Snatch Strap Damper

[AJ]

Any thoughts/comments on a proposed new training theory that involves attaching 5m of chain to a snatch strap as a damper in case of recovery point failure. Air brake dampers have been tested and shown to be useless, so this is the new idea.
My preference would be using a tree trunk protector wrapped around the axle as a safety lanyard, though this is impracticle in some situations.

[RobertM]

most 4bs have 2 recovery points so couldn't you just use the other one to attach the tree protectoper to, would be a lot easier.

I think these are a good idea as I have had quite a bit of damage to the back of a previous 4wd and only the strap hit and the spare tyre luckily took most of the hit but you could see where the strap hit the back door like a snake

[flappan]

Why not just use the same sort of damper you use for a winch cable.

Drape it over the strap . . . if you use one like the ARB ones, with velcro , it can't fall off.

[baby_troupe]

My opinion on using something solid as a damper.


NO WAY. I want to keep living.

Ever heard of a sling shot with a heavy projectile :o

Gotta agree with BT. Attaching a (hard & solid) weight to the snatch is counter-intutive, isn't it?

[FREAK]

I read an article about this recently somewhere but can't remember where. Can't say i like the chain idea either but i think a tree protector or another shorter snatch strap looped around the original strap and then shackled at the opposite end to the second recovery point or the diff as AJ suggested has merit.

In the very least if the loaded snatch strap lets go the second one would have to stop the recoil from hitting either vehicle.

But yes people really do need to just slow down in recovery operations, it's not a race, and just think it through and then keep spectators way back out of the way, the unforseen has a nasty habit of coming back to bite you on the arse. ::)

[RobertM]

Tree trunk protector would be the way to go.

Didn't 4wd monthly do somthing on the a while ago?

[oiler]

I use the old blanket method laid over the snatch strap and on the one occasion a strap has broken (it turned out to be faulty and was replaced free of charge) it did no damage.

I also think that the manufacturers should place a weak point in the straps so if they break you know where they will break. The weak point would still need to be stronger than the SWS. Without this point you don't know where the strap is likely to break.

rant mode on >:(

How will knowing where the break will happen help? You would still need to account for the likelyhood of the strap breaking anywhere anyhow, and now the strap is weaker by design!

It's crazy talk - imagine a rigger using a piece of kit with an inbuilt weak point...build the damn things right in the first place I say! For us users, use the things as intended and take the required safety precautions and toss any strap that you even suspect could be suspicious!


Damn things break with a stated minimum breaking strain of (normally) 8000kg. That is, by the way, very different to industry standards. In industry, a 8000kg snatch strap would be lucky to be rated at 1000kg SWL. Can't believe some folk think their 15000kg minimum breaking strength polyester winch extender or tree protector is stronger than a rated shackle!

And lastly, regulate the industry, create and enforce a recognised standard that straps are measured against, for that matter, all 4WD recovery gear should be rated in a meaning-full manner. An 8000kg snatch strap means that is the minumum load before breakage (sometimes!), as stated by the manufacturer against whatever criterion they choose, and only when everything else is perfect, and thats bullshit! "Rate" the things like the lifting industry where a SWL of 4.2 tonnes means exactly that, no surprises, the shackle won't break at 4.0 tonnes because the shackle is wet...samples from each batch are tested according to defined procedures, we know exactly what the performance of these devices are!

That won't solve all the problems but it will stop the likes of Supercheap selling "15000kg" snatch straps because mug punters assume 15000kg is better than 8000kg. Those crap 15000kg jobbies from supercheap are just accidents waiting to happen - they get away with it because there are no standards by which to rate these appliances, and there damn well should be >:( Do the test in a lab in Thailand and whalla - 15000kg breaking strain is the number that appears on the pack. Consider, a reputable company uses "minimum breaking strain" as a reference point, other companies can define the load rating in anyway they think will enhance sales.

rant mode off :)

Hi Oiler - rant wan't aimed at you ;D

[BLU-125]

HT, you obviously have rigging experience. How would you test & rate straps. Is there any relevant Aus. Standards? How do we really make sure we are buying safe products?

Your input requested, please.

[FREAK]

The thing with doing this process of setting a standard in place would then be the repercussion of doing so.

