How Rope Slings and I Have Changed
by Dennis J. O’Rourke, CSP
Let me take you back a few years to discuss sling changes. Three underlying causes for this evolution are reliability, handleability, and cost. OSHA, in 1971, left sling capacity up to the users. Tables listed; construction of rope, type of Eye end connections, rope diameter, and how used (choker, etc.) for the user to ponder capacity!
The picture above presents a poor rigging practice. The “fold-back” Aluma-Grip fabricated splice forming the sling’s eye is unreliable (yellow arrow) and not recommended for hoisting, but still in use.
Why? When fabricating a fold-back eye splice, 100% of the eye strength is dependent on the wall thickness of the fitting, and its clamping force. These non-steel fittings are softer, and when swaged, they are less likely to hold a uniform wall thickness, thus strength. Examinations show cracks, voids, and wall thickness to a sliver! This method is less expensive but lacks reliability.
The Flemish eye, Molly Hogan, or a Farmer’s eye – all synonyms – are preferred, reliable. To construct, untwist (open) the rope with three strands on one side and three strands plus the core on the opposite. The loop develops (eye) when the “two-sides” are re-formed back together. A sling with a thimble is shown in Fig. 2. Reliable is achieved when the sleeve is swaged up to 300 tons pressure.
Tests show the frictional grip of the strands, under tension, will hold up to 22% of the breaking strength of the rope before failure. Even with no fitting applied, that would equal a 5 to 1 factor of safety. Today, the two-sides are cut off at the “crotch” of the eye. A tapered steel sleeve is slipped over the crotch and crimped to hold the eye, to as much as 90% of the ropes breaking strength, keeping the ends locked.
Back in the day, the two-sides were closed to the crotch; the excess is pulled over the rope and re-laid together, forming a “pigtail.” Fig. 3. Wire rope clips were used to secure the live end to the pigtail dead-end, locking the eye just as the swaged fitting today. Thus, a rigger, on the tailgate of his truck, in 15 minutes, could fabricate a completed “field splice.” A thimble clip-on was available.
A field splice is a term I first heard around the Post Korean Wartime era. It meant precisely that. The sling, being very adaptable, were used for many types of work – in the “field.” A guy would form an eye in a wire rope, when and where it was needed! Those eyes were hand tuck, wire rope clips, wedged sockets, Flemish eyes, and pored zinc spelter sockets. These slings were used to pull equipment, haul logs, tie-down trucks, and lift loads.
We realized, there was little consistency of technique or any quality control procedures in place. The resulting accidents exposed numerous errors. Pulling a tree could tolerate some mistakes (perhaps), but deaths, lifting construction material, was unacceptable. Field-splice became a dirty word!
“The times they are a-changing boys.” And how! The building is faster, higher, heavier, and more costly. People in the field are erecting towers, build skyscrapers, or replacing heat exchangers. It’s just too expensive to be sitting around, whittling away on wood or forming make-shift eye splices from used rope, ah, the good ole days.
So, the “certified sling” with a capacity tag attached to the rope, load tested to 150% of capacity, and the quality control program of the fabricator was born. (But the kids should know, some of us could do it right – out in the field.)
DENNIS J. O’ROURKE, CSP, is the Director of National Crane Services, Inc. He has over fifty years’ experience in the industrial, maritime, and construction fields working with heavy equipment and material handling devices. As a safety engineer, Mr. O’Rourke has developed and/or presented over 300 safety-training programs for all representative elements of government and industry. (firstname.lastname@example.org)