capacity rate calculation for flat webbing slings

[Guideon]

Hi,

I have read Lift-it catalogue and I found out it is very professional and useful . I have some questions in the subject and I'll be grateful for professional answers.

I found out that there is a direct relationship between SWL and the width of the strip (1600 lbs/in of width for nylon, 1000 lbs/in for polyester ), and I wonder what about the thickness of the strip? Does the thickness not affect SWL? Looking in manufacturers catalogues shows that it does affect SWL!?
Also, I have read WSTDA std and even there I realized that the material strength is expressed in the same manner/units (values 9800 lbs/in of width & 6800 lbs/in of width or 121.43/175 in SI units) without taking in account the thickness.
1) Could enyone explain to me why/where the thickness is hiding ? Also, what is the meaning of these values (9800/6800) in compare to the above 1600/1000 values?
2) At the above std (WSTDA-WS-1 2.10.3) there is an expression for rated capacity determination, and I'll be glad to get some typical values using this expression. By the way, is there a difference in the safety factor (5) used in USA according to the mentioned expression, and that used in the EUROPEAN (7)?
3) Is there any similar expression for round slings?

Thank you,
Guideon

 

[RebelBrill]

The thickness is hiding in the number of plys. Slings are designated as single ply, double ply, etc. More plys provide a higher SWL, all other things being equal.

 

[Guideon]

RebelBrill Hi,

Thank you for your reply, but I think that you are wrong, because even in one ply (as you can see in several catalogues)there is a significant difference in SWL when the thickness increases(what I am trying to say is that SWL for one ply 4 mm thickness is greater than one ply 3 mm thickness).

Regards,
Guideon

 

[unclesyd]

The SWL of a synthetic fiber sling is determined by the T.S. of the webbing or rope/tow. There are some in-house aids but the final number is still determine by the tested strength.
In the (WSTDA-WS-1 standard there is a list of available publications which I believe has some better information. If you look at a single rope of a particular construction you will see how the strengths are addable.
I would not try to redesign a synthetic fiber sling. Most companies give sufficient information to safely use said sling.

http://www.slingmax.com/pdf/new_composite_yarn.pdf

http://www.ropecentre.co.nz/chain19.html

http://www.americansling.com/grabiq.html

I had a friend that was in the wire rope sling business and as the synthetic slings came on the market he tested a great number of them for different companies and the government. His drawbar had 1.5 million pounds pull.

 

[rb1957]

i thought the catalogues defined slings by their product code, 3mm thk nylon is a different product than 4mm thk nylon and so has a different strength/in width. in any case it's usually the stitching that's the weakest link.

 

[unclesyd]

There was an interesting show on Discovery's "how do they make that " that concerned making multi strand synthetic fiber rope and slings. There was an actual test of piece of a tow/braid to be used as rope or sling. It showed it being tested to destruction on a one million lb. drawbar. The technician that was doing the test stated that the test for establishing a safe working load for that particular lay of rope. The rope was strength 3 times before it align the fibers before the final pull..
One thing that I had never seen in failure of synthetic fiber was that when it failed it failed in a least four places with resulting bangs.

http://www.samsonrope.com/index.cfm