Recovery System Design HELP and the Weak Link

[Islandnomad]

Since joining AULRO I have read with great interest this recovery section. It has made me rethink my system, which I use mainly for beach recoveries. It seems to me that for a Snatch Recovery all items used and their SWL's should be considered.




If you assume that the system consists of:
1. Recovery points on bogged vehicle, lets assume a front recovery so 2 recovery points
2. 2 x shackles
2. An Equaliser strap
3. A Snatch Strap
4. Another single shackle
5. A Tow bar recovery hitch


Shouldn't the design of this system be designed to break, so that debris doesn't fly everywhere and kill someone? I would have thought that if the equaliser was designed to break first that the safest outcome would be achieved ie. steel shrapnel would not be flying all over the place.


I have a D2 and am wondering when I buy this system what ratings I need to be specifying prior to purchase?


I think I'm on the right track with this, any help would be appreciated.


I am thinking of buying Philco's (a member on here) recovery points or similar.

[BadCo.]

Rating consistency might be an issue.

I'm not super experienced at recoveries but in my head I would use as big as you can fit (or 1.5x GVM) and if you are worried you can get those straps for the shackles to stop them flying.

Also people tend not use shackles between the equaliser and snatch, just thread the bridle through the snatch. They won't bind together because the eye is strengthened/hardened/extra stitching/what ever, from what I have experienced.

I also use a bridle/equaliser front and back if possible.

Bad

[Tank]

Don't use recovery points that orientate the shackle pins in the horizontal plane as the shackles will become the weakest point if they are not pulled on from directly ahead, shackles aren't designed to be pulled off centre.
A good recovery point will have the shackle pin hole orientated vertically, allowing the shackle to align itself with the direction of pull. if you use a bridle strap, use the longest you can buy, the angle created in the legs of the bridle strap should be kept as low as possible, a 90 degree angle will double the load on the bridle strap. With the legs of the strap together the SWL doubles the further apart the SWL decreases.
If you must snatch prepare the groundwork around the bogged wheels.
The IDEAL snatch/recovery should be mounted in a boxed and reinforced end of chassis rail with a swivel collared eye bolt going through (but not attached to) the bull bar, you can buy "RATED" shackles and eyebolts through Rigging Supply outlets, usually cheaper than 4WD stores, try Bullivants for a start.

[Islandnomad]

Tank,
With the "recovery points that orientate the shackle pins in the horizontal plane" issue if you reverse the shackle and have the bow end at the recovery point, and the pin in the equaliser loop, doesn't the "pulling off centre" issue go away? If that's the case then the 12mm plate or 20mm plate recovery points that people are selling would be ok wouldn't they?


Otherwise a swivel rigging point would be the other option or maybe a JATE ring. I think the military have been using JATE rings for a long time. See photo of a D2 with these fitted.



Haven't heard anyone talk about the weak link idea of mine, maybe I'm getting off the track with that.


Most standard Snatch straps are 8000kg SWL I think, not sure what rating shackles are correct to use?

[Blknight.aus]

Bow shackles can be pulled on almost any which way so get them.

The weak link in the recovery system should ALWAYS be the pulling source. (the winch, the drawbar pull or deadweight) of the vehicle or the rated effort of your recovery device. This means that every part of your system should be below its maximum limit when the winch stalls from overloading.

So if you have a winch with a bottom line pull of 4000kg the shackle and chain that you use to anchor the end of the winch rope should be rated to 4500kg (4001kg would work but they don't make a shackle that size) your snatch block should be rated to a minimum of 8000kg and the equipment that you use to attach the block to anything else should also be rated to at least 8000kg.

Bridle straps and shackles are another story, on paper each end should be rated to about 2/3 of the pull that's going to be placed on the center but for various reasons I personally recommend that each end should be able to handle the full pull of what Ever is going to be applied.

Snatch straps should be rated to a minimum of 2* the gvm of the vehicle doing the pulling and ideally less than 5* the weight of the vehicle being recovered (insert caveats for all other safety requirments and inspection regimes for snatch type recovery gear here.)

[Islandnomad]

Dave
If the weak link in the recovery system should ALWAYS be the pulling source, then shouldn't it read "This means that every part of your system should be above its maximum limit when the winch stalls from overloading"?

I don't have a winch so following this logic it means that the Tow bar recovery hitch which is rated at 4.75 ton (its hard to find a rating on any of them see an example below) should be the weakest link in the chain, or alternatively it's single shackle.


The reason I suggested that the equaliser should break, is that I thought that it would be a safer event than a shackle breaking and flying back at the recovered vehicle???? Why does the pulling source always have to be the weak link?

[FeatherWeightDriver]

^^^ What they said

Winch recoveries the winch will stall once you get to a certain load, so all of the other links in the chain should be at least as strong as that stall force. Note if you use a pulley that effectively doubles all of the ratings you need.

Tank's "vertical shackle attachment" is to minimise the shackle twisting across the plane of the pin - as this only decreases the breaking strengths of the shackle and attachment point. What you are trying to avoid is the scenario shown here: Recovery 101 including acronyms and definitions

As for snatch recoveries, use the strongest gear you can afford, start out slow and gradually increase the snatch forces involved.

By the time any piece of gear break in a snatch recovery the forces involved are almost certainly measured in tonnes, and there is no hope in hell of controlling the mode of failure. Even a snatch strap breaking and hitting the back of your truck (without a shackle) will break glass and/or punch a sizeable dent in the panel work.

See 10 minutes and 10 seconds in to this clip:

[ame]https://www.youtube.com/watch?v=ZSlOH7LHVxk&feature=player_detailpage#t=50 1[/ame]

I have seen snatch straps which have secondary attachment points in case the main eye fails, but I would guess there would be 10s or 100s of mid line failures for every eye failure. They also claim to help with shackle and attachment point failure, but if you just broke a rated shackle and your strongest attachment point, your backup is probably just going to get torn apart as well.

[Islandnomad]

I do understand Tank's vertical shackle attachment attachment issue (see photo below), I question whether in that photo whether the shackle is placed correctly though, as I have said twice, if you inverted or reversed the shackle and had the shackle pin at the web loop and the bow at the recovery point, doesn't his issue go away??????

[FeatherWeightDriver]

I think you will find that on most recovery points, only the pin fits through not the bow.

My understanding is that on a connection where one side is metal, and the other is webbing, the pin should be on the metal side. But having said that I don't remember why I think that is the case, nor find anything to back up my theory so maybe someone else in the brains trust can help out...

[Blknight.aus]

Dave
If the weak link in the recovery system should ALWAYS be the pulling source, then shouldn't it read "[I]This means that every part of your system should be above its maximum limit when the winch stalls from overloading.

Nope.

Below.

If the winch stalls at 4000kg and you have a 4500kg shackle then the shackle has not reached its limit. If you had a three thousant kg shackle then when the winch hits 4000kg of pull the shackle broke because it was rated below the load out on it.