Snatching and recovery points.
Submitted: Monday, Jan 09, 2006 at 16:27
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Scoey
OK so I know that looping the snatch over the towball is bad and a big no-no. I've also noticed that a lot of fourby's have a recovery point that they can slide into the provision for their towball tongue. I've also noticed that my shackles are rated at 4.7t "SWL" (I'm assuming this is 4700kg??)
My question is this:-
If my snatch strap is rated at 8000kg and my towball at 3000kg then mathematics tells me this is a bad idea, but to hook the snatch onto a proper tow point with a shackle rated at only 4700kgs still leaving me a defecit on the snatch's rating of 3300kgs doesn't make feel a whole lot safer! Unless the "SWL" stamped on the shackle is some sort of different measurement???
I've also seen recovery gear rated at a "massive" 10,000lbs!!! Which is approx 4500kgs? Am I right? Is this just a marketing gimmick and a trap for
young players?
Anyone that can explain all this mess in idiots terms would be appreciated! I'm one of those people that need to understand how it works before I'll do it! :-)
Cheers
Scoey!
Reply By: garrycol - Monday, Jan 09, 2006 at 16:49
Monday, Jan 09, 2006 at 16:49
I have been wondering the same thing - the responses will be interesting
Gazzz
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Reply By: Bonz (Vic) - Monday, Jan 09, 2006 at 16:52
Monday, Jan 09, 2006 at 16:52
Scoey your 8000 kg snatch will fail at around 6700-11500kg, your 4.7t shackle will fail at around 27t (don't quote me, I cant find the 4WDM that they tested them in.) and your tree protector anywhere from 4500to17000kg
SWL is Safe Working Load, it means just that its safe to work at that load, shackles have a high redundancy about 6 times SWL to fail, straps much less.
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Follow Up By: Scoey - Monday, Jan 09, 2006 at 17:28
Monday, Jan 09, 2006 at 17:28
That makes a lot of sense! Cheers! Do you know wat the rating on the towball is meant to indicate? I would think it would be a safe sort of constant pulling capacity, not a quick yank (for want of a better expression!)...? And if used for snatching what sort of loads they would fail at??
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Follow Up By: Scoey - Monday, Jan 09, 2006 at 17:34
Monday, Jan 09, 2006 at 17:34
Also what the difference in loads would be for a towball and a recovery point that slides into the towball tongue provision??
Scoey
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Follow Up By: Bonz (Vic) - Monday, Jan 09, 2006 at 17:35
Monday, Jan 09, 2006 at 17:35
Tow balls arent rated, for yanking or anything, thats the problem with them. Anyone who tells you theyre OK for say 3000kg's is taking a stab in the dark. I would love to see them tested but boy how would u do it safely.
Balls fail for many reasons in many ways varied by their construction process
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Follow Up By: garrycol - Monday, Jan 09, 2006 at 17:36
Monday, Jan 09, 2006 at 17:36
So a 8000kg snatch strap is not really an 8000kg snatch strap - it is really a 6700kg snatch strap - I would have thought that would break some trade laws - surely the SWL of a 8,000 kg snatch strap is 8,000kg and its actual break load would be something greater.
Cheers
Garry
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Follow Up By: Bonz (Vic) - Monday, Jan 09, 2006 at 17:42
Monday, Jan 09, 2006 at 17:42
its the ball that fails Scoey, it breaks or shoots off somehow. The slide in unit it rated and relies on a shear force across two 18mm surfaces. I have done the calcs somewhere else on EO and the failure calcs at a VERY high level hence the largely increased safety margin.
Garrycol the straps failed at differing loads depending on their make/manufacturer, go get 4WDM Issue 82 Aug 2005, all the good oil is in there.
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Follow Up By: Scoey - Monday, Jan 09, 2006 at 17:46
Monday, Jan 09, 2006 at 17:46
Cheers for the info Bonz! ;-)
Scoey!
