Joined
·
2,917 Posts
In once more before the lock!!!!!!!!!!!!!!!!!:goodfinge
Straps VS. Chain
101 - 120 of 124 Posts
Joined
·
2,917 Posts
That a cable breaking under load could not cut you in half. But it would sure fawk you up.
Joined
·
4,279 Posts
Rifle VS. Pistol
Strap VS. Chain
Rifle VS. Pistol to me. It depends on the use and the user. Either one will save you or kill you depends on you.
When I served in the USN we were told stories of men being cut in half by arresting cables that broke during aircraft recovery.
I was told similar stories about mooring lines (huge ropes).
I worked in the weight test shop aboard one ship and pull tested chains, shackles, slings, all types of heavy rigging equipment and broke some of it. It's scary as hell.
My point of view is that chains, straps, ropes are all an assembly of moving parts. They have to be maintained and worked within their limits, and scrapped on the first signs of wear or failure.
And respected.
Our dilemma is the forces at work aren't measurable or easy to control. If a 5000# vehicle is taking a run to yank out a stuck buddy's rig it ain't a 5000# vehicle anymore... it's WAY more (dunno the math but huge more).
If a 5000# vehicle is stuck in the mud or against an obstacle or downhill it ain't a 5000# vehicle anymore either... it's SOME more (dunno the math for that either). The two combined is STUPID more than 10k I'm sure.
So how could a 30k strap or chain survive that? It's all in the manner they are used.
There is no such thing as overkill, but I pulled a stuck rig out with 3/8" rope (doubled) after a guy broke 2 3" straps trying to yank it out. Technique can make all the difference.
It looked like 2 gay dudes can't cut a dead pig in half.
Strap VS. Chain
Rifle VS. Pistol to me. It depends on the use and the user. Either one will save you or kill you depends on you.
When I served in the USN we were told stories of men being cut in half by arresting cables that broke during aircraft recovery.
I was told similar stories about mooring lines (huge ropes).
I worked in the weight test shop aboard one ship and pull tested chains, shackles, slings, all types of heavy rigging equipment and broke some of it. It's scary as hell.
My point of view is that chains, straps, ropes are all an assembly of moving parts. They have to be maintained and worked within their limits, and scrapped on the first signs of wear or failure.
And respected.
Our dilemma is the forces at work aren't measurable or easy to control. If a 5000# vehicle is taking a run to yank out a stuck buddy's rig it ain't a 5000# vehicle anymore... it's WAY more (dunno the math but huge more).
If a 5000# vehicle is stuck in the mud or against an obstacle or downhill it ain't a 5000# vehicle anymore either... it's SOME more (dunno the math for that either). The two combined is STUPID more than 10k I'm sure.
So how could a 30k strap or chain survive that? It's all in the manner they are used.
There is no such thing as overkill, but I pulled a stuck rig out with 3/8" rope (doubled) after a guy broke 2 3" straps trying to yank it out. Technique can make all the difference.
Well what did you think rocket science was for? surely not rockets.:wackoapparently we need rocket science to recover a vehicle.
Well shit...with the amount of vehicles I have recovered...I must be a Rocket Scientist....:histericaapparently we need rocket science to recover a vehicle.
A friend of mine got ahold of some crane straps, not sure what they are rated at, at least a 100k pounds. They have a little stretch but not much. I have seen trailer balls go flying no matte what you use I have straightened receiver hooks, broken chains, had straps break the back windows out, and seen the rubber band effect of ripping front tow how hooks off and slinging them into the upper atmosphere only to burn up on the return to earth. No matter what you are doing take precautions and DUCK!!! It is as simple as this.
I am not saying this to condone stupidity, moreover I am saying this to tell the inexperienced to not rely on that "unbreakable strap or chain" , because sometime when you arent looking or paying attention your going to get a reality check right up side your head.
