Rated capacity for webbing flat slings

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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
 

Answers and Replies

  • #2
AlephZero
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I don't claim any particular knowledge of slings, but thinking about this from first principles:

Effectively, the sling has no stiffness in bending. Whatever its thickness, it is very flexible.

Therefore, the pressure load acting on the surface of the sling depends mainly on the local radius of curvature of the sling. If the sling is wrapped round a sharp corner of a component, this pressure will be very high.

I would guess critical failure mechanism is that the high surface pressure can "cut through" the sling at a sharp corner. The load required to make the sling fail in tension by over-stretching it is probably much higher (at least for slings with realistic practical thicknesses) so that does not limit the SWL.

The thickness might affect the length of time it takes to cut through the sling, but not the fact that a given pressure will start to cut through it. Clearly for safe working you don't want any damage done to the sling, so the extra time to failure doesn't change the SWL.

The force required to produce a given pressure is proportional to the width, because a wider sling distributes the force over a bigger area. Therefore the SWL is proportional to the width as in the catalog.
 
  • #3
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Well conditioned rope will break at its adjoining knot. The breaking force of a knot typically varies from 50% to 75% of the rope's capacity. So, as stated above, the webbing will break at its fold. The Swl has a safety factor of about 5, but this varies.
 
  • #4
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Thank you for the replies.
Alef theory is interesting, but I expect (as far as possible) to get a definite answer/s from someone who has an experience in that field.
Thanks again, Guideon
 
  • #5
2,017
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Slings are manufactures to standard thickness. So you can get 1 ply, 2 ply, etc. As they all have the same thickness, you can take that term out of your definition of strength. You'll notice 2 ply webbing have a higher load capacity per unit width than an 1 ply (should be approximately double).

It's as simple as that really, it's one of those things you just discount because the answer seems too simple to be correct.
 
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  • #6
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xxChrisxx Hi,
You are right that there are standard thicknesses, but I think that you misunderstood what I ment..I'll try to clarify.
One ply strip could be manufactured in several standard thicknesses, and their coresponding SWL is different (SWL increases with thickness increase, in other words, one ply 4 mm thickness has higher strength than one ply 3 mm thickness).
If so, how come the thickness is not a parameter in the correlation (SWL=width * number of plies * 1600 lbs/in for nylon, or *1000 for polyester) and the strength material I mentioned in my original question (their units are also lbs/in of width)?!
Thanks,
Guideon
 
  • #7
2,017
85
xxChrisxx Hi,
You are right that there are standard thicknesses, but I think that you misunderstood what I ment..I'll try to clarify.
One ply strip could be manufactured in several standard thicknesses, and their coresponding SWL is different (SWL increases with thickness increase, in other words, one ply 4 mm thickness has higher strength than one ply 3 mm thickness).
If so, how come the thickness is not a parameter in the correlation (SWL=width * number of plies * 1600 lbs/in for nylon, or *1000 for polyester) and the strength material I mentioned in my original question (their units are also lbs/in of width)?!
Thanks,
Guideon
If a manufacturer makes 1 ply 4mm, all 1 ply will be 4mm from that manufacturer. The ply thickness may change between manufacturers, but it's still irrelevent. It only becomes relevement if you have to specify a strap or tether with a certain CSA (which almost never happens, which is consequently why they don't specify a braking load in terms of stress).

To be honest it's irrlevent the thickness of the strap, all you need to know is the breaking load. You'll find many manufacturers sell slings and straps with the daN. Thats a pure load rating.

Also a link to the catalogue where you are having trouble would be helpful.
 
Last edited:

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