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Follow Up By: Bonz (Vic) - Monday, Jan 09, 2006 at 17:59
Monday, Jan 09, 2006 at 17:59
I found the calcs they are here. The shear force to break the hayman reece pin is 1267 tonnes
Thats a tad more than the snatch strap.
I used to be anal but now I am not so sure
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Follow Up By: Des Lexic - Monday, Jan 09, 2006 at 23:03
Monday, Jan 09, 2006 at 23:03
Bonz, does that mean your Pre-anal or Post-anal? Please explain!!!!
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Follow Up By: Bonz (Vic) - Thursday, Jan 12, 2006 at 21:31
Thursday, Jan 12, 2006 at 21:31
just b-anal Des
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Reply By: Scubaroo - Monday, Jan 09, 2006 at 16:56
Monday, Jan 09, 2006 at 16:56
4.7t is the safe working limit (SWL) - the actual load before it breaks will be 4 or 5 times this figure - I recall one of the mags tested shackles and they
broke somewhere in the region of 20-28 tons.
Most straps advertised limit though is the actual breaking load! So you could expect an 8000kg snatch strap in NEW UNDAMAGED condition to break at around 8000kg. 4WD Monthly have done their famous series of tests on snatch and winch straps in the past, and found some straps actually
broke at lower loads than advertised. NO SAFETY FACTOR in the advertised rating!
So the discrepancies come in from the safety factor - if shackles had a lower safety factor, you could advertise the same shackle as a 20t shackle!
Winches are often advertised in imperial - just an advertising gimmick I think. Some poor sod is running around out there thinking his 9000lb winch is a 9000kg winch I bet.
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Reply By: Member - Beatit (QLD) - Monday, Jan 09, 2006 at 17:03
Monday, Jan 09, 2006 at 17:03
I guess, leaving myself a little exposed here, but whilst it is good to understand these tolerances there must be a large number of variables that can cause failure beyond the working loads of the equipment used. Such as the number of times the strap has been used, its general condition, the condition of the 2 vehicle attaching points (not just your own), the length of the snatch and run up, the extent of the bogging. My point is that we all have to play it safe and it works but some failures do occur. When you are stuck you will not be as fussed about the the question of physics and potential for failure. What is important is that you use good equipment (and it looks like you do) and you keep people away that don't need to be involved and use as little effort as is required.
Kind regards
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Follow Up By: Scoey - Monday, Jan 09, 2006 at 17:32
Monday, Jan 09, 2006 at 17:32
At the risk of exposing myself as
well (if you can, I can too! ;) ) then I would agree, using common sense when recovering is paramount! My snatch is ARB and my shackles Black Rat and are always clean and stored properly.
I just hear a lot on this site (and others) about the dangers of tow balls and given the weight ratings I can't help but cringe when recovering as vehicle even using shackles! This info goes a long way to easing my mind!
Cheers!
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Follow Up By: Member - Beatit (QLD) - Tuesday, Jan 10, 2006 at 10:25
Tuesday, Jan 10, 2006 at 10:25
No problems wasn't looking! I guess a lot of us (me included) have been guilty of using tow balls before we were educated and very little goes wrong. BUT it can happen and I have seen it twice (not on mine) it resulted in severe damage and it is was easy to see how it could have ended up in serious injury or worse.
Kind regards
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Reply By: Member - Zach S (SA) - Monday, Jan 09, 2006 at 17:23
Monday, Jan 09, 2006 at 17:23
just to tack on an additional question to this thread, where is the best two point in the back of a GQ?
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Follow Up By: Trev88 - Tuesday, Jan 10, 2006 at 12:56
Tuesday, Jan 10, 2006 at 12:56
The loop that all patrols have is not rated and when I did my 4WD course the instuctor did not want to use it to recover the bogged demo instead opting for the hayman resse reciver with the pin in the strap.
The trainer said he has seen many patrols recovered using
the loop but he has also seen the effects of one that did not hold (Details unknown) but it happened to one of his co instructors during a demo.
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Reply By: Scoey - Monday, Jan 09, 2006 at 17:26
Monday, Jan 09, 2006 at 17:26
Great! I thought there would be a pretty simple answer and that makes a lot of sense.