I have done some of these stupid things and as much as we try to do things the safest way possible, sometimes you just have to get the job done. The one thing you can do to help minimize the risks is make sure the people have a barrier, ie.. tree, bystanding vehicle or a rock between them and the vehicle being tugged.
I have seen and read alot of the disagreements in this thread but the thing that needs to be stressed most of all is DUCK!!!!!
Cody
I am not saying this to condone stupidity, moreover I am saying this to tell the inexperienced to not rely on that "unbreakable strap or chain" , because sometime when you arent looking or paying attention your going to get a reality check right up side your head.
I have done some of these stupid things and as much as we try to do things the safest way possible, sometimes you just have to get the job done. The one thing you can do to help minimize the risks is make sure the people have a barrier, ie.. tree, bystanding vehicle or a rock between them and the vehicle being tugged.
I have seen and read alot of the disagreements in this thread but the thing that needs to be stressed most of all is DUCK!!!!!
Cody
Joined
·
3,450 Posts
apparently we need rocket science to recover a vehicle.
what did ya think was going to happen when you started this thread, it was asking for lots of debate
Joined
·
1,215 Posts
:stupid :toothless
You know, I think this is actually where we've gotten stuck, you're right. Most of what I've said above is related to the forces involved in snatching a vehicle. The situation you're describing is basically towing out a vehicle that doesn't have enough traction. I would agree that as long as it's used with some sense, chain is perfectly fine for that -- since there's really no reason you'd actually intentionally snatch a vehicle that's not actually stuck. These situations usually don't require a lot of force anyway (heck, sometimes half a dozen guys on the other end of the strap can give the vehicle enough oomph to get traction and get moving).
To me, "stuck" means you have the vehicle mired in such a way that it can't move under its own power at all. Either the wheels are too far down in mud or sand, or worse, hard parts are buried in it. There's a zone there where the vehicle isn't too stuck to remove without digging, yet can't be towed out because the pulling vehicle can't generate enough force to break it loose. You can pull the vehicle out once it's moving, but the static drag is more than you have enough traction for. That's the problem I'm considering, because I think that's where most accidents usually occur.
I got curious enough on this after I read your reply this morning, I decided to wait until after work and actually run some numbers just to satisfy my own curiosity. Sometimes it's the only way to get my head off a topic. As physics problems go this one actually isn't too bad as long as you make some reasonable assumptions, and you can simplify the equations in a way that gives you a rough numerical solution using nothing more complicated than a spreadsheet. I did the basic setup of the problem on paper, and then translated it onto a spreadsheet that would let me vary initial speed, strap/chain stiffness, vehicle masses, traction of the pulling vehicle, force needed to "unstuck" the vehicle you're recovering, and a constant drag on the "stuck" vehicle once you get it moving. This basically assumes you've got the stuck vehicle in some mud, and once you get it moving you're still going to have to keep force on it to get it out. I'm confident enough in this set of calcs being fairly realistic that I'd be willing to base a design on it if I needed to (though I'd want better information on the stiffnesses and strengths of the chain and strap, of course -- better input would be needed, but the calc itself works).
The results are actually pretty informative, and I'll post up my different assumptions and the basics. Common to all the cases:
- Pulling and stuck vehicle both weigh 5,000lb.
- Static drag of 5,000lb must be overcome to get the stuck truck moving.
- 3,000lb of drag on the stuck truck once you get it started. This is net, so if the stuck truck can help some, this is the force after you subract any help from the stuck truck from the drag.
- Lead vehicle pulling speed when the strap/chain tensions is 5ft/sec. That's about 3.5mph. Go higher and the strap forces get very big, very fast. This combination of mass and speed means you have a starting energy of about 23,300 in-lb to work with.
- I tried to keep my strap and chain fairly equivalent in breaking strength.
The first case is a 30' strap. I had to make a reasonable guess on the stiffness, so my made-up strap is a 20,000lb strap that will stretch about 2' at its breaking force. This is a spring force of about 833lb/in of stretch. I'm a lot less confident in this number than I am of the chain numbers for stiffness. The first couple of cases assume the driver of the pulling vehicle doesn't gas it, and relies on his own momentum to do the work.