Imagine a manufacturer putting this standard in place after it had been developed, what fall back have they got when it their product fails, the only way they can cover their arse is to then put more legislation in place to make periodical testing mandatory, exactly the same as what is required of all rigging equipment now eg slings, chains etc.

Don't get me wrong there does need to be a more reliable avenue of insurance for people investing in this equipment, but if we're not careful, it will end up being the same as recreational fishing eg safety equipment carried in boats, expiry dates, fines in place, court appearances where the hell does it stop. Personally i am sick and tired of limitations being put on my fun and i can only see it getting worse. >:(

Aawww geezz my blood pressures going through the roof now, i wish i wasn't at work so i could bring it down with a few beers :)

By the way i'm not having a shot at anyone here either, just fed up with beauracratic crap.

[oiler]

Freak,

I accept your point on break points. I work in the aviation industry and it is common to build weak points into equipment so the failure can be controlled. If you built a snatch strap with 8000kg SWL and over-engineered it to 10000kg, then placed an 8000kg section in the middle, you could place your dampers on either side of the weak point and reasonably expect the strap to break there. This is the same logic behind using higher rated D shackles and attachment points than the strap itself, if anything breaks you want it to be the strap.

As for an Australian Standard, I think there was an article in a recent test issue of the magazine that explained that there is no standard and that manufacturers apply them as they see fit to try and sell there articles. In the end the conclusion was to buy well known local brands.

[4wd26]

you should consider using 2 dampers placed at 1/3, 2/3 rd's along the strap. If something breaks this method will stop either end reaching the vehicle. Murphys law 1 damper in the middle (approx) strap usually breaks at the ends and the long end could reach the vehicle, usually your own :-[...

The standards thing would be great, have you tried getting an answer from the vehicle manufactures re: recovery points load ratings.

This whole idea (recovery) is a grey area if things go wrong whilst working. Because of no ratings on recovery points we can not use snatch straps for recovery on our mine site.

[FREAK]

Freak,

I accept your point on break points. I work in the aviation industry and it is common to build weak points into equipment so the failure can be controlled. If you built a snatch strap with 8000kg SWL and over-engineered it to 10000kg, then placed an 8000kg section in the middle, you could place your dampers on either side of the weak point and reasonably expect the strap to break there. This is the same logic behind using higher rated D shackles and attachment points than the strap itself, if anything breaks you want it to be the strap.

As for an Australian Standard, I think there was an article in a recent test issue of the magazine that explained that there is no standard and that manufacturers apply them as they see fit to try and sell there articles. In the end the conclusion was to buy well known local brands.



Oiler,

i'm glad your not offended by the break point comments mate as it wasn't actually me who mentioned them ;) as with any issue like this it is bound to stir up some emotion from people who feel strongly about what they believe is right.

Personally i think it's great to get it all out without getting personal with each other, especially if you can keep a reasonably open mind, it's surprising what you can learn from these debates ;D

[Mid73]

There was a write up about rating snatch straps in the September issue (no.57)

[rickwagupatrol]

probably get shot down in flames for this but here goes.
we all know that a strap stretches when under load right, and as a result of that load, things may break.
so how bout this idea.
use the strap as intended. ie. rated recovery points, shackles, etc.
then use a piece of decent rope in conjuction, fixed to both vehicles that is longer than the streched lengh of the strap, loosely tape in place along length of strap( with decent tape)
if strap breaks,,,a few of the tape sections will break,taking the force out of the strap , then stopping it.
dumb idea??????
maybe,,,but we need to come up with something that will stop us getting hurt in a recovery situation.
anyone else got an idea, hopefully better than mine.:):):)

rick.

[oiler]

RickW

Snatch straps should not break if they are used correctly and are kept in good order. I have never seen a good condition strap that was being used properly actually break. Every broken strap I have witnessed has been due to poor practices such as trying to move the immovable or not padding the strap at the attachment points. I reckon the best we can do is to learn how to use them properly and how to inspect them for damage.

[baby_troupe]

RickW

Snatch straps should not break if they are used correctly and are kept in good order. I have never seen a good condition strap that was being used properly actually break. Every broken strap I have witnessed has been due to poor practices such as trying to move the immovable or not padding the strap at the attachment points. I reckon the best we can do is to learn how to use them properly and how to inspect them for damage.


