Breaking strength, SWL, WLL, safety factor...

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I realize that my first post could easily be mis construed. In spite that I don't think about the numbers per say I do know in the back of my mind the overall strengths of the rigging I use. I do my inspections on the gear and know the strengths in the back of my mind, and apply it at will.

When we did the drop load test I found out that I am very conservative in the real world of rigging work. I have never come close to what that rigging in the video was taking without breaking.

Though it didn't change how I did my rigging after wards. The drop load test only affirmed to me that I play it very safe.

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I think it simply shows that you are a seasoned professional. I bet you've got all sorts of stuff "dialed in" in the back of your mind. One of these days, I'll be there. One day...
 
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MBS, means none will be less than the MBS.


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Not quite. See below.



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[a]Tensile strength/Breaking Strength[/b] - The average static force required to break a piece of gear if you pull on it.

Safe Working Load - Breaking strength with a safety factor applied. 10:1 for life support, 5:1 for rigging in most industries, but we seem to set our own in this industry.

[c]Working Load Limit[/b] - Same as SWL.

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A. The average force required to break a piece of gear when pulled to destruction.

B. Ditto; consult the manufacturer

C. Ditto; consult the manufacturer

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A) Agreed, but tensile strength/breaking strength can be expressed as either average breaking strength (ABS) or minimum breaking strength (MBS), which are two different measurements.

NOTE: the following tests and numbers are for new products in good condition!

ABS is simply the average of the loads at which the tested samples failed. Thus, statistically, 50% of those items will fail above the ABS and 50% will break below the ABS. It's worth restating that half of the product will break at something less than the ABS.

MBS is found by starting with the ABS and then applying some more calculations to find the standard deviation, denoted by the Greek letter sigma. The MBS is three standard deviations below the average/mean, and is also referred to as the ‘three sigma rating’. Statistically, there is less than a 0.5% chance (i.e. less than one half of one percent chance) that something will break below the MBS—more than 99.5% of the product will break above the MBS. Put another way, the MBS is the lowest load at which you can expect the product to fail. Most (about 99.5%) will fail above the MBS. A very small percent (about 0.5%) will fail below the MBS.

B) Ditto ditto.

C) Ditto ditto.

Perhaps worth noting—WLL and SWL are generally synonymous, but Pete Donzelli encouraged the use of WLL rather than SWL. He thought that SWL would lead workers to think that that load was ‘safe’ and they would not consider other factors such as cycles to failure, abrasion, dynamics, etc.

Also, few, if any, other industries publish the tensile strength/breaking strength in supply catalogues. Every industrial rigging catalogue that I have examined has shown the WLL/SWL or used a similar term, and may or may not show the safety factor that was used to arrive at the WLL/SWL.

All that being said, like Jerry, I know the numbers, but most of the time I chose gear through experience. I generally don't crunch numbers unless things are fairly technical.
 
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