Thread: Another snatch strap death

Originally Posted by rick130

A bolt in shear (a sideways force across the shank or in bolt terms, grip) isn't as strong as a bolt in tension, (clamping force) either.
The minimum specifications are for a bolt in tension.
It's been waaay too long, and I can't recall the numbers but Google should help.

Yes you are right.

I should have only looked at shear strength.

Google says it usually 0.6 of its tensile strength.

So say m20*1.5
So the stress area 272mm²
This makes the minimal ultimate tensile load of 332k N

Convert this to shear load it will be 199K N

And this mean 20 tons?
I have one on each chassis rail so 40 tons?

So I will need to pull 40 ton to shear both 12.9 bolts?
Oh is my calculation correct?

Originally Posted by 350RRC

From my days doing physics and pure & applied maths : Force = Mass x Acceleration (F=MA).

The mass of the bogged vehicle is constant.

If you just drag or winch it out there is very little acceleration involved so there is not a lot of force involved, just enough to overcome the resistance of the bog [...]
DL

Thats a bit of a logic fail... what if "just enough [force needed] to overcome the resistance of the bog" is a freaking lot of force? i.e. That the stuck vehicle is not moving even with huge pull applied. (regardless of the means of applying pull)

Yes there is little force involved when there is little stopping the bogged from moving. But this is a sliding scale continuum. e.g:
- a vehicle on a level blue ice patch cannot get moving... the pull needed to assist involves little force. You could use a skipping rope anchored by cable ties attached to the stuck vehicle's spotlight bracket. Provided its a nice slow pull, little acceleration, using any equipment to apply pull (KERR, tow, winch).
- at the other end of the scale though, where the stuck vehicle is theoretically "immovable", as is the winching vehicle, for the sake of the thought experiment. Huge forces can be involved regardless of equipment used (KERR, tow, winch). Even with the slowest, gentlest, slow spool in with a winch - it won't take long for the forces to build (theoretically to infinite) as the winch takes up tension. (immovable object meets irresistible force)

All recoveries fall somewhere between the the two ends of the sliding scale described above. Regardless whether it is a snatch or winch or tow recovery.

The differences are (just focussing on the stuck vehicle being as close as possible to the "immovable" end of the scale):
- in a snatch recovery, it is the mass of the recovering vehicle, magnified by its acceleration, that is the mechanism to get to excessive (breaking point of something) forces. And gets to that breaking point faster than a slow winch or tow options.
- in a winching recovery it may take longer to get to a level where the forces involved can reach breaking point of something, or self limits (clutch slippage etc)
- in a tow recovery typically the limitations of traction available to the recovery vehicle (ground surface and tyre interface) is what will ultimately cap the level that recovery forces can reach. Typically much lower than winch or snatch. Of course a gutless towing vehicle will further lower the level of force that can be reached - but assuming the tow vehicle is powerful enough, its where the rubber meets the road/gravel that caps forces on the tow equipment.

Originally Posted by martnH

Yes you are right.

I should have only looked at shear strength.

Google says it usually 0.6 of its tensile strength.

So say m20*1.5
So the stress area 272mm²
This makes the minimal ultimate tensile load of 332k N

Convert this to shear load it will be 199K N

And this mean 20 tons?
I have one on each chassis rail so 40 tons?

So I will need to pull 40 ton to shear both 12.9 bolts?
Oh is my calculation correct?

Cool, I had in my head 2/3 but it's been far too long since I've been involved in that stuff.

Originally Posted by trout1105

I agree that these factory towing points are NOT Recovery points and would doubtfully withstand the forces involved in a "Snatch" recovery.
However they are designed to enable the vehicle to be "Towed" So using this as an anchor point when doing a double line pull with a pulley block/snatch block is acceptable as far as I am concerned.

Where there is bugger all rolling resistance, on road for example - I agree.

however - If the vehicle is stuck such that rolling resistance is substantially increased. No thank you.

Originally Posted by trout1105

[...].
Remember you are not trying to Lift 3T of vehicle off the ground you are dragging it along the ground So 6T of force isn't usually necessary.

Tombie hit the nail on the head (when he mentioned a sticky bog and hung up on a rock...).

You are ignoring that, depending on what is impeding the stuck vehicle (see previous post up a few from here - sliding scale), the force needed to recover the stuck vehicle may well exceed the force needed to lift a vehicle off the ground vertically.

Originally Posted by tact

Tombie hit the nail on the head (when he mentioned a sticky bog and hung up on a rock...).

You are ignoring that, depending on what is impeding the stuck vehicle (see previous post up a few from here - sliding scale), the force needed to recover the stuck vehicle may well exceed the force needed to lift a vehicle off the ground vertically.

If the vehicle is that badly bogged/hung up then a 12,000lb winch won't be able to pull you out and won't be able to impart enough pull to destroy the towing point anyway.
Seriously guys if there is a rock or anything else stopping a recovery you would either remove the offending item or jack the vehicle off it, Simply using maxtracks/treads under each wheel will "Lift" the vehicle up and out of the bog or you could even go "Old School" and do the spare wheel trick to lift the truck whilst winching
Honestly if you have to use 6T of pull to recover a D2 then it is time to re think your recovery method.

Originally Posted by trout1105

If the vehicle is that badly bogged/hung up then a 12,000lb winch won't be able to pull you out and won't be able to impart enough pull to destroy the towing point anyway.
Seriously guys if there is a rock or anything else stopping a recovery you would either remove the offending item or jack the vehicle off it, Simply using maxtracks/treads under each wheel will "Lift" the vehicle up and out of the bog or you could even go "Old School" and do the spare wheel trick to lift the truck whilst winching
Honestly if you have to use 6T of pull to recover a D2 then it is time to re think your recovery method.

Fair enough - if you are no longer bring hypotheticals into the mix (comments like “remember you are not trying to lift a 3T vehicle in the air) then theoretically correct comments like “a stuck vehicle may require way more effort than a simple dead lift” can also be parked.

You quite reasonably span across the hypothetical/theoretical gulf when you suggest that if a stuck vehicle is that badly stuck, and requires that much force to extract - then a rethink of the recovery methodology is needed.

Sadly, my own observation is that such sensible thinking often doesn’t occur in recovery situations until after the power smoke starts leaking out of electric winches, motor vehicle clutches, etc. Then only someone might twig that it’s time to do a bit of thinking and manual labour.

In the situation where a person with a winch and tow points is deciding whether to recover a bogged vehicle that has tow points, only a single line pull is likely to be sensible, in which case the tow point of the recovery vehicle does not come into question.

If the bogged vehicle has a recovery point you can use, and a double line pull is deemed necessary, then it becomes a matter of judgement to not overload the recovery vehicle's tow point. I think by this time you are out of the realms of most recoveries and into the realm of a serious recovery needing careful thought. Maybe the line of pull can be improved to reduce the loads involved. Maybe the hook can be taken to a sling around a tree instead.

[My current vehicle has 'semi' recovery points at the front, it has a winch bar with mounts designed to take bow shackles, so although i am confident its not fully properly rated, i know it has been designed for the task.]