When conducting work site inspections, audits, or periodic rigging inspections, it is not uncommon to have to take anywhere from 80% -100% of the synthetic web slings out of service. There are a large variety of reasons a synthetic sling, “gets itself” into such a state of disrepair, but one thing that comes up from time to time when you ask a sling’s user why it is still on the rack in a clearly damaged state, a typical response might be:
“Oh, well you can’t see the red marker threads, so I thought they were still good…?”
Unbeknownst to some, many synthetic slings can be cut completely in twain and there will be no evidence of the aforementioned “red marker threads” whatsoever.
Backstory: some sling manufacturers would weave a different colored thread into the web sling material as a way to warn users of excess wear/abrasion/etc. The idea was that sling users would be able to more quickly identify a worn out sling by seeing the different colored thread.
So now we can see the issue that I have with this: the manufacturers added a feature to help sling users identify worn web slings and then sling users eventually turned it into a Go/No-Go gauge. This is made even worse by the fact that many, many slings do not have red marker threads of any kind. And even when slings do have them there can be many other reasons to take a synthetic sling out of service before they become visible.
If you’re noticing a trend like this on your worksite inspections follow this link http://www.northernstrands.com/training.aspx and we can discuss how the Northern Strands Below the Hook Rigging Training program could help you and/or your workforce learn where to draw the line and take slings and other rigging out of service before they become a hazard.
Why does rigging need a safety factor?
Have you wondered why rigging experts always suggest a sling that has a significantly higher breaking strength than the actual weight of the load you are lifting? The manufacturers know that the rigging used in overhead applications need to have room for error. This is known as the Safety Factor.
Northern Strands manufactures wire rope slings rated up to 36,000 lbs and sells round synthetic slings that are rated up to 140,000 lb capacity. This capacity is the Working Load Limit of the sling, which is the maximum amount of weight or force that the sling's user is allowed to put on the sling. Note: These slings do not break at the working load limit. These slings are designed with a safety factor of 5:1. This means that 5 times as much force as the working load limit has to be applied to the sling before it potentially fails. This means the wire rope slings have a Breaking Strength of up to 180,000 lbs and the round synthetic slings can withhold up to 700,000 lbs.
Why are these capacities reduced by such a drastic degree?
Among others, here are 3 good reasons:
- Shock Loading - Unexpected drops where the load can accelerate and then must be 'caught' by the slings.
- Wear - Working load limits are based on slings in brand new condition and a safety factor can help account for normal wear and tear until it is deemed unfit for further use.
- Uneven loading - Slings are made up of either wires or fibers that must all share the weight of the load evenly. If any situation arises where the sling is bent or wrapped around an object, there is potential that some of the wires or fibers will be taking on a greater share of the load than others.
Visit Northern Strands website to use the sling tension calculator. The Northern Strands Sling Calculator has been designed to assist you in selecting slings with enough load carrying capacity for your lifting applications. It is your responsibility to assure that the slings you use are appropriate for your application. http://www.northernstrands.com/sling-calculator.aspx
To inquire about Rigging and Safety Factors, contact:
Northern Strands @ 306-242-7073
or Email firstname.lastname@example.org
Northern Strands is proudly Saskatoon, Saskatchewan owned and operated.