What brought about yesterday’s post was that I got my hands on a copy of Harry Donovan‘s excellent book Entertainment Rigging. We’ve started finals, so I can keep it out of the university library for a month or so without much fear of depriving anyone. And within the first few minutes of skimming through the book I learned something.
In rigging, whether for sails, construction lifting, trapeze, or suspending line-array loudspeakers, we use screw-pin shackles. The standard training has been to use the “bell side” towards any rope that could possibly move, because the last thing you want is the some movement causing the pin to unscrew. Here is an example from the DOE Hanford rigging manual:
Improper use of a screw-pin shackle
Now a standard choke hitch that we use over a burlap-padded steel beam to make a point is just such a circumstance. You want the pin in the sling-eye, and the bow around the sling. But I’ve been seeing more and more people do it the other way around — with the theoretically movable rope against the pin. All is made clear in Donovan’s book, however, because if you are an up-rigger on a beam and you have to make the “correct” hitch, it requires two hands, but if you start with the bell already in the eye of the sling you can unscrew the pin, catch the free end in the shackle, and screw the shackle back together using only one hand, leaving one hand for holding on. Falls are the leading cause of casualties in this job, and I’m not aware of any incidents involving loads falling from pins unscrewing in choke hitches, so this seems like a really smart tradeoff.
The choke hitch I’m describing is clear in this video from Rigstar, which also shows why you probably want to use a basket-hitch instead of a choke anyway.