When the strap force reaches 5,000lb, enough to break the stuck truck free and start it accelerating, the strap will have stretched 6", and will have a stored energy of 15,000 in-lb, which is about 2/3 of the energy of the pulling truck, slowing the pulling truck from 60in/s to 36in/s.
The maximum force in the strap will be about 5900lb, and this will occur about 56 milliseconds after tensioning. This is less than 1/3 of the breaking capacity of the strap, no problem. At this point the strap is storing 20,840 lb-in, which is almost 90% of the total energy in the system. The pulling truck is down to about 11in/s, and the stuck truck has sped up to match.
Game over occurs at about 1/3 second, when the drag force has eaten up all the energy you started with, and the stuck truck stops moving. It has moved 5" foward.
Case two has all of the assumptions identical, except the strap has been replaced with a 30' length of 3/8" Gr70 transport chain. The chain has a SWL of about 6600lb, which means it probably has a breaking strength of somewhere on the order of 20,000lb or so. This chain will also weigh around 45lb. It would be a good size and weight for light recovery and towing, where you're not snatching. If you just wanted to do a steady pull, it should work fine. However, if you try and snatch, even at the reasonable speed we're assuming (5ft/sec) ...
When the chain force reaches 5,000lb, it will have stretched 0.28", and will have a stored energy of 703 in-lb, which is about 3% of the energy of the pulling truck, only slowing the pulling truck from 60in/s to 59.1in/s.
The maximum force in the chain will be about 22,600lb, and this will occur about 27 milliseconds after tensioning. This is near or over our estimated breaking strength. At this point the chain is storing 14,374 lb-in, which is 62% of the total energy in the system. At least when it breaks it's storing less energy than the strap. The pulling truck is down to about 26in/s, and the stuck truck has sped up to match.
Game over occurs at about 0.3 second, when the stuck truck stops moving. It has moved 8 1/4" foward. 3.25" inches gained at a significant increase in risk.
Cases three and four assumed you had the throttle on when you tensioned the chain, enough to produce 3500lb of continuous pulling force. This is about what's required to pull the stuck truck all the way out in one go. You get a maximum strap tension of 6540lb, or a max chain tension of 28660lb. Yowzer.
Case five answers the question of, "We have a lot more capacity in the strap, just how hard can we yank?" I went on the assumption of pure momentum, no throttle to speak of once the line is tensioned. Turns out you can hit the assumed strap with about 20.8ft/s, or just over 14mph, or a whopping 404,770 in-lb of total energy. This is basically backing up 20', and hitting the gas enough to get 31.75 continuous HP (sort of a simplified assumption, but it's definitely do-able).
That serious yank will move the stuck truck a whopping 134 3/4" just on momentum and stored energy, and it'll be rolling for over a second. That's more than enough time to find the gas and complete the pull. However, you'd better make damn sure what you're pulling on is more than strong enough, because the energy stored in the strap is enough to launch a broken 5lb shackle at over 300mph. :shocked
Anyways -- just thought that was interesting. I think I'll stick with a winch whenever possible, which would have done this example recovery at 1/2 capacity at a nice, controlled speed. :duh
Joined
·
11,531 Posts
call out people that were using hitch balls and chain in another thread about tow hooks. this thread was started to clean up anothers post.
The difference in stored energy is about the scale I imagined, but I had never attempted to crunch the numbers. I'm terrible at ID-ing & ignoring the right variables to allow me to get to the real calculations, but I'm OK at intuitive analysis.Chuck said:
Did you ever try to figure out what the chain would do? I know the strap's mass is negligible for most calculations, but the semifluid nature of the chain along with its overwhelming density make it much more difficult for me to guess its behavior, to a quantitative degree. I also think the difference in fracture behavior (brittle vs. plastic) is significant.
