Impact force of a broken cable

[michaelGT06]

I am designing a winch guard to protect the operator in case the winch cable snaps. The structure is going to be built with angle iron and the open faces are to be covered with expanded metal. My question is, how do I calculate the impact force that the expanded metal will experience if the cable does break. I personally don't think the cable will break the expanded metal and injure the operator, but I need some numbers to prove it. It is a 3/8" metal cable with a breaking strength of 12,000 lbs. I think I have to find the acceleration of the cable when it snaps but I am not sure.

Any help would be appreciated. Thanks in advance.

 

[MiketheEngineer]

A very hard thing to predict....You actually need to know how fast it will de-accelerate. Then you can find the force. That number is very hard to predict. But imagine something that breaks at 12,000 lbs coming at you.... I would not trust grated metal - what if the tip just happened to make it through a "hole". Something more along the lines of a 1/2'' plate might be more reasonable. Testing might even be required...

At that force - the cable could easily cut you in half....Talk to anyone who has worked on an aircraft carrier. While that me be an exageration - they ALWAYS duck when a plane lands.

 

[GregLocock]

You've got two decent sized research projects there, if you really want an analytical solution.

You might be able work out the speed of the flailing end, but I think being able to work back to a mesh size based on that would be tricky.

If you were to abandon pencil and paper and go to simulation then LS DYNA would be able to model both parts of the problem. Without testwork to validate the model the consultant you employ will learn a lot and enjoy himself. If you have a physical test, then don't bother with the FEA.

If this is for a one off rig just use something as thick as the competition does, or a bit better.

Incidentally Kevlar sounds ideal for this application, if weight is important.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[arunmrao]

I thought the falling load from a snapped wire was more critical than the flailing wire rope.


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[KENAT]

Wasn't there a not entirely disimalr question a while ago, but about a pneumatic fitting coming off under pressure or something? Maybe there was an answer on that one you could use.

Perhaps an energy approach could get you a rough conservative estimate.? F=kX, E = 1/2 kx^2 and all that to find the energy in the cable. Although how that might be transferred to the frame may affect things.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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What is Engineering anyway: faq1088-1484

 

[prex]

I would calculate the energy stored in the cable under load (P²L/2EA, including any other loaded parts that would transfer energy to the broken cable, and compare it with with the toughness of the protecting frame, determining first the load path of minimum strength (fasteners or welds going away?).
Shouldn't give a totally unreasonable result.

prex

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[Ron]

You can calculate values ad nauseum for something like this and never know if you're right until the next one breaks. This is an application for prototype testing.

Be careful mixing expanded metal with cables. If the whip action is high, you can cut the cable on the plate edges, thus having shards of steel wire flying all over. I would use solid plate and have a round or hemmed leading edge.

 

[gwolf2]

You could try this:

Assume all of the strain energy in the rope and fittings is converted to kinetic energy. Assume a point impact in the centre of your largest plate. Using a point load assumption use FE or hand calcs to predict the load required to put enough strain energy into the plate to equal the kinetic energy of the rope. Then stress the enclosure and fittings based on this load.

Perform a local punch-through calc based on the cross-section area of the rope as if the tip struck the plate like a bullet. The punch-through load can be estimated by looking at the shear area of a perfect hole and doing some energy calcs related to standard specimen impact data.

A perforated plate sounds like it might let some shrapnel (or more) through - I'd go for solid if possible.

 

[bentwings2]

I have to agree with Ron on the flying shareds of metal and testing. In the field I've seen a winch cable come loose from a stuck truck and the cable with the hook penetrated the grill and radiator of the tow truck. Another had 2 cables connected together and the broken end of one cable and the hook from the other went through the back of the the truck cab.

Usually a chain draped over the cable and tied down or eevn a tree branch will deflect the cable enough to prevent it from going very far.

I'm not sure if you could reliably protect an operator standing near a loaded winch. A better choice would be to make a remote winch operating control. This is what we use on the trucks.

 

[amorrison]

Thirty years ago I was "near" a CAT pulling a self powered vibrating plow that was installing gas (CH4)piping six feet under the ground at a rate of 2 mph.
The tension on the steel fiber tow cable between the machines (both tracked) was "fantastic".
Actually there was no safety problem because if the cable snapped it would just fall to the ground as there was some type of energy dissipating device built into the cable just for this type of problem - it is probably required by Code.
Considering all the broken legs and heads (read lawsuits) that have been caused by snapped cables over the last century I believe it was in industries best economic interest to get safe cables on the market.

 

[L17Aurora]

Have a word with a trawler crew or tug. I'm sure they've seen it a hundred times

 

[MTPipeliner]

A while back there was an episode of Myth Busters that dealt with the idea of getting cut in half with a cable. They were trying to cut a pig carcass.

I don't think that you'll be able to get a direct answer from the show, but it might interest you if you can dig up a copy of it somewhere.

As I reacall they had a hard time to even break the skin on the pig and they never did cut one in half.

 

[GregLocock]

http://news.google.com/newspapers?n...N4VAAAAIBAJ&sjid=shIEAAAAIBAJ&pg=6901,2749148

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[amorrison]

Some posts here

http://mythbusterresults.com/episode62

I think there are two very different situations:

1. The cable is tensioned between two massive end anchors with no loads (sideways or potentially sideways) along its length then the cable largely just drops when it breaks

2. If the cable has sideloads between massive end anchors then when the cable breaks (into two pieces) the sideload will produse a massive acceleration in each of the individual cables which pivot (whip) around each of the anchor points. The mass of the cable rotation around the end anchors produces the damage when it contacts something.

An example of this is when an aircraft carrier landing cable breaks. If the cable does not break exactly (middle?)at the aircraft hook the still moving aircraft hook itself will produce a very high horizontal acceleration on the cable as it threads through the "hook".

 

[zekeman]

I think worst case scenario is you take the energy stored in the rope (see Prex's equation) and assume that it is stored in the mass ( the mass is actually distributed) of the rope and let it impact the frame.
If the frame "spring constant" is k, then the energy transferred (very conservatively at 100%) is
.5*F^2/k=energy stored in rope
where F is the impact force.
The trick is to get k . It should be fairly easy to do that knowing the frame structure.
This is fairly conservative since the actual case is that following the break, a pressure wave is set up which travels along the rope toward the fixed point and also spreads laterally and probably losses a good deal of the energy before impact and also the rope acts as a spring storage device which further mitigates the effect.
As a curiosity, why don't you get a spring , stretch it and let it go on one end to see the impact effect on the other end.I don't think you will see much impact.

 

[michaelGT06]

Thank you for all the responses and suggestions. I've made a lot of conservative assumptions and did some calculations to try and come up with a reasonable estimate but it seems kind of high and unrealistic.

Prex, can you tell me what all the variables are in the equation you presented?

 

[corrosionman]

michaelGT06 My tractor winch pulls out tree stumps and I know your problem - good visibility + good safety. Additional to all the very sound ideas, please mount the expanded grid on soft rubber blocks and it absorbs the energy without puncture.
Corrosionman.

 

[prex]

P=tension in the cable
L=length of cable under tension
E=elastic modulus
A=area of cable
Note however, as also observed by others, that a great deal of energy could be transferred to the snapping cable by a falling or otherwise travelling mass.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads