Monday, Apr 25, 2016 at 21:30
Sorry to get agro about this matter, but there is a lot of spurious bunkum, going around as arguments against using equaliser straps.
Sorry none of them hold any water. Most of us should be using equaliser straps on all recoveries.
Firtsly in any snatch recovery there may
well be non straight line forces on the recovery points.
Secondly, if you have an snatch strap with a minimum breaking strain of 8 tonnes and a recovery point with a minumim breaking strain of 4.5 tonnes ( 9000 pounds), your system is badly compromised from the get go as the recovery point is a little more than half as strong as the snatch strap. HALF AS STRONG.
These popular 4.5 tonne hook type recovery points are commonly known to fail, either by straightening or fracturing when used as a single attachment point.
Simple maths no surprises there.
A friend of
mine is a working motor engineer and 4wd competitor, he has seen many of these hooks fail.
His professional opinion is that the stamped rating is a fair estimate of the performance of the item.
In our recovery systems we do not want failures in hard tackle, a failed recovery point may result in a hard metal missile or at the least an attachment point that no longer works for another attempt.
In a snatch system, we should be making sure, it is the snatch strap that fails, firtsly because it is soft, secondly because it can be replaced and finally with repeated use a snatch strap WILL fail anyway.
Now to the equaliser strap.
While there are some "
home made" equalisers that are way too short, ( like 1.5 meters) the commercial straps are at least 2.5 meters, I use a 3 meter strap.
If ya realy obsessed about the included angle and off line pull, use a 5 meter tree trunk protector.
With my 3 meter equaliser strap and my recovery hooks being on 640mm centres, that puts the included angle in the sling( equaliser strap) at 23 degrees and the off line pull on the recovery point at 11.5 degrees. ( assuming straight line pull)
These are the sort of angles that ya simply don't bother calculating derating for, because the derating is so low.
In a lifting sling, a 2 leg sling with an included angle of 60 degrees is rated at 1.7 times the strength of a straight single leg sling ...... my rig is around 20 degrees.
Nobody bothers calculating sling derating below 30 degrees.
So, if we use a long enough equaliser strap (for most 2.5 or 3 meters is plenty), neither the derating of the strap,nor the off line pull on the recovery points is an issue.
AND most importantly we end up with an attachment arangement using both recovery points that is TWICE AS STRONG and stronger than the snatch strap.
Now the third issue that does a lots of peoples heads in ...... the equaliser strap does not need to be as strong as the snatch strap .... a 6 tonne equaliser strap, will be good for around 12 tonnes of main line pull, and very much stronger than an 8 tonne snatch strap. As long as the included angle is less than 30 degrees.
There are some other arguments, for using equaliser straps.
There is the question of how
well the recovery points are attached to the chassis ...... Yeh it is a question, but still many of those popular 4.5 tonne hooks get straightened or break with no damage to the chassis or fasteners.
But however strong the chassis and fasteners are ..... two attachment points will be twice as strong as one.
There is an argument that pulling on a single chassis rail, may twist the chassis ...... I recon that too is a bit of a long conclusion to draw ...... but still best to use an equaliser strap and two recovery points.
There has been some discussion about using towbars as a recovery point .
It is plain and clear that the tow ball should never be used.
Aparantly the tow bar manufacturers are happy with us using hitch reciever tow bars as recovery points. Although I can see some may be apprehensive.
Many times there are lower rated tongue only type towbars fitted to vehicles ...... Yeh I would not use one of those for anything other than a gentle tug.
I will use a recovery block in my hitch reciever tow bar ....... but if it became a tough recovery I'd be using, my equaliser strap and both rear recovery hooks mounted to my chassis.
cheers
FollowupID:
868352