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post #8 of Old 06-21-2009
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You'll find more strength in a shackle of some sort versus a ring, or steel grommet, if you will.

You should though be aware of some basic cargo gear factors before you attempt the suspension of such a heavy load.

If I understand your goal, it is to suspend a thousand pound weight by way of two lines, one to either side of the weight, and have the lines as nearly horizontal as possible. My first, and facetious, question is; have you ever seen a twelve inch mooring line (circ.)? (g) The non-facetious point is that, when suspending a load between two lines in such a manner one must be conversant with the stresses involved in the geometry of the two lines.

On board ship we'd call each of your lines a "fall". If we were suspending a load of 1000 lb between two falls where the angle between the falls was zero, both vertical, the strain on each fall would be 500 lbs. This is the lowest stress possible on each fall. As we separate the hauling part of each fall, and we'll assume that the fall angle for each is an equal distance off the vertical, the strain on the falls increases. At a fall angle of 90 degrees, each fall 45 degrees off from the vertical, the strain on each fall is 750 lbs. At a fall angle of 120 degrees, each fall 60 degrees off the vertical, the strain on each individual fall would be equal to the load suspended, 1000lbs. That 120 degree fall angle is regarded as the maximum safe working fall angle for cargo operations, btw. Anything after that is regarded as "tightlining", your objective, I believe. The danger in that is that at, say, a 150 degree fall angle, each fall 75 degrees off the vertical, the strain on each fall would be 1900lbs. Note though that the combined forces that you are subjecting the falls to, wherever to which the falls are attached is 3800lbs. As you approach a fall angle of 180 degrees, both falls at 90 degrees from the vertical, you reach a strain on each fall approaching infinity.

These problems can be solve either geometrically or trigonometrically but I lack the equipment to diagram it for you here. You might google the parallelogram method of strain analysis for more detailed information.

I think you'll now appreciate my facetious example. Btw, a 12" mooring line is a 4" diameter line and when constructed of nylon has a breaking strain of around 324,000lbs, and using a safety factor of 5, would give a working load of 64,800lbs. If it was to be contemplated that there might be shock loading, one would use a safety factor of 9, giving it a working load of 36,000lbs. At that fall angle of 150 degrees we'd be able to only support a load of 18,950lbs safely!

(by my quick calculations, to support a load of 1000lbs, you're going to need line with a breaking strength of 14,343lbs to suspend that load with a fall angle of 176 degrees, allowing no safety factor for working load. With a safety factor of 5, that's a line capable of 71,715lbs. 2" diameter nylon should do you.) I'd be looking for some 50 ton shackles and leave the climbing gear out! (g)

“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.
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