Just some more food for thought (I really wish I had an off switch for my brain!) Say my mate's Patrol (approx 2.7t) is stuck in the mud and I'm trying to snatch it out with my Cruiser. The downward weight of the vehicle plus the suction and resistance given by the mud must make for some incredible figures as far as pulling weight to retrieve said Patrol! Does anyone know of any reports/studies etc that have tested this???
I'm not really interested in the physics or what other constraints come into play just the higher end numbers. Simply for curiosties sake!
Cheers again,
Scoey!
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Follow Up By: cokeaddict - Monday, Jan 09, 2006 at 17:39
Monday, Jan 09, 2006 at 17:39
Thats an Impossible situation Scoey,
We all know patrols dont get bogged..epecially in the presence of cruisers :-)
Just kidding all u yota fans.
But an excellent question to begin with and some fantastic replies as expected guys.
On a serious note, I have seen the result of a snatch gone wrong, I cant remember seeing things fly by but i sure as hell heard it with devestating results. sounded like a bomb went off on impact.
Cheers.
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Follow Up By: Scoey - Monday, Jan 09, 2006 at 17:45
Monday, Jan 09, 2006 at 17:45
Yeah, I couldn't resist! I'm still smiling from the weekend playin' in the rain when my very loud trol driving mate got hung up on his rear bumper, and stuck proper, as we all do! He even tried to convince another driver to recover him than let me! After I pulled him free, he was rather quiet for the rest of the arvo! haha! But being the mate I am I won't mention it - and only keep the photo's and video for insurance! ;-)
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Follow Up By: Bonz (Vic) - Monday, Jan 09, 2006 at 17:50
Monday, Jan 09, 2006 at 17:50
well we had one break at work and drove the connection point which was half inch steel 6"x3" straight thru the back of the truck bucket like a knife thru butter
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Reply By: Member - David 0- Monday, Jan 09, 2006 at 21:43
Monday, Jan 09, 2006 at 21:43
The trick is the rating are different. The snatch strap is rated for the kind of work it does. Shackles are rated for the kind of work they normally do= lifting. Somewhere in the grand scheme of things SWL (safe working load for lifting) was determined to be (frome memory) 25% of the failure load. I guess this was to ensure large parts of buildings and containers etc wouldn't be falling on people as cranes worked in our cities and workplaces. The upshot of it is that the shackles are plenty strong enough.
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Follow Up By: Member - Tony G (ACT) - Tuesday, Jan 10, 2006 at 10:06
Tuesday, Jan 10, 2006 at 10:06
As you quite rightly point out, shackles are rated as a static load, ie Lifting/ Lowering, Under tension, all static loads.
Now in recoveries we use then for connecting snatch straps etc and subject them to shock loads. What does that say for the SWL then?
I have seen shackles fail, and it is usually the ones that have been used in the wrong application. A straight shackle instead of a bow shackle, and it's always been the pin that has let go, and usually under a shock load.
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Follow Up By: Member - David 0- Tuesday, Jan 10, 2006 at 10:15
Tuesday, Jan 10, 2006 at 10:15
I am no physicist so I can't answer the question with regard to whether a shock load is any different to a static load. All I can say is there is plenty of room for much higher load than the rated shackle is "rated" at.
You are right to point out the very real dangers of using shackles that aren't rated. Even rated shackles have failed for whatever reason. The general rule should be, don't use a shackle if you don't have to, and the best way to ensure you don't have to is to have purpose made recovery points that will accept the strap withour shackles.
If you do use a shackle, be sure to put a dampener on the strap closest to the end with the shackle.
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Reply By: Kiwi Kia - Tuesday, Jan 10, 2006 at 15:36
Tuesday, Jan 10, 2006 at 15:36
Don't use shackles when snatching.
There is no difference between straight and bow shackles other then as follows;
The bow shackle was designed for attaching multiple straps (slings) when lifting.
The straight shackle is for direct line pull with one strap.
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