope.

I've seen large diameter wire rope cut with a welding rod where the ends were welded together as the wire was cut.

Have you requested help from the wire rope manufacturer?

 

[samanchek]

A couple items to address all the recent posts.

-It is in an area that is difficult to access and there may not even be enough room to let it relax and straighten completely.

-Heating it may not be a bad option, compromising the wire rope is not a problem as it is not supporting anything and will not be used again.

-We will only have direct access to the wire rope for a short period of time so taking it apart 1 strand at a time would probably be too time consuming.

-The rope was installed 30 years ago and the original manufacturer has shut down since then. Tried contacting other wire rope manufacturers but they all said they had no information on this type of situation.

-Using a torch or welding rod to cut is not a possibility due to fire hazards.

-We believe it is a single strand.

Thanks for all the help

 

[IRstuff]

You're scenario is making less sense to me.

What is the rest of the rope doing and do you have access to it?

Would dumping LN2 on it help?

TTFN

FAQ731-376

 

[racookpe1978]

I think I need to make a sketch of this before anybody can make sense of this idea, but then again, we don't a sketch from the OP to know what is really going on either.... 8<)

Apparently, the very thick, 4 inch diameter wire rope is bent around a 50 inch diameter post or pin, and is secured (or was secured ?) at both ends. These ends apparently no longer have a load on them, so I'd expect only limited "bend-back force to be "stored" as potential energy in the bent wire rope.

On the other hand, if there is no restraining force on both ends of the wire that is forcing the wire to bend, then it would have straightened out on its own. Even friction and gravity forcing the ends of the wire out against the walls of the tower/shaft/tunnel (?) might be enough to force the wire to stay bent.

Regardless, we must assumed there is some force on both ends of the wire: So, when the wire is cut you need to (1) hold the wire so it doesn't unsafely drop on somebody's head below, or equipment below, or back down the shaft/tube/pipe/column and jam someplace else. (2) You want to make sure it doesn't "sprong" back into the cutter/grinder/workers' face either.

So wrap two nylon strap rigging hangers around the wire - one on each side of the pin. (Nylon will naturally grip the wire, you don't need clamps or bolts or anything else.) Take the two straps back around the pin to their opposite sides and use two chainfalls (or come-a-longs) to take a tension on the nylon straps.

Now, you are "pulling" the ends of the wire towards the pin, and the part of the wire bending around the pin will be unloaded - or as close to unloaded as you can get in a reasonable way.

Now, cut the wire as described above, and use the two straps to pull the ends of the wire out of the column/beam/tower/shaft/whatever.

 

[dicer]

Have you thought of making a holding fixture, that is adjustable so after the cut is made it can be let down easy?