Doubling a strap weakens it? (1 Viewer)

[Buckru]

I had a situation where I needed to shorten the strap on an easy tug. I ran it though my shackle and then shackled both end loops to the rig needing a tug. I evened out the length and gave a light tug.

Someone mentioned that doubling the strap like that weakens it. Any validity to that statement? I had not heard this before.

Buck

 

[MANUCHAO]

My understading is that webbing (used for climbing anchors) could loose their rated tensile strength by as much as 20% for every bend or knot..... I would imagine that straps would too...

Im sure someone with more knowledge will chime in....

 

[Buckru]

Would this be a permanent tensile strenght reduction or only while the strap is doubled?

Buck

 

[roalco]

Normally when you are working with nylon web slings (same thing as straps) the rated strength is rated on a straight pull (single line run). If you double it back on itself (both eyes together and the middle of the strap (the bight) run around an object (tree, pin, bar, shackle...) you double the strength of the member. You do have to make sure that the middle of the strap (the bight) is run around a reasonably large (say at least a diamater of 4 to 6 times the webbing thickness) smooth object, (the strap wants to lay pretty flat here and not be subject to sharp edges!) So depending the size of the shackle you were running it through you should be fine. just don't kink or tie a knot in the strap.
Check out the info on this page, it'll show you the strengths of a sling under different configutraions.
NOTE the reduction in strength when it is used as a choker!
Tony Beal Limited - Lifting Systems
Hope this helps!
Ross

 

[jasonmt]

Depending upon the width of your sling and the width and type of shackle used there is no huge loss of efficiency doing what you did. Keep in mind that the thickness at the eye of the sling or the entire sling (if a type 5 endless sling) is also being subjected to the same small radius around the shackle or pin.

That being said if you are talking about a 3" or 4" wide strap and using a 4.75 or 6.5 ton bow shackle this is a poor practice which will impair the efficiency of the sling, you should be using a "sling saver" shackle of appropriate width for these purposes as it allows 100% of the slings rated Working Load Limit to be achieved.

Use this:

Not this:

 

[AdventureBuddies]

Climbers routinely double up slings (basically a nylon strap sewn into a loop) to adjust their length. It's common knowledge amounst climbers that a knot will reduce strength but as long as the double-back bend is over a rounded surface it shouldn't have too much impact. I think the same concepts apply to tow straps.

As stated above, it would not be ideal to for a 3" wide strap through a normal shackle because it will distort the strap shape which could weaken it.

 

[Buckru]

Great information and I appreciate the responses and web link. In hindsight, what I did was the wrong approach. The shackle did cause the strap to curve which reduced it's strength. My strap is rated to 20k and I guesstimate the amount of force used was 2k, if that.

The best approach will be to purchase a short strap so I have both if need be. I sure don't want something breaking loose while in use.

Thanks again,

Buck