|Topic Review (Newest First)|
|06-22-2009 09:06 PM|
Have a look at the bottom of this page:
Hamilton Marine 2009 iCatalog - page 133
I think the "weldless rings" or the "weldless sling links" might be what you're looking for. 3/8" sling link will handle 1000 # and cost about $7.00
|06-22-2009 07:18 PM|
|sailaway21||Any knot or hitch will part before the line does. Any time you place so much as an overhand knot in a line you weaken it. If memory serves me right you can expect to lose about 50% of the strength of a line with a knot, hitch, or bend. A properly made splice will retain 90% of the line's strength.|
|06-22-2009 07:09 PM|
If you want something that's easily opened, D-shaped screw gate carabiners will be stronger for the same weight that any other shape.
There's a long and interesting history behind the design of carabiners, some of which is covered here:
ATS Resources for the Recreational Canyoneer - Carabiners
Originally Posted by wind_magic View Post
|06-22-2009 06:53 PM|
Thank you all for the great responses.
Sway, the math really helped, thanks for taking the time to write all of that out.
I think I may be okay, the load is actually less than 1000lbs, more like 600lbs, I was being conservative and wanted it to use 1000lbs for a safety margin. Now I see that the working load on the equipment already has a safety margin built into it at 5-to-1 or even 10-to-1 to estimated break, I just hadn't read the fine print and didn't know they were already being conservative. The load won't be lifted more than about 60 degrees from vertical, and the rings I am looking at now are forged steel rated over 3000lbs WLL and are 10-to-1, rated over 30000lbs to break, that should more than do it (famous last words).
Edit - follow-up question, is there a resources somewhere that says what the estimated holding power of various knots and splices, etc, are for lines ? I would assume that the goal in using most knots would be to have the rope break before the knot, but I don't like assuming things, so it would be nice to find out what chances there are of various knots slipping.
Edit #2 - I forgot to say why rings since that might come up, the reason is that I need to attach the load to the rings in multiple places, not with a single line, so I need some surface area to work with and the rings would let me attach the load with a number of lines. I think the shackles might be too cramped though you guys have totally convinced me that is the better route to go if possible.
|06-21-2009 11:31 PM|
|patrickrea||Back in my days of theatre rigging and flying large PA stacks, fabric or wire rope slings were good and quick links were never allowed because of they way they can loosen when vibrating. A shackle is the proper device as you can safety wire them.|
|06-21-2009 11:06 PM|
Solid welded links are probably stronger than screw links, if less convenient. I'd agree with Sway that holding a load up by two essentially horizontal lines is not a wise thing and quite dangerous. The forces involved are quite significant and unless you're really aware of them, you're going to get someone killed. His point about the loads being enormously magnified by the lines being so close to horizontal is one you have to pay attention to.
This is kind of why I was wondering what your purpose for the rings was.
Just for yucks and giggles, take some small stuff, say 1/8" rope and try lifting a 50 lb. weight so that the two cords are horizontal as you've indicated you want to do. I know a 1/8" rope will hold 50 lbs. pretty easily, since the stuff I use has a breaking load of almost 450 lbs. I bet the rope breaks before you can support the 50 lb. weight with the lines almost horizontal.
Originally Posted by wind_magic View Post
|06-21-2009 10:41 PM|
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)
|06-21-2009 10:37 PM|
|jgeissinger||It's just a stainless O ring. WM has them up to about 2 1/2 in. They are used regularly for rope bridles on hoist launched boats such as Lido 14s, CFJs, etc. Sorry but I don't know where you would get a bigger one. Also, as scottyt said, cast is not really stronger. Forged and welded would be my choice.|
|06-21-2009 08:55 PM|
Not sure but maybe what you describe is a "figure 8" they are available made from a variety of materials and sizes.
|06-21-2009 08:22 PM|
Originally Posted by sailingdog View Post
I'm starting to think that screw links like the ones that climbers use might be better able to hold a load instead of something round like a metal ring.
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