Sometimes a bit of time on the shovel too ;)

[oiler]

Thanks for the reminder BT I need to buy a new long handle shovel. They break when Nissans run over them.

Thanks for the reminder BT I need to buy a new long handle shovel. They break when Nissans run over them.


Bloody Nissans >:(

[desertfox]

Hi guys

Some interesting discussions going on !!

One question nobody was able to answer so far:

What force is applied to the equippment when snatching ?

Since this questions will remain open, there is a good chance something will break. What will it be ?
The 8000 kg strap?
One of the 4700 kg rated shakles?
Or the recovery point?

On my troopie I haven't found a SWL for the recovery point and nobody (so far) could tell me.

Therfore I try to avoid snatching if possible and use the winch or my NON elastic 6400 kg strap. In 95% of all cases one of the two will do the job. This action will take place with pretty well controlled forces. The risk is minimised to a point where the whole discussion about what happens if.. is absolete.

I have done most of my 4WD'ing in Europe and Africa and snatchum straps are not very well known over there. Nevertheless so far everybody has been recovered and I never witnessed any damage nor injuries. What else do we want ?

I'm not writing to play the smartass but would like to here you opinions ;)

Desertfox

[Humpy]

Making some big assumptions, but here go’s

Assumptions
Snatching 4wd weighs 2 ton
Stuck 4wd doesn’t move
Strap stretch is proportional to the speed of the car
Deceleration is constant once the strap is tight.

Formulas
Acceleration = (Finish Velocity squared - Start Velocity squared) / (2 x Distance)
Force = Mass x Acceleration

Notes
Rated shackles have a safety factor of around 5, so a 3.2t “safe working load” shackle will probably break at about 16t, unless its twisted up and loaded wrong.
Straps are quoting the breaking load usually in pounds (lb).
Acceleration due to gravity or G is 9.81m/s^2
Stretch distance is a guess at how long the snatch vehicle takes to stop, from when the strap first gets tight.
Calculation for acceleration gives negative numbers, this just means its accelerating backwards or slowing down.

4wd travelling at 10km/h = 2.77m/s
Stretch is 0.6m or 6% of 10m strap
(0^2 - 2.77^2) / (2 x 0.75) = -7.71m/s^2 or 0.79G
2.0 x 0.79 = 1.57 ton force = 2890 lb

4wd travelling at 20km/h = 5.55m/s
Stretch is 1.2m or 12% of 10m strap
(0^2 – 5.55^2) / (2 x 1.2) = -12.86m/s^2 or 1.31G
2.0 x 1.31 = 2.62 ton force = 5780 lb

4wd travelling at 30km/h = 8.33m/s
Stretch is 1.8m or 18% of 10m strap
(0^2 – 8.33^2) / (2 x 1.8) = -19.29m/s^2 or 1.96G
2.0 x 1.96 = 3.93 ton force = 8670 lb

4wd travelling at 40km/h = 11.11m/s
Stretch is 2.4m or 24% of 10m strap
(0^2 – 11.11^2) / (2 x 2.4) = -25.72m/s^2 or 2.62G
2.0 x 2.62 = 5.24 ton force = 11560 lb

AARRGHHHHHHHHHHHHHH i got a migraine reading that!!! Thanks humpy! :-X :-\ :P

Forget all the details, will reach deep back into the neuron’s and try to find my old notes from my Army days but it all relates to rolling resistance. I do remember that the rolling resistance for a 100 L/R on a hard surface is 1/20th of the weight of the vehicle, and there are all sorts of other tabulated data that relates to rolling resistance in sand, mud etc. There is also tabulated data in terms of tractive effort that a vehicle can apply. Again, for some strange reason I recall a Leopard has a tractive force of 25 tons, so it pulls the Landie out of the mud without any drama! (but it can't pull an APC out if bogged to the turrent in black soil plains ;D)

Seriously though, I had a rant on this thread earlier. Army will not allow the use of snatch straps because they are not rated. I would be surprised in fact if any (sensible) employer would allow the use of snatch straps for that very reason, the risk in terms of OH&S, duty of care and all that stuff simply precludes them from a workplace environment.

