Thread: Recovery System Design HELP and the Weak Link

Originally Posted by philco

Our Gold Coast Club do not recommend the use of hooks to any of our members.

First time you have ever replied to me, I have posted PM's to you asking why your recovery points don't allow the shackle to orientate itself to the line of pull, which could be made possible if your recovery points had the shackle pin hole in the vertical plane rather than the horizontal.
I know it is easier to stamp or profile cut your recovery points, but a bit of heat and some twisting or forging could align the pin hole correctly.
I would prefer to use a hook, esp. on top of the chassis rail than a bit of plate bolted to the side of the chassis rail, some distance back from the bull bar.
If a hook straightens out it will not have a recovery point and shackle attached travelling at supersonic speeds towards another vehicle or person, Regards Frank.

Originally Posted by workingonit

Better than typing a lot...

See link, particulary 'calculating the force of a recovery' about quarter way down - don't know how valid calcs are but food for thought.

Pirate4x4.Com - Extreme Four Wheel Drive

Workingonit, good article, in Australia the use of "Bulldog Clamps" on working cables are totally banned, they may be used on Standing Rigging, such as Guy Wires as used to hold up tall masts that don't move. I have seen bulldog clamps used on tow trucks and 4WD winches, they are prone to slip under load and they decrease the SWL/WLL of any cable they are applied to. Will get you out of trouble if you break a cable, but make sure if you use them to have the "U" bolt loop on the tail of the cable. Good clamps have the profile of the cable in the body of the clamp, which always goes on the lead part of the cable, to give better grip.
The only other thing I noted was that the article didn't mention that 6 strand rope core wire cable should not be used on a winch or run over a sheave (snatch block) as it will crush flat and damage the cable, only 7 strand wire with a wire core should be used on a winch/sheave/tirfor, would have liked to have seen formulas for determining SWL of wire cables, I use the Dia. squared (in mm) x 10 and add 12% for wire core cable, Regards Frank.

So how do we work out the load when a snatching vehicle accelerates off and takes up the slack? Are there different loads at play between a slow winch increasing the load on the rope until say 3 x LW to get a vehicle moving off its belly compared to the snatch recovery - my old maths II school brain says Force (F) = Mass(M) x acceleration (A). So in each recovery the mass remains same (say 3t for loaded disco) but the acceleration in a winch recovery is much lower - so in old mates theory of 1x, 2x, 3x LW depending on the degree of stuck, then if you say the acceleration in a snatch recovery is 2x that of a winch recovery the force could be 6x LW (18t) ?

Here we go I found an answer for my question, interesting tables of test results of snatch straps about middle to bottom of page ... Super cheap straps by name....

http://www.landroverclub.za.org/snatch_straps.htm