How do you rate a sling when using an adjustable hitch?
Question: In your Did You Know videos you show an “adjustable hitch” using a web sling and sling saver shackle. Very useful. My question is how do you rate the sling in this configuration? It seems the lifting capacity is dependent on the friction formed at the shackle so it would be dependent on the load. We also teach that if a sling does not show a capacity on the nametag for a hitch then it cannot be used for that hitch. So, what should we teach if the adjustable hitch isn’t listed on the nametag?
Answer: The WSTDA gives this hitch a vertical capacity rating. However, in my “Rigger’s Reference Handbook” and in the “Did You Know” video http://www.iandisling.com/Did_You_Know/009/DYK_009.htm I recommend using the choker hitch capacity of the sling. I have done several break tests with web slings in this configuration and found that the break without hardware meets a 5:1 from the vertical rating, but barley. In the video I show the uses of a synthetic sling savor shackle, but in my testing since that video a standard anchor shackle works the best. The curvature creates more friction and it will break before running. Once again, this break was barely at a 5:1 from vertical. The synthetic sling saver shackle hook up barley reaches a 5:1 from the choker capacity before it runs, no break. So this our reason for recommending a choker hitch rating for the adjustable hitch.
As far as the hitches used with any particular sling, I don’t believe it is a good practice to limit the hitch used based on the hitches identified on the tag. As per the latest version of ASME B30.9 a sling only needs to have one legible rating on the tag to meet criteria for use. Furthermore the vertical, choker and basket hitches normally shown on most slings are reference points to be use for maximum capacity for that sling in that particular hitch configuration as well as the starting point for calculating sling reduction or increase of capacity for double basket and choker hitches. If the hitches recognized on the sling tag where the only hitches used then a choker hitch would probably be used for a load such as bundled pipe. When in fact the safest hitches would be a double wrap choker or double wrap basket hitch for such a load.
Not to be lost in this discussion, is the fact that some sling types such as wide body web slings and Type 2 web slings are not designed to be used in a choker hitch configuration. In fact a Type 2 web sling cannot be made into a choker hitch if attempted. The key to safe use and capacity verification for riggers, when it comes to sling use, is good positive training on the subject.