Not saying they aren't a good idea, they are. But if safety is your primary concern then the only way to control the situation is to only use rated equipment (as in SWL, not breaking strength).

Rated gear as used in the lifting industry (or aviation which is where I learned all that stuff during my army days, making up slinged loads for rotary wing transportation mainly, but the occasional loading Herc's & boos) is rated as an order of magnitude, which of course means a factor 10. So, a 3.2 ton shackle would more than likely have a breaking load of 32 tons. Reverse the math and a 8000 kg strap would have a SWL of 800kg - having said that it not that simple, because straps are not "rated", and are affected by all sorts of environmental factors including wear, moisture content, exposure to UV and chemicals (POL etc) and so on.

Anybody got a copy of that tabulated data the military once put together? Looks like mine is lost, and I tossed the loadmaster stuff (rigging, dangerous cargo etc) when I left the service, reckoned I wouldn’t be loading any more slings for choppers or tying L/R's to hercules floors anymore... :)

Has the proper use of snatch strap ever actually killed or seriously injured anybody?


Edit: my spelling really, and man really, sucks :(

[Humpy]

Forgot about traction :-[

Add "No additional force is provided by traction of the snatching vehicle" to my assumptions

...and isn't the basic principle of the snatch is its ability to "store" energy from the mobile vehicle and apply it gracefully to the recovered vehicle? That means it's entirely likely that the energy transmitted into the strap is an accumulation of the kinetic energy applied by the tractor - how else could a zook pluck a cruiser from the mud, his tractive effort at any instant of time is almost certainly less than that required to pluck the cruiser, but his accumulated energy is stored in the strap until (hopefully) exceeding the pull required.

Consider the same scenario of a zook plucking a cruiser from the mud with a chain. A chain is direct link in terms of transfer of energy. Chances are the zook would simply spin its wheels and get nowhere because there is no way to amass the required energy; the energy is lost spinning up the tyres. A chain between a zook and stranded cruiser is not only much harder on the zook than the snatch, but is also not likely to work is it it?

Now here's a challenge - to calculate the problem you probably need to consider the kinetic energy generated by the tractor and how efficiently that kinetic energy is converted into potential energy within the snatch strap. The potential energy now building up within the strap must be transferred equally to both vehicles; the trick is get to the situation where the tractor’s kinetic energy (momentum) is always higher than the pull required to pluck the stranded vehicle. And then there is time, a snatch does not occur in an instant of time after all, the energy is built up, stored and dissipated over a measurable period, and just how much does the immobile vehicles efforts in driving affect (reduce?)the loading on the strap?

Of course, even the accumulated energy from a zook still not might pluck a bemoth like a cruiser from the mud simply because a zook is really light and a cruiser is really heavy- but I've seen some amazing snatch results.

Point is, it’s real poindexter stuff…no wonder there isn’t standards established, the problem is to elastic ;D

[4wd26]

"I would be surprised in fact if any (sensible) employer would allow the use of snatch straps for that very reason, the risk in terms of OH&S, duty of care and all that stuff simply precludes them from a workplace environment."

This is correct, My employer (a large mining company) will not let us use a snatch strap for this reason. Also (as stated previously) trying to find a rating on OME hooks is imposible. How stupid does this senario end in practice? A Patrol ute requireing a D11 dozer (with rated slings) to remove it from a bog. other 4wd's could not budge the vehicle using a static rated strap, BUT i think a snatch strap would have removed the vehicle.

[kitch76]

Thanks for the reminder BT I need to buy a new long handle shovel. They break when Nissans run over them.


Went to bunnings the other day and picked up one for $10. Seams to be strong enough and has a good thick handel. on the snach-um-strap side of things, u say not to buy Super Cheep type ones and stick with the brands, well I bought an ARB strap and from memmory they didn't rate well!!! What to do?????

[Humpy]

The snatch strap test the mag did didnt have a huge sample, theres a good chance the winners were out of a good batch and the loosers were out of a bad batch.

But as they said you dont know which batch yours came from.

Use the one youve got if it breaks, you may or may not choose to buy that brand again