Will I kill us all? How do I estimate the strain I'll be putting on a stainless steel wire rope?

March 8, 2006 9:20 AM

I'm making a fence using 4x4's and 3/16" stranded stainless steel wire rope. The cable is made of six bundles of wires surrounding a center bundle. Each bundle looks like it has a similar structure. The tag on the spool says “Not for overhead lifting.” Each spool holds 125’, which I plan to join using loops and ferrules.

I'll use eyebolts at either end of the cable and a turnbuckle near the center to provide tension. I'd like to make the cable run with only two terminations, each about 250' long, between 5 fence posts.

My question: do I have to worry about the breaking point of the cable? It doesn’t seem likely that I’ll do anything that will come close to approaching the breaking strength of the wire rope, but I’d hate to be wrong and have it snap suddenly. How do I calculate what kind of load I’ll be putting on the cable?

posted by RelentlesslyOptimistic to home & garden (24 comments total)

The cable you're using apparently has a breaking strength of 3,700 lb. So I think you're OK on that.

I can't find a strength rating on the swaged cable sleeves, although that might be a CYA move because the vendor can't control the quality of a joint you make. But in my experience, swaged joints are very strong, and I don't think you'll be reaching a 3,700 lb load.

That said, I don't know how to calculate the load you will be applying. Perhaps if you turn the turnbuckle with a torque wrench...?
posted by adamrice at 9:34 AM on March 8, 2006

Go to Wire Rope Industries. It looks like yours will hold up to 1900 lbs of pull strength and possibly more. I'll buy the beer if you can put that much stress on the cable with a manual turnbuckle without pulling the stanchions first.

Most cities have a wire rope distributor; they sell the cable for hoists, car-pullers, winches, and other industrial uses. Call them for advice if you are unsure. As a matter of fact, I believe that Wire Rope Corporation of America's HQ is in KC, MO.
posted by beelzbubba at 9:39 AM on March 8, 2006

You can buy tension meters, but they look a bit pricey. I'm assuming you can get the maximum load for which the cable is rated from the manufacturer.

This high school lesson plan includes some math for calculating tension by measuring the deflection of the cable when a given amount of force is applied.
posted by IshmaelGraves at 9:40 AM on March 8, 2006

Well, the Wire rope table in the back of the Backstage Handbook says:

With a safety factor of 5, 3/16" 6x19 galvanized steel wire rope (6 bundles with 19 strands in each bundle) has a safe working load of 520 to 640 pounds, depending on the quality of steel.

Safety factor of five means that the actual breaking strength is five times the safe working load, or in the neighborhood of 2500 pounds.

Note that this is pounds of weight applied as a static force. Pulling, tugging, or dropping even lightweight dynamic loads can exert far greater force on the cable.

My guess is that you will approach nowhere near the limit by hand-tightening a turnbuckle. And you are not using this for "overhead lifting" The weakness in this system will be in the termination points: how the cable is fastened to itself, and then to the 4 x 4's. I'd suggest beefy rated forged and shouldered eye bolts with big fat washers. The wood and bolt hole would most likely be the weakest part of this system, the hole will splinter out long before the cable breaks.

Check out the bolts and turnbuckles at Sapsis Rigging. Go into the online catalog, and look under Stage Equipment and Hardware and Wire Rope and Cable Fittings categories

Cable termination is critical, the selection of ferrules and the adjustment of the swaging tool is another point where the strength could be weakened.

Same goes for quality of turnbuckles, they should be one piece forged with jaw ends, not bent eye-bolt ends.

Given that the termination points will be weakest, I'd advise against running one long cable back and forth to make a fence with multiple horizontal cable runs. Each horizontal run should have it's own terminations, turnbuckle, etc. That way if one cable comes loose, the others do not slack as well.
posted by sol at 9:47 AM on March 8, 2006

Yeah, my experience is that the termination points are where everything goes wrong. I always end up using hardware that looks about twice as big as it needs to be (after the stuff that looks right breaks).

On the other other hand it is quite possible to snap a steel cable like that if you use a come-along or anything similar, in which case you can be in big trouble since they store a heck of a lot of potential energy.

