More wenching stuff… sorry… winching….

The chain may have a high rating but it's useful to remember when using a grab hook the safe working load must be reduced by 25% or 50% if choking the chain. A claw hook is the better option for shortening with no SWL reduction in a straight pull but they are rarely fitted to drag chains.
Same goes for winch straps. Double one over & you reduce the breaking strain by 50%. I wouldn't do that personally as that reduces a 5 tonne strap to just 2.5 tonne, well below the pulling capacity of many winches particually double blocked.
Cheers Craig.............

How does doubling a winch strap over half it's breaking strain?

Scuby I was wrong with the strap being half as strong. I got mixed up with wire rope which is halved if choked or fed around a tight hook etc.
By running the unprotected part of the strap around a D shakell or hook you are concentrating the force on the weakest part of the webbing. Depending on how tight the radius of the D pin is you can reduce the breaking point by around 20% turning a 5 tonne strap into a 4 tonne. Should idealy always use the reinforced eyes to attach a strap. Sorry for the confusion.
Cheers Craig............

Sorry, I don't follow that logic - the loops on a strap aren't reinforced, they're just stitched into a fold, and covered with a wear sleeve. They would be subject to the same pinching effects etc from shackles and the like.

Seeing as in the stretch tests, most straps failed either by stitching failing, or the strap failing where weakened by stitching - I don't think any failed by the strap just "parting" somewhere in the middle - I still reckon that a doubled strap would increase the load capacity. Probably not by double, as the load to "tear" the strap at the point where it doubles around a shackle is probably less than double the single line pull rating, but I still reckon it would go up than down. Sounds like an argument that can only be settled with some similar testing in a lab.

>I still reckon that a doubled strap would increase the load capacity.

I don't see how it could _increase_ the load capacity because at the point where the strap is in contact with the hook there is only a single piece of strap being subject to the full strain.

I think two straps together would increase load capacity but I doubt it would be doubled because the straps are not identical one would assume more of the strain than the other.

Any mechanical engineers care to comment?

Mike Harding

The testing seemed to indicate that the load capacity of a strap was determined by the weakest point - i.e either the stitching breaking and coming undone, or the webbing where stitches had penetrated (and there's a ton of stitching there to make holes and damage the webbing fibres). Think of the sewing holes as "damaging" the strap webbing at that point.

So imagine if you will a "perfect" strap - no sewn weakpoints - the failure point of the webbing itself is probably much higher. If you could somehow loadtest just the webbing (dunno, giant flat clamps holding each end), 60mm webbing used to make an 4000kg winch strap probably fails at 6000kg - it's the damaged sewn portion that fails at 4000kg and gives the strap it's rating.

So I reckon the "loop" formed by doubling a piece of webbing through a shackle is much stronger than a sewn loop on the ends, because there's no stitching to either weaken the strap or tear out and fail at that point. But it wouldn't double the capacity of the strap - doubling a 4000kg strap to make an 8000kg strap simply won't work. But maybe it would get you 5-6000kg, and a shorter strap. The point is, the strap's rating would increase to whatever the rating of the webbing itself is, rather than what the rating of a stitched part of the webbing is.

I see where you're coming from now. Sounds reasonable - the strap must have been weakened below it's original strength by the action of stitching so you're load sharing across the two eyes.

Yep, I can buy that you would get a stronger strap but by how much, as you mention, is a bit of a guessing game.

Mike Harding

The weakest point on a winch strap is at the first stitching on the eye. When doubled the two eyes share 1/2 the load each but at the other end the unprotected middle of the strap is wraped tightly around the D or hook & now becomes the weak point. When lifting with fibre slings the safe working load of the strap is reduced by 20% if choked or doubled over.(as per ratings on the lifting gear or load charts) If looped around a larger pipe that is called a basket hitch & the SWL is doubled. It all comes down to how tight the radius is where it's secured in the middle.
A winch cable is similar too. You should never bend a cable tighter than 6 times it's diameter or you derate it's strength.
Cheers Craig..............

Oops, meant 2.5m. There is a 3m strap rated at 8000kg as well.

yep drag chains are the go. If you don't have one you can as pointed out above use them for other things.

Leroy