I don't really agree with that characterization - IMO, if a vehicle can't go home under its own power &/or traction, I consider it "stuck", even if it's able to move a few feet. But if you use "stuck" to mean "absolutely immobile", I'd say your definition is at least as valid. That might be another reason there are so many disagreements in this thread. :shrug Different interpretations of terms.Chuck said:
That's something else I have to disagree with - I've participated among "half a dozen guys" to lift the back end of a partially-loaded pickup with the automatic trans in P & the doors locked, turn the truck on its front wheels like a wheelbarrow, then roll it up onto a curb & into a yard. So IMO, "half a dozen guys" can produce "A LOT OF FORCE".Chuck said:
Chuck,
Would you please review this and offer your opinion of it's practicality and accuracy, as well as Bill's math in relation to the calculations you used:
http://www.pirate4x4.com/tech/billavista/Recovery/index.html
Scroll down to: "Calculating the force of a recovery operation:" (it's about mid-page I think).
. . . I'm not ignoring you Steve. I just know that you have seen the link before.
Would you please review this and offer your opinion of it's practicality and accuracy, as well as Bill's math in relation to the calculations you used:
http://www.pirate4x4.com/tech/billavista/Recovery/index.html
Scroll down to: "Calculating the force of a recovery operation:" (it's about mid-page I think).
. . . I'm not ignoring you Steve. I just know that you have seen the link before.
Joined
·
1,215 Posts
I just made it back to the comp, so it'll take me a bit to look through things. I've read BV's recovery article before, but not necessarily with a very critical eye.
Steve -- with definition of "stuck" out of the way, I think we're mostly saying the same thing, then. My vote is, use chain or strap if you just need to give the hapless vehicle a tow up a hill or out of a hole, and stick with strap if you're doing something you should really be using a winch for instead. :thumbup
I could make a pretty reasonable guess on the chain assuming a brittle fracture that absorbed little energy, but if the chain actually managed to yield significantly before fracture, a guess I make would be high. It's really hard to estimate the amount of energy absorbed accurately, because the only thing numbers-wise you usually have to go on is the tabulated percent elongation per six inches, and that's really only valid for a 6" test sample. It really doesn't mean much numerically for any other shape, it's just an effective seat-of-the-pants reference number for ductility. I'll try and take a look at it this evening to see what I get, just for curiosity's sake ...
Steve -- with definition of "stuck" out of the way, I think we're mostly saying the same thing, then. My vote is, use chain or strap if you just need to give the hapless vehicle a tow up a hill or out of a hole, and stick with strap if you're doing something you should really be using a winch for instead. :thumbup
I could make a pretty reasonable guess on the chain assuming a brittle fracture that absorbed little energy, but if the chain actually managed to yield significantly before fracture, a guess I make would be high. It's really hard to estimate the amount of energy absorbed accurately, because the only thing numbers-wise you usually have to go on is the tabulated percent elongation per six inches, and that's really only valid for a 6" test sample. It really doesn't mean much numerically for any other shape, it's just an effective seat-of-the-pants reference number for ductility. I'll try and take a look at it this evening to see what I get, just for curiosity's sake ...
That was always my impression. But remember: our misunderstandings aside, this whole thing started because someone said chain is a killer & should NEVER be used for recovery, and I disagreed. That was the original object of this thread, anyway.Chuck said:
My assumption was always that there WOULD be significant elongation before & during the fracture, which is why I THINK chain won't fly as much as strap, assuming they both break when used properly, but simply overloaded.
I don't really want to get in this argument, so I'll just add a comment....My friends and I use both straps and chains.. And if someone doesn't have a proper recovery point, we have used the tow ball before with good luck.. But we always take a shirt, tie off the sleeves, and fill them with sand or rock or whatever, to slow the chain or strap from doing any damage... and I can tell you that worked really well.. Just my .02:toothless
101 - 120 of 124 Posts
Join the discussion