Check out this excellent PDF about Fence wire tensioning, joining, stapling, grounding etc from the Ag & Fisheries ministry of the BC government, which includes details of a nifty home-made wire tension gauge.

I think that about 250 lbs of tension is the most you will need.
posted by unSane at 10:37 AM on March 8, 2006

You can guesstimate the tension from a turnbuckle by counting the number of threads per inch. Say it's 16, each rotation gives you 1/8" (assuming it's one with a left hand and a right hand threaded rod). So if you use a a 12" bar to twist it, you've got a 12"*2*3.14159 : 1/8" ratio or a ~600:1 increase in force (neglecting friction, so oil it up). So, yes you can over tension that cable with a turnbuckle. The thing is, the smaller the turnbuckle, the finer the thead pitch, so its easier (with the same length handle) to overtension a small turnbuckle than a large one.
posted by 445supermag at 11:02 AM on March 8, 2006

I've put my life in the hands of cable that was less than 1/4" inch thick. And I don't mean that figuratively: wired stoppers for rock climbing have arrested falls and saved me from almost certain death.

Granted, the cable used in my stopper was made to save human life, so it was engineered with absolute redundancy, and your cable was not. But I'd expect yours to be able to hold a (fully static) small car without breaking.

(The "not for overhead use" warning is primarily about liability -- it's not made in such a way that they can be certain it will do it's job 99.9% of the time in life-threatening situations, but it's still plenty strong).
posted by teece at 11:23 AM on March 8, 2006

Your weakest point is the crimp, they are generally good for about 70-80% of the working load of the cable. More secure are the saddle type (remember never saddle a dead horse) at 80-90%. The wedge style can be good for over 95%.

You'll bust the turnbuckle way before cable but you don't even want to be close to that tight anyways. Remember that in the case of a failure of your cable (either at the connectors or because of failure of the attachment point) all that tension will be released, often in fractions of a second. A cable with a thousand pounds of tension in it can take your leg off.

Use eyelet forms to protect your joining loops. Thery are about $0.20 each and make for a professional looking job.
posted by Mitheral at 1:26 PM on March 8, 2006

I would consider using crosby clips instead of "crimpers."
The big advantage will be if you need to take it apart for any reason. Crosby clips are used in theatrical rigging all of the tiem (including permanent instalations.)
posted by leafwoman at 1:26 PM on March 8, 2006

I would wear eye protection regardless.
posted by smackfu at 1:57 PM on March 8, 2006

And heavy leather gloves. A broken strand of wire rope will slice your hand open with little effort.
posted by Mitheral at 2:22 PM on March 8, 2006

I've seen a horse break that kind of cable. It was used to delineate a parking lot at a state park and strung at about knee height. The horse spooked and stumbled into the cable, it tried to pick up it's front feet and jump it at the last second but instead caught the cable pretty much dead on the cannon bone (lower leg) and the cable snapped (Horse was fine). The horse was moving fairly slowly and was actually a pony, no more than 900 lbs and that particular part of the leg is not heavy or strong. If a horse hit a plain thin wire fence in a similar fashion I would expect the horse to fall and be badly cut and the wire to survive or only be broken at the posts, so I was very suprised and happy that the heavy cable broke.

The cable wasn't new but I've no idea how old it was.
posted by fshgrl at 7:54 PM on March 8, 2006

I've seen a horse break that kind of cable.

The cable was under tension already -- so adding the shock of the horse hitting it pushed the cable beyond its breaking point. It's amazing what tension can do. A climbing rope is a royal pain in the ass to cut with a knife -- but hang 300 pounds on it, and it will snap like a rubber band with a light chop.
posted by teece at 9:43 PM on March 8, 2006

Nope, it wasn't under tension. The cable was hanging in loops between the posts, probably 8-10" lower in the middle.
posted by fshgrl at 11:01 PM on March 8, 2006

And it was thicker than the OPs cable, more like 3/4" I'd guess.
posted by fshgrl at 11:01 PM on March 8, 2006

No tension and about 3/4 of an inch? Well, then it was radically different that the cable I'm used to. A 3/4" cable could stop a 20 foot whipper pretty easily on my climbing rack, and that's going to easily generate a 1000 lbs of force. If the cables on my rack were that easy to break, I'd be long dead. I wonder what made it so fragile?

Strange.
posted by teece at 11:10 PM on March 8, 2006

My guess would be that fencing cable is completely different: more decorative, less strong. I've seen medium size cranes use the same diameter cable that the pony broke through to move large loads so it must be in the construction. And to a lesser degree maintenance.

I guess my point is that I wouldn't compare a fencing cable to other steel cables if there is some concern about breakage. As an aside my experience with running fencing tells me that the OP will yank his fence posts long before he breaks the cable! 50' is a long distance between posts too, so sagging will likely be an issue in the long run.
posted by fshgrl at 11:32 PM on March 8, 2006

Thanks everybody! I now know that:
1) There's such a thing as the "Backstage Handbook." I love it! (I used to want to do backstage work as a career, so I might get a copy just for curiousity)
2) That ferrules may not be the best choice for me, because I don't have the tools to make a good crimp (I was just going to rig something up with a hydraulic jack). Crosby clips will be more tedious, but a better choice.
3) I'm considering doing a series of runs that will start and end on the same post (i.e., up and back). That will hide the potentially ugly connection ends and eyebolts at one end, limit the length of each run, and lose only two strands if a connector fails.
4) I'll use the HS lesson to do some tension figuring - thanks!
5) I need bigger and sturdier turnbuckles.
6) I need to figure the tension and strength of my posts, too (duh!). They're anchored two feet deep in concrete, so they probably won't pull out, but they might break.
7) Thanks for the online stores, too - I couldn't find much when I searched.

Thanks again!
I'll be posting the construction/assembly on Flickr when I'm thru. Email me at relenlesslyoptimistic at gmail.com if you want to be notified when it's posted.
posted by RelentlesslyOptimistic at 8:30 AM on March 9, 2006

Oh yeah, and I'll keep horses out of my back yard!
posted by RelentlesslyOptimistic at 8:31 AM on March 9, 2006

RO--There's a tool for swaging nicopress cable sleeves called a Swage-It. It's basically a hand-held clamp that you tighten with a wrench. The one I've got is only good for three sizes up to 1/8" cable and it cost about $30; there's a larger version good up to 1/4" that looks to cost $60 or so. It's much much cheaper than any other swaging tool out there.
posted by adamrice at 8:47 AM on March 9, 2006

RO, are all the posts anchored or just the corner posts? For comparison every fence I've ever built (and it's been a few) has been off a 12' spacing for the posts. We might adjust up and down a bit from that but never to 50'. We sink the corner posts about 4' , concrete and cross-brace them. The other posts are sunk to the same depth but not normally concreted. Corner posts could be as small as 4"x4" for board fencing but are typically 6"x6" pressure treated for any kind of tensile fence.

I've never strung cable as heavy as the stuff you describe though.
posted by fshgrl at 7:01 PM on March 9, 2006

All the posts are 4x4 cedar sunk 2 feet deep in concrete.

Hmnn...I guess there's no reason I couldn't terminate some runs on center posts to equalize the load. I'll have to think about that.
posted by RelentlesslyOptimistic at 4:05 AM on March 10, 2006

teece writes "I wonder what made it so fragile?"

A fibre core steel cable will quickly rust from the inside out[1] if not maintained properly[2]. You end up with a bunch of short, say an inch or two, strands of cable held together by rust and the fibre core. Result: a 3/4" wildly strong looking cable that can only hold a few hundred pounds.

This is one of the reasons you should never use discarded cable and when discarding cable it should be cut up into very short lengths. We coil it up into about a 2-3' per loop coil and then make a cut with a rail saw.

[1] steel core cable rusts like this as well but it takes longer and then starts disintegrating because it doesn't have the non rusting fibre to keep it together.
[2] cable is a machine. When exposed to the elements it needs regular lubrication, careful handling and regular inspection. Which rarely happens when used for fencing.
posted by Mitheral at 7:22 AM on March 10, 2006

Mitheral - what about with stainless steel? Would a periodic (annual?) application of oil or grease to the cable prevent long term decay?
I'd also thought about caulking the holes in the post, but I suspect that the cable will stretch over time and need to be tightened, which would pull the caulk out. What about squirting grease into the holes with the cable?
posted by RelentlesslyOptimistic at 6:52 AM on March 11, 2006